Cell Wall Deposition Research Articles

Glutathione mitigates aluminum toxicity in root-apex transition zone of rice through reducing aluminum absorption and maintaining redox balance

Aluminium (Al) toxicity is recognized as a major constraint on crop growth and production in acidic soils, and the transition zone (TZ) of plant root apex emerges as the major perception site of Al toxicity. Glutathione (GSH) is reported to be involved in plant responses to various abiotic stresses, but its role and mechanism under Al stress remain unknown. Here, we found that GSH significantly mitigated Al toxicity on rice as revealed by the promotion of root elongation, reduction of oxidative stress and Al absorption. GSH application scavenged Al-induced H2O2 burst by activating the ascorbate (AsA)-GSH cycle and proline synthesis in root-apex TZ, thereby alleviating oxidative stress. GSH effectively reduced Al-induced pectin increment and inhibits the H2O2-induced pectin methylesterase (PME) activity and demethylesterification degree in root-apex TZ, leading to a reduction in Al binding sites and subsequently Al deposition in cell walls, thereby attenuating the inhibitory effect of Al toxicity on cell elongation. In addition, GSH-derived phytochelatins (PCs) promoted the vacuolar Al sequestration in root-apex TZ, which alleviated Al toxicity to the cytoplasm. Taken together, our results indicate a mechanism underlying how GSH alleviates Al toxicity through influencing redox state and Al absorption in rice root-apex TZ.

Spatiotemporal bifurcation of HY5-mediated blue-light signaling regulates wood development during secondary growth

Plants have evolved photoreceptors to optimize their development during primary growth, including germination, hypocotyl elongation, cotyledon opening, and root growth, allowing them to adapt to challenging light conditions. The light signaling transduction pathway during seedling establishment has been extensively studied, but little molecular evidence is available for light-regulated secondary growth, and how light regulates cambium-derived tissue production remains largely unexplored. Here, we show that CRYPTOCHROME (CRY)-dependent blue light signaling and the subsequent attenuation of ELONGATED HYPOCOTYL 5 (HY5) movement to hypocotyls are key inducers of xylem fiber differentiation in Arabidopsis thaliana. Using grafted chimeric plants and hypocotyl-specific transcriptome sequencing of light signaling mutants under controlled light conditions, we demonstrate that the perception of blue light by CRYs in shoots drives secondary cell wall (SCW) deposition at xylem fiber cells during the secondary growth of hypocotyls. We propose that HY5 is a blue light–responsive mobile protein that inhibits xylem fiber formation via direct transcriptional repression of NAC SECONDARY WALL THICKENING PROMOTING 3 ( NST3 ). CRYs retain HY5 in the nucleus, impede its long-distance transport from leaf to hypocotyl, and they initiate NST3- driven SCW gene expression, thereby triggering xylem fiber production. Our findings shed light on the long-range CRYs-HY5-NST3 signaling cascade that shapes xylem fiber development, highlighting the activity of HY5 as a transcriptional repressor during secondary growth.

Application risk and value of Cd-enriched poplar wood: Wood properties, leaching characteristics and brown rot resistance

This study investigated the application risk and value of Cd-enriched poplar wood, focusing on its wood properties, leaching characteristics, and brown rot resistance. The results indicated that Cd deposition in cell walls significantly inhibited brown rot fungi, thereby enhancing decay resistance. Furthermore, the extent of improvement in brown rot resistance was linked to wood density: the higher the density of Cd-enriched poplar wood, the stronger its resistance to brown rot. As the Cd concentration increased, the Cd distribution abundance and the wood crystallinity gradually increased. Structural changes were observed, including fluctuating microfibril angle and double wall thickness of fibers and vessels. Cd concentrations exceeding 50 mg/kg altered the chemical composition of the cell walls. The binding form of Cd in wood cell wall showed a trend of bound Cd ＞ free Cd ＞ residual Cd. Cd leaching occurred under cyclic soaking in water, which may lead to secondary contamination. However, under the condition of 75 % relative humidity, Cd leaching was negligible, suggesting potential for safer use in controlled environments. These findings provide valuable insights into the management and application of Cd-enriched wood, especially in contexts where decay resistance is critical or in water-exposed environments.

