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Phytohormones are organic compounds produced in trace amounts within plants that regulate their physiological processes. Their physiological effects are highly complex and diverse. They influence processes ranging from cell division, elongation, and differentiation to plant germination and rooting. Therefore, phytohormones play a crucial regulatory role in plant growth and development. Recently, various studies have highlighted the role of PHs, such as auxin, cytokinin (CK), and abscisic acid (ABA), and newer classes of PHs, such as brassinosteroid (BR) and peptide hormone, in the plant responses toward environmental stresses. These hormones not only have distinct roles at different stages of plant growth but also interact to promote or inhibit each other, thus effectively regulating plant development. Roots are the primary organs for water and mineral absorption in plants. During seed germination, the radicle breaks through the seed coat and grows downward to form the primary root. This occurs because the root needs to quickly penetrate the soil to absorb water and nutrients, providing essential support for the plant’s subsequent growth. Root development is a highly complex and precisely regulated process influenced by various signals. Changes in root architecture can affect the plant’s ability to absorb nutrients and water, which in turn impacts crop yield. Thus, studying the regulation of root development is of great significance. Numerous studies have reported on the role of phytohormones, particularly auxins, in root regulation. This paper reviews recent studies on the regulation of root development by various phytohormones, both individually and in combination, providing a reference for researchers in this field and offering perspectives on future research directions for improving crop yields.

In vitro cultivation of Melaleuca could contribute to the cloning of superior genotypes. Studies of factors affecting micropropagation are needed, such as the interaction with light-emitting diodes (LEDs) and plant growth regulators added to the culture media. This study aimed at better understanding the effects of spectra on the development and physiology of melaleuca cultivated in vitro, as well as the interaction of LEDs with the main cytokinin used in micropropagation, N6-Benzylaminopurine (6-BAP). 6-BAP, spectra, and their interaction had a significant effect on most of the variables analyzed, altering the in vitro development and chlorophyll concentrations in the plants, as well as changing different variables in the culture medium, such as pH, EC, and levels of Ca2+, Mg2+, and P, and nutrient accumulation in the shoots. The results demonstrate that the main effects of adding BAP to the in vitro cultivation of melaleuca are an increase in the number of shoots, which resulted in greater fresh and dry masses; a reduction in height and chlorophyll content; complete inhibition of adventitious rooting; higher consumption of Mg, and lower consumption of Ca and P from the culture medium; higher content of Fe, and lower content of P, S, Mn, Cu and B in the in vitro shoot tissues.

This study addresses the propagation challenges faced by ‘Shine Muscat’, a newly introduced premium grapevine cultivar in South Korea, where multiple viral infections pose considerable economic loss. The primary objective was to establish a robust in vitro propagation method for producing disease-free grapes and to identify effective plant growth regulators to facilitate large-scale mass cultivation. After experimentation, 2.0 µM 6-benzyladenine (BA) exhibited superior shoot formation in the Murashige and Skoog medium compared with kinetin and thidiazuron. Conversely, α-naphthaleneacetic acid (NAA) hindered shoot growth and induced callus formation, while indole-3-butyric acid (IBA) and indole-3-acetic acid (IAA) demonstrated favorable root formation, with IBA showing better results overall. Furthermore, inter simple sequence repeat analysis confirmed the genetic stability of in vitro-cultivated seedlings using 2.0 μM BA and 1.0 μM IBA, validating the suitability of the developed propagation method for generating disease-free ‘Shine Muscat’ grapes. These findings offer promising prospects for commercial grape cultivation, ensuring a consistent supply of healthy grapes in the market.

Blossom end enlargement (BEE) is a postharvest deformation that may be related to the influx of photosynthetic assimilates before harvest. To elucidate the mechanism by which BEE occurs, expression marker genes that indicate the physiological condition of BEE-symptomatic fruit are necessary. First, we discovered that preharvest treatment with a synthetic cytokinin, N-(2-Chloro-4-pyridyl)-N’-phenylurea (CPPU), promoted fruit growth and suppressed BEE occurrence. This suggests that excessive assimilate influx is not a main cause of BEE occurrence. Subsequently, the expression levels of seven sugar-starvation marker genes, CsSEF1, AS, CsFDI1, CsPID, CsFUL1, CsETR1, and CsERF1B, were compared among symptomatic and asymptomatic fruits, combined with and without CPPU treatment. Only CsSEF1 showed a higher expression level in asymptomatic fruits than in symptomatic fruits, regardless of CPPU treatment. This was then tested using fruits stored via the modified-atmosphere packaging technique, which resulted in a lower occurrence of BEE, and the asymptomatic fruits showed a higher CsSEF1 expression level than symptomatic fruits, regardless of the packaging method. CsSEF1 codes a CCCH-type zinc finger protein, and an increase in the expression of CsSEF1 was correlated with a decrease in the fruit respiration rate. Thus, CsSEF1 may be usable as a BEE expression marker gene.

Some citrus orchards in China often experience nitrogen (N) deficiency. For the first time, targeted metabolomics was used to examine N-deficient effects on hormones in sweet orange (Citrus sinensis (L.) Osbeck cv. Xuegan) leaves and roots. The purpose was to validate the hypothesis that hormones play a role in N deficiency tolerance by regulating root/shoot dry weight ratio (R/S), root system architecture (RSA), and leaf and root senescence. N deficiency-induced decreases in gibberellins and indole-3-acetic acid (IAA) levels and increases in cis(+)-12-oxophytodienoic acid (OPDA) levels, ethylene production, and salicylic acid (SA) biosynthesis might contribute to reduced growth and accelerated senescence in leaves. The increased ethylene formation in N-deficient leaves might be caused by increased 1-aminocyclopropanecarboxylic acid and OPDA and decreased abscisic acid (ABA). N deficiency increased R/S, altered RSA, and delayed root senescence by lowering cytokinins, jasmonic acid, OPDA, and ABA levels and ethylene and SA biosynthesis, increasing 5-deoxystrigol levels, and maintaining IAA and gibberellin homeostasis. The unchanged IAA concentration in N-deficient roots involved increased leaf-to-root IAA transport. The different responses of leaf and root hormones to N deficiency might be involved in the regulation of R/S, RSA, and leaf and root senescence, thus improving N use efficiency, N remobilization efficiency, and the ability to acquire N, and hence conferring N deficiency tolerance.

Here, cytosine methylation in the whole genome of pear flower buds was mapped at a single-base resolution. There was 19.4% methylation across all sequenced C sites in the Pyrus pyrifolia cultivar ‘Sucui 1’ flower bud genome. Meantime, the CG, CHG, and CHH sequence contexts (where H = A, T or C) exhibited 47.4%, 33.3%, and 11.9% methylation, respectively. Methylation in different gene regions was revealed through combining methylome and transcriptome analysis, which presented various transcription trends. Genes with methylated promoters exhibited lower expression levels than genes with non-methylated promoters, while body-methylated genes displayed an obvious negative correlation with their transcription levels. The methylation profiles of auxin- and cytokinin-related genes were estimated. And some of them proved to be hypomethylated, with increased transcription levels, in wizened buds. More specifically, the expression of the genes PRXP73, CYP749A22, and CYP82A3 was upregulated as a result of methylation changes in their promoters. Finally, auxin and cytokinin concentrations were higher in wizened flower buds than in normal buds. The exogenous application of paclobutrazol (PP333) in the field influenced the DNA methylation status of some genes and changed their expression level, reducing the proportion of wizened flower buds in a concentration-dependent manner. Overall, our results demonstrated the relationship between DNA methylation and gene expression in wizened flower buds of P. pyrifolia cultivar ‘Sucui 1’, which was associated with changes in auxin and cytokinin concentrations.

