Search

Radial growth has long been a subject of interest in tree biology research. Recent studies have brought a significant change in the understanding of some basic processes characteristic to the vascular cambium, a meristem that produces secondary vascular tissues (phloem and xylem) in woody plants. A new hypothesis regarding the mechanism of intrusive growth of the cambial initials, which has been ratified by studies of the arrangement of cambial cells, negates the influence of this apical cell growth on the expansion of the cambial circumference. Instead, it suggests that the tip of the elongating cambial initial intrudes between the tangential (periclinal) walls, rather than the radial (anticlinal) walls, of the initial(s) and its(their) derivative(s) lying ahead of the elongating cell tip. The new concept also explains the hitherto obscure mechanism of the cell event called ‘elimination of initials’. This article evaluates these new concepts of the cambial cell dynamics and offers a new interpretation for some curious events occurring in the cambial meristem in relation to the radial growth in woody plants.

High-throughput sequencing and Degradome sequencing revealed that miRNAs play crucial roles during the juvenile-to-mature stages in Chinese fir. In woody plants, the vascular cambium is a continuous cylinder of meristematic cells in the stem that produces secondary xylem (wood) and secondary phloem via cell division, expansion, secondary wall formation, lignification, and programmed cell death. MicroRNAs (miRNAs) are endogenous single-stranded non-coding RNAs that play important roles in plant growth and development. However, little is known about the roles of miRNAs in age-related changes in the vascular cambium, especially in conifers. Here, we compared age-related miRNAs in the vascular cambium of 3-(juvenile), 13-(transition), and 35-year-old (mature) Chinese fir (Cunninghamia lanceolata) trees. Transmission electron microscopy revealed changes in the morphology and cell wall thickness of the vascular cambium during the juvenile-to-mature phase transition. In addition, high-throughput sequencing and subsequent analyses identified 700 miRNAs, including 651 known and 49 novel miRNAs. We identified 105, 120, and 94 differentially expressed miRNAs at the three stages of growth. These miRNAs included various differentially expressed development-related and phytohormone-related miRNAs involved in cambium activity, cell division, and cell wall modification. Degradome sequencing revealed 38 target genes that were cleaved by 88 known miRNAs and 16 target genes that were cleaved by seven novel miRNAs. Additionally, miR159, miR164, and miR166 were highly expressed in the 13-transition stage; these miRNAs are involved in the auxin-signaling pathway. Moreover, miR156 levels were abundant in VC13 along with miR172 levels decreased during the juvenile-to-mature stages. These results provide insight into the roles of age-specific miRNAs in the growth of C. lanceolata and shed light on vascular cambium development in conifers.

In Hancornia speciosa, the latex is synthesized in the cytosol of the ground meristem cells and in plastids of the fusiform derivatives of secondary phloem. Laticifer development is related to climatic seasonality. Hancornia speciosa is a latescent species that grows in the Brazilian Cerrado (neotropical savanna) and shows significant potential for rubber production and the extraction of bioactive compounds from its latex. We examined the development of laticifers in its stem apex and secondary phloem in relation to climate seasonality. Morphometric evaluations of elongating branches and micromorphometric evaluations of the cambial zone were carried out monthly for 1 year, with structural and ultrastructural analyses of the stem apex and secondary phloem. Laticifer development in both the stem apex and secondary phloem is related to increasing day length and maximum temperature and humidity. Laticifers are formed by anastomosis of the transverse and longitudinal walls of the ground meristem cells of the stem apex, and the fusiform derivatives of secondary phloem in the cambial zone. The process of latex secretion involves the synthesis of terpenic droplets in the cytoplasm of the ground meristem cells of the stem apex and in the plastids of the fusiform derivatives of secondary phloem. Latex is an emulsion formed by the cytosol terpenic droplets engulfed by vacuoles contaning mucilages and proteins secreted by dictyosomes and rough endoplasmic reticulum, and alkaloids secreted by the phloem cells. This work expands our knowledge concerning the development of laticifers in Apocynaceae and contributes to our better understanding of the influence of environmental factors on latex secretion.

International audience; Tree stems undergo a massive secondary growth in which secondary xylem and phloem tissues arise from the vascular cambium. Vascular cambium activity is driven by endogenous developmental signalling cues and environmental stimuli. Current knowledge regarding the genetic regulation of cambium activity and secondary growth is still far from complete. The tropical Cannabaceae tree Parasponia andersonii is a non-legume research model of nitrogen-fixing root nodulation. Parasponia andersonii can be transformed efficiently, making it amenable for CRISPR-Cas9-mediated reverse genetics. We considered whether P. andersonii also could be used as a complementary research system to investigate tree-related traits, including secondary growth. We established a developmental map of stem secondary growth in P. andersonii plantlets. Subsequently, we showed that the expression of the co-transcriptional regulator PanNODULE ROOT1 (PanNOOT1) is essential for controlling this process. PanNOOT1 is orthologous to Arabidopsis thaliana BLADE-ON-PETIOLE1 (AtBOP1) and AtBOP2, which are involved in the meristem-to-organ-boundary maintenance. Moreover, in species forming nitrogen-fixing root nodules, NOOT1 is known to function as a key nodule identity gene. Parasponia andersonii CRISPR-Cas9 loss-of-function Pannoot1 mutants are altered in the development of the xylem and phloem tissues without apparent disturbance of the cambium organization and size. Transcriptomic analysis showed that the expression of key secondary growth-related genes is significantly down-regulated in Pannoot1 mutants. This allows us to conclude that PanNOOT1 positively contributes to the regulation of stem secondary growth. Our work also demonstrates that P. andersonii can serve as a tree research system.

The OsRGLP1 gene was overexpressed under the control of CaMV 35S promoter in tomato (Solanum lycopersicum L.) plants using Agrobacterium-mediated transformation. The transgene expression was validated using RT-qPCR (real-time quantitative polymerase chain reaction) in the transgenic plants. Noticeable morpho-physiological traits, higher SOD activity, and elevated histological H2O2 were manifested by transgenic tomato lines. An increase in total chlorophyll and relative water content (RWC) in the leaves of transgenic lines was noted. The electrolytic leakage/conductivity (EC) in leaves of transgenic lines was lower than the control. The transgenic tomato lines showed dwarf phenotypic features in vegetative and floral traits. Interestingly, the vascular cambium regions in cross sections of stem showed secondary growth in radial symmetry. The transgenic lines also produced higher density and diversity of trichomes. The differences observed in transgenic lines are quite interesting which need to be further explored referring to the functional aspects of the transgene with reference to stress.

During secondary growth, the thickening of plant organs, wood (xylem) and bast (phloem) is continuously produced by the vascular cambium. In Arabidopsis hypocotyl and root, we can distinguish two phases of secondary growth based on cell morphology and production rate. The first phase, in which xylem and phloem are equally produced, precedes the xylem expansion phase in which xylem formation is enhanced and xylem fibers differentiate. It is known that gibberellins (GA) trigger this developmental transition via degradation of DELLA proteins and that the cambium master regulator BREVIPEDICELLUS/KNAT1 (BP/KNAT1) and receptor like kinases ERECTA and ERL1 regulate this process downstream of GA. However, our understanding of the regulatory network underlying GA-mediated secondary growth is still limited. Here, we demonstrate that DELLA-mediated xylem expansion in Arabidopsis hypocotyl is mainly achieved through DELLA family members RGA and GAI, which promote cambium senescence. We further show that AUXIN RESPONSE FACTOR 6 (ARF6) and ARF8, which physically interact with DELLAs, specifically repress phloem proliferation and induce cambium senescence during the xylem expansion phase. Moreover, the inactivation of BP in arf6 arf8 background revealed an essential role for ARF6 and ARF8 in cambium establishment and maintenance. Overall, our results shed light on a pivotal hormone cross-talk between GA and auxin in the context of plant secondary growth.