Environmental Impacts on Plant Cell Wall Lignification

Lignin is a natural resource used for energy production and a widely applied basis in the chemical industry. Physiological and wood anatomical evidence now suggests that the degree of lignin deposition in plant cell walls is constrained by low temperature and enhanced by increased temperature. Placing these findings in an industrial setting implies planning lignin supply in the forecasted global warming scenario.

PagNAC2a promotes phloem fiber development by regulating PagATL2 in poplar

Phloem fiber is a key component of phloem tissue and is involved in supporting its structural integrity. NAC domain transcription factors are master switches that regulate secondary cell wall (SCW) biosynthesis in xylem fibers, but the mechanism by which NACs regulate phloem fiber development remains unexplored. Here, a NAC2-like gene in poplar, PagNAC2a, was shown to be involved in phloem fiber differentiation. qRT-PCR and GUS staining revealed that PagNAC2a was specifically expressed in the phloem zone of poplar stems. The overexpression of PagNAC2a in poplar increased plant biomass by increasing plant height, stem diameter, and leaf area. Stem anatomy analysis revealed that overexpression of PagNAC2a resulted in enhanced phloem fiber differentiation and cell wall deposition. In addition, PagNAC2a directly upregulated the expression of PagATL2, a gene involved in phloem development, as revealed by yeast one hybrid (Y1H) and electrophoretic mobility shift assay (EMSA) assays. Overall, we proposed that the PagNAC2a was a positive regulator of phloem fiber development in poplar, and these results provided insights into the molecular mechanisms involved in the differentiation of phloem fibers.

Identification of BpEXP family genes and functional characterization of the BpEXPA1 gene in the stems development of Betula platyphylla

Expansins (EXPs) are unique plant cell wall proteins with the ability to induce cell wall expansion and play potential roles in xylem development. In the present study, a total of 25 BpEXP genes were identified in Betula platyphylla. Results of bioinformatics analysis described that BpEXP gene family was highly conserved in the process of evolution. All these genes were clustered into four groups, EXPA (Expansin A), EXPB (Expansin B), EXLA (Expansin-like A) and EXLB (Expansin-like B), according to phylogenetic analysis and BpEXPA1 was highly homologous to PttEXP1 and PttEXP2. The results of RT-qPCR showed that BpEXPA1 was expressed higher in stems and preferentially expressed in the first internodes, followed by apical buds and the third internodes, promoter expression analysis with GUS assay demonstrated that it was expressed in developing xylem, suggesting that BpEXPA1 might be involved in the development of the primary stems of birch. Overexpression of BpEXPA1 can promote cortex cell expansion and then enlarge the cortex cell area and layer, however inhibit the secondary cell wall deposition and result in the thinner cell wall and larger lumens of xylem fiber in transgenic plants. This study will provide information for investigating the regulation mechanism of BpEXP family genes and gene resources for birch genetics improvement.

Septin Organization and Dynamics for Budding Yeast Cytokinesis.

Cytokinesis, the process by which the cytoplasm divides to generate two daughter cells after mitosis, is a crucial stage of the cell cycle. Successful cytokinesis must be coordinated with chromosome segregation and requires the fine orchestration of several processes, such as constriction of the actomyosin ring, membrane reorganization, and, in fungi, cell wall deposition. In Saccharomyces cerevisiae, commonly known as budding yeast, septins play a pivotal role in the control of cytokinesis by assisting the assembly of the cytokinetic machinery at the division site and controlling its activity. Yeast septins form a collar at the division site that undergoes major dynamic transitions during the cell cycle. This review discusses the functions of septins in yeast cytokinesis, their regulation and the implications of their dynamic remodelling for cell division.

