Involvement of VNS NAC-domain transcription factors in tracheid formation in Pinus taeda

Isolation and functional analysis of the Larix olgensis LoNAC3 transcription factor gene

BACKGROUND

Larch is an important timber tree species. The traditional methods of tree genetic breeding have been progressing slowly. It is necessary to carry out gene function analysis and genetically modified breeding research. The NAC transcription factor family is a plant-specific transcription factor family with various biological functions, as shown in recent research. However, there are few studies on the NAC gene among gymnosperm coniferous species.

RESULTS

LoNAC3 with complete cds was identified and isolated from the cDNA of Larix olgensis based on transcriptome data. The cDNA length of LoNAC3 is 1185 bp, encoding 394 amino acids, with a conserved NAM domain located at the N-terminus, and subcellular localization in the nucleus. The results of real-time quantitative PCR analysis showed that at different growth stages and in different tissues of L. olgensis, the relative expression level of LoNAC3 was highest in the needles. After drought, salt, alkali stress and hormone treatment, expression was induced to different degrees. The expression level of LoNAC3 was significantly increased under drought and salt conditions. The relative expression level changed under methyl jasmonate (MeJA) and abscisic acid (ABA) treatment. By observing the phenotype of overexpressed LoNAC3 tobacco, it was found that overexpressed tobacco is shorter and blooms earlier than wild-type tobacco. Under abiotic stress, LoNAC3 overexpressed tobacco has lower germination rates and poorer growth status. Transgenic tobacco under stress treatment has a higher malondialdehyde (MDA) content than wild-type tobacco, while peroxidase (POD) activity is lower than wild-type tobacco.

CONCLUSIONS

Through the analysis of LoNAC3 sequence and promoter expression, it can be concluded that LoNAC3 is involved in the drought and salt stress response processes of L. olgensis, and is induced by ABA and MeJA expression. Overexpression of LoNAC3 leads to stunted tobacco growth and negatively regulates its tolerance to drought and salt stress through the reactive oxygen species pathway. The preliminary analysis of the expression pattern and function of the LoNAC3 can provide a theoretical basis and high-quality materials for genetic improvement of larch in later stages.

Active protein ubiquitination regulates xylem vessel functionality

Xylem vessels function in the long-distance conduction of water in land plants. The NAC transcription factor VASCULAR-RELATED NAC-DOMAIN7 (VND7) is a master regulator of xylem vessel cell differentiation in Arabidopsis (Arabidopsis thaliana). We previously isolated suppressor of ectopic xylem vessel cell differentiation induced by VND7 (seiv) mutants. Here, we report that the responsible genes for seiv3, seiv4, seiv6, and seiv9 are protein ubiquitination-related genes encoding PLANT U-BOX46 (PUB46), an uncharacterized F-BOX protein (FBX), PUB36, and UBIQUITIN-SPECIFIC PROTEASE1 (UBP1), respectively. We also found decreased expression of genes downstream of VND7 and abnormal xylem transport activity in the seiv mutants. Upon VND7 induction, ubiquitination levels from 492 and 180 protein groups were upregulated and downregulated, respectively. VND7 induction resulted in the ubiquitination of proteins for cell wall biosynthesis and protein transport, whereas such active protein ubiquitination did not occur in the seiv mutants. We detected the ubiquitination of three lysine residues in VND7: K94, K105, and K260. Substituting K94 with arginine significantly decreased the transactivation activity of VND7, suggesting that the ubiquitination of K94 is crucial for regulating VND7 activity. Our findings highlight the crucial roles of target protein ubiquitination in regulating xylem vessel activity.