Aims Exposure of Eucalyptus tree stems to the radiant heat of forest fires can kill cambial cells and their embedded regenerative meristems, thus preventing epicormic resprouting and recovery of the tree. Currently, there is no tissue-level method to quantify the viability of cambial cells in Eucalyptus following heat exposure. The first aim of this study was to adapt and validate the tetrazolium reduction method of testing for cell viability in Eucalyptus. The second aim was to apply the method to establish a threshold level of cambium cell viability in Eucalyptus obliqua to enable the identification of a critical temperature. Methods The study used the tetrazolium reduction method to quantitatively determine phloem–cambium cell viability in Eucalyptus. Circular sections of bark with underlying phloem and cambium were cut from mature E. obliqua and samples ranging in mass from 1 to 30 mg were exposed for 1 min to temperature treatments ranging from 20 to 85 °C and kept for 20–22 h at room temperature in 0.8% 2,3,5-triphenyl tetrazolium chloride (TTC) to test for cell viability. The 1,3,5-triphenyl tetrazolium formazan (TPF) formed was cold extracted with ethanol and quantified as absorbance at 485 nm. Important Findings The TTC reduction method reliably quantified a decline in cell viability with rising temperature in tissue sections that included vascular cambium, and identified 60 °C as the critical temperature for cambium–phloem cells of Eucalyptus species. Cell viability, calculated as [TPF Treatment °C]/[TPF 20 °C], declined by 90% between 20 and 85 °C. The cell viability results confirmed that significant tissue necrosis occurred in Eucalyptus at temperatures between 50 and 70 °C, after 1 min of in vitro tissue heating. The decline in cell viability with increasing temperature shown by the TTC method was consistent with an independently derived count of live cells following temperature treatment and neutral red staining.

Like communities of people, communities of cells must continuously communicate to thrive. Polypeptide signaling molecules that act as mobile ligands are widely used by eukaryotic organisms to transmit information between cells to coordinate developmental processes and responses to environmental cues. In plants, the CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) genes encode a large family of extracellular signaling peptides that stimulate receptor-mediated signal transduction cascades to modulate diverse developmental and physiological processes. This review highlights the emerging roles of Arabidopsis thaliana CLE peptide signaling pathways in shoot stem cell homeostasis and root xylem development, as well as in root protophloem cell differentiation, vascular cambium activity, and stomatal formation and closure.

Chinquapin (Castanea henryi) is a dual-purpose tree species in China valued for as a source of timber and starch. We investigated the effect of four cutting mediums (pure vermiculite; peat:river sand at 3:1 v/v; peat:krasnozem at 1:1 v/v; and pure krasnozem) and three stem cutting periods (March, May, and July) on rooting performance of C. henryi cuttings. Different cutting periods and cutting mediums greatly influenced the rooting rate of C. henryi, ranging from 3.35 to 77.31%. Principal component analysis indicated that the best combination of cutting period and cutting medium was semi-hardwood cuttings (May cuttings) + krasnozem. Histological evidence indicated that adventitious root initials were present by week 5–6, and that the site of root primordia initiation was observed in the vascular cambium. Stem anatomical structures observed at different periods indicated that a xylem/radius ratio of 29.90–37.42% and a fractured phloem fiber ring are indicative of rooting success. The relational model between rooting index and medium properties indicated that nutrient content and porosity significantly influenced callus production. However, pH strongly affected C. henryi root formation, with the Pearson correlation coefficients for May and July cuttings of − 0.856 and − 0.947, respectively. Our protocol is helpful to achieve mass clone propagation of improved C. henryi genotypes, thus overcoming a common hurdle in chinquapin breeding programs.

Secondary growth is a key characteristic of trees, which requires the coordination of multiple regulatory mechanisms including transcriptional regulators and microRNAs (miRNAs). However, the roles of microRNAs in the regulation of secondary growth need to be explored in depth. Here, the role of miR319a and its target, PtoTCP20, in the secondary growth of Populus tomentosa stem was investigated using genetic and molecular analyses. The expression level of miR319a gradually decreased from primary to secondary growth in P. tomentosa, while that of PtoTCP20 gradually increased. MiR319a overexpression in seedlings resulted in delayed secondary growth and decreased xylem production, while miR319a knockdown and PtoTCP20 overexpression promoted secondary growth and increased xylem production. Further analysis showed that PtoTCP20 interacted with PtoWOX4a and activated PtoWND6 transcription in vitro and in vivo. Our data show that PtoTCP20 controls vascular cambium proliferation by binding to PtoWOX4a, and promotes secondary xylem differentiation by activating PtoWND6 transcription, thereby regulating secondary growth in P. tomentosa. Our findings provide insight into the molecular mechanisms underlying secondary growth in trees.

Variation in susceptibility of 28 Prunus rootstock genotypes to the causal agents of Armillaria root rot, Armillaria mellea, A. solidipes, and Desarmillaria tabescens, were studied by conducting in vitro root screening assays. Root segments with wounded and intact periderm were placed next to and on the top of the fungal cultures. At day 21, the percent success of fungal penetration and the circumferential and longitudinal lengths of fungal growth were measured. A parallel investigation using the inoculated root segments was carried out to characterize the active host defense mechanisms involved, including anatomical responses in bark and wood. Overall, the success of penetration and the longitudinal and circumferential spread of Armillaria spp. and D. tabescens were significantly different among various Prunus rootstock genotypes. None of the tested rootstock genotypes were completely resistant to infection. However, plum and plum derived rootstocks, and some of the cherry genotypes were less susceptible to infection compared to the peach genotypes. The host’s ability to limit infection by Armillaria spp. and D. tabescens was not limited to a single mechanism but appeared to be regulated by several collective nonspecific host responses acting together. Differential levels of a series of non-specific coordinated events were triggered in Prunus genotypes such as the formation of new callus tissue on the root surface, a colored reaction zone, necrophylactic periderm, new cells, and new vascular cambium to compartmentalize the pathogen. The host responses were elevated in the genotypes with low level of infection as compared to highly infected genotypes.