Lr34/Yr18/Sr57/Pm38 confers broad-spectrum resistance to fungal diseases via sinapyl alcohol transport for cell wall lignification in wheat

The widely recognized pleiotropic adult plant resistance gene Lr34 encodes an ATP-binding cassette transporter and plays an important role in breeding wheat for enhanced resistance to multiple fungal diseases. Despite its significance, the mechanisms underlying Lr34-mediated pathogen defense remain largely unknown. Our study demonstrates that wheat lines carrying the Lr34res allele exhibit thicker cell walls and enhanced resistance to fungal penetration compared to those without Lr34res. Transcriptome and metabolite profiling revealed that the lignin biosynthetic pathway is suppressed in lr34 mutants, indicating a disruption in cell wall lignification. Additionally, we discovered that lr34 mutant lines are hypersensitive to sinapyl alcohol, a major monolignol crucial for cell wall lignification. Yeast accumulation and efflux assays confirmed that the LR34 protein functions as a sinapyl alcohol transporter. Both genetic and virus-induced gene silencing experiments demonstrated that the disease resistance conferred by Lr34 can be enhanced by incorporating the TaCOMT-3B gene, which is responsible for the biosynthesis of sinapyl alcohol. Collectively, our findings provide novel insights into the role of Lr34 in disease resistance through mediating sinapyl alcohol transport and cell wall deposition, and highlight the synergistic effect of TaCOMT-3B and Lr34 against multiple fungal pathogens by mediating cell wall lignification in adult wheat plants.

The transcription factor PagLBD4 represses cell differentiation and secondary cell wall biosynthesis in Populus

LBD (LATERAL ORGAN BOUNDARIES DOMAIN) transcription factors are key regulators of plant growth and development. In this study, we functionally characterized the PagLBD4 gene in Populus (Populus alba × Populus glandulosa). Overexpression of PagLBD4 (PagLBD4OE) significantly repressed secondary xylem differentiation and secondary cell wall (SCW) deposition, while CRISPR/Cas9-mediated PagLBD4 knockout (PagLBD4KO) significantly increased secondary xylem differentiation and SCW deposition. Consistent with the functional analysis, gene expression analysis revealed that SCW biosynthesis pathways were significantly down-regulated in PagLBD4OE plants but up-regulated in PagLBD4KO plants. We also performed DNA affinity purification followed by sequencing (DAP-seq) to identify genes bound by PagLBD4. Integration of RNA sequencing (RNA-seq) and DAP-seq data identified 263 putative direct target genes (DTGs) of PagLBD4, including important regulatory genes for SCW biosynthesis, such as PagMYB103 and PagIRX12. Together, our results demonstrated that PagLBD4 is a repressor of secondary xylem differentiation and SCW biosynthesis in Populus, which possibly lead to the dramatic growth repression in PagLBD4OE plants.

Auxins differentially affect growth in Scots pine and Norway spruce in spring and autumn

Trees adapt to changing environmental conditions largely by regulating growth processes, and ongoing climate aridization puts pressure on growth processes during spring and summer. However, trees can be limited in their ability to utilize favourable early-season and late-season conditions for growth, and many of these limitations can be due to shortages in auxin transport and/or signalling. We studied whether leaf spraying with auxins in spring and autumn can aid in the growth of Pinus sylvestris and Picea abies saplings. In spring, spruce demonstrated prominent positive effects of auxin supplementation on the growth of current-year shoot and xylem growth, with increased xylem cell formation, whereas a limited effect was observed in pine. Autumnal auxin treatment had prominent positive effects on the growth of the root system and xylem area in pine, activating xylem cell enlargement and increasing carbon deposition in cell walls. In spruce, the effects of auxin spraying on xylem growth were less prominent, and the effects on root growth were absent compared to those in pine. The metabolome analysis of pine organs demonstrated that the activation of growth processes in the xylem and roots was accompanied by a decrease in nonstructural carbohydrates in the cambial zone but an increase in roots, indicating that auxin influenced the link between growth and carbon balance in an organ-specific manner. The major amino acid content was not influenced by auxin treatment. The observed results indicate that changes in the auxin balance during the autumnal period can activate growth processes to increase plant adaptation to changing climate conditions.