PagMYB128 regulates secondary cell wall formation by direct activation of cell wall biosynthetic genes during wood formation in poplar

The biosynthesis of cellulose, lignin, and hemicelluloses in plant secondary cell walls (SCWs) is regulated by a hierarchical transcriptional regulatory network. This network features orthologous transcription factors shared between poplar and Arabidopsis, highlighting a foundational similarity in their genetic regulation. However, knowledge on the discrepant behavior of the transcriptional-level molecular regulatory mechanisms between poplar and Arabidopsis remains limited. In this study, we investigated the function of PagMYB128 during wood formation and found it had broader impacts on SCW formation compared to its Arabidopsis ortholog, AtMYB103. Transgenic poplar trees overexpressing PagMYB128 exhibited significantly enhanced xylem development, with fiber cells and vessels displaying thicker walls, and an increase in the levels of cellulose, lignin, and hemicelluloses in the wood. In contrast, plants with dominant repression of PagMYB128 demonstrated the opposite phenotypes. RNA sequencing and reverse transcription - quantitative polymerase chain reaction showed that PagMYB128 could activate SCW biosynthetic gene expression, and chromatin immunoprecipitation along with yeast one-hybrid, and effector-reporter assays showed this regulation was direct. Further analysis revealed that PagSND1 (SECONDARY WALL-ASSOCIATED NAC-DOMAIN PROTEIN1) directly regulates PagMYB128 but not cell wall metabolic genes, highlighting the pivotal role of PagMYB128 in the SND1-driven regulatory network for wood development, thereby creating a feedforward loop in SCW biosynthesis.

Evaluation of novel promoters for vascular tissue-specific gene expression in Populus

Due to the extended generation cycle of trees, the breeding process for forest trees tends to be time-consuming. Genetic engineering has emerged as a viable approach to expedite the genetic breeding of forest trees. However, current genetic engineering techniques employed in forest trees often utilize continuous expression promoters such as CaMV 35S, which may result in unintended consequences by introducing genes into non-target tissues. Therefore, it is imperative to develop specific promoters for forest trees to facilitate targeted and precise design and breeding. In this study, we utilized single-cell RNA-Seq data and co-expression network analysis during wood formation to identify three vascular tissue-specific genes in poplar, PP2-A10, PXY, and VNS07, which are expressed in the phloem, cambium/expanding xylem, and mature xylem, respectively. Subsequently, we cloned the promoters of these three genes from '84K' poplar and constructed them into a vector containing the eyGFPuv visual selection marker, along with the 35S mini enhancer to drive GUS gene expression. Transgenic poplars expressing the ProPagPP2-A10::GUS, ProPagPXY::GUS, and ProPagVNS07::GUS constructs were obtained. To further elucidate the tissue specificity of these promoters, we employed qPCR, histochemical staining, and GUS enzyme activity. Our findings not only establish a solid foundation for the future utilization of these promoters to precisely express of specific functional genes in stems but also provide a novel perspective for the modular breeding of forest trees.

Convergent evolution of water-conducting cells in Marchantia recruited the ZHOUPI gene promoting cell wall reinforcement and programmed cell death

A key adaptation of plants to life on land is the formation of water-conducting cells (WCCs) for efficient long-distance water transport. Based on morphological analyses it is thought that WCCs have evolved independently on multiple occasions. For example, WCCs have been lost in all but a few lineages of bryophytes but, strikingly, within the liverworts a derived group, the complex thalloids, has evolved a novel externalized water-conducting tissue composed of reinforced, hollow cells termed pegged rhizoids. Here, we show that pegged rhizoid differentiation in Marchantia polymorpha is controlled by orthologs of the ZHOUPI and ICE bHLH transcription factors required for endosperm cell death in Arabidopsis seeds. By contrast, pegged rhizoid development was not affected by disruption of MpNAC5, the Marchantia ortholog of the VND genes that control WCC formation in flowering plants. We characterize the rapid, genetically controlled programmed cell death process that pegged rhizoids undergo to terminate cellular differentiation and identify a corresponding upregulation of conserved putative plant cell death effector genes. Lastly, we show that ectopic expression of MpZOU1 increases production of pegged rhizoids and enhances drought tolerance. Our results support that pegged rhizoids evolved independently of other WCCs. We suggest that elements of the genetic control of developmental cell death are conserved throughout land plants and that the ZHOUPI/ICE regulatory module has been independently recruited to promote cell wall modification and programmed cell death in liverwort rhizoids and in the endosperm of flowering plant seed.

