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Abstract Background Plants show developmental plasticity with variations in environmental nutrients. Considering low-cost rock dust has been identified as a potential alternative to artificial fertilizers for more sustainable agriculture, the growth responses of Arabidopsis seedlings on three rock meals (basalt, granite, and marlstone) were examined for the different foraging behavior, biomass accumulation, and root architecture. Results Compared to ½ MS medium, basalt and granite meal increased primary root length by 13% and 38%, respectively, but marlstone caused a 66% decrease, and they all drastically reduced initiation and elongation of lateral roots but lengthened root hairs. Simultaneous supply of organic nutrients and trace elements increased fresh weight due to the increased length of primary roots and root hairs. When nitrogen (N), phosphorus (P), and potassium (K) were supplied individually, N proved most effective in improving fresh weight of seedlings growing on basalt and granite, whereas K, followed by P, was most effective for those growing on marlstone. Unexpectedly, the addition of N to marlstone negatively affected seedling growth, which was associated with repressed auxin biosynthesis in roots. Conclusions Our data indicate that plants can recognize and adapt to complex mineral deficiency by adjusting hormonal homeostasis to achieve environmental sensitivity and developmental plasticity, which provide a basis for ecologically sound and sustainable strategies to maximize the use of natural resources and reduce the production of artificial fertilizers.

Potato virus Y (PVY) is an economically important plant pathogen that reduces the productivity of several host plants. To develop PVY-resistant cultivars, it is essential to identify the plant-PVY interactome and decipher the biological significance of those molecular interactions. We performed a yeast two-hybrid (Y2H) screen of Nicotiana benthamiana cDNA library using PVY-encoded NIa-pro as the bait. The N. benthamiana Indole-3-acetic acid-amido synthetase (IAAS) was identified as an interactor of NIa-pro protein. The interaction was confirmed via targeted Y2H and bimolecular fluorescence complementation (BiFC) assays. NIa-pro interacts with IAAS protein and consequently increasing the stability of IAAS protein. Also, the subcellular localization of both NIa-pro and IAAS protein in the nucleus and cytosol was demonstrated. By converting free IAA (active form) to conjugated IAA (inactive form), IAAS plays a crucial regulatory role in auxin signaling. Transient silencing of IAAS in N. benthamiana plants reduced the PVY-mediated symptom induction and virus accumulation. Conversely, overexpression of IAAS enhanced symptom induction and virus accumulation in infected plants. In addition, the expression of auxin-responsive genes was found to be downregulated during PVY infection. Our findings demonstrate that PVY NIa-pro protein potentially promotes disease development via modulating auxin homeostasis.

Auxin is a central phytohormone and controls almost all aspects of plant development and stress response. Auxin homeostasis is coordinately regulated by biosynthesis, catabolism, transport, conjugation, and deposition. Endoplasmic reticulum (ER)-localized MAIGO2 (MAG2) complex mediates tethering of arriving vesicles to the ER membrane, and it is crucial for ER export trafficking. Despite important regulatory roles of MAG2 in vesicle trafficking, the mag2 mutant had mild developmental abnormalities. MAG2 has one homolog protein, MAG2-Like (MAL), and the mal-1 mutant also had slight developmental phenotypes. In order to investigate MAG2 and MAL regulatory function in plant development, we generated the mag2-1 mal-1 double mutant. As expected, the double mutant exhibited serious developmental defects and more alteration in stress response compared with single mutants and wild type. Proteomic analysis revealed that signaling, metabolism, and stress response in mag2-1 mal-1 were affected, especially membrane trafficking and auxin biosynthesis, signaling, and transport. Biochemical and cell biological analysis indicated that the mag2-1 mal-1 double mutant had more serious defects in vesicle transport than the mag2-1 and mal-1 single mutants. The auxin distribution and abundance of auxin transporters were altered significantly in the mag2-1 and mal-1 single mutants and mag2-1 mal-1 double mutant. Our findings suggest that MAG2 and MAL regulate plant development and auxin homeostasis by controlling membrane trafficking, with functional redundancy.

Abstract Background Coffee production relies on plantations with varieties from Coffea arabica and Coffea canephora species. The first, the most representative in terms of coffee consumption, is mostly propagated by seeds, which leads to management problems regarding the plantations maintenance, harvest and processing of grains. Therefore, an efficient clonal propagation process is required for this species cultivation, which is possible by reaching a scalable and cost-effective somatic embryogenesis protocol. A key process on somatic embryogenesis induction is the auxin homeostasis performed by Gretchen Hagen 3 (GH3) proteins through amino acid conjugation. In this study, the GH3 family members were identified on C. canephora genome, and by performing analysis related to gene and protein structure and transcriptomic profile on embryogenic tissues, we point a GH3 gene as a potential regulator of auxin homeostasis during early somatic embryogenesis in C. arabica plants. Results We have searched within the published C. canephora genome and found 17 GH3 family members. We checked the conserved domains for GH3 proteins and clustered the members in three main groups according to phylogenetic relationships. We identified amino acids sets in four GH3 proteins that are related to acidic amino acid conjugation to auxin, and using a transcription factor (TF) network approach followed by RT-qPCR we analyzed their possible transcriptional regulators and expression profiles in cells with contrasting embryogenic potential in C. arabica. The CaGH3.15 expression pattern is the most correlated with embryogenic potential and with CaBBM, a C. arabica ortholog of a major somatic embryogenesis regulator. Conclusion Therefore, one out of the GH3 members may be influencing on coffee somatic embryogenesis by auxin conjugation with acidic amino acids, which leads to the phytohormone degradation. It is an indicative that this gene can serve as a molecular marker for coffee cells with embryogenic potential and needs to be further studied on how much determinant it is for this process. This work, together with future studies, can support the improvement of coffee clonal propagation through in vitro derived somatic embryos.

