Vascular Cambium: The Source of Wood Formation

A cytokinin response factor PtCRF1 is involved in the regulation of wood formation in poplar

Wood formation is a complex developmental process under the control of multiple levels of regulatory transcriptional network and hormone signals in trees. It is well known that cytokinin (CK) signaling plays an important role in maintaining the activity of the vascular cambium. The CK response factors (CRFs) encoding a subgroup of AP2 transcription factors have been identified to mediate the CK-dependent regulation in different plant developmental processes. However, the functions of CRFs in wood development remain unclear. Here, we characterized the function of PtCRF1, a CRF transcription factor isolated from poplar, in the process of wood formation. The PtCRF1 is preferentially expressed in secondary vasculature, especially in vascular cambium and secondary phloem, and encodes a transcriptional activator. Overexpression of PtCRF1 in transgenic poplar plants led to a significant reduction in the cell layer number of vascular cambium. The development of wood tissue was largely promoted in the PtCRF1-overexpressing lines, while it was significantly compromised in the CRISPR/Cas9-generated double mutant plants of PtCRF1 and its closest homolog PtCRF2. The RNA sequencing (RNA-seq) and quantitative reverse transcription PCR (RT-qPCR) analyses showed that PtCRF1 repressed the expression of the typical CK-responsive genes. Furthermore, bimolecular fluorescence complementation assays revealed that PtCRF1 competitively inhibits the direct interactions between histidine phosphotransfer proteins and type-B response regulator by binding to PtHP protein. Collectively, these results indicate that PtCRF1 negatively regulates CK signaling and is required for woody cell differentiation in poplar.

Cultivating potential: Harnessing plant stem cells for agricultural crop improvement

Meristems are stem cell-containing structures that produce all plant organs and are therefore important targets for crop improvement. Developmental regulators control the balance and rate of cell divisions within the meristem. Altering these regulators impacts meristem architecture and, as a consequence, plant form. In this review, we discuss genes involved in regulating the shoot apical meristem, inflorescence meristem, axillary meristem, root apical meristem, and vascular cambium in plants. We highlight several examples showing how crop breeders have manipulated developmental regulators to modify meristem growth and alter crop traits such as inflorescence size and branching patterns. Plant transformation techniques are another innovation related to plant meristem research because they make crop genome engineering possible. We discuss recent advances on plant transformation made possible by studying genes controlling meristem development. Finally, we conclude with discussions about how meristem research can contribute to crop improvement in the coming decades.

Genome-wide identification and expression analysis of GDP-D-mannose pyrophosphorylase and KATANIN in Corymbia citriodora

The GDP-D-mannose pyrophosphorylase (GMP) and microtubule severing enzyme KATANIN (KTN) are crucial for wood formation. Although functional identification has been performed in Arabidopsis, few comprehensive studies have been conducted in forest trees. In this study, we discovered 8 CcGMP and 4 CcKTN genes by analyzing the whole genome sequence of Corymbia citriodora. The chromosomal location, genome synteny, phylogenetic relationship, protein domain, motif identification, gene structure, cis-acting regulatory elements, and protein-interaction of CcGMP and CcKTN were all investigated. KTN has just one pair of segmentally duplicated genes, while GMP has no duplication events. According to gene structure, two 5' UTRs were identified in CcGMP4. Furthermore, there is no protein-interaction between KTN and GMP. Based on real-time PCR, the expression of most genes showed a positive connection with DBH diameters. In addition, the expression of CcGMP4 and CcKTN4 genes were greater in different size tree, indicating that these genes are important in secondary xylem production. Overall, this findings will enhance our comprehension of the intricacy of CcGMP&CcKTN across diverse DBHs and furnish valuable insights for future functional characterization of specific genes in C. citriodora.