Woody plant cell walls: Fundamentals and utilization

Cell walls in plants, particularly forest trees, are the major carbon sink of the terrestrial ecosystem. Chemical and biosynthetic features of plant cell walls were revealed early on, focusing mostly on herbaceous model species. Recent developments in genomics, transcriptomics, epigenomics, transgenesis, and associated analytical techniques are enabling novel insights into formation of woody cell walls. Here, we review multilevel regulation of cell wall biosynthesis in forest tree species. We highlight current approaches to engineering cell walls as potential feedstock for materials and energy and survey reported field tests of such engineered transgenic trees. We outline opportunities and challenges in future research to better understand cell type biogenesis for more efficient wood cell wall modification and utilization for biomaterials or for enhanced carbon capture and storage.

PbBPC4 involved in a xylem-deficient dwarf phenotype in pear by directly regulating the expression of PbXND1

Dwarfing is an important agronomic trait in fruit breeding. At present, dwarf cultivars or dwarfing rootstocks are used for high-density planting. Although some dwarf rootstocks have been used in the cultivation of pear (Pyrus bretschneideri Rehd), the breeding of dwarf pear rootstocks or cultivars is still sorely lacking. A previous study reported that PbXND1 results in a xylem-dwarf phenotype in pear trees. However, the regulatory mechanism upstream of PbXND1 is unclear. In this study, we identified PbBPC4 as an upstream regulatory factor of PbXND1 in yeast one-hybrid assays. In β-glucuronidase staining and dual-luciferase assays, PbBPC4 enhanced the activity of the PbXND1 promoter. Tobacco plants overexpressing PbBPC4 showed decreased plant height because of a reduced xylem size. Similar changes in the xylem was observed in transgenic pear roots; those overexpressing PbBPC4 showed reduced xylem size, and those with silencing PbBPC4 expression showed increased xylem size, greater density of xylem vessels, and a larger proportion of the xylem out of the total cross-section area. Expression analyses showed that PbBPC4 increases the transcription of PbXND1, leading to reduced transcript levels of genes involved in the positive regulation of xylem development, ultimately resulting in a xylem-deficient dwarf phenotype. Taken together, our results reveal the mechanism by which PbBPC4 participates in the regulation of xylem development via directly altering the expression of PbXND1, thus leading to the dwarf phenotype in pear. These findings have reference value for the breeding of dwarf pear trees.

Comparative functional analysis of PdeNAC2 and AtVND6 in the tracheary element formation

Tracheary elements (i.e., vessel elements and tracheids) are highly specialized, non-living cells present in the water-conducting xylem tissue. In angiosperms, proteins in the VASCULAR-RELATED NAC-DOMAIN (VND) subgroup of the NAC transcription factor family (e.g., AtVND6) are required for the differentiation of vessel elements through transcriptional regulation of genes responsible for secondary cell wall (SCW) formation and programmed cell death (PCD). Gymnosperms, however, produce only tracheids, the mechanism of which remains elusive. Here, we report functional characteristics of PdeNAC2, a VND homolog in Pinus densiflora, as a key regulator of tracheid formation. Interestingly, our molecular genetic analyses show that PdeNAC2 can induce the formation of vessel element-like cells in angiosperm plants, demonstrated by transgenic overexpression of either native or NAC domain-swapped synthetic genes of PdeNAC2 and AtVND6 in both Arabidopsis and hybrid poplar. Subsequently, genome-wide identification of direct target genes of PdeNAC2 and AtVND6 revealed 138 and 174 genes as putative direct targets, respectively, but only 17 genes were identified as common direct targets. Further analyses have found that PdeNAC2 does not control some AtVND6-dependent vessel differentiation genes in angiosperm plants, such as AtVRLK1, LBD15/30, and pit-forming ROP signaling genes. Collectively, our results suggest that different target gene repertoires of PdeNAC2 and AtVND6 may contribute to the evolution of tracheary elements.

