Comparative genomic analysis of NAC transcriptional factors to dissect the regulatory mechanisms for cell wall biosynthesis

BACKGROUND: NAC domain transcription factors are important transcriptional regulators involved in plant growth, development and stress responses. Recent studies have revealed several classes of NAC transcriptional factors crucial for controlling secondary cell wall biosynthesis. These transcriptional factors mainly include three classes, SND, NST and VND. Despite progress, most current analysis is carried out in the model plant Arabidopsis. Moreover, many downstream genes regulated by these transcriptional factors are still not clear. METHODS: In order to identify the key homologue genes across species and discover the network controlling cell wall biosynthesis, we carried out comparative genome analysis of NST, VND and SND genes across 19 higher plant species along with computational modelling of genes regulated or co-regulated with these transcriptional factors. RESULTS: The comparative genome analysis revealed that evolutionarily the secondary-wall-associated NAC domain transcription factors first appeared in Selaginella moellendorffii. In fact, among the three groups, only VND genes appeared in S. moellendorffii, which is evolutionarily earlier than the other two groups. The Arabidopsis and rice gene expression analysis showed specific patterns of the secondary cell wall-associated NAC genes (SND, NST and VND). Most of them were preferentially expressed in the stem, especially the second internodes. Furthermore, comprehensive co-regulatory network analysis revealed that the SND and MYB genes were co-regulated, which indicated the coordinative function of these transcriptional factors in modulating cell wall biosynthesis. In addition, the co-regulatory network analysis revealed many novel genes and pathways that could be involved in cell wall biosynthesis and its regulation. The gene ontology analysis also indicated that processes like carbohydrate synthesis, transport and stress response, are coordinately regulated toward cell wall biosynthesis. CONCLUSIONS: Overall, we provided a new insight into the evolution and the gene regulatory network of a subgroup of the NAC gene family controlling cell wall composition through bioinformatics data mining and bench validation. Our work might benefit to elucidate the possible molecular mechanism underlying the regulation network of secondary cell wall biosynthesis

Secondary Wall Regulating NACs Differentially Bind at the Promoter at a CELLULOSE SYNTHASE A4 Cis-eQTL

Arabidopsis thaliana CELLULOSE SYNTHASE A4/7/8 (CESA4/7/8) are three non-redundant subunits of the secondary cell wall cellulose synthase complex. Transcript abundance of these genes can vary among genotypes and expression quantitative trait loci (eQTL) were identified in a recombinant population of the accessions Bay-0 and Shahdara. Genetic mapping and analysis of the transcript levels of CESAs between two distinct near isogenic lines (NILs) confirmed a change in CESA4 expression that segregates within that interval. We sequenced the promoters and identified 16 polymorphisms differentiating CESA4Sha and CESA4Bay. In order to determine which of these SNPs could be responsible for this eQTL, we screened for transcription factor protein affinity with promoter fragments of CESA4Bay, CESA4Sha, and the reference genome CESA4Col. The wall thickening activator proteins NAC SECONDARY WALL THICKENING PROMOTING FACTOR2 (NST2) and NST3 exhibited a decrease in binding with the CESA4Sha promoter with a tracheary element-regulating cis-element (TERE) polymorphism. While NILs harboring the TERE polymorphisms exhibited significantly different CESA4 expression, cellulose crystallinity and cell wall thickness were indistinguishable. These results suggest that the TERE polymorphism resulted in differential transcription factor binding and CESA4 expression; yet A. thaliana is able to tolerate this transcriptional variability without compromising the structural elements of the plant, providing insight into the elasticity of gene regulation as it pertains to cell wall biosynthesis and regulation. We also explored available DNA affinity purification sequencing data to resolve a core binding site, C(G/T)TNNNNNNNA(A/C)G, for secondary wall NACs referred to as the VNS element

Understanding the transcriptional regulation of secondary cell wall biosynthesis in the model grass Brachypodium distachyon

Secondary cell wall synthesis occurs in specialized cell types following completion of cell enlargement. By virtue of mechanical strength provided by a wall thickened with cellulose, hemicelluloses, and lignin, these cells can function as water-conducting vessels and provide structural support. Several transcription factor families regulate genes encoding wall synthesis enzymes. Certain NAC and MYB proteins directly bind upstream of structural genes and other transcription factors. The most detailed model of this regulatory network is established predominantly for a eudicot, Arabidopsis thaliana. In grasses, both the patterning and the composition of secondary cell walls are distinct from that of eudicots. These differences suggest transcriptional regulation is similarly distinct. Putative rice and maize orthologs of several eudicot cell wall regulators genetically complement mutants of A. thaliana or result in wall defects when constitutively over-expressed; nevertheless, aside from maize ZmMYB31, switchgrass PvMYB4, and Brachypodium BdSWN5, function has not been tested in a grass. Similar to the seminal work conducted in A. thaliana, gene expression profiling in maize, rice, and other grasses implicates additional genes as regulators. Characterization of these genes in a grass species will continue to elucidate the relationship between the transcription regulatory networks of eudicots and grasses. In the context of this dissertation two cell wall genes responsible for synthesizing cellulose in the secondary cell walls were characterized. Several MYBs, a NAC and a bZIP protein was found to interact with the cellulose gene promoters. A reverse genetics approach was used to functionally characterize two of those regulators, MYB48 and GNRF. MYB48 is the first grass specific cell wall regulator found to positively regulate cell wall biosynthesis by binding to the cellulose and lignin gene promoters. It regulates above ground biomass in B. distachyon. GNRF, on the hand, unlike the characterized NAC proteins, was shown to repress cell wall biosynthesis. GNRF is also repressing flowering in B. distachyon, a novel regulatory function that has not been associated with the characterized NAC proteins to date. Further characterization of GNRF is likely to provide new insights into the pleiotropic regulatory roles of this protein

