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.

Ubiquitinated DA1 negatively regulates vascular cambium activity through modulating the stability of WOX4 in Populus.

Activity of the vascular cambium gives rise to secondary xylem for wood formation in trees. The transcription factor WUSCHEL-related HOMEOBOX4 (WOX4) is a central regulator downstream of the hormone and peptide signaling pathways that maintain cambial activity. However, the genetic regulatory network underlying WOX4-mediated wood formation at the post-transcriptional level remains to be elucidated. In this study, we identified the ubiquitin receptor PagDA1 in hybrid poplar (Populus alba × Populus glandulosa clone 84K) as a negative regulator of wood formation, which restricts cambial activity during secondary growth. Overexpression of PagDA1 in poplar resulted in a relatively reduced xylem due to decreased cambial cell division. By contrast, mutation of PagDA1 by CRISPR/Cas9 resulted in an increased cambial cell activity and promoted xylem formation. Genetic analysis demonstrated that PagDA1 functions antagonistically in a common pathway as PagWOX4 to regulate cambial activity. We propose that PagDA1 physically associates with PagWOX4 and modulates the degradation of PagWOX4 by the 26S proteasome. Moreover, genetic analysis revealed that PagDA1 exerts its negative effect on cambial development by modulating the stability of PagWOX4 in a ubiquitin-dependent manner mediated by the E3 ubiquitin ligase PagDA2. In sum, we have identified a cambial regulatory protein complex, PagDA1-PagWOX4, as a potential target for wood biomass improvement.

Genome-Wide Identification and Expression Profiling of Velvet Complex Transcription Factors in Populus alba × Populus glandulosa.

The discovery of new genes with novel functions is a major driver of adaptive evolutionary innovation in plants. Especially in woody plants, due to genome expansion, new genes evolve to regulate the processes of growth and development. In this study, we characterized the unique VeA transcription factor family in Populus alba × Populus glandulosa, which is associated with secondary metabolism. Twenty VeA genes were characterized systematically on their phylogeny, genomic distribution, gene structure and conserved motif, promoter binding site, and expression profiling. Furthermore, through ChIP-qPCR, Y1H, and effector-reporter assays, it was demonstrated that PagMYB128 directly regulated PagVeA3 to influence the biosynthesis of secondary metabolites. These results provide a basis for further elucidating the function of VeAs gene in poplar and its genetic regulation mechanism.

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.

Hierarchical Transcription Factor and Chromatin Binding Network for Wood Formation in Black Cottonwood (Populus trichocarpa).

Wood remains the world's most abundant and renewable resource for timber and pulp and is an alternative to fossil fuels. Understanding the molecular regulation of wood formation can advance the engineering of wood for more efficient material and energy productions. We integrated a black cottonwood (Populus trichocarpa) wood-forming cell system with quantitative transcriptomics and chromatin binding assays to construct a transcriptional regulatory network (TRN) directed by a key transcription factor (TF), PtrSND1-B1 (secondary wall-associated NAC-domain protein). The network consists of four layers of TF-target gene interactions with quantitative regulatory effects, describing the specificity of how the regulation is transduced through these interactions to activate cell wall genes (effector genes) for wood formation. PtrSND1-B1 directs 57 TF-DNA interactions through 17 TFs transregulating 27 effector genes. Of the 57 interactions, 55 are novel. We tested 42 of these 57 interactions in 30 genotypes of transgenic P. trichocarpa and verified that ∼90% of the tested interactions function in vivo. The TRN reveals common transregulatory targets for distinct TFs, leading to the discovery of nine TF protein complexes (dimers and trimers) implicated in regulating the biosynthesis of specific types of lignin. Our work suggests that wood formation may involve regulatory homeostasis determined by combinations of TF-DNA and TF-TF (protein-protein) regulations.

Growth-regulating factor 5 (GRF5)-mediated gene regulatory network promotes leaf growth and expansion in poplar.