Cellulose synthesis in land plants

All plant cells are surrounded by a cell wall that provides cohesion, protection, and a means of directional growth to plants. Cellulose microfibrils contribute the main biomechanical scaffold for most of these walls. The biosynthesis of cellulose, which typically is the most prominent constituent of the cell wall and therefore Earth's most abundant biopolymer, is finely attuned to developmental and environmental cues. Our understanding of the machinery that catalyzes and regulates cellulose biosynthesis has substantially improved due to recent technological advances in, for example, structural biology and microscopy. Here, we provide a comprehensive overview of the structure, function, and regulation of the cellulose synthesis machinery and its regulatory interactors. We aim to highlight important knowledge gaps in the field, and outline emerging approaches that promise a means to close those gaps.

Secondary cell wall patterning-connecting the dots, pits and helices

All plant cells are encased in primary cell walls that determine plant morphology, but also protect the cells against the environment. Certain cells also produce a secondary wall that supports mechanically demanding processes, such as maintaining plant body stature and water transport inside plants. Both these walls are primarily composed of polysaccharides that are arranged in certain patterns to support cell functions. A key requisite for patterned cell walls is the arrangement of cortical microtubules that may direct the delivery of wall polymers and/or cell wall producing enzymes to certain plasma membrane locations. Microtubules also steer the synthesis of cellulose-the load-bearing structure in cell walls-at the plasma membrane. The organization and behaviour of the microtubule array are thus of fundamental importance to cell wall patterns. These aspects are controlled by the coordinated effort of small GTPases that probably coordinate a Turing's reaction-diffusion mechanism to drive microtubule patterns. Here, we give an overview on how wall patterns form in the water-transporting xylem vessels of plants. We discuss systems that have been used to dissect mechanisms that underpin the xylem wall patterns, emphasizing the VND6 and VND7 inducible systems, and outline challenges that lay ahead in this field.

The flying spider-monkey tree fern genome provides insights into fern evolution and arborescence

To date, little is known about the evolution of fern genomes, with only two small genomes published from the heterosporous Salviniales. Here we assembled the genome of Alsophila spinulosa, known as the flying spider-monkey tree fern, onto 69 pseudochromosomes. The remarkable preservation of synteny, despite resulting from an ancient whole-genome duplication over 100 million years ago, is unprecedented in plants and probably speaks to the uniqueness of tree ferns. Our detailed investigations into stem anatomy and lignin biosynthesis shed new light on the evolution of stem formation in tree ferns. We identified a phenolic compound, alsophilin, that is abundant in xylem, and we provided the molecular basis for its biosynthesis. Finally, analysis of demographic history revealed two genetic bottlenecks, resulting in rapid demographic declines of A. spinulosa. The A. spinulosa genome fills a crucial gap in the plant genomic landscape and helps elucidate many unique aspects of tree fern biology.

Enhancement of Secondary Cell Wall Formation in Poplar Xylem Using a Self-Reinforced System of Secondary Cell Wall-Related Transcription Factors

