Abstract
Plant cells biosynthesize primary cell walls (PCW) in all cells and produce secondary cell walls (SCWs) in specific cell types that conduct water and/or provide mechanical support, such as xylem vessels and fibers. The characteristic mechanical stiffness, chemical recalcitrance, and hydrophobic nature of SCWs result from the organization of SCW-specific biopolymers, i.e., highly ordered cellulose, hemicellulose, and lignin. Synthesis of these SCW-specific biopolymers requires SCW-specific enzymes that are regulated by SCW-specific transcription factors. In this review, we summarize our current knowledge of the transcriptional regulation of SCW formation in plant cells. Advances in research on SCW biosynthesis during the past decade have expanded our understanding of the transcriptional regulation of SCW formation, particularly the functions of the NAC and MYB transcription factors. Focusing on the NAC-MYB-based transcriptional network, we discuss the regulatory systems that evolved in land plants to modify the cell wall to serve as a key component of structures that conduct water and provide mechanical support.
Highlights
The cell wall, a characteristic feature of plant cells, consists of biopolymers, such as polysaccharides, phenolic compounds, and various proteins, which impart mechanical strength and rigidity
These findings revealed the regulation of secondary cell wall (SCW) biosynthesis at the molecular level, showing that plants have specific transcriptional switches that regulate SCW biosynthesis, and these factors belong to the NAC family, including VND and NST transcription factors (Yamaguchi and Demura, 2010; Zhong et al, 2010a; Wang and Dixon, 2011; Hussey et al, 2013)
Other VND-related Arabidopsis proteins, namely SOMBRERO (SMB), BEARSKIN1 (BRN1), and BRN2, induce ectopic SCW deposition when overexpressed, in wild type cells, they are expressed in root cap regions where SCW is not deposited (Willemsen et al, 2008; Bennett et al, 2010; Figure 3A, Table 1). These results indicate that the capacity to induce SCW biosynthesis is conserved among the VND, NST, SMB, and BRN proteins, and that these genes likely evolved from a common ancestral gene, acquiring the capacity to regulate wall modification during the differentiation of specific cell types
Summary
The cell wall, a characteristic feature of plant cells, consists of biopolymers, such as polysaccharides, phenolic compounds, and various proteins, which impart mechanical strength and rigidity. The structure of the cell wall determines the characteristics of plant cells, directly affecting organ development and responses to environmental stimuli (Hamant and Traas, 2010; Wolf et al, 2012). Plant cells have two types of cell wall, primary cell wall (PCW) and secondary cell wall (SCW), based on their biosynthetic composition and cellular location (Figure 1). Every plant cell has a PCW, a relatively thin and extensible wall that the cell synthesizes during cell division. The relatively thick and rigid SCW forms in specific types of cells, such as xylem cells, and cells of valve margin and anther endothecium. The cell produces the SCW between the PCW and the plasma membrane during cell differentiation, and the SCW imparts additional
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