The dual effect of tubulin manipulation on Populus wood formation and drought tolerance
The main objectives were to generate transgenic Populus with altered alpha- (TUA) and beta-tubulin (TUB) expression and/or post-translational modifications (PTMs) in a plant-wide, guard cell-specific or xylem-specific manner, and then to characterize the effects of the modifications on tree growth and wood properties. In short, strong effects on cell wall composition were noted in leaves where transgene expression was strong (Harding et al. 2017; Hu et al. in review). Subtle, but noteworthy effects were observed in wood (developing xylem) where transgene expression was weak (Swamy et al. 2015). Any effect of PTM manipulation was functionally recessive and/or confounded by tubulin overexpression. It was anticipated that PTM manipulation would affect tubulin half-life, and this is the question that was being addressed, but overall tubulin protein levels correlated with transgene expression levels. We also demonstrated that a dynamic C-terminal PTM known as the tyrosination/detyrosination cycle, so common in animal systems, and purportedly operational in plants, does not occur in wood-forming tissues of Populus (Hu et al. 2016). Contrary to the commonly hypothesized role of microtubules in directing microfibril deposition, cellulose content and microfibril angle were not changed in tubulin-overexpressing plants, nor were hemicellulose and lignin contents, in either wood or leaves. Instead, this project demonstrated that the extractability of pectin and xylan polysaccharides was sensitive to tubulin perturbations, with consequences in stomatal stress response and lignocellulosic biomass utilization. The work also revealed a complex interplay by multi-compartmental (cell wall and cytoskeleton) signaling that impinged upon the control of phenylpropanoid metabolic flux to modulate stomatal response to environmental stimuli.