The secondary cell wall (SCW) in the xylem is one of the largest sink organs of carbon in woody plants, and is considered a promising sustainable bioresource for biofuels and biomaterials. To enhance SCW formation in poplar (Populus sp.) xylem, we developed a self-reinforced system of SCW-related transcription factors from Arabidopsis thaliana, involving VASCULAR-RELATED NAC-DOMAIN7 (VND7), SECONDARY WALL-ASSOCIATED NAC-DOMAIN PROTEIN 1/NAC SECONDARY WALL THICKENING-PROMOTING FACTOR3 (SND1/NST3), and MYB46. In this system, these transcription factors were fused with the transactivation domain VP16 and expressed under the control of the Populus trichocarpa CesA18 (PtCesA18) gene promoter, creating the chimeric genes PtCesA18pro::AtVND7:VP16, PtCesA18pro::AtSND1:VP16, and PtCesA18pro::AtMYB46:VP16. The PtCesA18 promoter is active in tissues generating SCWs, and can be regulated by AtVND7, AtSND1, and AtMYB46; thus, the expression levels of PtCesA18pro::AtVND7:VP16, PtCesA18pro::AtSND1:VP16, and PtCesA18pro::AtMYB46:VP16 are expected to be boosted in SCW-generating tissues. In the transgenic hybrid aspens (Populus tremula × tremuloides T89) expressing PtCesA18pro::AtSND1:VP16 or PtCesA18pro::AtMYB46:VP16 grown in sterile half-strength Murashige and Skoog growth medium, SCW thickening was significantly enhanced in the secondary xylem cells, while the PtCesA18pro::AtVND7:VP16 plants showed stunted xylem formation, possibly because of the enhanced programmed cell death (PCD) in the xylem regions. After acclimation, the transgenic plants were transferred from the sterile growth medium to pots of soil in the greenhouse, where only the PtCesA18pro::AtMYB46:VP16 aspens survived. A nuclear magnetic resonance footprinting cell wall analysis and enzymatic saccharification analysis demonstrated that PtCesA18pro::AtMYB46:VP16 influences cell wall properties such as the ratio of syringyl (S) and guaiacyl (G) units of lignin, the abundance of the lignin β-aryl ether and resinol bonds, and hemicellulose acetylation levels. Together, these data indicate that we have created a self-reinforced system using SCW-related transcription factors to enhance SCW accumulation.

Functional Analysis of Poplar Sombrero-Type NAC Transcription Factors Yields a Strategy to Modify Woody Cell Wall Properties

Woody cells generate lignocellulosic biomass, which is a promising sustainable bioresource for wide industrial applications. Woody cell differentiation in vascular plants, including the model plant poplar (Populus trichocarpa), is regulated by a set of NAC family transcription factors, the VASCULAR-RELATED NAC-DOMAIN (VND), NAC SECONDARY CELL WALL THICKENING PROMOTING FACTOR (NST)/SND, and SOMBRERO (SMB) (VNS)-related proteins, but the precise contributions of each VNS protein to wood quality are unknown. Here, we performed a detailed functional analysis of the poplar SMB-type VNS proteins PtVNS13-PtVNS16. PtVNS13-PtVNS16 were preferentially expressed in the roots of young poplar plantlets, similar to the Arabidopsis thalianaSMB gene. PtVNS13 and PtVNS14, as well as the NST-type PtVNS11, suppressed the abnormal root cap phenotype of the Arabidopsis sombrero-3 mutant, whereas the VND-type PtVNS07 gene did not, suggesting a functional gap between SMB- or NST-type VNS proteins and VND-type VNS proteins. Overexpressing PtVNS13-PtVNS16 in Arabidopsis seedlings and poplar leaves induced ectopic xylem-vessel-like cells with secondary wall deposition, and a transient expression assay showed that PtVNS13-16 transactivated woody-cell-related genes. Interestingly, although any VNS protein rescued the pendant stem phenotype of the Arabidopsis nst1-1 nst3-1 mutant, the resulting inflorescence stems exhibited distinct cell wall properties: poplar VNS genes generated woody cell walls with higher enzymatic saccharification efficiencies compared with Arabidopsis VNS genes. Together, our data reveal clear functional diversity among VNS proteins in woody cell differentiation and demonstrate a novel VNS-based strategy for modifying woody cell wall properties toward enhanced utilization of woody biomass.

Full-length transcriptomic identification of R2R3-MYB family genes related to secondary cell wall development in Cunninghamia lanceolata (Chinese fir)

BACKGROUND

R2R3-MYB is a class of transcription factor crucial in regulating secondary cell wall development during wood formation. The regulation of wood formation in gymnosperm has been understudied due to its large genome size. Using Single-Molecule Real-Time sequencing, we obtained full-length transcriptomic libraries from the developmental stem of Cunninghamia lanceolata, a perennial conifer known as Chinese fir. The R2R3-MYB of C. lanceolata (hereafter named as ClMYB) associated with secondary wall development were identified based on phylogenetic analysis, expression studies and functional study on transgenic line.