Although polyploid plants have larger leaves than their diploid counterparts, the molecular mechanisms underlying this difference (or trait) remain elusive. Differentially expressed genes (DEGs) between triploid and full-sib diploid poplar trees were identified from two transcriptomic data sets followed by a gene association study among DEGs to identify key leaf growth regulators. Yeast one-hybrid system, electrophoretic mobility shift assay, and dual-luciferase assay were employed to substantiate that PpnGRF5-1 directly regulated PpnCKX1. The interactions between PpnGRF5-1 and growth-regulating factor (GRF)-interacting factors (GIFs) were experimentally validated and a multilayered hierarchical regulatory network (ML-hGRN)-mediated by PpnGRF5-1 was constructed with top-down graphic Gaussian model (GGM) algorithm by combining RNA-sequencing data from its overexpression lines and DAP-sequencing data. PpnGRF5-1 is a negative regulator of PpnCKX1. Overexpression of PpnGRF5-1 in diploid transgenic lines resulted in larger leaves resembling those of triploids, and significantly increased zeatin and isopentenyladenine in the apical buds and third leaves. PpnGRF5-1 also interacted with GIFs to increase its regulatory diversity and capacity. An ML-hGRN-mediated by PpnGRF5-1 was obtained and could largely elucidate larger leaves. PpnGRF5-1 and the ML-hGRN-mediated by PpnGRF5-1 were underlying the leaf growth and development.

Single-cell RNA sequencing reveals a high-resolution cell atlas of xylem in Populus.

High-throughput single-cell RNA sequencing (scRNA-seq) has advantages over traditional RNA-seq to explore spatiotemporal information on gene dynamic expressions in heterogenous tissues. We performed Drop-seq, a method for the dropwise sequestration of single cells for sequencing, on protoplasts from the differentiating xylem of Populus alba × Populus glandulosa. The scRNA-seq profiled 9,798 cells, which were grouped into 12 clusters. Through characterization of differentially expressed genes in each cluster and RNA in situ hybridizations, we identified vessel cells, fiber cells, ray parenchyma cells and xylem precursor cells. Diffusion pseudotime analyses revealed the differentiating trajectory of vessels, fiber cells and ray parenchyma cells and indicated a different differentiation process between vessels and fiber cells, and a similar differentiation process between fiber cells and ray parenchyma cells. We identified marker genes for each cell type (cluster) and key candidate regulators during developmental stages of xylem cell differentiation. Our study generates a high-resolution expression atlas of wood formation at the single cell level and provides valuable information on wood formation.

METHIONINE SYNTHASE1 Is Involved in Chromatin Silencing by Maintaining DNA and Histone Methylation.

DNA methylation and histone modification are important epigenetic marks that coregulate gene expression and genome stability. To identify factors involved in chromatin silencing, we carried out a forward genetic screen for mutants that release the silenced Pro-35S:LUCIFERASE (35SP-LUC) in Arabidopsis (Arabidopsis thaliana). We identified an epigenetic regulator, METHIONINE SYNTHASE1 (ATMS1), which catalyzes the synthesis of methionine (Met) in the one-carbon metabolism pathway. The ATMS1 mutation releases the silenced 35SP-LUC and the majority of endogenous genes and transposons. The effect of ATMS1 on chromatin silencing is related to decreased levels of DNA methylation (CG, CHG, and CHH) and histone-3 lysine-9 dimethylation. The ATMS1 mutation caused a significant decrease in the ratio of S-adenosylmethionine to S-adenosylhomocysteine. Exogenous application of Met rescued the phenotype of atms1-1 ATMS1 plays a predominant role in DNA and histone methylations among the three Met synthetase homologs. These results suggest that ATMS1 is required for DNA and histone methylations through its function in the one-carbon metabolism pathway, indicating the complex interplay between metabolism and epigenetic regulation.

MicroRNA comparison between poplar and larch provides insight into the different mechanism of wood formation.

KEY MESSAGE: MiRNA transcriptome analysis of different tissues in poplar and larch suggests variant roles of miRNAs in regulating wood formation between two kinds of phyla. Poplar and larch belong to two different phyla. Both are ecological woody species and major resources for wood-related industrial applications. However, wood properties are different between these two species and the molecular basis is largely unknown. In this study, we performed high-throughput sequencing of microRNAs (miRNAs) in the three tissues, xylem, phloem and leaf of Populus alba × Populus glandulosa and Larix kaempferi. Differentially expressed miRNA (DEmiRNA) analysis identified 85 xylem-specific miRNAs in P. alba × P. glandulosa and 158 xylem-specific miRNAs in L. kaempferi. Among 36 common miRNAs, 12 were conserved between the two species. GO and KEGG analyses of the miRNA target genes showed similar metabolism in two species. Through KEGG and BLASTN, we predicted target genes of xylem differentially expressed (DEmiRNA) in the wood formation-related pathways and located DEmiRNAs in these pathways. A network was built for wood formation-related DEmiRNAs, their target genes and orthologous genes in Arabidopsis thaliana. Comparison of DEmiRNA and target gene annotation between P. alba × P. glandulosa and L. kaempferi suggested the different functions of DEmiRNAs and divergent mechanism in wood formation between two species, providing knowledge to understand wood formation mechanism in gymnosperm and angiosperm woody plants.