In vitro mass propagation of apple plants plays an important role in the rapid multiplication of genetically uniform, disease-free scions and rootstocks with desired traits. Successful micropropagation of apple using axillary shoot cultures is influenced by several factors, the most critical of which is the cytokinin included in the culture medium. The impact of medium composition from single added cytokinins on shoot proliferation of apple scion Húsvéti rozmaring cultured on agar-agar gelled Murashige and Skoog medium fortified with indole butyric acid and gibberellic acid was investigated. The optimum concentration for efficient shoot multiplication differs according to the type of cytokinin. The highest significant multiplication rate (5.40 shoots/explant) was achieved using 2.0 μM thidiazuron while the longest shoots (1.80 cm) were observed on the medium containing benzyladenine at a concentration of 2.0 μM. However, application of either thidiazuron or benzyladenine as cytokinin source in the medium resulted in shoots of low quality, such as stunted and thickened shoots with small leaves. In the case of benzyladenine riboside, the 8 μM concentration was the most effective in increasing the multiplication rate (4.76 shoots/explant) but caused thickened stem development with tiny leaves. In the present study, meta-topolin was shown to be the most effective cytokinin that could be applied to induce sufficient multiplication (3.28 shoots/explant) and high-quality shoots along with shoot lengths of 1.46 cm when it was applied at concentrations of 4 μM. However, kinetin was the least active cytokinin; it practically did not induce the development of new shoots. The superior cytokinin for in vitro axillary shoot development of apple scion Húsvéti rozmaring with high-quality shoots was the meta-topolin, but it may be different depending on the variety/genotype under study.

Defoliation is an inevitable abiotic stress for forage and turf grasses because harvesting, grazing, and mowing are general processes for their production and management. Vegetative regrowth occurs upon defoliation, a crucial trait determining the productivity and persistence of these grasses. However, the information about the molecular regulation of this trait is limited because it is still challenging to perform molecular analyses in forage and turf grasses. Here, we used rice as a model to investigate vegetative regrowth upon defoliation at physiological and molecular levels. This study analyzed stubble and regrown leaves following periodic defoliation using two rice varieties with contrasting regrowth vigor. Vigorous regrowth was associated with maintained chlorophyll content and photosystem II performance; a restricted and promoted mRNA accumulation of sucrose synthase (SUS) I and III subfamilies, respectively; and reduced enzymatic activity of SUS. These results suggest that critical factors affecting vegetative regrowth upon defoliation are de novo carbohydrate synthesis by newly emerged leaves and proper carbohydrate management in leaves and stubble. Physiological and genetic analyses have demonstrated that the reduced sensitivity to and inhibited biosynthesis of cytokinin enhance regrowth vigor. Proper regulation of these metabolic and hormonal pathways identified in this study can lead to the development of new grass varieties with enhanced regrowth vigor following defoliation.

Phytohormones, particularly cytokinin trans-zeatin (tZ), were studied for their impact on the green alga Desmodesmus armatus under cadmium (Cd) stress, focusing on growth, metal accumulation, and stress response mechanisms. Using atomic absorption spectroscopy for the Cd level and high-performance liquid chromatography for photosynthetic pigments and phytochelatins, along with spectrophotometry for antioxidants and liquid chromatography–mass spectrometry for phytohormones, we found that tZ enhances Cd uptake in D. armatus, potentially improving phycoremediation of aquatic environments. Cytokinin mitigates Cd toxicity by regulating internal phytohormone levels and activating metal tolerance pathways, increasing phytochelatin synthase activity and phytochelatin accumulation essential for Cd sequestration. Treatment with tZ and Cd also resulted in increased cell proliferation, photosynthetic pigment and antioxidant levels, and antioxidant enzyme activities, reducing oxidative stress. This suggests that cytokinin-mediated mechanisms in D. armatus enhance its capacity for Cd uptake and tolerance, offering promising avenues for more effective aquatic phycoremediation techniques.

Numerous biotechnological applications require a fast and efficient clonal propagation of whole plants under controlled laboratory conditions. For most plant species, the de novo regeneration of shoots from the cuttings of various plant organs can be obtained on nutrient media supplemented with plant hormones, auxin and cytokinin. While auxin is needed during the early stages of the process that include the establishment of pluripotent primordia and the subsequent acquisition of organogenic competence, cytokinin-supplemented media are required to induce these primordia to differentiate into developing shoots. The perception of cytokinin through the receptor ARABIDOPSIS HISTIDINE KINASE4 (AHK4) is crucial for the activation of the two main regulators of the establishment and maintenance of shoot apical meristems (SAMs): SHOOTMERISTEMLESS (STM) and the WUSCHEL-CLAVATA3 (WUS-CLV3) regulatory circuit. In this review, we summarize the current knowledge of the roles of the cytokinin signaling cascade in the perception and transduction of signals that are crucial for the de novo establishment of SAMs and lead to the desired biotechnological output—adventitious shoot multiplication. We highlight the functional differences between individual members of the multigene families involved in cytokinin signal transduction, and demonstrate how complex genetic regulation can be achieved through functional specialization of individual gene family members.

Cucumber (Cucumis sativus L.) lateral branch elongation is influenced by a variety of environmental signals, including light [e.g., far-red (FR) light] and hormones. In this experiment, the effect of FR light on the lateral branch elongation of cucumber (‘Zhongnong No. 26’) seedlings was investigated. The results showed that FR light significantly inhibited the lateral branch elongation of cucumber seedlings. In addition, FR light significantly increased the auxin (indole-3-acetic acid, IAA) content, decreased the cytokinin (CTK; Zeatin) content, and suppressed the expression of most CTK synthetic-related genes, such as IPTs, in cucumber seedlings. The lateral branch elongation of cucumber seedlings was assessed in response to decapitation and exogenous 6-BA treatment to further investigate the relationship between IAA and CTK on the lateral branch elongation of cucumber seedlings under FR light. Both decapitation and exogenous 6-BA treatment eliminated the inhibitory effect of FR light on the lateral branch elongation of cucumber seedlings. In conclusion, these results indicated that IAA and CTK were involved in the regulatory effects of FR light on cucumber seedling lateral branch elongation.

Cassava breeding faces obstacles due to late flowering and poor flower and seed set. The acceleration of breeding processes and the reduction in each cycle’s duration hinge upon efficiently conducting crosses to yield ample progeny for subsequent cycles. Our primary objective was to identify methods that provide tools for cassava breeding programs, enabling them to consistently and rapidly generate offspring from a wide array of genotypes. In greenhouse trials, we examined the effects of the anti-ethylene silver thiosulfate (STS) and the cytokinin benzyladenine (BA). STS, administered via petiole infusion, and BA, applied as an apical spray, combined with the pruning of young branches, significantly augmented the number of flowers. Controls produced no flowers, whereas treatments with pruning plus either BA or STS alone produced an average maximum of 86 flowers per plant, and the combination of pruning, BA and STS yielded 168 flowers per plant. While STS had its primary effect on flower numbers, BA increased the fraction of female flowers from less than 20% to ≥87%, thus increasing the number of progeny from desired parents. Through field studies, we devised an optimal protocol that maintained acceptable levels of phytodamage ratings while substantially increasing seed production per plant compared to untreated plants. This protocol involves adjusting the dosage and timing of treatments to accommodate genotypic variations. As a result, cassava breeding programs can effectively leverage a diverse range of germplasm to develop cultivars with the desired traits.

In Eastern Europe, the traditional marketing of cyclamen is in the period from the middle of February to the middle of March. The poor light of winter months and the higher number of plants (because of the reduction in heating costs) often result in elongated plants. To avoid this, it is recommended to use plant growth retardants. At the same time, another problem is that flowers do not rise from the level of rosette due to the unfavorable cultivation conditions. This can be solved with growth stimulants. In the experiment, we tested the effect of growth regulators on the growth of a frequently used variety of cyclamen. We used daminozide and paclobutrazol plant growth retardants for height control and gibberellic acid (GA) and benzyladenine (BAP) as growth promoters for increasing the number of flowers and the length of stems. The results show that daminozide and paclobutrazol are both effective for height control in cyclamen production.