RESULTS

The evolutionary relationship of 52 ClMYBs with those from Arabidopsis thaliana, Eucalyptus grandis, Populus trichocarpa, Oryza sativa, two gymnosperm species, Pinus taeda, and Picea glauca were established by neighbour-joining phylogenetic analysis. A large number of ClMYBs resided in the woody-expanded subgroups that predominated with the members from woody dicots. In contrast, the woody-preferential subgroup strictly carrying the members of woody dicots contained only one candidate. The results suggest that the woody-expanded subgroup emerges before the gymnosperm/angiosperm split, while most of the woody-preferential subgroups are likely lineage-specific to woody dicots. Nine candidates shared the same subgroups with the A. thaliana orthologs, with known function in regulating secondary wall development. Gene expression analysis inferred that ClMYB1/2/3/4/5/26/27/49/51 might participate in secondary wall development, among which ClMYB1/2/5/26/27/49 were significantly upregulated in the highly lignified compression wood region, reinforcing their regulatory role associated with secondary wall development. ClMYB1 was experimentally proven a transcriptional activator that localised in the nucleus. The overexpression of ClMYB1 in Nicotiana benthamiana resulted in an increased lignin deposition in the stems. The members of subgroup S4, ClMYB3/4/5 shared the ERF-associated amphiphilic repression motif with AtMYB4, which is known to repress the metabolism of phenylpropanoid derived compounds. They also carried a core motif specific to gymnosperm lineage, suggesting divergence of the regulatory process compared to the angiosperms.

CONCLUSIONS

This work will enrich the collection of full-length gymnosperm-specific R2R3-MYBs related to stem development and contribute to understanding their evolutionary relationship with angiosperm species.

Xylem vessel cell differentiation: A best model for new integrative cell biology?

Xylem vessels transport water and essential low-molecular-weight compounds throughout vascular plants. To achieve maximum performance as conductive tissues, xylem vessel cells undergo secondary cell wall deposition and programmed cell death to produce a hollow tube-like structure with a rigid outer shell. This unique process has been explored in detail from a cell biology and molecular biology perspective, culminating in the identification of the master transcriptional switches of xylem vessel cell differentiation, the VASCULAR-RELATED NAC-DOMAIN (VND) proteins. High-resolution analyses of xylem vessel cell differentiation have since accelerated and are now moving toward single cell-level dissection from a variety of directions. In this review, we introduce the current model of xylem vessel cell differentiation and discuss possible future directions in this field.

Prior secondary cell wall formation is required for gelatinous layer deposition and posture control in gravi-stimulated aspen

Cell walls, especially secondary cell walls (SCWs), maintain cell shape and reinforce wood, but their structure and shape can be altered in response to gravity. In hardwood trees, tension wood is formed along the upper side of a bending stem and contains wood fiber cells that have a gelatinous layer (G-layer) inside the SCW. In a previous study, we generated nst/snd quadruple-knockout aspens (Populus tremula × Populus tremuloides), in which SCW formation was impaired in 99% of the wood fiber cells. In the present study, we produced nst/snd triple-knockout aspens, in which a large number of wood fibers had thinner SCWs than the wild type (WT) and some had no SCW. Because SCW layers are always formed prior to G-layer deposition, the nst/snd mutants raise interesting questions of whether the mutants can form G-layers without SCW and whether they can control their postures in response to changes in gravitational direction. The nst/snd mutants and the WT plants showed growth eccentricity and vessel frequency reduction when grown on an incline, but the triple mutants recovered their upright growth only slightly, and the quadruple mutants were unable to maintain their postures. The mutants clearly showed that the G-layers were formed in SCW-containing wood fibers but not in those lacking the SCW. Our results indicate that SCWs are essential for G-layer formation and posture control. Furthermore, each wood fiber cell may be able to recognize its cell wall developmental stage to initiate the formation of the G-layer as a response to gravistimulation.