Breeding grasses for capacity to biofuel production or silage feeding value: an updated list of genes involved in maize secondary cell wall biosynthesis and assembly

In the near future, maize, sorghum, or switchgrass stovers and cereal straws will be a significant source of carbohydrates for sustainable biofuel production, in addition to the current use of grass silage in cattle feeding. However, cell wall properties, including the enzymatic degradability of structural polysaccharides in industrial fermenters or animal rumen, is greatly influenced by the embedding of cell wall carbohydrates in lignin matrix, and the linkages between lignins, p-hydroxycinnamic acids, and arabinoxylans. Breeding for higher and cheaper biofuel or silage production will thus be based on the discovery of genetic traits involved in each cell wall component biosynthesis and deposition in each lignified tissue. Due to its considerable genetic and genomic backgrounds, maize is the relevant model species for identifying traits underlying cell wall degradability variations in grasses. Maize genes involved or putatively involved in the biosynthesis of cell wall phenolic compounds, cell wall carbohydrates and regulation factors were therefore searched for using data available in grass, Arabidopsis, and woody species (mostly poplar and eucalyptus). All maize ortholog genes were searched for using protein sequences and a “blastp” strategy against data available in the www.maizesequence.org database. Genes were also mapped in silico considering their physical position in the same database. Finally, 409 candidate genes putatively involved in secondary cell wall biosynthesis and assembly were shown in the maize genome, out of which 130 were related to phenolic compound biosynthesis, 81 were related to cell wall carbohydrate biosynthesis, and 198 were involved in more or less known regulation mechanisms. Most probable candidate genes involved in regulation and assembly of secondary cell wall belonged to the MYB (45 genes) and NAC (38 genes) families, but also included zinc finger and HDZipIII encoding genes. While genes involved in ferulic acid cross-linkages with other cell wall components were little known, several families putatively involved in (arabino)-xylan chain biosynthesis and in feruloyl transfer were shown, including especially arabinosyl-CoA-acyltransferases, feruloyl-AX b-1,2-xylosyl transferases, and xylan-O-3-arabinosyl transferases. This candidate gene list, which focused on genes and orthologs known to be involved in cell wall component biosynthesis and regulation, cannot be considered as exhaustive. Other genes, whose role in cell wall lignification and deposition have not yet been defined, should very likely be added to the list of candidates required for secondary cell wall assembly. Genes encoding proteins of still unknown function should also be added to the list, as several of the latter are probably involved in lignified tissue biosynthesis and deposition

Brachypodium distachyon GNRF, SWAM1 and SWAM4 are transcriptional regulators of secondary cell wall biosynthesis

Plant cell walls are complex structures that contain a matrix of cellulose, lignin and hemicellulose. The regulation of the biosynthesis of these components has been well-studied in the eudicot plant Arabidopsis thaliana, and a transcriptional network has been elucidated. Several NAC and MYB family transcription factors are key regulators of secondary cell wall biosynthesis, and their functional characterization provides significant insight into the complex underlying transcriptional network. Genetic and structural evidence suggests that genes controlling this process might be different between eudicots and monocots. Here, the model grass Brachypodium distachyon has been selected to characterize the function of GNRF (GRASS NAC REPRESSOR OF FLOWERING), SWAM1 (SECONDARY WALL ASSOCIATED MYB1), and SWAM4 in the regulation of secondary cell wall biosynthesis. Phylogenetic analysis identified that GNRF and SWAM4 as the respectively AtSND2 and AtMYB61 transcription factors in B. distachyon. Co-expression analysis showed that both, GNRF and SWAM4, clustered with putative cell-wall-associated genes. Functional characterization was performed by using the overexpression plants GNRF-OE and SWAM4-OE; sodium azide mutant plants from a TILLING (Targeting Induced Local Lesion IN Genome) collection for gnrf-1, gnrf-2, gnrf-3, gnrf-4, gnrf-5, swam4-1, and swam4-2; a T-DNA insertional mutant plant, gnrf-6; and a dominant repressor plant, SWAM4-DR. GNRF-OE plants remained at juvenile stage and exhibited persistent vegetative growth, and some gnrf mutant plants were late flowering. SWAM4-DR plants were severely dwarfed. Stems of all genotypes were subjected to lignin quantification, cell wall thickness measurements, Q-RT-PCR, and RNA-seq analysis. Cell wall and transcriptomic analysis revealed that GNRF is a repressor of SWAM1, a MYB activator of cell wall thickening, and represses genes encoding cellulose, lignin, and xylan biosynthetic enzymes. GNRF was found to function as a pleiotropic repressor of cell wall biosynthesis, flowering, and transport proteins. Protein-DNA interactions were revealed in yeast by yeast-one-hybrid assays; the GNRF binding site (CT/GTA/G/CA/TNNNNT/G/CAA/CA/T/GA/TA/T) was identified by DNA affinity purification sequencing (DAP-seq) assay. SWAM4 is a putative regulator of cell wall biosynthetic genes (CESA4, CESA7, CESA8, CAD, and COMT), and other proteins associated with cell wall formation. Collectively, GNRF, SWAM4, and SWAM1 were characterized as secondary cell wall regulators in B. distachyon