A micropropagation protocol for growing Clematis ‘Warszawska Nike’ was developed. The MS medium supplemented with 1 mg∙dm−3 BAP showed good results in the case of microshoot initiation (80%). The addition of BAP to the medium at higher concentrations resulted in the formation of a large amount of callus tissue at the base of the explant. Of the explants growing on the medium with the lowest cytokinin concentration, 8% flowered. Very quickly, after just 14 days, the explants began to die: some of the leaves that developed in in vitro cultures began to turn yellow and wither. The propagation of shoots was performed in two steps. In the first step, cytokinin BAP and Kin in various concentrations (0.5–2 mg∙dm−3) were added to the MS medium. In the second step, MS medium with the combinations of BAP (0.5 and 1 mg∙dm−3) with IAA or GA3 (1 and 2 mg∙dm−3) was used. The MS medium with 0.5 mg∙dm−3 BAP and 2 mg∙dm−3 GA3 was the best medium for the multiplication stage of clematis. Plants growing on this medium had the largest number of leaves, shoots, and internodes, and were also heavier compared to plants propagated on other media. The proliferated clematis explants were rooted on MS medium with the addition of IAA or IBA in different concentrations (0.5 to 4 mg∙dm−3). Of the concentrations tested, 0.5 mg∙dm−3 IAA was the most effective one for in vitro root induction. The highest percentage of acclimatized plants (75%) was observed when the shoots were rooted on MS medium with 0.5 mg∙dm−3 IAA.

Tillering is an important part in strawberry growth, and strawberries can reproduce nutritionally through stolons to generate genetically stable offspring. However, excessive tillering during the fruit-growing stage can negatively impact fruit yield and quality. In this study, different concentrations of exogenous rac-GR24 (GR24) are used to treat the strawberry plants. It was found that GR24 effectively inhibited the sprouting of strawberry stolons, while promoting the growth of the stems and leaves. Among the treatments, the most effective concentration was found to be 5 μmol/L GR24. This treatment resulted in a decrease in the glucose content in the strawberry crowns and also caused changes in the contents of two endogenous phytohormones, gibberellic acid (GA3) and trans-zeatin riboside (tZR). Transcriptome data further suggested that exogenous GR24 may inhibit strawberry plant tillering by affecting various phytohormone signaling pathways and the sugar metabolism pathway. In 5 μmol/L GR24-treated plants, the expression level of type-B response regulator (B-ARR) was down-regulated and the expression level of CYTOKININ RESPONSE 1 (CRE1), histidine-containing phosphotransfer protein (AHP), and type-A response regulator (A-ARR) were up-regulated, suggesting the inhibition of the cytokinin (CTK) signaling pathway. The down-regulation of auxin (AUX) and auxin response factor (ARF), as well as the up-regulation of auxin/indole-3-acetic acid (AUX/IAA), led to the inhibition of the indole-3-acetic acid (IAA) signaling pathway. Additionally, the up-regulation of pyrabactin resistance 1/ pyrabactin resistance 1-like (PYR/PYL), non-fermenting 1-related protein kinase 2 (SnRK2), and ABRE binding factors (ABF) and the down-regulation of protein phosphatase 2C (PP2C) were observed in the up-regulated abscisic acid (ABA) signaling pathways. In the sugar metabolism pathway, the up-regulation of invertase (INV), hexokinase (HK), and fructokinase (FRK) and the down-regulation of trehalase (TREH) and beta-amylase (BMY) led to a decreased glucose synthesis and an increased glucose consumption. Therefore, GR24 can effectively inhibit strawberry plant tillering through these pathways, making it an effective reagent for tillering inhibition.

In potatoes, tuber secondary growth, especially sprouting, deforms the tubers and severely lowers their commercial value. Tuber sprouting is induced by signal substances, such as gibberellin (GA), which are transported to the tuber from the plant body. The molecular mechanism underlying GA-induced sprouting remains ambiguous. Here, we tried to recreate tuber secondary growth using in vitro stemmed microtubers (MTs) (with the nodal stem attached) and MT halves (with the nodal stem entirely removed). Our experiments showed that GA alone could initiate the sprouting of stemmed microtubers; however, GA failed to initiate MT halves unless 6-benzyladenine, a synthetic cytokinin CK, was co-applied. Here, we analyzed the transcriptional profiles of sprouting buds using these in vitro MTs. RNA-seq analysis revealed a downregulation of cytokinin-activated signaling but an upregulation of the “Zeatin biosynthesis” pathway, as shown by increased expression of CYP735A, CISZOG, and UGT85A1 in sprouting buds; additionally, the upregulation of genes, such as IAA15, IAA22, and SAUR50, associated with auxin-activated signaling and one abscisic acid (ABA) negative regulator, PLY4, plays a vital role during sprouting growth. Our findings indicate that the role of the nodal stem is synonymous with CK in sprouting growth, suggesting that CK signaling and homeostasis are critical to supporting GA-induced sprouting. To effectively control tuber sprouting, more effort is required to be devoted to these critical genes.

Somatic embryogenesis (SE) has many applications in grapevine biotechnology including micropropagation, eradicating viral infections from infected cultivars, mass production of hypocotyl explants for micrografting, as a continuous source for haploid and doubled haploid plants, and for germplasm conservation. It is so far the only pathway for the genetic modification of grapevines through transformation. The single-cell origin of somatic embryos makes them an ideal explant for mutation breeding as the resulting mutants will be chimera-free. In the present research, two combinations of plant growth regulators and different explants from flower buds at two stages of maturity were tested in regard to the efficiency of callusing and embryo formation from the callus produced in three white grape cultivars. Also, the treatment of somatic embryos with the chemical mutagen ethyl methanesulfonate (EMS) was optimised. Medium 2339 supplemented with β-naphthoxyacetic acid (5 μM) and 6-benzylaminopurine (BAP—9.0 μM) produced significantly more calluses than medium 2337 supplemented with 2,4-dichlorophenoxyacetic acid (4.5 µM) and BAP (8.9 µM) in all explants. The calluses produced on medium 2337 were harder and more granular and produced more SEs. Although the stage of the maturity of floral bud did not have a significant effect on the callusing of the explants, calluses produced from immature floral bud explants in the premeiotic stage produced significantly more SEs than those from more mature floral buds. Overall, immature ovaries and cut floral buds exposing the cut ends of filaments, style, etc., tested for the first time in grapevine SE, produced the highest percentage of embryogenic calluses. It is much more efficient to cut the floral bud and culture than previously reported explants such as anthers, ovaries, stigmas and styles during the short flowering period when the immature flower buds are available. When the somatic embryos of the three cultivars were incubated for one hour with 0.1% EMS, their germination was reduced by 50%; an ideal treatment considered to obtain a high frequency of mutations for screening. Our research findings will facilitate more efficient SE induction in grapevines and inducing mutations for improving individual traits without altering the genetic background of the cultivar.

Lemongrass (Cymbopogon citratus) essential oil (EO) is a major source of bioactive compounds (BC) with anticancer activity such as α-citral, limonene, geraniol, geranyl acetate, and β-caryophyllene. Comparative studies about cytokinin effects on BC profiles in lemongrass are missing. Here, we evaluated four cytokinins (2iP, tZ, BAP, and KIN) in two different osmotic media, MS-N (3% sucrose, 3 g L−1 Gelrite™) and MS-S (5% sucrose, 5 g L−1 Gelrite™). It results in a higher multiplication rate in BAP containing medium compared to tZ, KIN, and 2iP (p ≤ 0.05). While shoots grown on MS-N/BAP, tZ, and KIN exhibited a highly branching morphology, MS-N/2iP produced a less branching architecture. BC profile analysis of established plants in pots revealed that their maxima production depends on the in vitro shoot growth conditions: i.e., highest content (80%) of α-citral in plants that were cultured in MS-S/BAP (p ≤ 0.05), limonene (41%) in MS-N/2iP, or geranyl acetate (25.79%) in MS-S/2iP. These results indicate that it is possible to increase or address the production of BC in lemongrass by manipulating the cytokinin type and osmotic pressure in culture media. The culture protocol described here is currently successfully applied for somatic embryogenesis induction and genetic transformation in lemongrass.

