Abstract:
The secondary cell walls of woody plant species are a major source of cellulose. This biopolymer is among the most abundant on the planet and is very useful to industry, having many applications, such as in the paper industry and as a possible source of fermentable sugar for biofuel production. Therefore, much emphasis is placed on understanding the formation of woody biomass (mainly comprising of secondary cell walls) as a possible feedstock for cellulose extraction. Wood fibres are of particular interest as they are most useful for the paper industry and have thick, cellulose rich secondary cell walls. Transcriptional regulation of secondary cell wall biosynthesis is facilitated through a complex network of proteins and genes operating in a semi-hierarchical manner. In xylem, each cell type is controlled by a "master switch", a transcription factor (TF) which is able to activate the entire secondary cell wall biosynthetic program for that particular cell type. In Arabidopsis xylem fibres, SECONDARY WALL ASSOCIATED NAC DOMAIN PROTEIN1 (SND1) is the master regulator for secondary cell wall deposition. Overexpression of SND1 results in the upregulation of a number TFs important for secondary cell wall biosynthesis. Three of the upregulated TFs were able to induce expression of a reporter gene under the control of a cellulose synthase (AtCesA8) promoter, indicating these TFs may be involved in cellulose biosynthesis. Of these, only SECONDARY WALL ASSOCIATED NAC DOMAIN PROTEIN2 (SND2) is not a direct target of SND1. While much work has been done on the regulatory network in the model plant Arabidopsis, there are still major gaps in our understanding. Comparatively little work has been done to study this network in hardwood crop tree species such as Eucalyptus grandis. Properly characterising and understanding this network will allow us to manipulate hardwood crops to obtain trees with commercially desirable traits and will also provide a platform for future studies in the field of transcriptional regulation of wood formation.
