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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.

Background Wood provides an important biomass resource for biofuel production around the world. The radial growth of tree stems is central to biomass production for forestry and biofuels, but it is challenging to dissect genetically because it is a complex trait influenced by many genes. In this study, we adopted methods of physiology, transcriptomics and genetics to investigate the regulatory mechanisms of tree radial growth and wood development. Results Physiological comparison showed that two Populus genotypes presented different rates of radial growth of stems and accumulation of woody biomass. A comparative transcriptional network approach was used to define and characterize functional differences between two Populus genotypes. Analyses of transcript profiles from wood-forming tissue of the two genotypes showed that 1542, 2295 and 2110 genes were differentially expressed in the pre-growth, fast-growth and post-growth stages, respectively. The co-expression analyses identified modules of co-expressed genes that displayed distinct expression profiles. Modules were further characterized by correlating transcript levels with genotypes and physiological traits. The results showed enrichment of genes that participated in cell cycle and division, whose expression change was consistent with the variation of radial growth rates. Genes related to secondary vascular development were up-regulated in the faster-growing genotype in the pre-growth stage. We characterized a BEL1-like (BELL) transcription factor, PeuBELL15, which was up-regulated in the faster-growing genotype. Analyses of transgenic Populus overexpressing as well as CRISPR/Cas9-induced mutants for BELL15 showed that PeuBELL15 improved accumulation of glucan and lignin, and it promoted secondary vascular growth by regulating the expression of genes relevant for cellulose synthases and lignin biosynthesis. Conclusions This study illustrated that active division and expansion of vascular cambium cells and secondary cell wall deposition of xylem cells contribute to stem radial increment and biomass accumulation, and it identified relevant genes for these complex growth traits, including a BELL transcription factor gene PeuBELL15. This provides genetic resources for improving and breeding elite genotypes with fast growth and high wood biomass.

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.

The periderm acts as armor protecting the plant's inner tissues from biotic and abiotic stress. It forms during the radial thickening of plant organs such as stems and roots and replaces the function of primary protective tissues such as the epidermis and the endodermis. A wound periderm also forms to heal and protect injured tissues. The periderm comprises a meristematic tissue called the phellogen, or cork cambium, and its derivatives: the lignosuberized phellem and the phelloderm. Research on the periderm hasmainly focused on the chemical composition of the phellem due to its relevance as a raw material for industrial processes. Today, there is increasing interest in the regulatory network underlying periderm development as a novel breeding trait to improve plant resilience and to sequester CO2. Here, we discuss our current understanding of periderm formation, focusing on aspects of periderm evolution, mechanisms of periderm ontogenesis, regulatory networks underlying phellogen initiation and cork differentiation, and future challenges of periderm research.

The CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION (ESR)‐RELATED (CLE) peptide ligands in connection with their receptors are important players in cell‐to‐cell communications in plants. Here, we investigated the function of the Populus CLV3/ESR‐RELATED 47 (PttCLE47) gene during secondary growth and wood formation in hybrid aspen (Populus tremula × tremuloides) using an RNA interference (RNAi) approach. Expression of PttCLE47 peaks in the vascular cambium. Silencing of the PttCLE47 gene expression affected lateral expansion of stems and decreased apical height growth and leaf size. In particular, PttCLE47 RNAi trees exhibited a narrower secondary xylem zone with less xylem cells/cell file. The reduced radial growth phenotype also correlated with a reduced number of cambial cell layers. In agreement with these results, expression of several cambial regulator genes was downregulated in the stems of the transgenic trees in comparison with controls. Altogether, these results suggest that the PttCLE47 gene is a major positive regulator of cambial activity in hybrid aspen, mainly promoting the production of secondary xylem. Furthermore, in contrast to previously characterized CLE genes expressed in the wood‐forming zone, PttCLE47 appears to be active at its site of expression.