Poa pratensis is widely distributed in cold temperate regions and can be used as a species for stress restoration and as a forage for livestock. Studying the genetic characteristics of tillering occurrence in bluegrass provides a theoretical basis for studying plant yield formation, environmental adaptation, and improving survival competitiveness. The regulating effects of endogenous hormone IAA content and the expression of related genes ARF1, ARF12, ARF14, ZT content and the expression of related genes CKX2, CKX3, CKX4, SL content and the expression of related genes D14-like, D14.1-like and D14 in wild Kentucky bluegrass were investigated. Kentucky bluegrass from Sunan and Qingshui was used to evaluate the influence of hormone and gene expression on tillering behavior. Endogenous hormone contents and expression levels of related genes in stems and roots of both materials were measured at prophase, peak, and anaphase of tillering. The results showed that among the three materials, the Sunan material had a better tillering ability for Poa pratensis, while the Qingshui material had poorer tillering ability. The downregulation of CKX2, CKX3, and CKX4 gene expression levels promotes the synthesis of ZT, thereby improving the tillering ability of the germplasm. Upregulation of ARF14, D14, and D14.1-like gene expression levels enhances the synthesis of IAA and SL, thereby inhibiting tillering. More importantly, the interaction between hormones affects the tillering ability of bluegrass, and high levels of ZT/IAA, ZT/SL, and ZT/(IAA+SL) values promote tillering. In summary, this study reveals the mechanism by which hormones regulate the occurrence of tillering in Kentucky bluegrass, providing a theoretical basis for understanding the genetic characteristics of plant type, effectively regulating tillering, studying yield development, environmental adaptation, and improving survival rate.

Zelkova schneideriana is a fast-growing tree species endemic to China. Recent surveys and reports have highlighted a continued decline in its natural populations; therefore, it is included in the Red List of Threatened Species by The International Union for Conservation of Nature. A new variety “HenTianGao” (H) has been developed with smaller plant height, slow growth, and lower branching points. In this study, we attempted to understand the differences in plant height of Z. schneideriana (J) and its dwarf variety H. We determined the endogenous hormone content in the annual grafted branches of both J and H. J exhibited higher gibberellic acid (GA)-19 and trans-Zeatin (tZ) levels, whereas H had higher levels of indole-3-acetic acid (IAA) catabolite 2-oxindole-3-acetic acid (OxIAA), IAA-Glu conjugate, and jasmonic acid (JA) (and its conjugate JA-Ile). The transcriptome comparison showed differential regulation of 20,944 genes enriched in growth and development, signaling, and metabolism-related pathways. The results show that the differential phytohormone level (IAA, JA, tZ, and GA) was consistent with the expression of the genes associated with their biosynthesis. The differences in relative OxIAA, IAA-Glu, GA19, trans-Zeatin, JA, and JA-Ile levels were linked to changes in respective signaling-related genes. We also observed significant differences in the expression of cell size, number, proliferation, cell wall biosynthesis, and remodeling-related genes in J and H. The differences in relative endogenous hormone levels, expression of biosynthesis, and signaling genes provide a theoretical basis for understanding the plant height differences in Z. schneideriana.

Heterotrophic ammonia-oxidizing bacteria (HAOB), crucial for soil nitrification, have unclear benefits for crop water use. This study explored the impact of a novel HAOB strain, S2_8_1, on maize drought resilience via pot culturing. The experiment included various treatments: control with sufficient water (CK), sufficient water + HAOB strain (WI), limited rewatering (DL), sufficient rewatering (DH), sufficient rewatering + HAOB strain (DHI), and limited rewatering + HAOB strain (DLI). The results revealed below-compensatory growth with DL compared to CK. Interestingly, the HAOB strain displayed survival resilience with a 96% increase in its copy numbers in the rhizosphere soils compared to CK during rewatering. The DLI treatment exhibited equal to compensatory growth, showing a remarkable 169% surge in the water use efficiency versus CK. This improvement was attributed to heightened rhizosphere soil nitrification by HAOB, enhancing the cytokinin production in roots and its transference to leaves, leading to a 25% higher leaf cytokinin concentration with DLI compared to CK during rewatering. Additionally, HAOB DHI prompted overcompensatory growth after sufficient rewatering, boosting nitrification and facilitating cytokinin root-to-leaf transport. However, its water use efficiency was 39% lower than DLI. The study highlights HAOB’s importance in optimizing crop water use, particularly in scenarios of limited rewatering in cropland soils.

Cytokinins play a relevant role in flower and fruit development and plant yield. Strawberry fruits have a high commercial value, although what is known as the “fruit” is not a “true” botanical fruit because it develops from a non-reproductive organ (receptacle) on which the true botanical fruits (achenes) are found. Given cytokinins’ roles in botanical fruits, it is important to understand their participation in the development of a non-botanical or accessory “fruit”. Therefore, in this work, the role of cytokinin in strawberry flowers and fruits was investigated by identifying and exploring the expression of homologous genes for different families that participate in the pathway, through publicly available genomic and expression data analyses. Next, trans-zeatin content in developing flowers and receptacles was determined. A high concentration was observed in flower buds and at anthesis and decreased as the fruit approached maturity. Moreover, the spatio-temporal expression pattern of selected CKX genes was evaluated and detected in receptacles at pre-anthesis stages. The results point to an important role and effect of cytokinins in flower and receptacle development, which is valuable both from a biological point of view and to improve yield and the quality of this fruit.

Early in the history of cytokinins, it was clear that Zea mays seeds contained not just trans-zeatin, but its nucleosides and nucleotides. Subsequently, both pods and seeds of legumes and cereal grains have been shown to contain a complex of cytokinin forms. Relative to the very high quantities of cytokinin detected in developing seeds, only a limited amount appears to have been translocated from the parent plant. Translocation experiments, and the detection of high levels of endogenous cytokinin in the maternal seed coat tissues of legumes, indicates that cytokinin does not readily cross the maternal/filial boundary, indicating that the filial tissues are autonomous for cytokinin biosynthesis. Within the seed, trans-zeatin plays a key role in sink establishment and it may also contribute to sink strength. The roles, if any, of the other biologically active forms of cytokinin (cis-zeatin, dihydrozeatin and isopentenyladenine) remain to be elucidated. The recent identification of genes coding for the enzyme that leads to the biosynthesis of trans-zeatin in rice (OsCYP735A3 and 4), and the identification of a gene coding for an enzyme (CPN1) that converts trans-zeatin riboside to trans-zeatin in the apoplast, further cements the key role played by trans-zeatin in plants.

The aim of the study was to examine the effects of application of plant-growth regulators (PGR) on the growth of paulownia plants and evaluate their usefulness in paulownia nurseries. The experiment was carried out on the paulownia interspecific hybrid (Paulownia elongata × Paulownia fortunei) ‘Oxytree’. Micropropagated plants grown in pots were sprayed with PGR solutions. Gibberellins (GA3, GA4+GA7), 6-benzyladenine (BA), triclopyr (TPA), ethephon (ETH), daminozide (DA), prohexadione-Ca (PH), and trinexapac-ethyl (TE) were used. They were included in commercial products (Florgib 245 mg dm−3, Gibb plus 11 SL 5 cm3 dm−3, Globaryll 100 SL 1.5 cm3 dm−3, Topper 10 ST 200 mg dm−3, Agrostym 480 SL 2 cm3 dm−3, B-Nine 85 SG 2 g dm−3, Regalis plus 10 WG 2.5 g dm−3, Moddus 250 EC 1 cm3 dm−3, respectively). None of the studied preparations modified positive correlations between the longitudinal vs. transverse growth of stems and growth of leaves. Not only GA3, GA4+GA7, but also BA (cytokinin) and unexpectedly TE (retardant) stimulated stem elongation and thickening. Furthermore, the effect of TE lasted longer than influence of GAx and BA. Leaves of TE-treated plants were distinguished by a higher chlorophyll a/b ratio, and a lower relative chlorophyll content and efficiency of photosystem II (Fv/Fm, Fv/F0). TPA strongly deformed the stems and leaves of liners. Other regulators (ETH, DA, PH) retarded the diameter growth of stems and leaf expansion. Some results obtained by GAx, BA, and especially by TE treatment, might be beneficial for paulownia grown as ornamental and/or timber tree.