Regulation of regeneration in Arabidopsis thaliana

We employed several algorithms with high efficacy to analyze the public transcriptomic data, aiming to identify key transcription factors (TFs) that regulate regeneration in Arabidopsis thaliana. Initially, we utilized CollaborativeNet, also known as TF-Cluster, to construct a collaborative network of all TFs, which was subsequently decomposed into many subnetworks using the Triple-Link and Compound Spring Embedder (CoSE) algorithms. Functional analysis of these subnetworks led to the identification of nine subnetworks closely associated with regeneration. We further applied principal component analysis and gene ontology (GO) enrichment analysis to reduce the subnetworks from nine to three, namely subnetworks 1, 12, and 17. Searching for TF-binding sites in the promoters of the co-expressed and co-regulated (CCGs) genes of all TFs in these three subnetworks and Triple-Gene Mutual Interaction analysis of TFs in these three subnetworks with the CCGs involved in regeneration enabled us to rank the TFs in each subnetwork. Finally, six potential candidate TFs-WOX9A, LEC2, PGA37, WIP5, PEI1, and AIL1 from subnetwork 1-were identified, and their roles in somatic embryogenesis (GO:0010262) and regeneration (GO:0031099) were discussed, so were the TFs in Subnetwork 12 and 17 associated with regeneration. The TFs identified were also assessed using the CIS-BP database and Expression Atlas. Our analyses suggest some novel TFs that may have regulatory roles in regeneration and embryogenesis and provide valuable data and insights into the regulatory mechanisms related to regeneration. The tools and the procedures used here are instrumental for analyzing high-throughput transcriptomic data and advancing our understanding of the regulation of various biological processes of interest.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-023-00121-9.

Regulation of secondary growth by poplar BLADE-ON-PETIOLE genes in Arabidopsis

BLADE-ON-PETIOLE (BOP) genes are essential regulators of vegetative and reproductive development in land plants. First characterized in Arabidopsis thaliana (Arabidopsis), members of this clade function as transcriptional co-activators by recruiting TGACG-motif binding (TGA) basic leucine zipper (bZIP) transcription factors. Highly expressed at organ boundaries, these genes are also expressed in vascular tissue and contribute to lignin biosynthesis during secondary growth. How these genes function in trees, which undergo extensive secondary growth to produce wood, remains unclear. Here, we investigate the functional conservation of BOP orthologs in Populus trichocarpa (poplar), a widely-used model for tree development. Within the poplar genome, we identified two BOP-like genes, PtrBPL1 and PtrBPL2, with abundant transcripts in stems. To assess their functions, we used heterologous assays in Arabidopsis plants. The promoters of PtrBPL1 and PtrBPL2, fused with a β-glucuronidase (GUS) reporter gene showed activity at organ boundaries and in secondary xylem and phloem. When introduced into Arabidopsis plants, PtrBPL1 and PtrBPL2 complemented leaf and flower patterning defects in bop1 bop2 mutants. Notably, Arabidopsis plants overexpressing PtrBPL1 and PtrBPL2 showed defects in stem elongation and the lignification of secondary tissues in the hypocotyl and stem. Finally, PtrBPL1 and PtrBPL2 formed complexes with TGA bZIP proteins in yeast. Collectively, our findings suggest that PtrBPL1 and PtrBPL2 are orthologs of Arabidopsis BOP1 and BOP2, potentially contributing to secondary growth regulation in poplar trees. This work provides a foundation for functional studies in trees.

Genes expression profiles in vascular cambium of Eucalyptus urophylla × Eucalyptus grandis at different ages

BACKGROUND

Wood is a secondary xylem generated by vascular cambium. Vascular cambium activities mainly include cambium proliferation and vascular tissue formation through secondary growth, thereby producing new secondary phloem inward and secondary xylem outward and leading to continuous tree thickening and wood formation. Wood formation is a complex biological process, which is strictly regulated by multiple genes. Therefore, molecular level research on the vascular cambium of different tree ages can lead to the identification of both key and related genes involved in wood formation and further explain the molecular regulation mechanism of wood formation.

RESULTS

In the present study, RNA-Seq and Pac-Bio Iso-Seq were used for profiling gene expression changes in Eucalyptus urophylla × Eucalyptus grandis (E. urograndis) vascular cambium at four different ages. A total of 59,770 non-redundant transcripts and 1892 differentially expressed genes (DEGs) were identified. The expression trends of the DEGs related to cell division and differentiation, cell wall biosynthesis, phytohormone, and transcription factors were analyzed. The DEGs encoding expansin, kinesin, cycline, PAL, GRP9, KNOX, C2C2-dof, REV, etc., were highly expressed in E. urograndis at three years old, leading to positive effects on growth and development. Moreover, some gene family members, such as NAC, MYB, HD-ZIP III, RPK, and RAP, play different regulatory roles in wood formation because of their sophisticated transcriptional network and function redundantly.

