Two NAC domain transcription factors, SND1 and NST1, function redundantly in regulation of secondary wall synthesis in fibers of Arabidopsis

Secondary walls are the major component of wood, and studies of the mechanisms regulating secondary wall synthesis is important for understanding the process of wood formation. We have previously shown that the NAC domain transcription factor SECONDARY WALL-ASSOCIATED NAC DOMAIN PROTEIN1 (SND1) is a key regulator of secondary wall synthesis in fibers of Arabidopsis thaliana stems and dominant repression of SND1 leads to a reduction in secondary wall thickening in fibers. However, T-DNA knockout of the SND1 gene did not cause an alteration in secondary wall thickness, suggesting that other SND1 homologs may compensate for the loss of SND1 expression. Here, we studied the effects of simultaneous inhibition of SND1 and its homolog, NAC SECONDARY WALL THICKENING PROMOTING FACTOR1 (NST1), on secondary wall synthesis in fibers. We show that simultaneous RNA interference (RNAi) inhibition of the expression of both SND1 and NST1 genes results in loss of secondary wall formation in fibers of stems. The fiber cells in the stems of SND1/NST1-RNAi plants lack all three major secondary wall components, including cellulose, xylan, and lignin, which is accompanied by a severe reduction in the expression of genes involved in their biosynthesis. In addition, inhibition of SND1 and NST1 leads to down-regulation of several fiber-associated transcription factor genes. Double T-DNA knockout mutations of SND1 and NST1 genes cause the same effects, as does simultaneous RNAi inhibition of SND1 and NST1. Our results provide first line evidence demonstrating that SND1 and NST1 function redundantly in the regulation of secondary wall synthesis in fibers.