Upregulation of the glycine-rich protein-encoding gene GhGRPL enhances plant tolerance to abiotic and biotic stressors by promoting secondary cell wall development

Abiotic and biotic stressors adversely affect plant survival, biomass generation, and crop yields. As the global availability of arable land declines and the impacts of global warming intensify, such stressors may have increasingly pronounced effects on agricultural productivity. Currently, researchers face the overarching challenge of comprehensively enhancing plant resilience to abiotic and biotic stressors. The secondary cell wall plays a crucial role in bolstering the stress resistance of plants. To increase plant resistance to stress through genetic manipulation of the secondary cell wall, we cloned a cell wall protein designated glycine-rich protein-like (GhGRPL) from cotton fibers, and found that it is specifically expressed during the period of secondary cell wall biosynthesis. Notably, this protein differs from its Arabidopsis homolog, AtGRP, since its glycine-rich domain is deficient in glycine residues. GhGRPL is involved in secondary cell wall deposition. Upregulation of GhGRPL enhances lignin accumulation and, consequently, the thickness of the secondary cell walls, thereby increasing the plant's resistance to abiotic stressors, such as drought and salinity, and biotic threats, including Verticillium dahliae infection. Conversely, interference with GhGRPL expression in cotton reduces lignin accumulation and compromises that resistance. Taken together, our findings elucidate the role of GhGRPL in regulating secondary cell wall development through its influence on lignin deposition, which, in turn, reinforces cell wall robustness and impermeability. These findings highlight the promising near-future prospect of adopting GhGRPL as a viable, effective approach for enhancing plant resilience to abiotic and biotic stress factors.

Cotton sphingosine kinase GhLCBK1 participates in fiber cell elongation by affecting sphingosine-1-phophate and auxin synthesis

Sphingolipids serve as essential components of biomembrane and possess significant bioactive properties. Sphingosine-1-phophate (S1P) plays a key role in plant resistance to stress, but its specific impact on plant growth and development remains to be fully elucidated. Cotton fiber cells are an ideal material for investigating the growth and maturation of plant cells. In this study, we examined the content and composition of sphingosine (Sph) and S1P throughout the progression of fiber cell development. The content of S1P elevated gradually during fiber elongation but declined during the transition stage. Exogenous application of S1P promoted fiber elongation while using of FTY720 (an antagonist of S1P), and DMS (an inhibitor of LCBK) hindered fiber elongation. Cotton Long Chain Base Kinase 1 (GhLCBK1) was notably expressed during the fiber elongation stage, containing all conserved domains of LCBK protein and localized in the endoplasmic reticulum. Overexpression GhLCBK1 increased the S1P content and promoted fiber elongation while retarded secondary cell wall (SCW) deposition. Conversely, downregulation of GhLCBK1 reduced the S1P levels, and suppressed fiber elongation, and accelerated SCW deposition. Transcriptome analysis revealed that upregulating GhLCBK1 or applying S1P induced the expression of GhEXPANSIN and auxin related genes. Furthermore, the levels of IAA were elevated and reduced in the fibers when up-regulating or down-regulating GhLCBK1, respectively. Our investigation demonstrated that GhLCBK1 and its product S1P facilitated the elongation of fiber cells by affecting auxin biosynthesis. This study contributes novel insights into the intricate regulatory pathways involved in fiber cell elongation, identifying GhLCBK1 as a potential target gene and laying the groundwork for enhancing fiber quality via genetic manipulation.

The development of pleiotropic phenotypes in powdery mildew-resistant barley and Arabidopsis thaliana mlo mutants is linked to nitrogen availability.