CONCLUSIONS

These candidate genes are a potential resource to further study wood formation, especially in fast-growing and adaptable eucalyptus. The results may also serve as a basis for further research to unravel the molecular mechanism underlying wood formation.

Comparative Analysis of the Wood Metabolites of Three Poplar Clones Using UPLC-Triple-TOF-MS

Poplar, a woody tree species, is widely used for industrial production and as a protective forest belt. Different clones of poplar exhibit clear variation in terms of morphological and physiological features, however, the impact of the genetic variation on the composition and abundance of wood metabolite have not been fully determined. In this study, ultra-high pressure liquid chromatography-triple time of flight-mass spectrometer (UPLC-Triple-TOF-MS) was used to explore the metabolite changes in poplar wood from three clones, including Populus deltoides CL. '55/65', P. deltoides CL. 'Danhong', and P. nigra CL. 'N179'. A total of 699 metabolites were identified. Clustering analysis and principal component analysis display that the metabolic differences of wood have allowed distinguishing different species of poplar. Meanwhile, eight significantly different metabolites were screened between P. deltoides and P. nigra, which may be considered as valuable markers for chemotaxonomy. In addition, the highly discriminant 352 metabolites were obtained among the three clones, and those may be closely related to the distinction in unique properties (e.g., growth, rigidity and tolerance) of the poplar wood cultivars. This study provides a foundation for further studies on wood metabolomics in poplar, and offers chemotaxonomic markers that will stimulate the early screening of potentially superior trees.

Comprehensive Time-Course Transcriptome Reveals the Crucial Biological Pathways Involved in the Seasonal Branch Growth in Siberian Elm (Ulmus pumila)

Timber, the most prevalent organic material on this planet, is the result of a secondary xylem emerging from vascular cambium. Yet, the intricate processes governing its seasonal generation are largely a mystery. To better understand the cyclic growth of vascular tissues in elm, we undertook an extensive study examining the anatomy, physiology, and genetic expressions in Ulmus pumila. We chose three robust 15-year-old elm trees for our study. The cultivars used in this study were collected from the Inner Mongolia Autonomous Region in China and nurtured in the tree farm of Shandong Normal University. Monthly samples of 2-year-old elm branches were taken from the tree from February to September. Marked seasonal shifts in elm branch vascular tissues were observed by phenotypic observation: In February, the cambium of the branch emerged from dormancy, spurring growth. By May, elms began generating secondary xylem, or latewood, recognized by its tiny pores and dense cell structure. From June to August, there was a marked increase in the thickness of the secondary xylem. Transcriptome sequencing provides a potential molecular mechanism for the thickening of elm branches and their response to stress. In February, the tree enhanced its genetic responses to cold and drought stress. The amplified expression of CDKB, CYCB, WOX4, and ARF5 in the months of February and March reinforced their essential role in the development of the vascular cambium in elm. Starting in May, the elm deployed carbohydrates as a carbon resource to synthesize the abundant cellulose and lignin necessary for the formation of the secondary wall. Major genes participating in cellulose (SUC and CESA homologs), xylan (UGD, UXS, IRX9, IRX10, and IRX14), and lignin (PAL, C4H, 4CL, HCT, C3H, COMT, and CAD) biosynthetic pathways for secondary wall formation were up-regulated by May or/and June. In conclusion, our findings provided a foundation for an in-depth exploration of the molecular processes dictating the seasonal growth of elm timber.

The central role of stem cells in determining plant longevity variation

Vascular plants display a huge variety of longevity patterns, from a few weeks for several annual species up to thousands of years for some perennial species. Understanding how longevity variation is structured has long been considered a fundamental aspect of the life sciences in view of evolution, species distribution, and adaptation to diverse environments. Unlike animals, whose organs are typically formed during embryogenesis, vascular plants manage to extend their life by continuously producing new tissues and organs in apical and lateral directions via proliferation of stem cells located within specialized tissues called meristems. Stem cells are the main source of plant longevity. Variation in plant longevity is highly dependent on the activity and fate identity of stem cells. Multiple developmental factors determine how stem cells contribute to variation in plant longevity. In this review, we provide an overview of the genetic mechanisms, hormonal signaling, and environmental factors involved in controlling plant longevity through long-term maintenance of stem cell fate identity.