Phytohormones are organic compounds produced in trace amounts within plants that regulate their physiological processes. Their physiological effects are highly complex and diverse. They influence processes ranging from cell division, elongation, and differentiation to plant germination and rooting. Therefore, phytohormones play a crucial regulatory role in plant growth and development. Recently, various studies have highlighted the role of PHs, such as auxin, cytokinin (CK), and abscisic acid (ABA), and newer classes of PHs, such as brassinosteroid (BR) and peptide hormone, in the plant responses toward environmental stresses. These hormones not only have distinct roles at different stages of plant growth but also interact to promote or inhibit each other, thus effectively regulating plant development. Roots are the primary organs for water and mineral absorption in plants. During seed germination, the radicle breaks through the seed coat and grows downward to form the primary root. This occurs because the root needs to quickly penetrate the soil to absorb water and nutrients, providing essential support for the plant's subsequent growth. Root development is a highly complex and precisely regulated process influenced by various signals. Changes in root architecture can affect the plant's ability to absorb nutrients and water, which in turn impacts crop yield. Thus, studying the regulation of root development is of great significance. Numerous studies have reported on the role of phytohormones, particularly auxins, in root regulation. This paper reviews recent studies on the regulation of root development by various phytohormones, both individually and in combination, providing a reference for researchers in this field and offering perspectives on future research directions for improving crop yields.

In vitro cultivation of Melaleuca could contribute to the cloning of superior genotypes. Studies of factors affecting micropropagation are needed, such as the interaction with light-emitting diodes (LEDs) and plant growth regulators added to the culture media. This study aimed at better understanding the effects of spectra on the development and physiology of melaleuca cultivated in vitro, as well as the interaction of LEDs with the main cytokinin used in micropropagation, N6-Benzylaminopurine (6-BAP). 6-BAP, spectra, and their interaction had a significant effect on most of the variables analyzed, altering the in vitro development and chlorophyll concentrations in the plants, as well as changing different variables in the culture medium, such as pH, EC, and levels of Ca 2+ , Mg 2+ , and P, and nutrient accumulation in the shoots. The results demonstrate that the main effects of adding BAP to the in vitro cultivation of melaleuca are an increase in the number of shoots, which resulted in greater fresh and dry masses; a reduction in height and chlorophyll content; complete inhibition of adventitious rooting; higher consumption of Mg, and lower consumption of Ca and P from the culture medium; higher content of Fe, and lower content of P, S, Mn, Cu and B in the in vitro shoot tissues.

Cultivated tomato (Solanum lycopersicum L.) is one of the most important horticultural crops in the world. The optimization of culture media for callus formation and tissue regeneration of different tomato genotypes presents numerous biotechnological applications. In this work, we have analyzed the effect of different concentrations of zeatin and indole-3-acetic acid on the regeneration of cotyledon explants in tomato cultivars M82 and Micro-Tom. We evaluated regeneration parameters such as the percentage of callus formation and the area of callus formed, as well as the initiation percentage and the number of adventitious shoots. The best hormone combination produced shoot-like structures after 2–3 weeks. We observed the formation of leaf primordia from these structures after about 3–4 weeks. Upon transferring the regenerating micro-stems to a defined growth medium, it was possible to obtain whole plantlets between 4 and 6 weeks. This hormone combination was applied to other genotypes of S. lycopersicum, including commercial varieties and ancestral tomato varieties. Our method is suitable for obtaining many plantlets of different tomato genotypes from cotyledon explants in a very short time, with direct applications for plant transformation, use of gene editing techniques, and vegetative propagation of elite cultivars.

Plant organ growth results from the combined activity of cell division and cell expansion. The co-ordination of these two processes depends on the interplay between multiple hormones that determine the final organ size. Using the semidominant Hairy Sheath Frayed1 (Hsf1) maize mutant that hypersignals the perception of cytokinin (CK), we show that CK can reduce leaf size and growth rate by decreasing cell division. Linked to CK hypersignaling, the Hsf1 mutant has an increased jasmonic acid (JA) content, a hormone that can inhibit cell division. The treatment of wild-type seedlings with exogenous JA reduces maize leaf size and growth rate, while JA-deficient maize mutants have increased leaf size and growth rate. Expression analysis revealed the increased transcript accumulation of several JA pathway genes in the Hsf1 leaf growth zone. A transient treatment of growing wild-type maize shoots with exogenous CK also induced the expression of JA biosynthetic genes, although this effect was blocked by the co-treatment with cycloheximide. Together, our results suggest that CK can promote JA accumulation, possibly through the increased expression of specific JA pathway genes.

Biological modularity refers to the organization of living systems into separate functional units that interact in different combinations to promote individual well-being and species survival. Modularity provides a framework for generating and selecting variations that can lead to adaptive evolution. While the exact mechanisms underlying the evolution of modularity are still being explored, it is believed that the pressure of conflicting demands on limited resources is a primary selection force. One prominent example of conflicting demands is the trade-off between survival and reproduction. In this review, we explore the available evidence regarding the modularity of plant hormones within the context of the survival-reproduction trade-off. Our findings reveal that the cytokinin module is dedicated to maximizing reproduction, while the remaining hormone modules function to ensure reproduction. The signaling mechanisms of these hormone modules reflect their roles in this survival-reproduction trade-off. While the cytokinin response pathway exhibits a sequence of activation events that aligns with the developmental robustness expected from a hormone focused on reproduction, the remaining hormone modules employ double-negative signaling mechanisms, which reflects the necessity to prevent the excessive allocation of resources to survival.

The vegetative propagation of hops, despite being a reliable method, is not very common due to the unavailability of the plant material. In this study, the technique of in vitro propagation was applied to the Cascade variety of Humulus lupulus L. The plant material was collected from a private field in Sicily; the explants were subjected to sterilization before in vitro culture. Single-node explants were placed in in vitro culture in nine different culture media for multiplication. Thidiazuron (TDZ), Benzyladenine (BAP) and meta-Topoline (mT) were tested for multiplication phase. For the rooting phase, five types of different culture media were evaluated. Binodal cuttings coming from the previous multiplication test were placed in the culture. The rooting media differ from each other in the concentration and ratio of two auxin hormones: Indolo-3-acetic acid (IAA) and Indole-3-butyric acid (IBA). In vitro rooted plants obtained from the rooting phase were transferred to ex vitro conditions in a microbox with agri-perlite and a solution containing Murashige and Skoog (MS) basal medium at half concentration. With a culture medium containing the highest TDZ doses (H6) and combination with cytokinin (H8 and H9), the highest shoot percentage was obtained. After 3 months of in vitro culture, the highest shoot percentage was observed in the culture medium with 2 mL L−1 of BAP. The highest rooting percentage, roots numbers and root length were found when the culture medium was supplemented with 1 mL L−1 of IAA. The usage of agri-perlite and MS at half concentration, without PGR, allowed us to obtain a 99.1% survival rate. This micropropagation protocol is useful for obtaining virus-free plants and for the development of the brewery industry.

Research into molecular mechanisms of self-incompatibility (SI) in plants can be observed in representatives of various families, including Solanaceae. Earlier studies of the mechanisms of S-RNase-based SI in petunia (Petunia hybrida E. Vilm.) demonstrate that programmed cell death (PCD) is an SI factor. These studies suggest that the phytohormon cytokinin (CK) is putative activator of caspase-like proteases (CLPs). In this work, data confirming this hypothesis were obtained in two model objects—petunia and tomato (six Solanaceae representatives). The exogenous zeatin treatment of tomato and petunia stigmas before a compatible pollination activates CLPs in the pollen tubes in vivo, as shown via the intravital imaging of CLP activities. CK at any concentration slows down the germination and growth of petunia and tomato male gametophytes both in vitro and in vivo; shifts the pH of the cytoplasm (PHc) to the acid region, thereby creating the optimal conditions for CLP to function and inhibiting the F-actin formation and/or destructing the cytoskeleton in pollen tubes to point foci during SI-induced PCD; and accumulates in style tissues during SI response. The activity of the ISOPENTENYLTRANSFERASE 5 (IPT5) gene at this moment exceeds its activity in a cross-compatible pollination, and the levels of expression of the CKX1 and CKX2 genes (CK OXIDASE/DEHYDROGENASE) are significantly lower in self-incompatible pollination. All this suggests that CK plays a decisive role in the mechanism underlying SI-induced PCD.