Cucumber is an economically important horticultural crop that is highly dependent on nitrogen fertilizer. Nitrate is the main nitrogen source for cucumber; however, the effects of nitrogen signaling on the early-stage growth of cucumber seedlings and the related regulatory mechanisms are still unclear. To compare seedling growth status at different nitrate levels, we performed a growth experiment using cucumber seedlings that had nearly exhausted their nitrogen reserves under nitrogen deficiency conditions (NO3−-N/NH4+-N = 0 in MS medium). Using qPCR and in situ RNA hybridization localization of candidate CsNPF genes, we found that short-term nitrogen deficiency promoted changes in root vascular bundle morphology and xylem growth in cucumber seedlings, thereby enhancing their growth potential. Among the candidate genes, CsNPF7.2, a gene located in the vascular cambium was found to be induced by short-term nitrogen deficiency. Considering the abundance of vasculature development marker genes, we speculated that the function of CsNPF7.2 might relate to the development of vascular bundles in plants suffering from nitrogen stress. The objective of our study was to investigate the growth changes in cucumber seedlings in response to different nitrogen levels, and to examine the mRNA accumulation and expression patterns of nitrate transporter CsNPF genes, so that critical genes can be identified to improve nitrogen use efficiency in cucumber cultivation. The results of this study provides a novel theoretical basis for optimizing cultivation, regulating rational fertilization levels, and improving nitrogen use efficiency in production. In addition, our study also provides new avenues for the further study of the function of CsNPF7.2 on regulating vasculature development in response to nitrogen stress.

For more than 225 million y, all seed plants were woody trees, shrubs, or vines. Shortly after the origin of angiosperms ∼140 million y ago (MYA), the Nymphaeales (water lilies) became one of the first lineages to deviate from their ancestral, woody habit by losing the vascular cambium, the meristematic population of cells that produces secondary xylem (wood) and phloem. Many of the genes and gene families that regulate differentiation of secondary tissues also regulate the differentiation of primary xylem and phloem, which are produced by apical meristems and retained in nearly all seed plants. Here, we sequenced and assembled a draft genome of the water lily Nymphaea thermarum , an emerging system for the study of early flowering plant evolution, and compared it to genomes from other cambium-bearing and cambium-less lineages (e.g., monocots and Nelumbo ). This revealed lineage-specific patterns of gene loss and divergence. Nymphaea is characterized by a significant contraction of the HD-ZIP III transcription factors, specifically loss of REVOLUTA , which influences cambial activity in other angiosperms. We also found the Nymphaea and monocot copies of cambium-associated CLE signaling peptides display unique substitutions at otherwise highly conserved amino acids. Nelumbo displays no obvious divergence in cambium-associated genes. The divergent genomic signatures of convergent loss of vascular cambium reveals that even pleiotropic genes can exhibit unique divergence patterns in association with independent events of trait loss. Our results shed light on the evolution of herbaceousness—one of the key biological innovations associated with the earliest phases of angiosperm evolution.

New observations of radial sieve tubes in the secondary xylem of two genera and four species of agarwood — Aquilaria sinensis, A. crasna, A. malaccensis and Gyrinops versteeghii (Thymelaeaceae) — are presented in this study. The earliest radial sieve tubes in Gyrinops are formed in the secondary xylem adjacent to the pith. The radial sieve tubes originate from the vascular cambium and develop in both uniseriate and multiseriate ray tissue. In addition to sieve plates in lateral and end walls, scattered or clustered minute sieve pores are localized in the lateral wall of radial sieve tubes. There is a direct connection between radial sieve tubes in ray tissue and axial sieve tubes in interxylary phloem strands (IP), such as (i) connection by bending of radial sieve tube strands, (ii) connection of two IP strands by an oblique bridge, and (iii) connection of two IP strands at a right angle. The average number of radial sieve tubes and interxylary phloem was found to be 1.7 per mm3 and 9.1 per mm2 in the secondary xylem. Considering the higher frequency of radial sieve tubes with the increasing thickness of the secondary xylem, the direct connections between radial and axial sieve tubes could play a significant role in assisting the translocation of metabolites in Aquilaria and Gyrinops.

Our study investigated the effect of stem temperature increase on xylem formation in Robinia pseudoacacia tree-trunks, caused by direct exposure to solar radiation. It is important to determine factors which may improve the concentricity of deposited wood tissue and intensify xylogenesis because a strong irregularity of wood tissue deposited in the radial direction in mature trees of R. pseudoacacia reduces the commercial value of the wood. Samples of vascular cambium along with adjacent tissues were collected from the southern (illuminated) and northern (shaded) side of tree-trunks growing in the inner and peripheral (thus exposed to direct sunlight) zones of the research plot. Sampling was performed several times during the growing season. The collected material was examined by epifluorescence microscopy and the thickness of deposited tissue comprising cambial xylem derivatives was measured. Deposition of a markedly greater amount of xylem on the southern side of tree-trunks in the peripheral zone of the plot was observed before full leaf development. Instrumental climatic data confirmed that in the early stage of the growing season, temperature on the southern side of the peripheral zone tree-trunk was higher than on the northern side. No clear response in terms of directional deposition of xylem was noticed in the inner zone trees and in peripheral zone trees after full leaf development. This study highlights the importance of temperature increase, caused by solar radiation, for R. pseudoacacia xylogenesis, which may be considered as a factor that affects the course of the radial growth before full leaf development.

A new bacterial disease of sweetpotato was recently identified in Puning, Guangdong Province, China and is referred to bacterial internal browning of sweetpotato in this paper. One-third of the tubers of sweetpotato were infected in the field. These tubers in the field with bacterial inner brown displayed yellowing on the skin that turned into slightly roughened and cracked surface with progress in disease. Cross-sections of these roots revealed a yellowish brown or dark brown circle in the vascular cambium. Pathogenicity tests were conducted on different organs of two sweetpotato cultivars and revealed that the isolated bacteria are pathogenic on stems and roots. We confirmed the causal agent as Burkholderia gladioli based on biochemical and physiological characterization, carbon source utilization, and sequence analysis of partial 16S rDNA and 16S–23S rDNA internal transcribed spacer (ITS) region. To our knowledge, it is the first report on the occurrence of B. gladioli in sweetpotato as a pathogen.

Plant stem cells are mainly located in the shoot apical meristem, root apical meristem and vascular cambium, which possess self-renewal and diverse differentiational potential in plants. Stem cells can maintain the size of their own cell population through cell division, or differentiate into various tissues and organs in plants. Maintaining the balance between the division and differentiation of stem cells is the basis for plants to accumulate biomass through longitudinal elongation and radial thickening growth. This process can be regulated by some factors such as genes, microRNAs (miRNAs), and plant hormones. This paper summarizes the research progress of stem cells regulating plant growth and development in recent years, and prospects for future research.

The periderm acts as the first line of defence for a plant, protecting wood and phloem from abiotic and biotic stresses. During secondary growth, through the increase in girth of plant organs, the periderm replaces the epidermis as the outermost tissue. The phellogen, a bifacial post-embryonic meristem, forms the phelloderm inwards (toward the vasculature) and the suberized phellem outwards (toward the environment). These three tissues are collectively referred to as the periderm. Here, we summarize recent findings on the molecular mechanisms of periderm development by describing periderm formation in connection to the fate of the surrounding tissues, by discussing common regulatory hubs between the vascular cambium and the phellogen, and by highlighting transcription factors (TFs) controlling phellem differentiation.