Genome-wide identification of auxin-responsive microRNAs in the poplar stem

BACKGROUND

Wood (secondary xylem) of forests is a material of great economic importance. Wood development is strictly controlled by both the phytohormone auxin and microRNAs (miRNAs). Currently, the regulatory mechanisms underlying wood formation by auxin-associated miRNAs remain unclear.

OBJECTIVE

This report was designed to identify auxin-responsive miRNAs during wood formation.

METHODS

Morphological observation of wood development in the poplar stems was performed under the treatment of different concentrations (0 mg/L, CK; 5 mg/L, Low; 10 mg/L, High) of indol-3-butyric acid (IBA). Using a small RNA sequencing strategy, the effect of IBA treatment on miRNAs expression was genome-widely analyzed.

RESULTS

In this study, we found that wood development of poplar was promoted by low concentration of IBA treatment but inhibited by high concentration of IBA treatment. Stringent bioinformatic analysis led to identification of 118 known and 134 novel miRNAs candidates. Sixty-nine unique developmental-related miRNAs, corresponding to 269 target genes, exhibited specific expression patterns in response to auxin, as was consistent with the influence of auxin application on wood formation. Three novel miRNAs had the most number (≥ 9) of target genes, belonging to SPL, GRF and ARF gene families. The evolutionary relationships and tissue expression patterns of 41 SPL, GRF and ARF genes in poplar were thus analyzed. Of them, four representative members and corresponding miRNAs were confirmed using RT-qPCR.

CONCLUSIONS

Our results may be helpful for a better understanding of auxin-induced regulation of wood formation in tree species.

Fine-tuning brassinosteroid biosynthesis via 3'UTR-dependent decay of CPD mRNA modulates wood formation in Populus

Brassinosteroids (BRs) are plant hormones that regulate wood formation in trees. Currently, little is known about the post-transcriptional regulation of BR synthesis. Here, we show that during wood formation, fine-tuning BR synthesis requires 3'UTR-dependent decay of Populus CONSTITUTIVE PHOTOMORPHOGENIC DWARF 1 (PdCPD1). Overexpression of PdCPD1 or its 3' UTR fragment resulted in a significant increase of BR levels and inhibited secondary growth. In contrast, transgenic poplars repressing PdCPD1 3' UTR expression displayed moderate levels of BR and promoted wood formation. We show that the Populus GLYCINE-RICH RNA-BINDING PROTEIN 1 (PdGRP1) directly binds to a GU-rich element in 3' UTR of PdCPD1, leading to its mRNA decay. We thus provide a post-transcriptional mechanism underlying BRs synthesis during wood formation, which may be useful for genetic manipulation of wood biomass in trees.

Exploring the Seasonal Dynamics and Molecular Mechanism of Wood Formation in Gymnosperm Trees

Forests, comprising 31% of the Earth's surface, play pivotal roles in regulating the carbon, water, and energy cycles. Despite being far less diverse than angiosperms, gymnosperms account for over 50% of the global woody biomass production. To sustain growth and development, gymnosperms have evolved the capacity to sense and respond to cyclical environmental signals, such as changes in photoperiod and seasonal temperature, which initiate growth (spring and summer) and dormancy (fall and winter). Cambium, the lateral meristem responsible for wood formation, is reactivated through a complex interplay among hormonal, genetic, and epigenetic factors. Temperature signals perceived in early spring induce the synthesis of several phytohormones, including auxins, cytokinins, and gibberellins, which in turn reactivate cambium cells. Additionally, microRNA-mediated genetic and epigenetic pathways modulate cambial function. As a result, the cambium becomes active during the summer, resulting in active secondary xylem (i.e., wood) production, and starts to become inactive in autumn. This review summarizes and discusses recent findings regarding the climatic, hormonal, genetic, and epigenetic regulation of wood formation in gymnosperm trees (i.e., conifers) in response to seasonal changes.

