Abstract
Secondary cell walls, consisting of cellulose, hemicelluloses and lignin, make up the bulk of wood biomass. It is therefore expected that dissection of the molecular mechanisms underlying secondary wall biosynthesis and its regulation will be instrumental to unravel the process of wood formation in tree species. Wood formation requires the coordinated activation of genes in the secondary wall biosynthetic program that is essential for the biosynthesis and assembly of wood components. It has recently been discovered that a group of poplar (Populus trichocarpa) wood-associated NAC domain transcription factors, PtrWNDs, which are functional orthologs of the Arabidopsis SND1, are capable of turning on the entire secondary wall biosynthetic program when expressed in Arabidopsis. In addition, two of the PtrWNDs were found to be able to activate the promoters of poplar wood biosynthetic genes and a number of other poplar wood-associated transcription factors. Further testing reveals that the promoters of these poplar wood-associated transcription factors are also activated by other PtrWNDs. It is therefore proposed that PtrWNDs are master transcriptional switches regulating a cascade of downstream transcription factors and thereby mediate the coordinated activation of wood biosynthetic genes during wood formation.