Aging is a universal property of multicellular organisms. Although some tree species can live for centuries or millennia, the molecular and metabolic mechanisms underlying their longevity are unclear. To address this, we investigated age-related changes in the vascular cambium from 15- to 667-y-old Ginkgo biloba trees. The ring width decreased sharply during the first 100 to 200 y, with only a slight change after 200 y of age, accompanied by decreasing numbers of cambial cell layers. In contrast, average basal area increment (BAI) continuously increased with aging, showing that the lateral meristem can retain indeterminacy in old trees. The indole-3-acetic acid (IAA) concentration in cambial cells decreased with age, whereas the content of abscisic acid (ABA) increased significantly. In addition, cell division-, cell expansion-, and differentiation-related genes exhibited significantly lower expression in old trees, especially miR166 and HD-ZIP III interaction networks involved in cambial activity. Disease resistance-associated genes retained high expression in old trees, along with genes associated with synthesis of preformed protective secondary metabolites. Comprehensive evaluation of the expression of genes related to autophagy, senescence, and age-related miRNAs, together with analysis of leaf photosynthetic efficiencies and seed germination rates, demonstrated that the old trees are still in a healthy, mature state, and senescence is not manifested at the whole-plant level. Taken together, our results reveal that long-lived trees have evolved compensatory mechanisms to maintain a balance between growth and aging processes. This involves continued cambial divisions, high expression of resistance-associated genes, and continued synthetic capacity of preformed protective secondary metabolites.

Conservation of endangered plant species requires a prerequisite basic knowledge of their taxonomy and ecology. Unfortunately in the Cordillera Central Range, several species have not been studied, even though this area harbors a unique type of vegetation not found in other parts of the Philippines. This study documents the morpho-anatomical characteristics of Rhododendron subsessile Rendle, one of the unique and endangered flora in the area. The species exhibits a morphology typical of plants in high elevations, being a hardy shrub with small tough leaves, and showy pink flowers that rival those of commercial rhododendron species. New findings on the species morphology include a variable number of corolla lobes, stamens and a description of its fruits and seeds. The anatomical features of the species are reported here for the first time. The plant exhibits a typical dicot anatomy with dorsiventral leaves, woody stem and root system with the usual arrangement of periderm followed by secondary phloem, vascular cambium, secondary xylem with vessel elements and pith (in the case of the stem). A unique characteristic was identified as the biseriate upper epidermis of the leaf. Documenting the morphological and anatomical features of the species should prove valuable as input baseline information in formulating measures for its conservation.

Extensive histology of host organs revealed the early events in the vegetative growth of Rafflesia consueloae including initial infection site, endophyte distribution, and other developmental events prior to bud emergence. The early events in the vegetative development of the holoparasite Rafflesia have long remained a mystery. Because its entire vegetative growth occurs within the host body, very little is known about the developmental events prior to emergence of the floral shoot. The goal of this study was to describe the events that occur during the vegetative growth of R. consueloae, particularly in the early stages of infection. We performed extensive microtome sectioning of multiple root and stem segments from different Tetrastigma host individuals to examine the cytology, distribution, and development of the R. consueloae endophyte within the host tissues. We found that R. consueloae infection is restricted to the roots of its host. Infection begins within the vascular cambium where the endophyte appears to initially reside prior to their radial spread to the vascular tissues. The tissues obtained from different host individuals had varying degrees of infection alluding to a possible role of host resistance mechanisms and/or varying levels of parasite infectiousness. Endophyte presence in host vines without external manifestations of infection indicates that the parasite may dwell within the host tissues for prolonged periods as small cell clusters without transitioning to the reproductive stage. Furthermore, we found that floral shoots may develop in scarcely infected host tissues indicating that extensive endophyte growth within the host is not a prerequisite to the onset of reproductive development. Overall, our study describes for the first time the developmental events prior to emergence of R. consueloae buds from its host.

Wood is the most abundant biomass produced by land plants and is mainly used for timber, pulping, and paper making. Wood (secondary xylem) is derived from vascular cambium, and its formation encompasses a series of developmental processes. Extensive studies in Arabidopsis and trees demonstrate that the initiation of vascular stem cells and the proliferation and differentiation of the cambial derivative cells require a coordination of multiple signals, including hormones and peptides. In this mini review, we described the recent discoveries on the regulation of the three developmental processes by several signals, such as auxin, cytokinins, brassinosteroids, gibberellins, ethylene, TDIF peptide, and their cross talk in Arabidopsis and Populus. There exists a similar but more complex regulatory network orchestrating vascular cambium development in Populus than that in Arabidopsis. We end up with a look at the future research prospects of vascular cambium in perennial woody plants, including interfascicular cambium development and vascular stem cell regulation.

Cryptomeria fortunei Hooibrenk is an important fast-growing coniferous timber species that is widely used in landscaping. Recently, research on timber quality has gained substantial attention in the field of tree breeding. Wood is the secondary xylem formed by the continuous inward division and differentiation of the vascular cambium; therefore, the development of the vascular cambium is particularly important for wood quality. In this study, we analyzed the transcriptomes of the cambial zone in C. fortunei during different developmental stages using Illumina HiSeq sequencing, focusing on general transcriptome and microRNA (miRNA) data. We performed functional annotation of the differentially expressed genes (DEGs) in the different stages identified by transcriptome sequencing and generated 15 miRNA libraries yielding 4.73 Gb of clean reads. The most common length of the filtered miRNAs was 21nt, accounting for 33.1% of the total filtered reads. We annotated a total of 32 known miRNA families. Some miRNAs played roles in hormone signal transduction (miR159, miR160, and miR166), growth and development (miR166 and miR396), and the coercion response (miR394 and miR395), and degradome sequencing showed potential cleavage sites between miRNAs and target genes. Differential expression of miRNAs and target genes and functional validation of the obtained transcriptome and miRNA data provide a theoretical basis for further elucidating the molecular mechanisms of cellular growth and differentiation, as well as wood formation in the vascular cambium, which will help improve the wood quality of C. fortunei.

Main conclusionThe monocot cambium is semi-storied, and its cells do not undergo rearrangement.AbstractThe monocot cambium is a lateral meristem responsible for secondary growth in some monocotyledons of Asparagales. It is an unusual meristem, not homologous with the vascular cambia of gymnosperms and non-monocotyledonous angiosperms. Owing to the limited information available on the characteristics of this meristem, the aim of this study was to survey the structure of the monocot cambium in order to clarify the similarities and dissimilarities of this lateral meristem to the vascular cambium of trees. Using the serial sectioning analysis, we have studied the monocot cambium of three species of arborescent monocotyledons, i.e., Quiver TreeAloe dichotoma, Dragon TreeDracaena draco, and Joshua TreeYucca brevifolia, native to different parts of the world. Data showed that in contrast to the vascular cambium, the monocot cambium is composed of a single type of short initials that vary in shape, and in tangential view display a semi-storied pattern. Furthermore, the cells of the monocot cambium do not undergo rearrangement. The criteria used in identifying monocot cambium initial cell are also discussed.

