The poplar PtrWNDs are transcriptional activators of secondary cell wall biosynthesis

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.

Figure 1

Complexity of wood structure. (A) Cross section of a stem of poplar (Populus trichocarpa) showing the differentiation of vascular cambium (vc) into secondary xylem (wood) cells, including vessels (ve), xylary fibers (xf) and ray parenchyma cells (rp). Note the gradient in the degree of secondary wall thickness from developing to mature secondary xylem. (B) Transmission electron micrograph of secondary walls of the mature secondary xylem of poplar.

Figure 2

PtrWNDs are capable of activating the promoters of a number of poplar wood-associated transcription factors. (A) Diagram of the effector constructs consisting of the full-length cDNA of PtrWNDs driven by the CaMV 35S promoter. NosT, nopaline synthase terminator. (B) Diagram of the GUS reporter constructs driven by the 2-kb promoters of poplar transcription factor genes. (C) Transactivation analysis showing the PtrWNDs-activated expression of the GUS reporter gene driven by the promoters of poplar transcription factor genes. Poplar transcription factors are listed together with their corresponding Arabidopsis secondary wall-associated transcription factors regulated by SND1. The effector and reporter constructs were cotransfected into Arabidopsis protoplasts for the activation analysis. The GUS activity in protoplasts transfected with the reporter construct alone was used as a control and was set to 1. Error bars represent the SE of three biological replicates. (D) Wood-associated expression of the poplar transcription factors used in this study based on the PopGenExpress transcript abundance data. The expression level of each gene in the seedlings was taken as 1.