Aesculus hippocastanum trees are commonly infested by the leaf miner Cameraria ohridella, whose larval activity causes the destruction of the leaf parenchyma and induces defoliation. Pest attacks result in, e.g., production of smaller fruits and tree re-flowering in autumn. Concerning pest influence on stem structure only scarce information of narrower annual growth rings of wood has been published. Therefore, we determined the effect of the presence of the leaf miner infestation on intra-annual cambial activity and on differentiation of conductive tissues. These data were compared with phenological phases and pest activity. Pest feeding resulted in changes in onset, cessation and duration of cambial divisions, and differentiation of secondary xylem. The duration of cambial activity was about a month shorter in heavily infested trees and was connected with premature tree defoliation. Affected trees were characterised by a reduction in cambial divisions and earlier cessation of wood differentiation resulting in narrower wood rings. Furthermore, the infested trees exhibited altered wood structure, with more vessels of smaller diameters, however these changes did not affect its theoretical hydraulic conductivity. Interestingly, pest attack did not influence secondary phloem differentiation. The probable influence of long-term infestation on tree growth and condition was discussed. info:eu-repo/semantics/openAccess

Mechanical stress in tree roots induces the production of reaction wood (RW) and the formation of new branch roots, both functioning to avoid anchorage failure and limb damage. The vascular cambium (VC) is the factor responsible for the onset of these responses as shown by their occurrence when all primary tissues and the root tips are removed. The data presented confirm that the VC is able to evaluate both the direction and magnitude of the mechanical forces experienced before coordinating the most fitting responses along the root axis whenever and wherever these are necessary. The coordination of these responses requires intense crosstalk between meristematic cells of the VC which may be very distant from the place where the mechanical stress is first detected. Signaling could be facilitated through plasmodesmata between meristematic cells. The mechanism of RW production also seems to be well conserved in the stem and this fact suggests that the VC could behave as a single structure spread along the plant body axis as a means to control the relationship between the plant and its environment. The observation that there are numerous morphological and functional similarities between different meristems and that some important regulatory mechanisms of meristem activity, such as homeostasis, are common to several meristems, supports the hypothesis that not only the VC but all apical, primary and secondary meristems present in the plant body behave as a single interconnected structure. We propose to name this structure “meristematic connectome” given the possibility that the sequence of meristems from root apex to shoot apex could represent a pluricellular network that facilitates long-distance signaling in the plant body. The possibility that the “meristematic connectome” could act as a single structure active in adjusting the plant body to its surrounding environment throughout the life of a plant is now proposed.

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.

During secondary growth, forest trees can modify the anatomy of the wood produced by the vascular cambium in response to environmental conditions. Notably, the trees of the model angiosperm genus, Populus, reduce the risk of cavitation and hydraulic failure under water stress by producing water-conducting vessel elements with narrow lumens, which are more numerous and more interconnected with each other. Here, we determined the genetic architecture of vessel traits affecting hydraulic physiology and resilience to water stress. Vessel traits were measured for clonally replicated genotypes of a unique Populus deltoides x nigra population carrying genomically defined insertions and deletions that create gene dosage variation. We found significant phenotypic variation for all traits measured (mean vessel diameter, height-corrected mean vessel diameter, vessel frequency, height-corrected vessel frequency, vessel grouping index, and mean vessel circularity), and that all traits were under genetic control and showed moderate heritability values, ranging from 0.32 to 0.53. Whole-genome scans of correlations between gene dosage and phenotypic traits identified quantitative trait loci for tree height, mean vessel diameter, height-corrected mean vessel diameter, height-corrected vessel frequency, and vessel grouping index. Our results demonstrate that vessel traits affecting hydraulic physiology are under genetic control, and both pleiotropic and trait-specific quantitative trait loci are found for these traits.

Secondary cell wall (SCW) deposition is an important process during wood formation. Although aspartic proteases (APs) have been reported to have regulatory roles in herbaceous plants, the involvement of atypical APs in SCW deposition in trees has not been reported. In this study, we characterised the Populus trichocarpa atypical AP gene PtAP66, which is involved in wood SCW deposition. Transcriptome data from the AspWood resource showed that in the secondary xylem of P. trichocarpa, PtAP66 transcripts increased from the vascular cambium to the xylem cell expansion region and maintained high levels in the SCW formation region. Fluorescent signals from transgenic Arabidopsis plant roots and transiently transformed P. trichocarpa leaf protoplasts strongly suggested that the PtAP66-fused fluorescent protein (PtAP66-GFP or PtAP66-YFP) localised in the plasma membrane. Compared with the wild-type plants, the Cas9/gRNA-induced PtAP66 mutants exhibited reduced SCW thickness of secondary xylem fibres, as suggested by the scanning electron microscopy (SEM) data. In addition, wood composition assays revealed that the cellulose content in the mutants decreased by 4.90–5.57%. Transcription analysis further showed that a loss of PtAP66 downregulated the expression of several SCW synthesis-related genes, including cellulose and hemicellulose synthesis enzyme-encoding genes. Altogether, these findings indicate that atypical PtAP66 plays an important role in SCW deposition during wood formation.

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.