Wood (secondary xylem) formation in tree species is dependent on auxin-mediated vascular cambium activity in stems. However, the complex regulatory networks underlying xylem formation remain elusive. Xylem development in Populus was characterized based on microscopic observations of stem sections in transgenic plants. Transcriptomic, quantitative real-time PCR, chromatin immunoprecipitation PCR, and electrophoretic mobility shift assay analyses were conducted to identify target genes involved in xylem development. Yeast two-hybrid, pull-down, bimolecular fluorescence complementation, and co-immunoprecipitation assays were used to validate protein-protein interactions. PaC3H17 and its target PaMYB199 were found to be predominantly expressed in the vascular cambium and developing secondary xylem in Populus stems and play opposite roles in controlling cambial cell proliferation and secondary cell wall thickening through an overlapping pathway. Further, PaC3H17 interacts with PaMYB199 to form a complex, attenuating PaMYB199-driven suppression of its xylem targets. Exogenous auxin application enhances the dual control of the PaC3H17-PaMYB199 module during cambium division, thereby promoting secondary cell wall deposition. Dual regulation of xylem formation by an auxin-mediated PaC3H17-PaMYB199 module represents a novel regulatory mechanism in Populus, increasing our understanding of the regulatory networks involved in wood formation.

Vascular cambium, a lateral plant meristem, is a central producer of woody biomass. Although a few transcription factors have been shown to regulate cambial activity1, the phenotypes of the corresponding loss-of-function mutants are relatively modest, highlighting our limited understanding of the underlying transcriptional regulation. Here, we use cambium cell-specific transcript profiling followed by a combination of transcription factor network and genetic analyses to identify 62 new transcription factor genotypes displaying an array of cambial phenotypes. This approach culminated in virtual loss of cambial activity when both WUSCHEL-RELATED HOMEOBOX 4 (WOX4) and KNOTTED-like from Arabidopsis thaliana 1 (KNAT1; also known as BREVIPEDICELLUS) were mutated, thereby unlocking the genetic redundancy in the regulation of cambium development. We also identified transcription factors with dual functions in cambial cell proliferation and xylem differentiation, including WOX4, SHORT VEGETATIVE PHASE (SVP) and PETAL LOSS (PTL). Using the transcription factor network information, we combined overexpression of the cambial activator WOX4 and removal of the putative inhibitor PTL to engineer Arabidopsis for enhanced radial growth. This line also showed ectopic cambial activity, thus further highlighting the central roles of WOX4 and PTL in cambium development. The vascular cambium contains meristem cells that produce secondary xylem and phloem in the stems and roots of many plants. Its activity largely determines wood formation. Now, a genome-wide transcript profiling of Arabidopsis thaliana root cambium is presented to unlock the complex network that regulates cambium development and activity.

Wild ginseng (Panax ginseng) can survive in their natural habitat for hundreds of years, reflecting a remarkable plasticity. Plant stem cells (SCs) play a key role in the regenerative capacity and lifelong activity of these plants. WUSCHEL-RELATED HOMEOBOX (WOX) genes are master regulators of plant SC pluripotency, but their functions in medicinal plants have not been previously reported. To investigate whether these genes define different SC niches in ginseng, we cloned and analysed five WOX genes in ginseng (PgWOXs) and found that they might regulate root reconstruction. Then, the whole-mount RNA in situ hybridization was used to characterize the 3D gene expression pattern of PgWOXs in ginseng seedlings and cultured adventitious roots. PgWOX4 was expressed in vascular cambium SCs; PgWOX5 and PgWOX11 were mainly expressed in the tips of seedling and adventitious roots, which are the energetic centre of the meristem; and PgWOX13a and PgWOX13b were detected in the parenchyma cells of the main root of seedlings and cultured adventitious roots, suggesting that they are important for maintaining the balance between SC differentiation and self-renewal in the phloem and xylem. This is the first report of SC regulation in medical herbs; we expect that P. ginseng can serve as a model herb for investigating the relationship between SCs and their herbal morphological features, which would be a new research direction to improve the yield and quality of the medicinal materials by regulating the herbal SCs.

Tropical palms reach tree-like heights without a vascular cambium through sustained cell expansion and lignification of primary tissues, but only a fraction of palms have been explored in their allometric relationships. Here, our main question was to determine how palms depart from the traditional mechanical models developed for trees and how they approach the theoretical buckling limit. We analyzed the stem allometry of 1603 palms of 14 species from different strata at 10 sites in Costa Rica and Peru. We measured their fit to the stress, elastic, and geometric similarity models, and their position relative to the maximum theoretical buckling limit calculated for trees. We evaluated the slope of the linear and logarithmic regressions between stem diameter and height using logarithmic least squares, and standardized major axis regression (SMA), expecting segregation according to canopy position and geographic location. Seventeen out of 19 statistically significant models had SMA slopes > 1, and 11 had SMA slopes ≥ 2, departing from traditional mechanical models developed for trees. Many species varied their allometry relative to geographic location. Canopy palms showed the highest regression fit but had less steep slopes than understory and subcanopy species. Subcanopy and understory species were more underbuilt than canopy palms, increasing height faster than diameter. Some of the tallest canopy palms surpassed the maximum buckling limit whereas subcanopy and understory species were consistently below the buckling limit of record-size trees. Palm stem allometry changed in response to environmental conditions.

The early life history of Rafflesia R.Br. is hidden by its cryptic nature. Being a holoparasitic plant, early growth and development of species in this genus have been suggested to occur inside the host plant. In this study, using microscopic analyses, we examined how Rafflesia patma Blume grows from a simple clump of cells or protocorm inside the cambium tissue of Tetrastigma leucostaphylum (Dennst.) Alston, its host plant. We further show how the protocorm expands in about 2 months into an early form of a flower knob or bud, just before the flower begins to emerge from the host plant vine. Our study suggests that the endophyte, R. patma, spreads within T. leucostaphylum vascular cambium tissue in a linear manner, but not as a continuous strand. We suspect that the parasitic endophyte spreads inside the host vascular cambium and is pushed farther away from its point of origin to another part of the host as the host vine cambium fusiform initial cells divide and enlarge over months. Moreover, the protocorm matures in the cambium region of the T. leucostaphylum vine which lies close to the xylem, and grow outwards, bursting through the host phloem, then the host cortex, and finally towards the host periderm as it begins to emerge. Although we believe this might be the closest prediction on how Rafflesia develops at an early stage and is distributed within its host body, a future analysis of endophytic tissue growth in a controlled environment, such as the use of tissue culture, is required to strengthen our findings.

Clubroot, caused by the obligate biotrophic protist Plasmodiophora brassicae is a devastating disease of crucifers that causes substantial economic loss worldwide. The disease is characterized by the formation of galls in the root and hypocotyl of infected plants which restricts host vascular cambium development inhibiting efficient water and nutrient uptake by the plant. The pathogen- driven interference of hormonal homeostasis, particularly of cytokinin, in the root tissue is intricately linked with induction of hypertrophy and cell divisions leading to formation of galls. Levels of cytokinins and cell division generally increase at the onset of the disease which declines at the later stages of gall formation. The genes involved cytokinin biosynthesis such as cytokinin oxidase/ dehydrogenases and isopentenyl transferases shows differential expressions during clubroot infection and gall expansion in root tissues. Wider understanding of the roles of cytokinins and associated genes along the development of the disease will be helpful in unravelling plants defense mechanism against clubroot disease.