Reciprocal cross-regulation of VND and SND multigene TF families for wood formation in Populus trichocarpa.

Secondary cell wall (SCW) biosynthesis is the biological process that generates wood, an important renewable feedstock for materials and energy. NAC domain transcription factors, particularly Vascular-Related NAC-Domain (VND) and Secondary Wall-Associated NAC Domain (SND) proteins, are known to regulate SCW differentiation. The regulation of VND and SND is important to maintain homeostasis for plants to avoid abnormal growth and development. We previously identified a splice variant, PtrSND1-A2IR , derived from PtrSND1-A2 as a dominant-negative regulator, which suppresses the transactivation of all PtrSND1 family members. PtrSND1-A2IR also suppresses the self-activation of the PtrSND1 family members except for its cognate transcription factor, PtrSND1-A2, suggesting the existence of an unknown factor needed to regulate PtrSND1-A2 Here, a splice variant, PtrVND6-C1IR , derived from PtrVND6-C1 was discovered that suppresses the protein functions of all PtrVND6 family members. PtrVND6-C1IR also suppresses the expression of all PtrSND1 members, including PtrSND1-A2, demonstrating that PtrVND6-C1IR is the previously unidentified regulator of PtrSND1-A2 We also found that PtrVND6-C1IR cannot suppress the expression of its cognate transcription factor, PtrVND6-C1PtrVND6-C1 is suppressed by PtrSND1-A2IR Both PtrVND6-C1IR and PtrSND1-A2IR cannot suppress their cognate transcription factors but can suppress all members of the other family. The results indicate that the splice variants from the PtrVND6 and PtrSND1 family may exert reciprocal cross-regulation for complete transcriptional regulation of these two families in wood formation. This reciprocal cross-regulation between families suggests a general mechanism among NAC domain proteins and likely other transcription factors, where intron-retained splice variants provide an additional level of regulation.

The auxin receptor TIR1 homolog (PagFBL 1) regulates adventitious rooting through interactions with Aux/IAA28 in Populus.

Adventitious roots occur naturally in many species and can also be induced from explants of some tree species including Populus, providing an important means of clonal propagation. Auxin has been identified as playing a crucial role in adventitious root formation, but the associated molecular regulatory mechanisms need to be elucidated. In this study, we examined the role of PagFBL1, the hybrid poplar (Populus alba × P. glandulosa clone 84K) homolog of Arabidopsis auxin receptor TIR1, in adventitious root formation in poplar. Similar to the distribution pattern of auxin during initiation of adventitious roots, PagFBL1 expression was concentrated in the cambium and secondary phloem in stems during adventitious root induction and initiation phases, but decreased in emerging adventitious root primordia. Overexpressing PagFBL1 stimulated adventitious root formation and increased root biomass, while knock-down of PagFBL1 transcript levels delayed adventitious root formation and decreased root biomass. Transcriptome analyses of PagFBL1 overexpressing lines indicated that an extensive remodelling of gene expression was stimulated by auxin signalling pathway during early adventitious root formation. In addition, PagIAA28 was identified as downstream targets of PagFBL1. We propose that the PagFBL1-PagIAA28 module promotes adventitious rooting and could be targeted to improve Populus propagation by cuttings.

CAD1 and CCR2 protein complex formation in monolignol biosynthesis in Populus trichocarpa.