Xylem vessel-specific SND5 and its homologs regulate secondary wall biosynthesis through activating secondary wall NAC binding elements

Secondary cell wall biosynthesis has been shown to be regulated by a suite of transcription factors. Here, we identified a new xylem vessel-specific NAC domain transcription factor, secondary wall-associated NAC domain protein5 (SND5), in Arabidopsis thaliana and studied its role in regulating secondary wall biosynthesis. We showed that the expression of SND5 and its close homolog, SND4/ANAC075, was specifically associated with secondary wall-containing cells and dominant repression of their functions severely reduced secondary wall thickening in these cells. Overexpression of SND4/5 as well as their homologs SND2/3 fused with the activation domain of the viral protein VP16 led to ectopic secondary wall deposition in cells that are normally parenchymatous. SND2/3/4/5 regulated the expression of the same downstream target genes as do the secondary wall NAC master switches (SWNs) by binding to and activating the secondary wall NAC binding elements (SNBEs). Furthermore, we demonstrated that the poplar (Populus trichocarpa) orthologs of SND2/3/4/5 also activated SNBEs and regulated secondary wall biosynthesis during wood formation. Together, these findings indicate that SND2/3/4/5 and their poplar orthologs regulate the expression of secondary wall-associated genes through activating SNBEs and they are positioned at an upper level in the SWN-mediated transcriptional network.

Wood transcriptome analysis of Pinus densiflora identifies genes critical for secondary cell wall formation and NAC transcription factors involved in tracheid formation

Although conifers have significant ecological and economic value, information on transcriptional regulation of wood formation in conifers is still limited. Here, to gain insight into secondary cell wall (SCW) biosynthesis and tracheid formation in conifers, we performed wood tissue-specific transcriptome analyses of Pinus densiflora (Korean red pine) using RNA sequencing. In addition, to obtain full-length transcriptome information, PacBio single molecule real-time iso-sequencing was carried out using RNAs from 28 tissues of P. densiflora. Subsequent comparative tissue-specific transcriptome analysis successfully pinpointed critical genes encoding key proteins involved in biosynthesis of the major secondary wall components (cellulose, galactoglucomannan, xylan and lignin). Furthermore, we predicted a total of 62 NAC (NAM, ATAF1/2 and CUC2) family transcription factor members and identified seven PdeNAC genes preferentially expressed in developing xylem tissues in P. densiflora. Protoplast-based transcriptional activation analysis found that four PdeNAC genes, homologous to VND, NST and SND/ANAC075, upregulated GUS activity driven by an SCW-specific cellulose synthase promoter. Consistently, transient overexpression of the four PdeNACs induced xylem vessel cell-like SCW deposition in both tobacco (Nicotiana benthamiana) and Arabidopsis leaves. Taken together, our data provide a foundation for further research to unravel transcriptional regulation of wood formation in conifers, especially SCW formation and tracheid differentiation.

Expression of peat moss VASCULAR RELATED NAC-DOMAIN homologs in Nicotiana benthamiana leaf cells induces ectopic secondary wall formation

