Modified Expression of a HD-ZIP III Transcription Factor Impacts Cell Wall and Biomass Properties in Populus

We applied a multipronged, genetics- and genomics-based, research strategy to examine the regulation of cell wall and biomass properties in Populus. Here, we present results from studies of genes belonging to the Homeodomain-leucine zipper class III (HD-ZIP III) and NAC domain transcription factor families. HD-ZIP III and NAC domain transcription factor family members have been previously known to regulate a diverse array of plant pathways ranging from embryo, shoot, leaf and flower development to biotic and abiotic stress response and hormone signaling. We examined the roles of selected gene family members, with enhanced expression in secondary wall-enriched xylem tissue, in determining biomass properties including sugar release efficiency of Populus. Transgenic Populus deltoides plants were generated via over expression or down regulation under the control of either a vascular-specific 4-coumarate-CoA ligase (4CL) promoter or that of a ubiquitous promoter. Morpho-anatomical, metabolic, physiological, cell wall chemistry studies revealed a range of phenotypic impacts; from changes in cell wall chemistry with no apparent compromise on plant growth and performance to pleiotropic effects to favorable phenotypic combination of enhanced growth and reduced recalcitrance. We further performed SNP network analyses to extract genotypic and phenotypic correlations significant to the targeted genes. Using PdWND1B (NAC) and PdHB3 (HD-ZIP III) as examples, we discuss alternate modes of action of these transcription factor genes in impacting biomass formation and sugar release efficiency in Populus.