Transcriptional Activation of Secondary Wall Biosynthesis by Rice and Maize NAC and MYB Transcription Factors

Abstract

The bulk of grass biomass potentially useful for cellulose-based biofuel production is the remains of secondary wall-containing sclerenchymatous fibers. Hence, it is important to uncover the molecular mechanisms underlying the regulation of secondary wall thickening in grass species. So far, little is known about the transcriptional regulatory switches responsible for the activation of the secondary wall biosynthetic program in grass species. Here, we report the roles of a group of rice and maize NAC and MYB transcription factors in the regulation of secondary wall biosynthesis. The rice and maize secondary wall-associated NACs (namely OsSWNs and ZmSWNs) were able to complement the Arabidopsis snd1 nst1 double mutant defective in secondary wall thickening. When overexpressed in Arabidopsis, OsSWNs and ZmSWNs were sufficient to activate a number of secondary wall-associated transcription factors and secondary wall biosynthetic genes, and concomitantly result in the ectopic deposition of cellulose, xylan and lignin. It was also found that the rice and maize MYB transcription factors, OsMYB46 and ZmMYB46, are functional orthologs of Arabidopsis MYB46/MYB83 and, when overexpressed in Arabidopsis, they were able to activate the entire secondary wall biosynthetic program. Furthermore, the promoters of OsMYB46 and ZmMYB46 contain secondary wall NAC-binding elements (SNBEs), which can be bound and activated by OsSWNs and ZmSWNs. Together, our results indicate that the rice and maize SWNs and MYB46 are master transcriptional activators of the secondary wall biosynthetic program and that OsSWNs and ZmSWNs activate their direct target genes through binding to the SNBE sites.

A Gibberellin-Mediated DELLA-NAC Signaling Cascade Regulates Cellulose Synthesis in Rice

“…The downstream components of NAC29 and NAC31 are often MYBs (Zhong et al, 2011). Coexpression analysis identified several secondary wall CESA coexpressed R2R3-type MYBs (Supplemental Table 1), most of which were GA-responsive (Supplemental Figure 4A).…”

“…Currently, through protein-DNA screening, an elaborate hierarchy network of TFs and secondary wall metabolic genes has been revealed in Arabidopsis, offering an opportunity for understanding the regulatory complexity of cell wall formation under abiotic stresses (Taylor-Teeples et al, 2015). However, the networks for grasses, which possess a distinct patterning and composition of the secondary wall, are largely unknown (Zhong et al, 2011;Handakumbura and Hazen, 2012). The CESA genes that are responsible for synthesizing cellulose, a major component of secondary walls, have been proposed to be regulated by many TFs.…”

Genetic Determinants for Enzymatic Digestion of Lignocellulosic Biomass Are Independent of Those for Lignin Abundance in a Maize Recombinant Inbred Population

“…et al, 2010; Kim et al, 2013). MYB genes serve as inducers or repressors of secondary wall synthesis, and their downregulation is coordinated with the up-regulation of the No apical meristem/Arabidopsis transcription activation factor/Cup-shaped cotyledon (NAC)/NAC secondary wall thickening promoting factor (NST) genes (Zhao and Dixon 2011;Zhong et al, 2011). For lignin/4-vinylphenol traits, expression differences of 5-fold or greater are found with a NAC domain and a MYB gene in QTL5, with MYB30 in QTL6, and with MYB61 in QTL8 (Supplemental Table S5).…”

RICE SALT SENSITIVE3 binding to bHLH and JAZ factors mediates control of cell wall plasticity in the root apex

“…[18][19][20][21][22][23][24] Some rice NAC-type transcription factors have also been described to control secondary cell wall metabolism. 25 In rss3, several NAC genes that belong to the subfamily, in which Arabidopsis NST1 and SND2 are classified, were upregulated (Fig. 1B).…”