Stem cell populations in meristematic tissues at distinct locations in the plant body provide the potency of continuous plant growth. Primary meristems, at the apices of the plant body, contribute mainly to the elongation of the main plant axes, whereas secondary meristems in lateral positions are responsible for the thickening of these axes. The stem cells of the vascular cambium—a secondary lateral meristem—produce the secondary phloem (bast) and secondary xylem (wood). The sites of primary and secondary growth are spatially separated, and mobile signals are expected to coordinate growth rates between apical and lateral stem cell populations. Although the underlying mechanisms have not yet been uncovered, it seems likely that hormones, peptides, and mechanical cues orchestrate primary and secondary growth. In this review, we highlight the current knowledge and recent discoveries of how cambial stem cell activity is regulated, with a focus on mobile signals and the response of cambial activity to environmental and stress factors.

Xylem vessel structure changes as trees grow and mature. Age- and development-related changes in xylem structure are likely related to changes in hydraulic function. We examined whether hydraulic function, including hydraulic conductivity and vulnerability to water-stress-induced xylem embolism, changed over the course of cambial development in the stems of 17 tree species. We compared current-year growth of young (1-4 years), intermediate (2-7 years), and older (3-10 years) stems occurring in series along branches. Diffuse and ring porous species were examined, but nearly all species produced only diffuse porous xylem in the distal branches that were examined irrespective of their mature xylem porosity type. Vessel diameter and length increased with cambial age. Xylem became both more conductive and more cavitation resistant with cambial age. Ring porous species had longer and wider vessels and xylem that had higher conductivity and was more vulnerable to cavitation; however, these differences between porosity types were not present in young stem samples. Understanding plant hydraulic function and architecture requires the sampling of multiple-aged tissues because plants may vary considerably in their xylem structural and functional traits throughout the plant body, even over relatively short distances and closely aged tissues.

Tree bark is a highly specialized array of tissues that plays important roles in plant protection and development. Bark tissues develop from two lateral meristems; the phellogen (cork cambium) produces the outermost stem-environment barrier called the periderm, while the vascular cambium contributes with phloem tissues. Although bark is diverse in terms of tissues, functions and species, it remains understudied at higher resolution. We dissected the stem of silver birch (Betula pendula) into eight major tissue types, and characterized these by a combined transcriptomics and metabolomics approach. We further analyzed the varying bark types within the Betulaceae family. The two meristems had a distinct contribution to the stem transcriptomic landscape. Furthermore, inter- and intraspecies analyses illustrated the unique molecular profile of the phellem. We identified multiple tissue-specific metabolic pathways, such as the mevalonate/betulin biosynthesis pathway, that displayed differential evolution within the Betulaceae. A detailed analysis of suberin and betulin biosynthesis pathways identified a set of underlying regulators and highlighted the important role of local, small-scale gene duplication events in the evolution of metabolic pathways. This work reveals the transcriptome and metabolic diversity among bark tissues and provides insights to its development and evolution, as well as its biotechnological applications. Published version

Premise of the study New growth in the spring requires resource mobilization in the vascular system at a time when xylem and phloem function are often reduced in seasonally cold climates. As a result, the timing of leaf out and/or flowering could depend on when the vascular system resumes normal function in the spring. This study investigated whether flowering time is influenced by vascular phenology in plants that flower precociously before they have leaves. Methods Flower, leaf, and vascular phenology were monitored in pairs of precocious and non-precocious congeners. Differences in resource allocation were quantified by measuring bud dry mass and water content throughout the year, floral hydration was modelled, and a girdling treatment completed on branches in the field. Key results Precocious flowering species invested more in floral buds the year before flowering than did their non-precocious congeners, thus mobilizing less water in the spring, which allowed flowering before new vessel maturation. Conclusions A shift in the timing of resource allocation in precocious flowering plants allowed them to flower before the production of mature vessels and minimized the significance of seasonal changes in vascular function to their flowering phenology. The low investment required to complete floral development in the spring when the plant vascular system is often compromised could explain why flowers can emerge before leaf out.

Trees in temperate regions exhibit evident seasonal patterns, which play vital roles in their growth and development. The activity of cambial stem cells is the basis for regulating the quantity and quality of wood, which has received considerable attention. However, the underlying mechanisms of these processes have not been fully elucidated. Here we performed a comprehensive analysis of morphological observations, transcriptome profiles, the DNA methylome, and miRNAs of the cambium inPopulus tomentosaduring the transition from dormancy to activation. Anatomical analysis showed that the active cambial zone exhibited a significant increase in the width and number of cell layers compared with those of the dormant and reactivating cambium. Furthermore, we found that differentially expressed genes associated with vascular development were mainly involved in plant hormone signal transduction, cell division and expansion, and cell wall biosynthesis. In addition, we identified 235 known miRNAs and 125 novel miRNAs. Differentially expressed miRNAs and target genes showed stronger negative correlations than other miRNA/target pairs. Moreover, global methylation and transcription analysis revealed that CG gene body methylation was positively correlated with gene expression, whereas CHG exhibited the opposite trend in the downstream region. Most importantly, we observed that the number of CHH differentially methylated region (DMR) changes was the greatest during cambium periodicity. Intriguingly, the genes with hypomethylated CHH DMRs in the promoter were involved in plant hormone signal transduction, phenylpropanoid biosynthesis, and plant–pathogen interactions during vascular cambium development. These findings improve our systems-level understanding of the epigenomic diversity that exists in the annual growth cycle of trees.

(1) Research Highlights: Thermal damage to cambium cells of Eucalyptus held in vitro was recorded at sublethal temperatures (40 °C–50 °C) when the duration of exposure extends beyond 2.5 min up to 5 min. (2) Background and Objectives: During a forest fire, heat can be transferred through tree bark potentially impacting viability of vascular cambium cells and the perennial growth of the tree. With the increased temperature of the cambium, cells are known to lose viability at temperatures exceeding 60 °C. However, it is possible that extended exposure to temperatures below 60 °C may also impair cell viability. This study aimed to identify the effect of the temperature and exposure time interaction on the cambium cell viability of Eucalyptus, a genus widely distributed in natural forests and commercial plantations globally. (3) Methods: Excised cambium-phloem tissue sections from three Eucalyptus species (Messmate–E. obliqua L’Hér., Narrow-leaf peppermint–E. radiata Sieber ex DC. and Swamp gum–E. ovata Labill.) were exposed in vitro to a series of temperature–time treatments (40 °C, 50 °C, 60 °C, 70 °C for 1 min, 2.5 min, and 5 min) and tested for cell viability using a tetrazolium reduction method. (4) Results: Cell viability of cambium cells decreased with increased temperature and exposure times for all three Eucalyptus species. Longer exposure to sublethal temperatures of 40 °C to 50 °C showed statistically similar results to shorter exposure to lethal temperatures (&gt, 50 °C). (5) Conclusions: Longer exposure to sublethal temperatures (40 °C–50 °C) caused irreversible thermal damage to cambium cells of Eucalyptus when tested in vitro, further refining our understanding of raised temperature on cell viability.

