MYB Transcription Factors and Its Regulation in Secondary Cell Wall Formation and Lignin Biosynthesis during Xylem Development

Abstract

The secondary wall is the main part of wood and is composed of cellulose, xylan, lignin, and small amounts of structural proteins and enzymes. Lignin molecules can interact directly or indirectly with cellulose, xylan and other polysaccharide molecules in the cell wall, increasing the mechanical strength and hydrophobicity of plant cells and tissues and facilitating the long-distance transportation of water in plants. MYBs (v-myb avian myeloblastosis viral oncogene homolog) belong to one of the largest superfamilies of transcription factors, the members of which regulate secondary cell-wall formation by promoting/inhibiting the biosynthesis of lignin, cellulose, and xylan. Among them, MYB46 and MYB83, which comprise the second layer of the main switch of secondary cell-wall biosynthesis, coordinate upstream and downstream secondary wall synthesis-related transcription factors. In addition, MYB transcription factors other than MYB46/83, as well as noncoding RNAs, hormones, and other factors, interact with one another to regulate the biosynthesis of the secondary wall. Here, we discuss the biosynthesis of secondary wall, classification and functions of MYB transcription factors and their regulation of lignin polymerization and secondary cell-wall formation during wood formation.

Keywords: MYB transcription factors; MYB46/83; classification; lignification; secondary cell wall biosynthesis.

Figure 1

Developmental stages of wood formation and characteristic features of each stage. Note: PIN1, auxin eux carrier PIN-FORMED1 protein; MP/ARF5, the auxin-responsive transcription factor (TF) AUXIN RESPONSE FACTOR 5/MONOPTEROS; AtHB8, ARABIDOPSIS THALIANA HOMEOBOX 8; AHP6, an auxin that positively regulates the expression of an inhibitor of CK signaling; RVE, one of five HD-ZIP III genes in Arabidopsis thaliana; KNAT7, KNOTTED-LIKE HOMEOBOX OF ARABIDOPSIS THALIANA 7; ACAULIS5 (ACL5), encodes a thermospermine synthase. Solid arrows and flat-headed arrows represent positive and negative transcriptional regulation, respectively. Referred to [23,24,25,26].

Figure 2

Phenylpropane lignin biosynthesis pathway in Arabidopsis. In this model, MYB transcription factors control the expression of genes in the lignin synthesis pathway. MYB58 and MYB63, MYB4 can activate/inhibit almost all the enzymes in the lignin synthesis pathway, respectively. All TFs appear to be in ovals in the figure. Solid arrows and flat-headed arrows represent positive and negative transcriptional regulation between transcription factors and enzymes, respectively. The other solid arrows represent the direction of the regulatory network. Note: PAL, phenylalanine ammonia lyase; 4CL, 4-coumarate-CoA ligase; C4H, cinnamate 4-hydroxylase; CCR, cinnamyl-CoA reductase; CAD, cinnamyl alcohol dehydrogenase; C3H, p-coumaric acid 3-hydroxylase; COMT, catechol-O-methyltransferase; F5H, ferulic acid 5-hydroxylase; POX, peroxidase; LAC, laccase.

Figure 3

Proposed model of MYB-mediated secondary cell wall (SCW) regulation in Arabidopsis. Solid black and flat-headed arrows represent positive and negative transcriptional regulation, respectively. Dashed lines represent co-expression relationships, MYB26 and SND1, MYB46/83, and other TFs represent the first, second, and third layers of the transcriptional regulatory network, respectively. The expression of 9 SND1-regulated transcription factors, namely, MYB20, MYB42, MYB43, MYB52, MYB54, MYB69, MYB85, MYB103, was developmentally associated with cells undergoing secondary wall thickening (a, b, c, d, e; [72]). MYB46 and MYB83 serve as the second layer of the main switch for secondary cell wall biosynthesis, which activate downstream transcription factors (including MYB20, MYB42, MYB43 and MYB85) by binding to SMRE sequence in an SCW MYB-responsive element (f, g, h, r; [101,103]) and directly or indirectly regulate the biosynthesis of the secondary wall. MYB42, MYB43, MYB85 (j, k, l; [109]), MYB58, MYB63 (m; [110]), MYB103 (i; [37]) are transcriptional regulators that directly activate lignin biosynthesis genes during secondary wall formation in Arabidopsis. MYB4, MYB7, MYB32, MYB75 are inhibitors of lignin biosynthesis (n, o, p, q; [60]). The concerted actions of the MYB TFs in this network leads to a coordinated activation of SCW biosynthetic genes, which results in the synthesis of lignin, cellulose, xylan.