Cytokinin is widely involved in the regulation of plant growth, but its pathway-related genes have not been reported in Moso bamboo. In this study, a total of 129 candidate sequences were identified by bioinformatic methods. These included 15 IPT family genes, 19 LOG family genes, 22 HK family genes, 11 HP family genes and 62 RR family genes. Phylogenetic analysis revealed that the cytokinin pathway was closely related to rice, and evolutionary pattern analysis found that most of the genes have syntenic relationship with rice-related genes. The Moso bamboo cytokinin pathway was evolutionarily conservative and mainly underwent purifying selection, and that gene family expansion was mainly due to whole-gene duplication events. Analysis of transcriptome data revealed a tissue-specific expression pattern of Moso bamboo cytokinin family genes, with auxin and gibberellin response patterns. Analysis of co-expression patterns at the developmental stages of Moso bamboo shoots revealed the existence of a phytohormone co-expression pattern centered on cytokinin signaling genes. The auxin signaling factor PheARF52 was identified by yeast one-hybrid assay as regulating the PheRR3 gene through a P-box element in the PheRR3 promoter region. Auxin and cytokinin signaling crosstalk to regulate Moso bamboo growth. Overall, we systematically identified and analyzed key gene families of the cytokinin pathway in Moso bamboo and obtained key factors for auxin and cytokinin crosstalk, laying the foundation for the study of hormone regulation in Moso bamboo.

Nitrogen is an important macronutrient required for plant growth and development, thus directly impacting agricultural productivity. In recent years, numerous studies have shown that nitrogen-driven growth depends on pathways that control nitrate/nitrogen homeostasis and hormonal networks that act both locally and systemically to coordinate growth and development of plant organs. In this review, we will focus on recent advances in understanding the role of the plant hormones auxin and cytokinin and their crosstalk in nitrate-regulated growth and discuss the significance of novel findings and possible missing links.

Kohlrabi (Brassica oleracea var. gongylodes) is a vegetable cultivated for its edible stem tuber. Although valued for its nutritional properties and tolerance to abiotic stress, kohlrabi is one of the least studied brassicas. In this review, we summarize the results of our decade-long research on in vitro morphogenesis of kohlrabi, starting from 2013. Protocols for efficient in vitro regeneration with minimal requirements for external application of plant growth regulators (PGRs) have been developed, both by somatic embryogenesis (SE) and by de novo shoot organogenesis (DNSO). Efficient regeneration by direct SE was achieved using immature zygotic embryos as explants incubated on PGR-free media, and the production process was maintained in culture thanks to highly efficient secondary SE. Conversely, efficient regeneration by indirect DNSO was achieved using entire seedlings as explants and adding only cytokinin (CK) without the need for exogenous auxin. Comprehensive phytohormone analyses revealed that different exogenously applied CKs differentially affected the composition of endogenous phytohormones and induced changes in the expression of cell cycle-related genes and other genes involved in the organogenic response. The addition of high sucrose concentrations to the nutrient media failed to induce the formation of stem tubers in in vitro culture, but revealed a complex interaction with exogenously applied CKs, interfering with both the endogenous phytohormonome and the expression of organogenesis-related genes. Our work has provided substantial biotechnological advances in the field of in vitro regeneration of kohlrabi, as well as in understanding the underlying phytohormonal regulation. The review aims to introduce kohlrabi to the scientific community as a model system for both basic and applied research, while we continue seeking answers to the outstanding questions and trying to pave the way for the development of more resistant varieties.

As the human population grows rapidly, food shortages will become an even greater problem; therefore, increasing crop yield has become a focus of rice breeding programs. The maize gene, ZmDUF1645, encoding a putative member of the DUF1645 protein family with an unknown function, was transformed into rice. Phenotypic analysis showed that enhanced ZmDUF1645 expression significantly altered various traits in transgenic rice plants, including increased grain length, width, weight, and number per panicle, resulting in a significant increase in yield, but a decrease in rice tolerance to drought stress. qRT-PCR results showed that the expression of the related genes regulating meristem activity, such as MPKA, CDKA, a novel crop grain filling gene (GIF1), and GS3, was significantly changed in the ZmDUF1645-overexpression lines. Subcellular colocalization showed that ZmDUF1645 was primarily localized on cell membrane systems. Based on these findings, we speculate that ZmDUF1645, like the OsSGL gene in the same protein family, may regulate grain size and affect yield through the cytokinin signaling pathway. This research provides further knowledge and understanding of the unknown functions of the DUF1645 protein family and may serve as a reference for biological breeding engineering to increase maize crop yield.

Members of the TaCKX gene family (GFMs) encode the cytokinin oxygenase/dehydrogenase enzyme (CKX), which irreversibly degrades cytokinins in the organs of wheat plants; therefore, these genes perform a key role in the regulation of yield-related traits. The purpose of the investigation was to determine how expression patterns of these genes, together with the transcription factor-encoding gene TaNAC2-5A, and yield-related traits are inherited to apply this knowledge to speed up breeding processes. The traits were tested in 7 days after pollination (DAP) spikes and seedling roots of maternal and paternal parents and their F2 progeny. The expression levels of most of them and the yield were inherited in F2 from the paternal parent. Some pairs or groups of genes cooperated, and some showed opposite functions. Models of up- or down-regulation of TaCKX GFMs and TaNAC2-5A in low-yielding maternal plants crossed with higher-yielding paternal plants and their high-yielding F2 progeny reproduced gene expression and yield of the paternal parent. The correlation coefficients between TaCKX GFMs, TaNAC2-5A, and yield-related traits in high-yielding F2 progeny indicated which of these genes were specifically correlated with individual yield-related traits. The most common was expressed in 7 DAP spikes TaCKX2.1, which positively correlated with grain number, grain yield, spike number, and spike length, and seedling root mass. The expression levels of TaCKX1 or TaNAC2-5A in the seedling roots were negatively correlated with these traits. In contrast, the thousand grain weight (TGW) was negatively regulated by TaCKX2.2.2, TaCKX2.1, and TaCKX10 in 7 DAP spikes but positively correlated with TaCKX10 and TaNAC2-5A in seedling roots. Transmission of TaCKX GFMs and TaNAC2-5A expression patterns and yield-related traits from parents to the F2 generation indicate their paternal imprinting. These newly shown data of nonmendelian epigenetic inheritance shed new light on crossing strategies to obtain a high-yielding F2 generation.

The role of plant hormones in tension wood (TW) formation has been studied but is still unclear. IAA, ABA, ACC, tZ, tZR, iP, and iPR in cambial region tissues were identified and quantified by liquid chromatography/mass spectrometry (LC/MS). We examined the distribution of plant hormones and their precursors in the stems of Quercus myrsinifolia Blume and Castanopsis cuspidata var.sieboldii Nakai after bending the stems or applying an ethylene precursor (ACC). After 3 weeks of bending, though not after 1 week of bending, the auxin (IAA) and abscisic acid (ABA) amounts were larger on the TW side than on the opposite wood (OW) side and in upright trees. After 2 weeks of bending, the peak concentrations of IAA in cambium on the TW side were obviously higher than those on the OW side. After 1 week of bending, the ACC amounts on both sides were larger than in upright trees, but after 3 weeks of bending, they were smaller than in upright trees. Applied ACC did not enhance TW formation but induced axical parenchyma and phloem formation in C. cuspidata var.sieboldii. These results indicated that the distribution patterns of IAA and ABA might have important roles in TW formation in these two species. The role of ACC might be limited in the early stages of TW formation.

