Regulation Of Secondary Wall Formation Research Articles

Poplar PdMYB221 is involved in the direct and indirect regulation of secondary wall biosynthesis during wood formation.

Wood is formed by the successive addition of secondary xylem, which consists of cells with a conspicuously thickened secondary wall composed mainly of cellulose, xylan and lignin. Currently, few transcription factors involved in the direct regulation of secondary wall biosynthesis have been characterized in tree species. Here, we show that PdMYB221, a poplar ortholog of the Arabidopsis R2R3-MYB transcription factor AtMYB4, directly regulates secondary wall biosynthesis during wood formation. PdMYB221 is predominantly expressed in cells of developing wood, and the protein it encodes localizes to the nucleus and acts as a transcriptional repressor. Ectopic expression of PdMYB221 resulted in reduced cell wall thicknesses of fibers and vessels in Arabidopsis inflorescence stems. The amounts of cellulose, xylose, and lignin were decreased and the expression of key genes synthesizing the three components was suppressed in PdMYB221 overexpression plants. Transcriptional activation assays showed that PdMYB221 repressed the promoters of poplar PdCESA7/8, PdGT47C, PdCOMT2 and PdCCR1. Electrophoretic mobility shift assays revealed that PdMYB221 bound directly to the PdCESA8, PdGT47C, and PdCOMT2 promoters. Together, our results suggest that PdMYB221 may be involved in the negative regulation of secondary wall formation through the direct and indirect suppression of the gene expression of secondary wall biosynthesis.

The Arabidopsis LRR-RLK, PXC1, is a regulator of secondary wall formation correlated with the TDIF-PXY/TDR-WOX4 signaling pathway

BackgroundAlthough a number of leucine-rich repeat receptor-like kinase-encoding genes (LRR-RLKs) have been identified in plants, a functional role has been determined for only a few. Recent studies have demonstrated that an LRR-RLK, PXY/TDR, is important for the process of secondary vascular development. Other studies have indicated that PXY/TDR is unlikely to be the sole LRR-RLK involved in this complex process.ResultsIn this study, in silico analyses led to the identification of three Arabidopsis LRR-RLK genes (PXY-correlated; PXC1, 2, 3) with transcript accumulation profiles that correlated strongly with several key regulators of vascular development, including PXY/TDR, HB-8, REV, and CLE41. Expression profiling using qPCR and promoter:reporter lines indicated that all three PXC genes are associated with the vasculature. One in particular, PXC1 (At2g36570), had a strong correlation with PXY/TDR. Shifting pxc1 mutants from long-days to short-days showed that loss of the gene led to a dramatic reduction in secondary wall formation in xylem fibers. Transcript analysis of mutants for a variety of secondary cell wall-associated genes, including PXY/TDR indicated that the pathways mediated by PXC1 connect with those mediated by the TDIF-PXY/TDR-WOX4 system.ConclusionsThe data indicate that the LRR-RLK, PXC1 is involved in secondary cell wall formation in xylem fibers. Whereas further study is needed to identify the ligands and mode of action of the PXC1 protein, it is clear from this work that similarly to the shoot apical meristem (SAM), secondary vascular development requires contributions from a number of LRR-RLKs.

Mutation of WRKY transcription factors initiates pith secondary wall formation and increases stem biomass in dicotyledonous plants

Stems of dicotyledonous plants consist of an outer epidermis, a cortex, a ring of secondarily thickened vascular bundles and interfascicular cells, and inner pith parenchyma cells with thin primary walls. It is unclear how the different cell layers attain and retain their identities. Here, we show that WRKY transcription factors are in part responsible for the parenchymatous nature of the pith cells in dicotyledonous plants. We isolated mutants of Medicago truncatula and Arabidopsis thaliana with secondary cell wall thickening in pith cells associated with ectopic deposition of lignin, xylan, and cellulose, leading to an ∼50% increase in biomass density in stem tissue of the Arabidopsis mutants. The mutations are caused by disruption of stem-expressed WRKY transcription factor (TF) genes, which consequently up-regulate downstream genes encoding the NAM, ATAF1/2, and CUC2 (NAC) and CCCH type (C3H) zinc finger TFs that activate secondary wall synthesis. Direct binding of WRKY to the NAC gene promoter and repression of three downstream TFs were confirmed by in vitro assays and in planta transgenic experiments. Secondary wall-bearing cells form lignocellulosic biomass that is the source for second generation biofuel production. The discovery of negative regulators of secondary wall formation in pith opens up the possibility of significantly increasing the mass of fermentable cell wall components in bioenergy crops.