Understanding the changing patterns of vascular cambium during seasonal cycles is crucial to reveal the mechanisms that control cambium activity and wood formation, but this area has been underexplored, especially in conifers. Here, we quantified the changing cellular morphology patterns of cambial zones during the active, transition and dormant stages. With the help of toluidine blue and periodic acid–Schiff staining to visualize cell walls and identify their constituents, we observed decreasing cambial cell layers, thickening of newly formed xylem cell walls and increased polysaccharide granules in phloem from June to the following March over the course of our collecting period. Pectin immunofluorescence showed that dormant-stage cambium can produce highly abundant de-esterified homogalacturonan and (1–4)-β-d-galactan epitopes, whereas active cambium can strong accumulate high methylesterified homogalacturonan. Calcofluor white staining and confocal Raman spectroscopy analysis revealed regular changes in the chemical composition of cell walls, such as relative lower cellulose deposition in transition stage in vascular cambium, and higher lignin accumulation was found in dormant stage in secondary xylem. Moreover, real-time quantitative polymerase chain reaction analysis suggested that various IAA (Aux/IAA protein), CesA, CslA and HDZ genes, as well as NAC, PME3 and PME4, may be involved in cambium activities and secondary xylem formation. Taken together, these findings provide new information about cambium activity and cell differentiation in the formation, structure and chemistry in conifers during the active–dormant transition.

The tight regulation of local auxin homeostasis and signalling maxima in xylem precursor cells specifies the organising activity of the vascular cambium and consequently promotes xylem differentiation and wood formation. However, the molecular mechanisms underlying the local auxin signalling maxima in the vascular cambium are largely unknown. Here, we reveal that brassinosteroid (BR)-activated WALLS ARE THIN1 (WAT1) facilitates wood formation by enhancing local auxin signalling in the vascular cambium in Solanum lycopersicum. Growth defects and low auxin signalling readouts in the BR-deficient tomato cultivar, Micro-Tom, were associated with a novel recessive allele, Slwat1-copi, created by the insertion of a retrotransposon in the last exon of the SlWAT1 locus. Molecular and genetic studies by generating the gain-of-function and loss-of-function tomato mutants revealed that SlWAT1 is a critical regulator for fine tuning local auxin homeostasis and signalling outputs in vascular cambium to facilitate secondary growth. Finally, we discovered that BR-regulated SlBZR1/2 directly activated downstream auxin responses by SlWAT1 upregulation in xylem precursor cells to facilitate xylem differentiation and subsequent wood formation. Our data suggest that the BR-SlBZR1/2-WAT1 signalling network contributes to the high level of auxin signalling in the vascular cambium for secondary growth.

During infection process, small galls emerge from epidermal cells in the elongation zone of host roots, and abundant secondary plasmodia, and resting spores can be seen within the parenchymal cells of such infected galls. At early infection stages in infected host cells, primary plasmodia contain complex cellular components including nuclei, lipid droplets, and an endomembrane system which is visible in epidermal cells in the root elongation zone. Cortical invasion by P. brassicae shows disruption of cell wall, presence of vesicles or inclusion bodies inside the cell wall, thickening of cell wall in combination with plasmodesmata, as well as elongation and/or disorganization of host nuclei. Multinucleate, intracellular plasmodia are visible within the clubs in the cortex and pericycle cells. Gall development amplifies merismatic activity within the vascular cambium and phloem parenchyma cells in the region of the hypocotyl. It also perturbs vascular development with reduction in xylem, and increase in phloem parenchyma cells in infected plants. Scanning electron microscopy of infected host galls revealed the initial plasmodia of the pathogen as spherical, smooth surfaces with an average diameter of 3.7 μm with an average size of 4.2 μm in diameter. Young plasmodia are round with partial rough surfaces and an average 8 μm in diameter. Vegetative plasmodia are smooth surface, spherical, and 45 μm in diameters which almost fill the host cells. Young resting spores are connected by fine filamentous materials, and mature resting spores have spiny surfaces. In resting sporangia during primary zoosporogenesis, single zoospores are differentiated within each sporangium being initiated by the emergence of two flagella in the tight space formed by invagination of plasma membrane within the sporangium. A deep groove is formed on the zoospore cell body by further invagination of the plasma membrane. Two flagella appear to coil around the zoospore cell body in parallel along this groove. A nucleolus like region is visible in the mature zoospores. A partial opened germ pore is seen in the sporangium containing the mature zoospores. Zoospores have electron dense mitochondria and well-developed endoplasmic reticulum. Mitotic nuclear division is characterized by a persistent nucleolus, bipolar centrioles paired end-to-end, densely stained chromatin, and a complex array of membranes that surrounded and ramify through the spindle. Chromatin begins to condense in prophase, and is aligned at metaphase in a reticulate plate on the nuclear equator. Large amounts of membrane cisternae within the spindle are visible at metaphase. Several concentric sheets of perinuclear endoplasmic reticulum surround the spindle and give the mitotic nucleus the superficial appearance of having an intact nuclear envelope.

Woody perennials usually have two distinct growth phases: juvenile and adult. The transition from juvenile wood (core wood) to adult wood (outer wood), which occurs gradually during tree development, is regulated by hormonal signals and their gradients along the plant axis, with opposing mechanisms in conifers versus angiosperms. Gibberellin is a key controlling signal in juvenile-adult transition. GA promotes the transition from the juvenile to the adult stage in young conifer trees by inducing the reproductive phase and cone production; while GA can delay the reproductive phase in angiosperms and may promote rejuvenation of adult woody angiosperms. Root signals arriving to the shoot apical meristem (SAM) delay the transition from the juvenile to the adult phase. Grafting adult branches on young juvenile rootstocks can promote rejuvenation of both buds and cambium in the scion (adult branch) that start to produce typical juvenile features. Since the cambium is influenced and produces wood in response to the hormonal signals it receives, the vague term cambial age (i.e., young cambia produce juvenile wood and old cambia produce mature wood) should be avoided. The nature of the produced wood does not depend on the chronological age of the vascular cambium, but on the hormonal signals and carbohydrates supply that it received; with the appropriate hormonal stimulation, an adult cambium can start to produce juvenile wood.

During secondary growth, the thickening of plant organs, wood (xylem) and bast (phloem) are continuously produced by the vascular cambium. In Arabidopsis hypocotyl and root, we can distinguish two phases of secondary growth based on cell morphology and production rate. The first phase, in which xylem and phloem are equally produced, precedes the xylem expansion phase in which xylem formation is enhanced and xylem fibers differentiate. It is known that Gibberellins (GA) trigger this developmental transition via the degradation of DELLA proteins and that the cambium master regulator BREVIPEDICELLUS/KNAT1 (BP/KNAT1) and the receptor like kinases ERECTA and ERL1 regulate this process downstream of GA. However, our understandings on the regulatory network underlying GA-mediated secondary growth, are still limited.Here, we demonstrate that DELLA-mediated xylem expansion is mainly achieved through RGA and GAI and that RGA and GAI promote cambium senescence. We further show that AUXIN RESPONSE FACTOR (ARF6) and ARF8, which physically interact with DELLAs, specifically repress phloem proliferation and induce cambium senescence during the xylem expansion phase. Moreover, the inactivation ofBPinarf6 arf8background revealed an essential role for ARF6 and ARF8 in cambium establishment and maintenance. Overall, our results shed light on a pivotal hormone cross-talk between GA and auxin in the context of plant secondary growth.