Lignin is the major phenolic polymer in plant secondary cell walls and is polymerized from monomeric subunits, the monolignols. Eleven enzyme families are implicated in monolignol biosynthesis. Here, we studied the functions of members of the cinnamyl alcohol dehydrogenase (CAD) and cinnamoyl-CoA reductase (CCR) families in wood formation in Populus trichocarpa, including the regulatory effects of their transcripts and protein activities on monolignol biosynthesis. Enzyme activity assays from stem-differentiating xylem (SDX) proteins showed that RNAi suppression of PtrCAD1 in P. trichocarpa transgenics caused a reduction in SDX CCR activity. RNAi suppression of PtrCCR2, the only CCR member highly expressed in SDX, caused a reciprocal reduction in SDX protein CAD activities. The enzyme assays of mixed and coexpressed recombinant proteins supported physical interactions between PtrCAD1 and PtrCCR2. Biomolecular fluorescence complementation and pull-down/co-immunoprecipitation experiments supported a hypothesis of PtrCAD1/PtrCCR2 heterodimer formation. These results provide evidence for the formation of PtrCAD1/PtrCCR2 protein complexes in monolignol biosynthesis in planta.

Characteristics of the Water- and Alkali-Soluble Hemicelluloses Fractionated by Sequential Acidification and Graded-Ethanol from Sweet Maize Stems.

Sweet maize stems were treated with hot water and potassium hydroxide to fractionate hemicellulosic polymers. The results showed that the water-soluble hemicelluloses were mainly composed of glucose (27.83%), xylose (27.32%), and galactose (16.81%). In comparison, alkali-soluble hemicelluloses fractionated by acidification and a graded ethanol solution (10%, 20%, 35%, 50%, 65%, and 80%) were mainly composed of xylose (69.73 to 88.62%) and arabinose (5.41 to 16.20%). More highly branched hemicelluloses tended to be precipitated in a higher concentration of ethanol solution, as revealed by the decreasing xylose to arabinose ratio from 16.43 to 4.21. Structural characterizations indicated that alkali-soluble hemicelluloses fractionated from sweet maize stems were mainly arabinoxylans. The results provided fundamental information on hemicelluloses composition and structure and their potential utilization in the fields of biofuels, biochemicals, and biomaterials.

Tissue and cell-type co-expression networks of transcription factors and wood component genes in Populus trichocarpa.

MAIN CONCLUSION: Co-expression networks based on transcriptomes of Populus trichocarpa major tissues and specific cell types suggest redundant control of cell wall component biosynthetic genes by transcription factors in wood formation. We analyzed the transcriptomes of five tissues (xylem, phloem, shoot, leaf, and root) and two wood forming cell types (fiber and vessel) of Populus trichocarpa to assemble gene co-expression subnetworks associated with wood formation. We identified 165 transcription factors (TFs) that showed xylem-, fiber-, and vessel-specific expression. Of these 165 TFs, 101 co-expressed (correlation coefficient, r > 0.7) with the 45 secondary cell wall cellulose, hemicellulose, and lignin biosynthetic genes. Each cell wall component gene co-expressed on average with 34 TFs, suggesting redundant control of the cell wall component gene expression. Co-expression analysis showed that the 101 TFs and the 45 cell wall component genes each has two distinct groups (groups 1 and 2), based on their co-expression patterns. The group 1 TFs (44 members) are predominantly xylem and fiber specific, and are all highly positively co-expressed with the group 1 cell wall component genes (30 members), suggesting their roles as major wood formation regulators. Group 1 TFs include a lateral organ boundary domain gene (LBD) that has the highest number of positively correlated cell wall component genes (36) and TFs (47). The group 2 TFs have 57 members, including 14 vessel-specific TFs, and are generally less correlated with the cell wall component genes. An exception is a vessel-specific basic helix-loop-helix (bHLH) gene that negatively correlates with 20 cell wall component genes, and may function as a key transcriptional suppressor. The co-expression networks revealed here suggest a well-structured transcriptional homeostasis for cell wall component biosynthesis during wood formation.

Complete proteomic-based enzyme reaction and inhibition kinetics reveal how monolignol biosynthetic enzyme families affect metabolic flux and lignin in Populus trichocarpa.