Powdery mildew-resistant barley (Hordeum vulgare) and Arabidopsis thaliana mlo mutant plants exhibit pleiotropic phenotypes such as the spontaneous formation of callose-rich cell wall appositions and early leaf chlorosis and necrosis, indicative of premature leaf senescence. The exogenous factors governing the occurrence of these undesired side effects remain poorly understood. Here, we characterised the formation of these symptoms in detail. Ultrastructural analysis revealed that the callose-rich cell wall depositions spontaneously formed in A. thaliana mlo mutants are indistinguishable from those induced by the bacterial pattern epitope, flagellin 22 (flg22). We further found that increased plant densities during culturing enhance the extent of the leaf senescence syndrome in A. thaliana mlo mutants. Application of a liquid fertiliser rescued the occurrence of leaf chlorosis and necrosis in both A. thaliana and barley mlo mutant plants. Controlled fertilisation experiments uncovered nitrogen as the macronutrient whose deficiency promotes the extent of pleiotropic phenotypes in A. thaliana mlo mutants. Light intensity and temperature had a modulatory impact on the incidence of leaf necrosis in the case of barley mlo mutant plants. Collectively, our data indicate that the development of pleiotropic phenotypes associated with mlo mutants is governed by various exogenous factors.

Cortical microtubule dynamics during reaction wood formation ensures context-appropriate cellulose microfibril angle in woody trees

Key messageCortical microtubule arrays are the primary mechanism for guiding the re-orientation of cellulose microfibrils and determining MFA in secondary cell wall of wood fibre and tracheid cells in reaction wood.Microtubules are directly and indirectly involved in guiding cellulose synthase complexes (CSCs) through the plasma membrane. The angle of cellulose deposition is a critical response to environmental signals and/or stress conditions, and particularly crucial during reaction wood formation, a process in which woody plants deposit additional cell wall material to counteract gravitational forces. Tubulin genes are upregulated in response to gravitational stimulus during reaction wood formation, which can result in changes to microtubule assembly. In this study, microtubules were visualised in three woody tree species (two angiosperms: Eucalyptus globulus Labill., Populus alba L., and one gymnosperm: Pinus radiata D. Don.) using immunofluorescence to quantitatively evaluate microtubule organisation during reaction wood formation. Our results suggest that reorientation of the cortical microtubule array affects secondary cell wall deposition, even across different types of reaction wood, by ensuring context-appropriate orientation of cellulose microfibrils and determining MFA in wood cells. Pharmacological studies conducted on in vitro cultured stem segments or in vivo during reaction wood formation corroborated these important roles for microtubules during wood development. This study starts to unveil the role of tubulins during wood formation by exploring cortical microtubule array organisation in trees subjected to gravitational stimulus and it sheds light on cellular and molecular mechanisms behind cellulose deposition in tree species.

Mutating alfalfa COUMARATE 3-HYDROXYLASE using multiplex CRISPR/Cas9 leads to reduced lignin deposition and improved forage quality.

Alfalfa (Medicago sativa L.) forage quality is adversely affected by lignin deposition in cell walls at advanced maturity stages. Reducing lignin content through RNA interference or antisense approaches has been shown to improve alfalfa forage quality and digestibility. We employed a multiplex CRISPR/Cas9-mediated gene-editing system to reduce lignin content and alter lignin composition in alfalfa by targeting the COUMARATE 3-HYDROXYLASE (MsC3H) gene, which encodes a key enzyme in lignin biosynthesis. Four guide RNAs (gRNAs) targeting the first exon of MsC3H were designed and clustered into a tRNA-gRNA polycistronic system and introduced into tetraploid alfalfa via Agrobacterium-mediated transformation. Out of 130 transgenic lines, at least 73 lines were confirmed to contain gene-editing events in one or more alleles of MsC3H. Fifty-five lines were selected for lignin content/composition analysis. Amongst these lines, three independent tetra-allelic homozygous lines (Msc3h-013, Msc3h-121, and Msc3h-158) with different mutation events in MsC3H were characterized in detail. Homozygous mutation of MsC3H in these three lines significantly reduced the lignin content and altered lignin composition in stems. Moreover, these lines had significantly lower levels of acid detergent fiber and neutral detergent fiber as well as higher levels of total digestible nutrients, relative feed values, and in vitro true dry matter digestibility. Taken together, these results showed that CRISPR/Cas9-mediated editing of MsC3H successfully reduced shoot lignin content, improved digestibility, and nutritional values without sacrificing plant growth and biomass yield. These lines could be used in alfalfa breeding programs to generate elite transgene-free alfalfa cultivars with reduced lignin and improved forage quality.