Thidiazuron (TDZ) is a widely used chemical defoliant in cotton and can stimulate the production of ethylene in leaves, which is believed to be the key factor in inducing leaf abscission. Ethephon (Eth) can also stimulate ethylene production in leaves, but it is less effective in promoting leaf shedding. In this study, the enzyme-linked immunosorbent assays (ELISA) and RNA-seq were used to determine specific changes at hormonal levels as well as transcriptomic mechanisms induced by TDZ compared with Eth. The TDZ significantly reduced the levels of auxin and cytokinin in cotton leaves, but no considerable changes were observed for Eth. In addition, TDZ specifically increased the levels of brassinosteroids and jasmonic acid in the leaves. A total of 13 764 differentially expressed genes that specifically responded to TDZ were identified by RNA-seq. The analysis of KEGG functional categories suggested that the synthesis, metabolism, and signal transduction of auxin, cytokinin, and brassinosteroid were all involved in the TDZ-induced abscission of cotton leaves. Eight auxin transport genes (GhPIN1-c_D, GhPIN3_D, GhPIN8_A, GhABCB19-b_A, GhABCB19-b_D, GhABCB2-b_D, GhLAX6_A, and GhLAX7_D) specifically responded to TDZ. The pro35S::GhPIN3a::YFP transgenic plants showed lower defoliation than the wild type treated with TDZ, and YFP fluorescence in leaves was almost extinguished after treatment with TDZ rather than Eth. This provides direct evidence that GhPIN3a is involved in the leaf abscission induced by TDZ. We found that 959 transcription factors (TFs) specifically responded to TDZ, and a co-expression network analysis (WGCNA) showed five hub TFs (GhNAC72, GhWRKY51, GhWRKY70, GhWRKY50, and GhHSF24) during chemical defoliation with TDZ. Our work sheds light on the molecular basis of TDZ-induced leaf abscission in cotton.

Soil-borne beneficial microbes establish symbioses with plant hosts and play key roles during growth and development therein. In this study, two fungal strains, FLP7 and B9, were isolated from the rhizosphere microbiome associated with Choy Sum (Brassica rapa var. parachinensis) and barley (Hordeum vulgare), respectively. Sequence analyses of the internal transcribed spacer and 18S ribosomal RNA genes combined with colony and conidial morphology identified FLP7 and B9 to be Penicillium citrinum strains/isolates. Plant–fungus interaction assays revealed that isolate B9 showed significant growth promotion effects in Choy Sum plants cultivated in normal soil, as well as under phosphate-limiting conditions. In comparison to the mock control, B9-inoculated plants showed a 34% increase in growth in aerial parts and an 85% upsurge in the fresh weight of roots when cultivated in sterilized soil. The dry biomass of such fungus-inoculated Choy Sum increased by 39% and 74% for the shoots and roots, respectively. Root colonization assays showed that P. citrinum associates directly with the root surface but does not enter or invade the root cortex of the inoculated Choy Sum plants. Preliminary results also indicated that P. citrinum can promote growth in Choy Sum via volatile metabolites too. Interestingly, we detected relatively higher amounts of gibberellins and cytokinins in axenic P. citrinum culture filtrates through liquid chromatography–mass spectrometry analyses. This could plausibly explain the overall growth induction in P. citrinum-inoculated Choy Sum plants. Furthermore, the phenotypic growth defects associated with the Arabidopsis ga1 mutant could be chemically complemented by the exogenous application of P. citrinum culture filtrate, which also showed accumulation of fungus-derived active gibberellins. Our study underscores the importance of transkingdom beneficial effects of such mycobiome-assisted nutrient assimilation and beneficial fungus-derived phytohormone-like metabolites in the induction of robust growth in urban farmed crops.

Heat stress caused by rapidly changing climate warming has become a serious threat to crop growth worldwide. Exogenous cytokinin (CK) kinetin (KT) has been shown to have positive effects in improving salt and drought tolerance in plants. However, the mechanism of KT in heat tolerance in rice is poorly understood. Here, we found that exogenously adequate application of KT improved the heat stress tolerance of rice seedlings, with the best effect observed when the application concentration was 10−9 M. In addition, exogenous application of 10−9 M KT promoted the expression of CK-responsive OsRR genes, reduced membrane damage and reactive oxygen species (ROS) accumulation in rice, and increased the activity of antioxidant enzymes. Meanwhile, exogenous 10−9 M KT treatment significantly enhanced the expression of antioxidant enzymes, heat activation, and defense-related genes. In conclusion, exogenous KT treatment regulates heat tolerance in rice seedlings by modulating the dynamic balance of ROS in plants under heat stress.

The SHPRH (SNF2, histone linker, PHD, RING, helicase) subfamily belonging to ATP-dependent chromatin remodeling factor is the effective tumor-suppressor, which can polyubiquitinate PCNA (proliferating cell nuclear antigen) and participate in post-replication repair in human. However, little is known about the functions of SHPRH proteins in plants. In this study, we identified a novel SHPRH member BrCHR39 and obtained BrCHR39-silenced transgenic Brassica rapa. In contrast to wild-type plants, transgenic Brassica plants exhibited a released apical dominance phenotype with semi-dwarfism and multiple lateral branches. Furthermore, a global alteration of DNA methylation in the main stem and bud appeared after silencing of BrCHR39. Based on the GO (gene ontology) functional annotation and KEGG (Kyoto encyclopedia of genes and genomes) pathway analysis, the plant hormone signal transduction pathway was clearly enriched. In particular, we found a significant increase in the methylation level of auxin-related genes in the stem, whereas auxin- and cytokinin-related genes were hypomethylated in the bud of transgenic plants. In addition, further qRT-PCR (quantitative real-time PCR) analysis revealed that DNA methylation level always had an opposite trend with gene expression level. Considered together, our findings indicated that suppression of BrCHR39 expression triggered the methylation divergence of hormone-related genes and subsequently affected transcription levels to regulate the apical dominance in Brassica rapa.

A multi-step phosphorelay system is the main conduit of cytokinin signal transduction. However, several groups of additional factors that also play a role in this signaling pathway have been found—among them the Cytokinin Response Factors (CRFs). In a genetic screen, CRF9 was identified as a regulator of the transcriptional cytokinin response. It is mainly expressed in flowers. Mutational analysis indicates that CRF9 plays a role in the transition from vegetative to reproductive growth and silique development. The CRF9 protein is localized in the nucleus and functions as a transcriptional repressor of Arabidopsis Response Regulator 6 (ARR6)—a primary response gene for cytokinin signaling. The experimental data suggest that CRF9 functions as a repressor of cytokinin during reproductive development.

Plasmodiophora brassicae is an obligate biotrophic pathogen causing clubroot disease in cruciferous plants. Infected plant organs are subject to profound morphological changes, the roots form characteristic galls, and the leaves are chlorotic and abscise. The process of gall formation is governed by timely changes in the levels of endogenous plant hormones that occur throughout the entire life cycle of the clubroot pathogen. The homeostasis of two plant hormones, cytokinin and auxin, appears to be crucial for club development. To investigate the role of cytokinin and auxin in gall formation, we used metabolomic and transcriptomic profiling of Arabidopsis thaliana infected with clubroot, focusing on the late stages of the disease, where symptoms were more pronounced. Loss-of-function mutants of three cytokinin receptors, AHK2, AHK3, and CRE1/AHK4, were employed to further study the homeostasis of cytokinin in response to disease progression; ahk double mutants developed characteristic symptoms of the disease, albeit with varying intensity. The most susceptible to clubroot disease was the ahk3 ahk4 double mutant, as revealed by measuring its photosynthetic performance. Quantification of phytohormone levels and pharmacological treatment with the cytokinin antagonist PI-55 showed significant changes in the levels of endogenous cytokinin and auxin, which was manifested by both enhanced and reduced development of disease symptoms in different genotypes.

Land plants survive the challenges of new environments by evolving mechanisms that protect them from excess irradiation, nutrient deficiency, and temperature and water availability fluctuations. One such evolved mechanism is the regulation of the shoot/root growth ratio in response to water and nutrient availability by balancing the actions of the hormones auxin and cytokinin. Plant terrestrialization co-occurred with a dramatic expansion in secondary metabolism, particularly with the evolution and establishment of the flavonoid biosynthetic pathway. Flavonoid biosynthesis is responsive to a wide range of stresses, and the numerous synthesized flavonoid species offer two main evolutionary advantages to land plants. First, flavonoids are antioxidants and thus defend plants against those adverse conditions that lead to the overproduction of reactive oxygen species. Second, flavonoids aid in protecting plants against water and nutrient deficiency by modulating root development and establishing symbiotic relations with beneficial soil fungi and bacteria. Here, we review different aspects of the relationships between the auxin/cytokinin module and flavonoids. The current body of knowledge suggests that whereas both auxin and cytokinin regulate flavonoid biosynthesis, flavonoids act to fine-tune only auxin, which in turn regulates cytokinin action. This conclusion agrees with the established master regulatory function of auxin in controlling the shoot/root growth ratio.