Transcriptional regulation of secondary wall formation controlled by NAC domain proteins

Woody cells develop secondary wall structure that mainly consists of polysaccharides (cellulose and hemicellulose) and lignin. These components are expected to be new sources of biofuels and biomaterials. Therefore, it is important to understand the molecular mechanism underlying secondary wall formation and how it contributes to plant biomass. Plant-specific NAC domain transcription factor family has been shown to be involved in diverse biological functions. Recently, several studies reported that a subfamily of the NAC domain transcription factors plays pivotal roles in secondary wall formation. In this review, we have summarized the role of NAC domain transcription factors in controlling the secondary wall formation.

A chimeric NST repressor has the potential to improve glucose productivity from plant cell walls

Bioethanol might be produced more economically and with less ecological impact (with reduced exploitation of food crops) if we could increase the production of glucose from the cellulosic materials in plant cell walls. However, plant cell walls are relatively resistant to enzymatic and physicochemical hydrolysis and, therefore, it is necessary to develop methods for reducing such resistance. Changes in plant cell wall materials, by genetic engineering, that render them more easily hydrolyzable to glucose might be a valuable approach to this problem. We showed previously that, in Arabidopsis, NAC secondary wall thickening-promoting factor1 (NST1) and NST3 are key regulators of secondary wall formation. We report here that transgenic Arabidopsis plants that expressed a chimeric repressor derived from NST1 produced cell wall materials that were twice as susceptible to both enzymatic and physicochemical hydrolysis as those from wild-type plants. The yields of glucose from both fresh and dry biomass were increased in the chimeric repressor lines. Use of the NST1 chimeric repressor might enhance production of glucose from plant cell walls, by changing the nature of the cell walls themselves.

Cell Wall-Related Genes Involved in Supporting Tissue Formation and Transcriptional Regulation in Arabidopsis thaliana

The characteristic growth pattern of vascular plants largely depends on the intrinsic properties of their cell walls, which are flexible, but strong enough to support the plant body. The plant body is composed of various tissues each with a specific cell wall type. Different sets of enzymes are required for the construction of these individual cell wall types. The cell wall type-specific enzyme-set hypothesis has been described to explain the mechanisms underlying cell wall construction. This hypothesis suggests that specific sets of transcription factors are required for the construction of each of the cell-wall types. Recent reverse genetic studies investigating secondary wall formation in Arabidopsis thaliana have demonstrated the existence of a hierarchical transcriptional network that governs the regulation of secondary wall formation in cell wall types. The examination of the effects of mechanical stimuli on the expression of genes encoding a particular set of cell wall-related enzymes and transcriptional factors has shown that A. thaliana is able to perceive subtle changes in self-weight of the aerial portions, and use this information as a signal to regulate formation of cell walls in the supporting tissues. However, the mechanisms by which mechanical signals are perceived via sensors presumably located at the cell surface remain unknown. In addition, the pathways through which the signal is transmitted and integrated into the transcriptional network that governs the coordinated actions of cell wall-related genes are also yet to be described. Current reverse genetic approaches based on comprehensive expression analysis of cell wall-related genes may aid in the elucidation of the regulatory mechanisms underlying supporting tissue formation via mechanical signals. Such information may contribute not only to a further understanding of the molecular basis underlying evolution of the plant vascular system, but may also provide us with the knowledge required for the future development and utilization of plant cell walls as a sustainable resource.

Molecular characterisation of a novel plant homeobox gene expressed in the maturing xylem zone of Populus tremula x tremuloides.

In trees, wood is produced from the vascular cambium. However, little is known about how differentiation of the cambial derivatives is controlled at the molecular level. Hybrid aspen, Populus tremula x tremuloides, is a fast-growing, wood-forming plant from which an efficient transformation and regeneration protocol has been developed. We use this species as a model to study xylem cell differentiation. In a cross-section of the stem, cells in the cambial region at different stages of development can be seen in a radial distribution. We dissected out the cambial region and constructed a cDNA-library from this tissue. The library was screened with a degenerated primer mix complementary to the third helix region of the homeobox. A number of cDNAs hybridising to the probe were isolated, purified, cloned and sequenced. Characterisation of the expression pattern of two such homeobox cDNAs revealed that one of the cDNAs, denoted PttHB1, was specifically expressed in the xylem maturation zone, indicating a role in the regulation of secondary wall formation, while the other, denoted PttHB2 was expressed in differentiating xylem and phloem cells. Structurally, the two hybrid aspen homeobox genes are homologous to each other, but no close relationship could be identified between these genes and any other homeobox sequence described thus far, in plants or other eukaryotes. We therefore conclude that the PttHB1 and PttHB2 genes belong to a novel hitherto undescribed sequence class of homeobox genes. We designate this as PALE.