Interlocked grain occurs when the orientation of xylem fibres oscillates, alternating between left- and right-handed spirals in successive wood layers. The cellular mechanisms giving rise to interlocked grain, thought to involve the slow rotation of fusiform initials within the vascular cambium, remain unclear. We suggest that observations of wood structure at the cellular level, but over large areas, might reveal these mechanisms. We assayed timber from several commercially important tropical angiosperms from the genus Khaya (African mahogany) that exhibit interlocked grain using X-ray computed microtomography followed by orthogonal slicing and image processing in ImageJ. Reconstructed tangential longitudinal sections were processed with the ImageJ directionality plug-in to directly measure fibre orientation and showed grain deviations of more than 10° from vertical in both left- and right-handed directions. Grain changed at locally constant rates, separated by locations where the direction of grain change sharply reversed. Image thresholding and segmentation conducted on reconstructed cross sections allowed the identification of vessels and measurement of their location, with vessel orientations then calculated in Matlab and, independently, in recalculated tangential longitudinal sections with the directionality plug-in. Vessel orientations varied more than fibre orientations, and on average deviated further from vertical than fibres at the locations where the direction of grain change reversed. Moreover, the reversal location for vessels was shifted ~400 μm towards the pith compared with the fibres, despite both cell types arising from the same fusiform initials within the vascular cambium. We propose a simple model to explain these distinct grain patterns. Were an auxin signal to control both the reorientation of cambial initials, as well as coordinating the end-on-end differentiation and linkage of xylem vessel elements, then it would be possible for fibres and vessels to run at subtly different angles, and to show different grain reversal locations.

Separating out the different effects of ageing on long-lived trees remains challenging. Herein current approaches used to explore senescence in millennial trees are highlighted. Molecular and biochemical analyses of the vascular cambium provide novel insight into the extent to which millennial trees can withstand the wear and tear of ageing.

During post-embryonic development, the pericycle specifies the stem cells that give rise to both lateral roots (LRs) and the periderm, a suberized barrier that protects the plant against biotic and abiotic stresses. Comparable auxin-mediated signaling hubs regulate meristem establishment in many developmental contexts; however, it is unknown how specific outputs are achieved. Using the Arabidopsis root as a model, we show that while LR formation is the main auxin-induced program after de-etiolation, plants with age become competent to form a periderm in response to auxin. The establishment of the vascular cambium acts as the developmental switch required to trigger auxin-mediated periderm initiation. Moreover, distinct auxin signaling components and targets control LR versus periderm formation. Among the periderm-specific-promoting transcription factors, WUSCHEL-RELATED HOMEOBOX 4 (WOX4) and KNAT1/BREVIPEDICELLUS (BP) stand out as their specific overexpression in the periderm results in an increased number of periderm layers, a trait of agronomical importance in breeding programs targeting stress tolerance. These findings reveal that specificity in pericycle stem cell fate is achieved by the integration of developmental cues into distinct regulatory modules.

International audience; A key determinant of plant resistance to vascular infections lies in the ability of the host to successfully compartmentalize invaders at the xylem level. Growing evidence supports that the structural properties of the vascular system impact host vulnerability towards vascular pathogens. The aim of this study was to provide further insight into the impact of xylem vessel diameter on compartmentalization efficiency and thus vascular pathogen movement, using the interaction between Vitis and Phaeomoniella chlamydospora as a model system. We showed experimentally that an increased number of xylem vessels above 100 μm of diameter resulted in a higher mean infection level of host tissue. This benchmark was validated within and across Vitis genotypes. Although the ability of genotypes to restore vascular cambium integrity upon infection was highly variable, this trait did not correlate with their ability to impede pathogen movement at the xylem level. The distribution of infection severity of cuttings across the range of genotype's susceptibility suggests that a risk‐based mechanism is involved. We used this experimental data to calibrate a mechanistic stochastic model of the pathogen spread and we provide evidence that the efficiency of the compartmentalization process within a given xylem vessel is a function of its diameter.

Secondary or lateral growth, is the biological process that confers girth to stems and roots in plants, which is essential to structurally sustain organs, for water and nutrients transport and continuous plant growth. This process occurs at the expense of the cell division activity in two main lateral meristems—the vascular cambium and the cork cambium. From the activity of cambia, one of the main sources of biomass on Earth, wood, is produced. Given its economic, environmental and societal relevance a great deal of attention has been given to finding regulators of vascular cambium activity and wood formation. Some of the regulatory networks found are under post-transcriptional regulation of gene expression by microRNAs during vasculature formation, cambium activity, and wood formation and in several aspects of vascular development. In this chapter, we will briefly review the current knowledge on microRNAs roles during plant vascular development with a focus on recent work on miRNAs activity in secondary growth in plants.

Despite the large amounts of data available on lateral root formation, little is known about their initiation from secondary structures. In the present work, we applied a bending treatment to Populus nigra (L.) woody taproots to induce the formation of new lateral roots. The development of lateral roots was monitored by stereomicroscopic examination of cross-sections. Tissues were sampled from the bending zone in the proximity of the vascular cambium before (time 0) and after the application of bending at three different time points (24, 48, and 72 h) and analyzed for the expression of P. nigra WOX homologs. The initiation of new lateral roots was observed to originate from the vascular cambium zone and was followed by primordium formation and root emergence. PnWOX4a, PnWOX4b, PnWOX5a, PnWOX5b, PnWOX11/12a, and PnWOX11/12b were shown to be expressed during the formation of new lateral roots at different developmental stages. The mechanical stress simulated by bending treatment was shown to activate the molecular mechanism leading to the expression of WOX genes, which are hypothesized to control SLR formation in the cambium zone of poplar taproot.

Mg deficiency affected only the differentiation zone in Citrus sinensis roots. Phloem impairment in Mg-deficient leaves resulted in cell wall lignifications of both vascular cambium and spongy parenchyma cells. Decrease of Mg content in the roots hampered secondary lignification of endodermal and protoxylem cell walls. Magnesium (Mg) is a vital nutrient for plant photosynthesis and biochemical reactions. Mg deficiency is a widespread problem in citrus production, directly affecting yield and quality. With the objective of investigating the relationship among root and leaf anatomy, photosynthetic efficiency and mineral nutrient uptake and transport in Mg-deficient citrus, the effects of Mg deficiency in ‘Xuegan’ (Citrus sinensis) seedlings on plant biomass, leaf gas exchange and chlorophyll, nutrient contents, root and leaf anatomy and lignin biosynthesis were studied. Mg deficiency decreased root and shoot dry weight, leaf CO2 assimilation, chlorophyll and Mg, phosphorus (P), boron (B), cupper (Cu) and iron (Fe) concentration in roots, stems and leaves, and increased potassium (K) and calcium (Ca) concentration. Mg deficiency in the leaves resulted in phloem disorder that blocked phloem transport, leading to carbohydrate accumulation in chloroplasts and secondary lignifications of both vascular cambium and spongy parenchyma cell walls. In contrast, decrease of Mg content in the roots hampered lignin deposition on the protoxylem and endodermal cell walls at the root differentiation zone. Poorly lignified Casparian strip and xylem cell walls in Mg-deficient root tips might decrease xylem loading efficiency of nutrients, lowering their contents in the above ground parts, which ultimately inhibited whole plant growth.