Phytohormones are regulators of plant growth and development, which under different types of stress can play a fundamental role in a plant’s adaptation and survival. Some of these phytohormones such as cytokinin, gibberellin, salicylic acid, auxin, and ethylene are also produced by plant growth-promoting bacteria (PGPB). In addition, numerous volatile organic compounds are released by PGPB and, like bacterial phytohormones, modulate plant physiology and genetics. In the present work we review the basic functions of these bacterial phytohormones during their interaction with different plant species. Moreover, we discuss the most recent advances of the beneficial effects on plant growth of the phytohormones produced by PGPB. Finally, we review some aspects of the cross-link between phytohormone production and other plant growth promotion (PGP) mechanisms. This work highlights the most recent advances in the essential functions performed by bacterial phytohormones and their potential application in agricultural production.

Sages are medicinal and aromatic plants that constitute a large pool from which active compounds of great pharmaceutical potential can be derived, while at the same time, they also have ornamental value. The purpose of this study was to develop the micropropagation protocols of Salvia fruticosa, S. officinalis, S. ringens, S. tomentosa, and S. pomifera ssp. pomifera to facilitate their exploitation in the pharmaceutical and floriculture industries. In vitro cultures of S. ringens and S. pomifera ssp. pomifera was studied for the first time. Shoot tips and single node explants from in vitro seedlings were initially cultured on hormone free (Hf)-MS medium, followed by subcultures on MS medium supplemented with 6-benzyladenine (BA) for all species, as well as with zeatin (ZEA), kinetin (KIN), 6-(γ,γ-dimethylallyamino) purine (2iP), or meta-topolin (mT) for S. fruticosa and S. officinalis, at concentrations 0.0 to 3.2 mg L−1, in combination with 0.01 mg L−1 1-naphthaleneacetic acid (NAA). S. officinalis was the most efficient in shoot multiplication of all the studied species. The highest multiplication indices were found using 0.8 mg L−1 BA for S. fruticosa, 0.4 mg L−1 BA, or mT for S. officinalis, and lower than 0.8 mg L−1 BA for the other three species. Hyperhydricity was a problem at the multiplication stage, and was most pronounced in single node explants, increasing in proportion to cytokinin concentration. Microshoots rooted at high percentages (75–85%) on half-strength MS medium with 0.0 or 0.5 mg L−1 Indole-3-butyric acid (IBA), except for those of S. ringens, which rooted best at 1.0–2.0 mg L−1 IBA. Ex vitro acclimatization was highly successful (80–95%) on peat–perlite substrate (1:1 v/v). Thus, the present study resulted in efficient micropropagation protocols for five Mediterranean sage species native to Greece, which will facilitate breeding programs and the promotion of these species in the floriculture and pharmaceutical industries.

Fine-tuned interactions between melatonin (MT) and hormones affected by environmental inputs are crucial for plant growth. Under high light (HL) conditions, melatonin reduced photodamage in Arabidopsis thaliana and contributed to the restoration of the expression of the cytokinin (CK) synthesis genes IPT3, IPT5 and LOG7 and genes for CK signal transduction AHK2,3 and ARR 1, 4, 5 and 12 which were downregulated by stress. However, CK signaling mutants displayed no significant changes in the expression of CK genes following HL + MT treatment, implying that a fully functional cytokinin signaling pathway is a prerequisite for MT–CK interactions. In turn, cytokinin treatment increased the expression of the key melatonin synthesis gene ASMT under both moderate and HL in wild-type plants. This upregulation was further accentuated in the ipt3,5,7 mutant which is highly sensitive to CK. In this mutant, in addition to ASMT, the melatonin synthesis genes SNAT and COMT, as well as the putative signaling genes CAND2 and GPA1, displayed elevated transcript levels. The results of the study suggest that melatonin acts synergistically with CK to cope with HL stress through melatonin-associated activation or repression of the respective hormonal genes.

Potato tubers are reproductive and storage organs, enabling their survival. Unraveling the molecular mechanisms that regulate tuberization is crucial for understanding how potatorespond to environmental stress situations and for potato breeding. Previously, we did a transcriptomic analysis of potato microtuberization without light. This showed that important cellular processes like ribosomal proteins, cell cycle, carbon metabolism, oxidative stress, fatty acids, and phytosterols (PS) biosynthesis were closely connected in a protein-protein interaction (PPI) network. Research on PS function during potato tuberization has been scarce. PS plays a critical role in regulating membrane permeability and fluidity, and they are biosynthetic precursors of brassinosteroids (BRs) in plants, which are critical in regulating gene expression, cell division, differentiation, and reproductive biology. Within a PPI network, we found a module of 15 genes involved in the PS biosynthetic process. Darkness, as expected, activated the mevalonate (MVA) pathway. There was a tight interaction between three coding gene products for HMGR3, MVD2, and FPS1, and the gene products that synthetize PS, including CAS1, SMO1, BETAHSD, CPI1, CYP51, FACKEL, HYDRA1, SMT2, SMO2, STE1, and SSR1. Quantitative real-time polymerase chain reaction (qRT-PCR) confirmed the expression analysis of ten specific genes involved in the biosynthesis of PS. This manuscript discusses the potential role of genes involved in PS biosynthesis during microtuber development.

Some citrus orchards in China often experience nitrogen (N) deficiency. For the first time, targeted metabolomics was used to examine N-deficient effects on hormones in sweet orange ( Citrus sinensis (L.) Osbeck cv. Xuegan) leaves and roots. The purpose was to validate the hypothesis that hormones play a role in N deficiency tolerance by regulating root/shoot dry weight ratio (R/S), root system architecture (RSA), and leaf and root senescence. N deficiency-induced decreases in gibberellins and indole-3-acetic acid (IAA) levels and increases in cis(+)-12-oxophytodienoic acid (OPDA) levels, ethylene production, and salicylic acid (SA) biosynthesis might contribute to reduced growth and accelerated senescence in leaves. The increased ethylene formation in N-deficient leaves might be caused by increased 1-aminocyclopropanecarboxylic acid and OPDA and decreased abscisic acid (ABA). N deficiency increased R/S, altered RSA, and delayed root senescence by lowering cytokinins, jasmonic acid, OPDA, and ABA levels and ethylene and SA biosynthesis, increasing 5-deoxystrigol levels, and maintaining IAA and gibberellin homeostasis. The unchanged IAA concentration in N-deficient roots involved increased leaf-to-root IAA transport. The different responses of leaf and root hormones to N deficiency might be involved in the regulation of R/S, RSA, and leaf and root senescence, thus improving N use efficiency, N remobilization efficiency, and the ability to acquire N, and hence conferring N deficiency tolerance.

The formation of underground stem bulblets in lilies is a complex biological process which is key in their micropropagation. Generally, it involves a stem-to-bulblet transition; however, the underlying mechanism remains elusive. It is important to understand the regulatory mechanism of bulblet formation for the reproductive efficiency of Lilium. In this study, we investigated the regulatory mechanism of underground stem bulblet formation under different conditions regarding the gravity point angle of the stem, i.e., vertical (control), horizontal, and slanting. The horizontal and slanting group displayed better formation of bulblets in terms of quality and quantity compared with the control group. A transcriptome analysis revealed that sucrose and starch were key energy sources for bulblet formation, auxin and cytokinin likely promoted bulblet formation, and gibberellin inhibited bulblet formation. Based on transcriptome analysis, we identified the LoLOB18 gene, a homolog to AtLOB18, which has been proven to be related to embryogenic development. We established the stem bud growth tissue culture system of Lilium and silenced the LoLOb18 gene using the VIGS system. The results showed that the bulblet induction was reduced with down-regulation of LoLOb18, indicating the involvement of LoLOb18 in stem bulblet formation in lilies. Our research lays a solid foundation for further molecular studies on stem bulblet formation of lilies.