GhHB14_D10 and GhREV_D5, two HD-ZIP III transcription factors, play a regulatory role in cotton fiber secondary cell wall biosynthesis.

GhHB14_D10 and GhREV_D5 regulated secondary cell wall formation and played an important role in fiber development. Cotton serves as an important source of natural fiber, and the biosynthesis of the secondary cell wall plays a pivotal role in determining cotton fiber quality. Nevertheless, the intricacies of this mechanism in cotton fiber remain insufficiently elucidated. This study investigates the functional roles of GhHB14_D10 and GhREV_D5, two HD-ZIP III transcription factors, in secondary cell wall biosynthesis in cotton fibers. Both GhHB14_D10 and GhREV_D5 were found to be localized in the nucleus with transcriptional activation activity. Ectopic overexpression of GhHB14_D10 and GhREV_D5 in Arabidopsis resulted in changed xylem differentiation, secondary cell wall deposition, and expression of genes related to the secondary cell wall. Silencing of GhHB14_D10 and GhREV_D5 in cotton led to enhanced fiber length, reduced cell wall thickness, cellulose contents and expression of secondary cell wall-related genes. Moreover, GhHB14_D10's direct interaction with GhREV_D5, and transcriptional regulation of cellulose biosynthesis genes GhCesA4-4 and GhCesA7-2 revealed their collaborative roles in secondary cell wall during cotton fiber development. Overall, these results shed light on the roles of GhHB14_D10 and GhREV_D5 in secondary cell wall biosynthesis, offering a strategy for the genetic improvement of cotton fiber quality.

Saccharomyces cerevisiae CellWall Remodeling in the Absence of Knr4 and Kre6 Revealed by Nano-FourierTransform Infrared Spectroscopy.

The cell wall integrity (CWI) signaling pathway regulates yeast cell wall biosynthesis, cell division, and responses to external stress. The cell wall, comprised of a dense network of chitin, β-1,3- and β-1,6- glucans, and mannoproteins, is very thin, <100 nm. Alterations in cell wall composition may activate the CWI pathway. Saccharomyces cerevisiae, a model yeast, was used to study the role of individual wall components in altering the structure and biophysical properties of the yeast cell wall. Near-field Fourier transform infrared spectroscopy (nano-FT-IR) was used for the first direct, spectrochemical identification of cell wall composition in a background (wild-type) strain and two deletion mutants from the yeast knock-out collection: kre6Δ and knr4Δ. Killer toxin resistant 6 (Kre6) is an integral membrane protein required for biosynthesis of β-1,6-glucan, while Knr4 is a cell signaling protein involved in the control of cell wall biosynthesis, in particular, biosynthesis and deposition of chitin. Complementary spectral data were obtained with far-field (FF)-FT-IR, in transmission, and with attenuated total reflectance (ATR) spectromicroscopy with 3-10 μm wavelength-dependent spatial resolution. The FF-FT-IR spectra of cells and spectra of isolated cell wall components showed that components of the cell body dominated transmission spectra and were still evident in ATR spectra. In contrast, the nano-FT-IR at ∼25 nm spatial resolution could be used to characterize the yeast wall chemical structure. Our results show that the β-1,6-glucan content is decreased in kre6Δ, while all glucan content is decreased in the knr4Δ cell wall. The latter may be thinner than in wild type, since not only are mannan and chitin detectable by nano-FT-IR, but also lipid membranes and protein, indicative of cell interior.

Analyses of high spatial resolution datasets identify genes associated with multi-layered secondary cell wall thickening in Pinus bungeana.

