Secondary wall NAC binding element (SNBE), a key cis-acting element required for target gene activation by secondary wall NAC master switches.

The biosynthesis of secondary walls in vascular plants requires the coordinated regulation of a suite of biosynthetic genes, and this coordination has recently been shown to be executed by the secondary wall NAC (SWN)-mediated transcriptional network. In Arabidopsis, five SWNs, including SND1, NST1/2 and VND6/7, function as master transcriptional switches to activate their common targets and consequently the secondary wall biosynthetic program. A recent report by Zhong et al. revealed that SWNs bind to a common cis-acting element, namely secondary wall NAC binding element (SNBE), which is composed of an imperfect palindromic 19-bp consensus sequence, (T/A)NN(C/T)(T/C/G)TNNNNNNNA(A/C)GN(A/C/T) (A/T). Genome-wide analysis of direct targets of SWNs showed that SWNs directly activate the expression of not only many transcription factors but also a battery of genes involved in secondary wall biosynthesis, cell wall modification and programmed cell death, the promoters of which all contain multiple SNBE sites. The functional significance of the SNBE sites is further substantiated by our current in planta expression study demonstrating that representative SNBE sequences from several SWN direct target promoters are sufficient to drive the expression of the GUS reporter gene in secondary wall-forming cells. The identification of the SWN DNA binding element (SNBE) and the SWN direct targets marks an important step forward toward the dissection of the transcriptional network regulating the biosynthesis of secondary walls, the most abundant biomass produced by land plants.