Identification and Expression Profile of CLE41/44-PXY-WOX Genes in Adult Trees Pinus sylvestris L. Trunk Tissues during Cambial Activity

WUSCHEL (WUS)-related homeobox (WOX) protein family members play important roles in the maintenance and proliferation of the stem cells in the cambium, the lateral meristem that forms all the wood structural elements. Most studies have examined the function of these genes in angiosperms, and very little was known about coniferous trees. Pine is one of the most critical forest-forming conifers globally, and in this research, we studied the distribution of WOX4, WOX13, and WOXG genes expression in Pinus sylvestris L. trunk tissues. Further, we considered the role of TDIF(CLE41/44)/TDR(PXY) signaling in regulating Scots pine cambial activity. The distribution of CLE41/44-PXY-WOXs gene expression in Scots pine trunk tissues was studied: (1) depending on the stage of ontogenesis (the first group of objects); and (2) depending on the stage of cambial growth (the second group of objects). The first group of objects is lingonberry pine forests of different ages (30-, 80-, and 180-year-old stands) in the middle taiga subzone. At the time of selection, all the trees of the studied groups were at the same seasonal stage of development: the formation of late phloem and early xylem was occurring in the trunk. The second group of objects is 40-year-old pine trees that were selected growing in the forest seed orchard. We took the trunk tissue samples on 27 May 2022, 21 June 2022, and 21 July 2022. We have indicated the spatial separation expressed of PsCLE41/44 and PsPXY in pine trunk tissues. PsCLE41/44 was differentially expressed in Fraction 1, including phloem cells and cambial zone. Maximum expression of the PsPXY gene occurred in Fraction 2, including differentiating xylem cells. The maximum expression of the PsCLE41/44 gene occurred on 27 May, when the number of cells in the cambial zone was the highest, and then it decreased to almost zero. The PsPXY gene transcript level increased from May to the end of July. We found that the highest transcript level of the PsWOX4 gene was during the period of active cell proliferation in the cambial zone, and also in the trees with the cambial age 63 years, which were characterized by the largest number of cell layers in the cambial zone. In this study, we have examined the expression profiles of genes belonging to the ancient clade (PsWOXG and PsWOX13) in stem tissues in Scots pine for the first time. We found that, in contrast to PsWOX4 (high expression that was observed during the period of active formation of early tracheids), the expression of genes of the ancient clade of the WOX genes was observed during the period of decreased cambial activity in the second half of the growing season. We found that PsWOX13 expression was shifted to Fraction 1 in most cases and increased from the phloem side, while PsWOXG expression was not clearly bound to a certain fraction. Based on the data, the role of the CLE41/44-PXY-WOX signaling module in regulating P. sylvestris cambial growth is discussed.

Cell-type-specific PtrWOX4a and PtrVCS2 form a regulatory nexus with a histone modification system for stem cambium development in Populus trichocarpa

Stem vascular cambium cells in forest trees produce wood for materials and energy. WOX4 affects the proliferation of such cells in Populus. Here we show that PtrWOX4a is the most highly expressed stem vascular-cambium-specific (VCS) gene in P. trichocarpa, and its expression is controlled by the product of the second most highly expressed VCS gene, PtrVCS2, encoding a zinc finger protein. PtrVCS2 binds to the PtrWOX4a promoter as part of a PtrWOX13a-PtrVCS2-PtrGCN5-1-PtrADA2b-3 protein tetramer. PtrVCS2 prevented the interaction between PtrGCN5-1 and PtrADA2b-3, resulting in H3K9, H3K14 and H3K27 hypoacetylation at the PtrWOX4a promoter, which led to fewer cambium cell layers. These effects on cambium cell proliferation were consistent across more than 20 sets of transgenic lines overexpressing individual genes, gene-edited mutants and RNA interference lines in P. trichocarpa. We propose that the tetramer-PtrWOX4a system may coordinate genetic and epigenetic regulation to maintain normal vascular cambium development for wood formation.