Secondary cell wall (SCW) thickening is a major cellular developmental stage determining wood structure and properties. Although the molecular regulation of cell wall deposition during tracheary element differentiation has been well established in primary growth systems, less is known about the gene regulatory processes involved in the multi-layered SCW thickening of mature trees. Using third-generation [long-read single-molecule real-time (SMRT)] and second-generation [short-read sequencing by synthesis (SBS)] sequencing methods, we established a Pinus bungeana transcriptome resource with comprehensive functional and structural annotation for the first time. Using these approaches, we generated high spatial resolution datasets for the vascular cambium, xylem expansion regions, early SCW thickening, late SCW thickening and mature xylem tissues of 71-year-old Pinus bungeana trees. A total of 79 390 non-redundant transcripts, 31 808 long non-coding RNAs and 5147 transcription factors were annotated and quantified in different xylem tissues at all growth and differentiation stages. Furthermore, using this high spatial resolution dataset, we established a comprehensive transcriptomic profile and found that members of the NAC, WRKY, SUS, CESA and LAC gene families are major players in early SCW formation in tracheids, whereas members of the MYB and LBD transcription factor families are highly expressed during late SCW thickening. Our results provide new molecular insights into the regulation of multi-layered SCW thickening in conifers. The high spatial resolution datasets provided can serve as important gene resources for improving softwoods.

Transcription factor PagMYB31 positively regulates cambium activity and negatively regulates xylem development in poplar.

Wood formation involves consecutive developmental steps, including cell division of vascular cambium, xylem cell expansion, secondary cell wall (SCW) deposition, and programmed cell death. In this study, we identified PagMYB31 as a coordinator regulating these processes in Populus alba × Populus glandulosa and built a PagMYB31-mediated transcriptional regulatory network. PagMYB31 mutation caused fewer layers of cambial cells, larger fusiform initials, ray initials, vessels, fiber and ray cells, and enhanced xylem cell SCW thickening, showing that PagMYB31 positively regulates cambial cell proliferation and negatively regulates xylem cell expansion and SCW biosynthesis. PagMYB31 repressed xylem cell expansion and SCW thickening through directly inhibiting wall-modifying enzyme genes and the transcription factor genes that activate the whole SCW biosynthetic program, respectively. In cambium, PagMYB31 could promote cambial activity through TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR (TDIF)/PHLOEM INTERCALATED WITH XYLEM (PXY) signaling by directly regulating CLAVATA3/ESR-RELATED (CLE) genes, and it could also directly activate WUSCHEL HOMEOBOX RELATED4 (PagWOX4), forming a feedforward regulation. We also observed that PagMYB31 could either promote cell proliferation through the MYB31-MYB72-WOX4 module or inhibit cambial activity through the MYB31-MYB72-VASCULAR CAMBIUM-RELATED MADS2 (VCM2)/PIN-FORMED5 (PIN5) modules, suggesting its role in maintaining the homeostasis of vascular cambium. PagMYB31 could be a potential target to manipulate different developmental stages of wood formation.

A subclass II bHLH transcription factor in Marchantia polymorpha gives insight into the ancestral land plant trait of spore formation

Sporopollenin is often said to be one of the toughest biopolymers known to man. The shift in dormancy cell wall deposition from around the diploid zygotes of charophycean algae to sporopollenin around the haploid spores of land plants essentially imparted onto land plants the gift of passive motility, a key acquisition that contributed to their vast and successful colonization across terrestrial habitats.1,2 A putative transcription factor controlling the land plant mode of sporopollenin deposition is the subclass II bHLHs, which are conserved and novel to land plants, with mutants of genes in angiosperms and mosses divulging roles relating to tapetum degeneration and spore development.3,4,5,6,7 We demonstrate that a subclass II bHLH gene, MpbHLH37, regulates sporopollenin biosynthesis and deposition in the model liverwort Marchantia polymorpha. Mpbhlh37 sporophytes show a striking loss of secondary wall deposits of the capsule wall, the elaters, and the spore exine, all while maintaining spore viability, identifying MpbHLH37 as a master regulator of secondary wall deposits of the sporophyte. Localization of MpbHLH37 to the capsule wall and elaters of the sporophyte directly designates these tissue types as a bona fide tapetum in liverworts, giving support to the notion that the presence of a tapetum is an ancestral land plant trait. Finally, as early land plant spore walls exhibit evidence of tapetal deposition,8,9,10,11,12 a tapetal capsule wall could have provided these plants with a developmental mechanism for sporopollenin deposition.