Functional Characterization of Poplar Wood-Associated NAC Domain Transcription Factors

Abstract

Wood is the most abundant biomass produced by land plants. Dissection of the molecular mechanisms underlying the transcriptional regulation of wood formation is a fundamental issue in plant biology and has important implications in tree biotechnology. Although a number of transcription factors in tree species have been shown to be associated with wood formation and some of them are implicated in lignin biosynthesis, none of them have been demonstrated to be key regulators of the biosynthesis of all three major components of wood. In this report, we have identified a group of NAC domain transcription factors, PtrWNDs, that are preferentially expressed in developing wood of poplar (Populus trichocarpa). Expression of PtrWNDs in the Arabidopsis (Arabidopsis thaliana) snd1 nst1 double mutant effectively complemented the secondary wall defects in fibers, indicating that PtrWNDs are capable of activating the entire secondary wall biosynthetic program. Overexpression of PtrWND2B and PtrWND6B in Arabidopsis induced the expression of secondary wall-associated transcription factors and secondary wall biosynthetic genes and, concomitantly, the ectopic deposition of cellulose, xylan, and lignin. Furthermore, PtrWND2B and PtrWND6B were able to activate the promoter activities of a number of poplar woodassociated transcription factors and wood biosynthetic genes. Together, these results demonstrate that PtrWNDs are functional orthologs of SND1 and suggest that PtrWNDs together with their downstream transcription factors form a transcriptional network involved in the regulation of wood formation in poplar.

Comprehensive Analysis of NAC Domain Transcription Factor Gene Family in Populus trichocarpa

“…The expression levels of five NAC genes including PNAC070, 086, 121 and 122 were peaked exclusively in differentiating xylems supported by two independent microarray datasets (Figure 6C and 6G) [87,88]. Among them, PNAC070 (PtWND5A), PNAC086 (PtWND1A) and PNAC008 (PtWND3A) were demonstrated in previous studies as putative redundant homologs involved in Populus wood formation [19]. Interestingly, several genes such as PNAC013, 020 and 120 were preferentially expressed in tissues enriched of secondary cell walls, whereas a subset of genes including PNAC028, 052, 055 and 023 showed constitutively high expressions in both secondary and primary cell wall enriching tissues (Figure 6A-E).…”

“…For example, in accordance with previous findings, a subset of eight genes among the 25 NACs tested were exclusively expressed in differentiating xylem, indicative of their putative roles in secondary cell wall formation. Three genes namely PNAC009 (PtWND4B), PNAC071 (PtWND1B) and PNAC085 (PtWND2B) have been functionally characterized in a recent report [19]. In the present study, PNAC071 gene was shown to primarily express in old root, while microarray profiles indicated the highest transcript abundance in differentiating xylems (Figure 7).…”

“…The six well-characterized NAC genes involved in the secondary cell wall thickening in the previous studies were all included in this category [19]. The expression pattern of PNAC008 gene was inconsistent with the above microarray analysis (Figure 6).…”

Lignin and Biomass: A Negative Correlation for Wood Formation and Lignin Content in Trees

“…Several transcription factors have been implicated in the control of lignin biosynthesis during formation of secondary xylem (Raes et al, 2003;Zhou et al, 2009;Zhong et al, 2010). NAC domain transcription factors have been implicated in both Arabidopsis (Arabidopsis thaliana) and Populus, including wood-associated NAC domain factors that are functional orthologs of SND1 (for secondary wall-associated NAC domain protein 1; Zhou et al, 2009;Zhong et al, 2010).…”

“…NAC domain transcription factors have been implicated in both Arabidopsis (Arabidopsis thaliana) and Populus, including wood-associated NAC domain factors that are functional orthologs of SND1 (for secondary wall-associated NAC domain protein 1; Zhou et al, 2009;Zhong et al, 2010). One LIM factor, up to eight MYBs, and a KNOX transcription factor are direct positive and negative regulators of genes in the monolignol biosynthetic pathway, affecting transcript abundance, enzyme activity, and lignin content or composition (S/G ratio; Tamagnone et al, 1998;Kawaoka et al, 2000;Dixon et al, 2002; Mele et al, Figure 3.…”