We established a predictive kinetic metabolic-flux model for the 21 enzymes and 24 metabolites of the monolignol biosynthetic pathway using Populus trichocarpa secondary differentiating xylem. To establish this model, a comprehensive study was performed to obtain the reaction and inhibition kinetic parameters of all 21 enzymes based on functional recombinant proteins. A total of 104 Michaelis-Menten kinetic parameters and 85 inhibition kinetic parameters were derived from these enzymes. Through mass spectrometry, we obtained the absolute quantities of all 21 pathway enzymes in the secondary differentiating xylem. This extensive experimental data set, generated from a single tissue specialized in wood formation, was used to construct the predictive kinetic metabolic-flux model to provide a comprehensive mathematical description of the monolignol biosynthetic pathway. The model was validated using experimental data from transgenic P. trichocarpa plants. The model predicts how pathway enzymes affect lignin content and composition, explains a long-standing paradox regarding the regulation of monolignol subunit ratios in lignin, and reveals novel mechanisms involved in the regulation of lignin biosynthesis. This model provides an explanation of the effects of genetic and transgenic perturbations of the monolignol biosynthetic pathway in flowering plants.

Systems biology of lignin biosynthesis in Populus trichocarpa: heteromeric 4-coumaric acid:coenzyme A ligase protein complex formation, regulation, and numerical modeling.

As a step toward predictive modeling of flux through the pathway of monolignol biosynthesis in stem differentiating xylem of Populus trichocarpa, we discovered that the two 4-coumaric acid:CoA ligase (4CL) isoforms, 4CL3 and 4CL5, interact in vivo and in vitro to form a heterotetrameric protein complex. This conclusion is based on laser microdissection, coimmunoprecipitation, chemical cross-linking, bimolecular fluorescence complementation, and mass spectrometry. The tetramer is composed of three subunits of 4CL3 and one of 4CL5. 4CL5 appears to have a regulatory role. This protein-protein interaction affects the direction and rate of metabolic flux for monolignol biosynthesis in P. trichocarpa. A mathematical model was developed for the behavior of 4CL3 and 4CL5 individually and in mixtures that form the enzyme complex. The model incorporates effects of mixtures of multiple hydroxycinnamic acid substrates, competitive inhibition, uncompetitive inhibition, and self-inhibition, along with characteristic of the substrates, the enzyme isoforms, and the tetrameric complex. Kinetic analysis of different ratios of the enzyme isoforms shows both inhibition and activation components, which are explained by the mathematical model and provide insight into the regulation of metabolic flux for monolignol biosynthesis by protein complex formation.

SND1 transcription factor-directed quantitative functional hierarchical genetic regulatory network in wood formation in Populus trichocarpa.

Wood is an essential renewable raw material for industrial products and energy. However, knowledge of the genetic regulation of wood formation is limited. We developed a genome-wide high-throughput system for the discovery and validation of specific transcription factor (TF)-directed hierarchical gene regulatory networks (hGRNs) in wood formation. This system depends on a new robust procedure for isolation and transfection of Populus trichocarpa stem differentiating xylem protoplasts. We overexpressed Secondary Wall-Associated NAC Domain 1s (Ptr-SND1-B1), a TF gene affecting wood formation, in these protoplasts and identified differentially expressed genes by RNA sequencing. Direct Ptr-SND1-B1-DNA interactions were then inferred by integration of time-course RNA sequencing data and top-down Graphical Gaussian Modeling-based algorithms. These Ptr-SND1-B1-DNA interactions were verified to function in differentiating xylem by anti-PtrSND1-B1 antibody-based chromatin immunoprecipitation (97% accuracy) and in stable transgenic P. trichocarpa (90% accuracy). In this way, we established a Ptr-SND1-B1-directed quantitative hGRN involving 76 direct targets, including eight TF and 61 enzyme-coding genes previously unidentified as targets. The network can be extended to the third layer from the second-layer TFs by computation or by overexpression of a second-layer TF to identify a new group of direct targets (third layer). This approach would allow the sequential establishment, one two-layered hGRN at a time, of all layers involved in a more comprehensive hGRN. Our approach may be particularly useful to study hGRNs in complex processes in plant species resistant to stable genetic transformation and where mutants are unavailable.

Ptr-miR397a is a negative regulator of laccase genes affecting lignin content in Populus trichocarpa.