Sedentary endoparasites such as cyst and root-knot nematodes infect many important food crops and are major agro-economical pests worldwide. These plant-parasitic nematodes exploit endogenous molecular and physiological pathways in the roots of their host to establish unique feeding structures. These structures function as highly active transfer cells and metabolic sinks and are essential for the parasites’ growth and reproduction. Plant hormones like indole-3-acetic acid (IAA) are a fundamental component in the formation of these feeding complexes. However, their underlying molecular and biochemical mechanisms are still elusive despite recent advances in the field. This review presents a comprehensive overview of known functions of various auxins in plant-parasitic nematode infection sites, based on a systematic analysis of current literature. We evaluate multiple aspects involved in auxin homeostasis in plants, including anabolism, catabolism, transport, and signalling. From these analyses, a picture emerges that plant-parasitic nematodes have evolved multiple strategies to manipulate auxin homeostasis to establish a successful parasitic relationship with their host. Additionally, there appears to be a potential role for auxins other than IAA in plant-parasitic nematode infections that might be of interest to be further elucidated.

Halauxifen-methyl is a new auxin herbicide developed by Corteva Agriscience (Wilmington, DE, USA). It has been suggested that ABF5 may be the target of halauxifen-methyl, as AFB5 mutants of Arabidopsis thaliana are resistant to halauxifen-methyl, which preferentially binds to AFB5. However, the mode of action of halauxifen-methyl has not yet been reported. Therefore, the aim of the present study was to reveal the mode of action of halauxifen-methyl by exploring its influence on indole-3-acetic acid (IAA) homeostasis and the biosynthesis of ethylene and Abscisic Acid (ABA) in Galium aparine. The results showed that halauxifen-methyl could disrupt the homeostasis of IAA and stimulate the overproduction of ethylene and ABA by inducing the overexpression of the 1-aminocyclopropane-1-carboxylate synthase (ACS) and 9-cis-epoxycarotenoid dioxygenase (NCED) genes involved in ethylene and ABA biosynthesis, finally leading to senescence and plant death.

Several Prunus species are among the most important cultivated stone fruits in the Mediterranean region, and there is an urgent need to obtain rootstocks with specific adaptations to challenging environmental conditions. The development of adventitious roots (ARs) is an evolutionary mechanism of high relevance for stress tolerance, which has led to the development of environmentally resilient plants. As a first step towards understanding the genetic determinants involved in AR formation in Prunus sp., we evaluated the rooting of hardwood cuttings from five Prunus rootstocks (Adafuel, Adarcias, Cadaman, Garnem, and GF 677) grown in hydroponics. We found that auxin-induced callus and rooting responses were strongly genotype-dependent. To investigate the molecular mechanisms involved in these differential responses, we performed a time-series study of AR formation in two rootstocks with contrasting rooting performance, Garnem and GF 677, by culturing in vitro microcuttings with and without auxin treatment (0.9 mg/L of indole-3-butyric acid [IBA]). Despite showing a similar histological structure, Garnem and GF677 rootstocks displayed dynamic changes in endogenous hormone homeostasis involving metabolites such as indole-3-acetic acid (IAA) conjugated to aspartic acid (IAA-Asp), and these changes could explain the differences observed during rooting.

Adventitious roots (ARs) are formed de novo during post-embryonic development from non-root tissues, in processes that are highly dependent on environmental inputs. Whole root excision from young seedlings has been previously used as a model to study adventitious root formation in Arabidopsis thaliana hypocotyls. To identify novel regulators of adventitious root formation, we analyzed adventitious rooting in the hypocotyl after whole root excision in 112 T-DNA homozygous leaf mutants, which were selected based on the dynamic expression profiles of their annotated genes during hormone-induced and wound-induced tissue regeneration. Forty-seven T-DNA homozygous lines that displayed low rooting capacity as regards their wild-type background were dubbed as the less adventitious roots (lars) mutants. We identified eight lines with higher rooting capacity than their wild-type background that we named as the more adventitious roots (mars) mutants. A relatively large number of mutants in ribosomal protein-encoding genes displayed a significant reduction in adventitious root number in the hypocotyl after whole root excision. In addition, gene products related to gibberellin (GA) biosynthesis and signaling, auxin homeostasis, and xylem differentiation were confirmed to participate in adventitious root formation. Nearly all the studied mutants tested displayed similar rooting responses from excised whole leaves, which suggest that their affected genes participate in shared regulatory pathways required for de novo organ formation in different organs.