Comparing adventitious root-formation and graft-unification abilities in clones of Argania spinosa

Argania spinosa trees have attracted attention in recent years due to their high resistance to extreme climate conditions. Initial domestication activities practiced in Morocco. Here we report on selection and vegetative propagation of A. spinosa trees grown in Israel. Trees yielding relatively high amounts of fruit were propagated by rooting of stem cuttings. High variability in rooting ability was found among the 30 clones selected. In-depth comparison of a difficult-to-root (ARS7) and easy-to-root (ARS1) clone revealed that the rooted cuttings of ARS7 have a lower survival rate than those of ARS1. In addition, histological analysis of the adventitious root primordia showed many abnormal fused primordia in ARS7. Hormone profiling revealed that while ARS1 accumulates more cytokinin, ARS7 accumulates more auxin, suggesting different auxin-to-cytokinin ratios underlying the different rooting capabilities. The hypothesized relationship between rooting and grafting abilities was addressed. Reciprocal grafting was performed with ARS1/ARS7 but no significant differences in the success of graft unification between the trees was detected. Accordingly, comparative RNA sequencing of the rooting and grafting zones showed more differentially expressed genes related to rooting than to grafting between the two trees. Clustering, KEGG and Venn analyses confirmed enrichment of genes related to auxin metabolism, transport and signaling, cytokinin metabolism and signaling, cell wall modification and cell division in both regions. In addition, the differential expression of some key genes in ARS1 vs. ARS7 rooting zones was revealed. Taken together, while both adventitious root-formation and graft-unification processes share response to wounding, cell reprogramming, cell division, cell differentiation and reconnection of the vasculature, there are similar, but also many different genes regulating the two processes. Therefore an individual genotype can have low rooting capacity but good graft-unification ability.

The Regulation of Xylem Development by Transcription Factors and Their Upstream MicroRNAs

Xylem, as a unique organizational structure of vascular plants, bears water transport and supports functions necessary for plant survival. Notably, secondary xylem in the stem (i.e., wood) also has important economic and ecological value. In view of this, the regulation of xylem development has been widely concerned. In recent years, studies on model plants Arabidopsis and poplar have shown that transcription factors play important regulatory roles in various processes of xylem development, including the directional differentiation of procambium and cambium into xylem, xylem arrangement patterns, secondary cell wall formation and programmed cell death. This review focuses on the regulatory roles of widely and thoroughly studied HD-ZIP, MYB and NAC transcription factor gene families in xylem development, and it also pays attention to the regulation of their upstream microRNAs. In addition, the existing questions in the research and future research directions are prospected.

A Constitutively Active Cytokinin Receptor Variant Increases Cambial Activity and Stem Growth in Poplar

The cambial meristem is responsible for bark and wood formation in woody plants. The activity of the cambial meristem is controlled by various factors; one of them is the plant hormone cytokinin. Here, we have explored different approaches to genetically engineering cambial activity in poplar plants by the ectopic expression of a cytokinin biosynthesis gene with enhanced activity (named ROCK4) or of a gene encoding a constitutively active cytokinin receptor variant (ROCK3). Both genes are derived from Arabidopsis thaliana and were expressed in poplar trees under the control of their own promoter or the cambium-specific pHB8 promoter. pIPT3:ROCK4- and pHB8:ROCK4-expressing plants were smaller than wild-type plants and formed more lateral branches; pHB8:ROCK4 transgenic plants additionally showed an increased stem diameter. In contrast, pAHK3:ROCK3- and pHB8:ROCK3-expressing plants grew taller than wild type without an altered branching pattern and formed more cambial cells, leading to increased radial stem growth. The effectivity of ROCK3 when expressed in either secondary phloem cells or in cambial cells is consistent with a dual, tissue-autonomous and non-autonomous activity of cytokinin in regulating cambial activity. We propose ROCK3 as a novel gene to enhance biomass formation in woody plants.