Laccases, as early as 1959, were proposed to catalyze the oxidative polymerization of monolignols. Genetic evidence in support of this hypothesis has been elusive due to functional redundancy of laccase genes. An Arabidopsis double mutant demonstrated the involvement of laccases in lignin biosynthesis. We previously identified a subset of laccase genes to be targets of a microRNA (miRNA) ptr-miR397a in Populus trichocarpa. To elucidate the roles of ptr-miR397a and its targets, we characterized the laccase gene family and identified 49 laccase gene models, of which 29 were predicted to be targets of ptr-miR397a. We overexpressed Ptr-MIR397a in transgenic P. trichocarpa. In each of all nine transgenic lines tested, 17 PtrLACs were down-regulated as analyzed by RNA-seq. Transgenic lines with severe reduction in the expression of these laccase genes resulted in an ∼40% decrease in the total laccase activity. Overexpression of Ptr-MIR397a in these transgenic lines also reduced lignin content, whereas levels of all monolignol biosynthetic gene transcripts remained unchanged. A hierarchical genetic regulatory network (GRN) built by a bottom-up graphic Gaussian model algorithm provides additional support for a role of ptr-miR397a as a negative regulator of laccases for lignin biosynthesis. Full transcriptome-based differential gene expression in the overexpressed transgenics and protein domain analyses implicate previously unidentified transcription factors and their targets in an extended hierarchical GRN including ptr-miR397a and laccases that coregulate lignin biosynthesis in wood formation. Ptr-miR397a, laccases, and other regulatory components of this network may provide additional strategies for genetic manipulation of lignin content.

Elucidation of Xylem-Specific Transcription Factors and Absolute Quantification of Enzymes Regulating Cellulose Biosynthesis in Populus trichocarpa.

Cellulose, the main chemical polymer of wood, is the most abundant polysaccharide in nature.1 The ability to perturb the abundance and structure of cellulose microfibrils is of critical importance to the pulp and paper industry as well as for the textile, wood products, and liquid biofuels industries. Although much has been learned at the transcript level about the biosynthesis of cellulose, a quantitative understanding at the proteome level has yet to be established. The study described herein sought to identify the proteins directly involved in cellulose biosynthesis during wood formation in Populus trichocarpa along with known xylem-specific transcription factors involved in regulating these key proteins. Development of an effective discovery proteomic strategy through a combination of subcellular fractionation of stem differentiating xylem tissue (SDX) with recently optimized FASP digestion protocols, StageTip fractionation, as well as optimized instrument parameters for global proteomic analysis using the quadrupole-orbitrap mass spectrometer resulted in the deepest proteomic coverage of SDX protein from P. trichocarpa with 9,146 protein groups being identified (1% FDR). Of these, 20 cellulosic/hemicellulosic enzymes and 43 xylem-specific transcription factor groups were identified. Finally, selection of surrogate peptides led to an assay for absolute quantification of 14 cellulosic proteins in SDX of P. trichocarpa.

Splice variant of the SND1 transcription factor is a dominant negative of SND1 members and their regulation in Populus trichocarpa.

Secondary Wall-Associated NAC Domain 1s (SND1s) are transcription factors (TFs) known to activate a cascade of TF and pathway genes affecting secondary cell wall biosynthesis (xylogenesis) in Arabidopsis and poplars. Elevated SND1 transcriptional activation leads to ectopic xylogenesis and stunted growth. Nothing is known about the upstream regulators of SND1. Here we report the discovery of a stem-differentiating xylem (SDX)-specific alternative SND1 splice variant, PtrSND1-A2(IR), that acts as a dominant negative of SND1 transcriptional network genes in Populus trichocarpa. PtrSND1-A2(IR) derives from PtrSND1-A2, one of the four fully spliced PtrSND1 gene family members (PtrSND1-A1, -A2, -B1, and -B2). Each full-size PtrSND1 activates its own gene, and all four full-size members activate a common MYB gene (PtrMYB021). PtrSND1-A2(IR) represses the expression of its PtrSND1 member genes and PtrMYB021. Repression of the autoregulation of a TF family by its only splice variant has not been previously reported in plants. PtrSND1-A2(IR) lacks DNA binding and transactivation abilities but retains dimerization capability. PtrSND1-A2(IR) is localized exclusively in cytoplasmic foci. In the presence of any full-size PtrSND1 member, PtrSND1-A2(IR) is translocated into the nucleus exclusively as a heterodimeric partner with full-size PtrSND1s. Our findings are consistent with a model in which the translocated PtrSND1-A2(IR) lacking DNA-binding and transactivating abilities can disrupt the function of full-size PtrSND1s, making them nonproductive through heterodimerization, and thereby modulating the SND1 transcriptional network. PtrSND1-A2(IR) may contribute to transcriptional homeostasis to avoid deleterious effects on xylogenesis and plant growth.