KEY MESSAGE

The homologs of VASCULAR RELATED NAC-DOMAIN in the peat moss Sphagnum palustre were identified and these transcriptional activity as the VNS family was conserved. In angiosperms, xylem vessel element differentiation is governed by the master regulators VASCULAR RELATED NAC-DOMAIN6 (VND6) and VND7, encoding plant-specific NAC transcription factors. Although vessel elements have not been found in bryophytes, differentiation of the water-conducting hydroid cells in the moss Physcomitrella patens is regulated by VND homologs termed VND-NST-SOMBRERO (VNS) genes. VNS genes are conserved in the land plant lineage, but their functions have not been elucidated outside of angiosperms and P. patens. The peat moss Sphagnum palustre, of class Sphagnopsida in the phylum Bryophyta, does not have hydroids and instead uses hyaline cells with thickened, helical-patterned cell walls and pores to store water in the leaves. Here, we performed whole-transcriptome analysis and de novo assembly using next generation sequencing in S. palustre, obtaining sequences for 68,305 genes. Among them, we identified seven VNS-like genes, SpVNS1-A, SpVNS1-B, SpVNS2-A, SpVNS2-B, SpVNS3-A, SpVNS3-B, and SpVNS4-A. Transient expression of these VNS-like genes, with the exception of SpVNS2-A, in Nicotiana benthamiana leaf cells resulted in ectopic thickening of secondary walls. This result suggests that the transcriptional activity observed in other VNS family members is functionally conserved in the VNS homologs of S. palustre.

XND1 Regulates Secondary Wall Deposition in Xylem Vessels through the Inhibition of VND Functions

Secondary wall deposition in xylem vessels is activated by Vascular-Related NAC Domain proteins (VNDs) that belong to a group of secondary wall NAC (SWN) transcription factors. By contrast, Xylem NAC Domain1 (XND1) negatively regulates secondary wall deposition in xylem vessels when overexpressed. The mechanism by which XND1 exerts its functions remains elusive. We employed the promoter of the fiber-specific Secondary Wall-Associated NAC Domain1 (SND1) gene to ectopically express XND1 in fiber cells to investigate its mechanism of action on secondary wall deposition. Ectopic expression of XND1 in fiber cells severely diminished their secondary wall deposition and drastically reduced the expression of SWN-regulated downstream transcription factors and secondary wall biosynthetic genes but not that of the SWN genes themselves. Transactivation analyses revealed that XND1 specifically inhibited SWN-activated expression of these downstream genes but not their MYB46-activated expression. Both the NAC domain and the C-terminus of XND1 were required for its inhibitory function and its NAC domain interacted with the DNA-binding domains of SWNs. XND1 was shown to be localized in the cytoplasm and the nucleus and its co-expression with VND6 resulted in the cytoplasmic sequestration of VND6. Furthermore, the C-terminus of XND1 was indispensable for the XND1-mediated cytoplasmic retention of VND6 and its fusion to VND6 was able to direct VND6 to the cytoplasm and render it unable to activate the gene expression. Since the XND1 gene is specifically expressed in xylem cells, these results indicate that XND1 acts through inhibiting VND functions to negatively regulate secondary wall deposition in xylem vessels.

The Progression of Xylem Vessel Cell Differentiation is Dependent on the Activity Level of VND7 in Arabidopsis thaliana

Xylem vessels are important for water conduction in vascular plants. The VASCULAR-RELATED NAC-DOMAIN (VND) family proteins, master regulators of xylem vessel cell differentiation in Arabidopsis thaliana, can upregulate a set of genes required for xylem vessel cell differentiation, including those involved in secondary cell wall (SCW) formation and programmed cell death (PCD); however, it is not fully understood how VND activity levels influence these processes. Here, we examined the Arabidopsis VND7-VP16-GR line, in which VND7 activity is post-translationally activated by treatments with different concentrations of dexamethasone (DEX), a synthetic glucocorticoid. Our observations showed that 1 nM DEX induced weak SCW deposition, but not PCD, whereas 10 or 100 nM DEX induced both SCW deposition and PCD. The decreased chlorophyll contents and SCW deposition were apparent after 24 h of 100 nM DEX treatment, but became evident only after 48 h of 10 nM DEX treatment. Moreover, the lower DEX concentrations delayed the upregulation of VND7 downstream genes, and decreased their induction levels. They collectively suggest that the regulation of VND activity is important not only to initiate xylem vessel cell differentiation, but also regulate the quality of the xylem vessels through VND-activity-dependent upregulation of the PCD- and SCW-related genes.