Current Understanding of the Genetics and Molecular Mechanisms Regulating Wood Formation in Plants

Unlike herbaceous plants, woody plants undergo volumetric growth (a.k.a. secondary growth) through wood formation, during which the secondary xylem (i.e., wood) differentiates from the vascular cambium. Wood is the most abundant biomass on Earth and, by absorbing atmospheric carbon dioxide, functions as one of the largest carbon sinks. As a sustainable and eco-friendly energy source, lignocellulosic biomass can help address environmental pollution and the global climate crisis. Studies of Arabidopsis and poplar as model plants using various emerging research tools show that the formation and proliferation of the vascular cambium and the differentiation of xylem cells require the modulation of multiple signals, including plant hormones, transcription factors, and signaling peptides. In this review, we summarize the latest knowledge on the molecular mechanism of wood formation, one of the most important biological processes on Earth.

Changes in the Activity of the CLE41/PXY/WOX Signaling Pathway in the Birch Cambial Zone under Different Xylogenesis Patterns

The balance between cell proliferation and differentiation into other cell types is crucial for meristem indeterminacy, and both growth aspects are under genetic control. The peptide-receptor signaling module regulates the activity of the cambial stem cells and the differentiation of their derivatives, along with cytokinins and auxin. We identified the genes encoding the signaling module CLE41-PXY and the regulator of vascular cambium division WOX4 and studied their expression during the period of cambial growth in the radial row: the conducting phloem/cambial zone and the differentiating xylem in two forms of Betula pendula, silver birch and Karelian birch. We have shown that the expression maximum of the BpCLE41/44a gene precedes the expression maximum of the BpPXY gene. Non-figured Karelian birch plants with straight-grained wood are characterized by a more intensive growth and the high expression of CLE41/44-PXY-WOX4. Figured Karelian birch plants, where the disturbed ratio and spatial orientation of structural elements characterizes the wood, have high levels of BpWOX4 expression and a decrease in xylem growth as well as the formation of xylem with a lower vessel density. The mutual influences of CLE41-PXY signaling and auxin signaling on WOX4 gene activity and the proliferation of cambium stem cells are discussed.

Epigenetics at the crossroads of secondary growth regulation

The development of plant tissues and organs during post-embryonic growth occurs through the activity of both primary and secondary meristems. While primary meristems (root and shoot apical meristems) promote axial plant growth, secondary meristems (vascular and cork cambium or phellogen) promote radial thickening and plant axes strengthening. The vascular cambium forms the secondary xylem and phloem, whereas the cork cambium gives rise to the periderm that envelops stems and roots. Periderm takes on an increasingly important role in plant survival under climate change scenarios, but it is also a forest product with unique features, constituting the basis of a sustainable and profitable cork industry. There is established evidence that epigenetic mechanisms involving histone post-translational modifications, DNA methylation, and small RNAs play important roles in the activity of primary meristem cells, their maintenance, and differentiation of progeny cells. Here, we review the current knowledge on the epigenetic regulation of secondary meristems, particularly focusing on the phellogen activity. We also discuss the possible involvement of DNA methylation in the regulation of periderm contrasting phenotypes, given the potential impact of translating this knowledge into innovative breeding programs.

OsWAK112, A Wall-Associated Kinase, Negatively Regulates Salt Stress Responses by Inhibiting Ethylene Production

The wall-associated kinase (WAK) multigene family plays critical roles in various cellular processes and stress responses in plants, however, whether WAKs are involved in salt tolerance is obscure. Herein, we report the functional characterization of a rice WAK, WAK112, whose expression is suppressed by salt. Overexpression of OsWAK112 in rice and heterologous expression of OsWAK112 in Arabidopsis significantly decreased plant survival under conditions of salt stress, while knocking down the OsWAK112 in rice increased plant survival under salt stress. OsWAK112 is universally expressed in plant and associated with cell wall. Meanwhile, in vitro kinase assays and salt tolerance analyses showed that OsWAK112 possesses kinase activity and that it plays a negative role in the response of plants to salt stress. In addition, OsWAK112 interacts with S-adenosyl-L-methionine synthetase (SAMS) 1/2/3, which catalyzes SAM synthesis from ATP and L-methionine, and promotes OsSAMS1 degradation under salt stress. Furthermore, in OsWAK112-overexpressing plants, there is a decreased SAMS content and a decreased ethylene content under salt stress. These results indicate that OsWAK112 negatively regulates plant salt responses by inhibiting ethylene production, possibly via direct binding with OsSAMS1/2/3.