We explored the interplay between ethylene signals and the auxin pool in roots exposed to high salinity using Arabidopsisthaliana wild-type plants (Col-0), and the ethylene-signaling mutants ctr1-1 (constitutive) and ein2-1 (insensitive). The negative effect of salt stress was less pronounced in ctr1-1 individuals, which was concomitant with augmented auxin signaling both in the ctr1-1 controls and after 100 mM NaCl treatment. The R2D2 auxin sensorallowed mapping this active auxin increase to the root epidermal cells in the late Cell Division (CDZ) and Transition Zone (TZ). In contrast, the ethylene-insensitive ein2-1 plants appeared depleted in active auxins. The involvement of ethylene/auxin crosstalk in the salt stress response was evaluated by introducing auxin reporters for local biosynthesis (pTAR2::GUS) and polar transport (pLAX3::GUS, pAUX1::AUX1-YFP, pPIN1::PIN1-GFP, pPIN2::PIN2-GFP, pPIN3::GUS) in the mutants. The constantly operating ethylene-signaling pathway in ctr1-1 was linked to increased auxin biosynthesis. This was accompanied by a steady expression of the auxin transporters evaluated by qRT-PCR and crosses with the auxin transport reporters. The results imply that the ability of ctr1-1 mutant to tolerate high salinity could be related to the altered ethylene/auxin regulatory loop manifested by a stabilized local auxin biosynthesis and transport.

Commercial carnation (Dianthus caryophyllus) cultivars are vegetatively propagated from axillary stem cuttings through adventitious rooting; a process which is affected by complex interactions between nutrient and hormone levels and is strongly genotype-dependent. To deepen our understanding of the regulatory events controlling this process, we performed a comparative study of adventitious root (AR) formation in two carnation cultivars with contrasting rooting performance, “2101–02 MFR” and “2003 R 8”, as well as in the reference cultivar “Master”. We provided molecular evidence that localized auxin response in the stem cutting base was required for efficient adventitious rooting in this species, which was dynamically established by polar auxin transport from the leaves. In turn, the bad-rooting behavior of the “2003 R 8” cultivar was correlated with enhanced synthesis of indole-3-acetic acid conjugated to aspartic acid by GH3 proteins in the stem cutting base. Treatment of stem cuttings with a competitive inhibitor of GH3 enzyme activity significantly improved rooting of “2003 R 8”. Our results allowed us to propose a working model where endogenous auxin homeostasis regulated by GH3 proteins accounts for the cultivar dependency of AR formation in carnation stem cuttings.

Epigenetic regulation involves a myriad of mechanisms that regulate the expression of loci without altering the DNA sequence. These different mechanisms primarily result in modifications of the chromatin topology or DNA chemical structure that can be heritable or transient as a dynamic response to environmental cues. The phytohormone auxin plays an important role in almost every aspect of plant life via gradient formation. Auxin maxima/minima result from a complex balance of metabolism, transport, and signaling. Although epigenetic regulation of gene expression during development has been known for decades, the specific mechanisms behind the spatiotemporal dynamics of auxin levels in plants are only just being elucidated. In this review, we gather current knowledge on the epigenetic mechanisms regulating the expression of genes for indole-3-acetic acid (IAA) metabolism and transport in Arabidopsis and discuss future perspectives of this emerging field.

The clubroot disease, caused by the obligate biotrophic protist Plasmodiophora brassicae, affects cruciferous crops worldwide. It is characterized by root swellings as symptoms, which are dependent on the alteration of auxin and cytokinin metabolism. Here, we describe that two different classes of auxin receptors, the TIR family and the auxin binding protein 1 (ABP1) in Arabidopsis thaliana are transcriptionally upregulated upon gall formation. Mutations in the TIR family resulted in more susceptible reactions to the root pathogen. As target genes for the different pathways we have investigated the transcriptional regulation of selected transcriptional repressors (Aux/IAA) and transcription factors (ARF). As the TIR pathway controls auxin homeostasis via the upregulation of some auxin conjugate synthetases (GH3), the expression of selected GH3 genes was also investigated, showing in most cases upregulation. A double gh3 mutant showed also slightly higher susceptibility to P. brassicae infection, while all tested single mutants did not show any alteration in the clubroot phenotype. As targets for the ABP1-induced cell elongation the effect of potassium channel blockers on clubroot formation was investigated. Treatment with tetraethylammonium (TEA) resulted in less severe clubroot symptoms. This research provides evidence for the involvement of two auxin signaling pathways in Arabidopsis needed for the establishment of the root galls by P. brassicae.