Tubulins, rhythms and cell walls in poplar leaves: it's all in the timing.

Plant cell walls exhibit architectural and compositional changes throughout their development and in response to external cues. While tubulins are involved in cell wall biogenesis, much remains unknown about the scope of their involvement during the orchestration of this resource-demanding process. A transgenic approach coupled with cell wall compositional analysis, RNA-seq and mining of publicly available diurnal gene expression data was used to assess the involvement of tubulins in poplar leaf cell wall biogenesis. Leaf cell walls of transgenic poplar lines with constitutive overexpression of α-tubulin (TUA) exhibited an increased abundance of homogalacturonan, along with a reduction in xylose. These changes were traced to altered expression of UDP-glucuronic acid decarboxylase (GADC) in the transgenic leaves. A model is postulated by which altered diurnal control of TUA through its constitutive overexpression led to a metabolic tradeoff affecting cellular utilization of GADC substrate UDP-glucuronic acid. While there were no effects on cellulose, hemicellulose or lignin abundance, subtle effects on hemicellulose composition and associated gene expression were noted. In addition, expression and enzymatic activity of pectin methylesterase (PME) decreased in the transgenic leaves. The change is discussed in a context of increased levels of PME substrate homogalacturonan, slow stomatal kinetics and the fate of PME product methanol. Since stomatal opening and closing depend on fundamentally contrasting microtubule dynamics, the slowing of both processes in the transgenic lines as previously reported appears to be directly related to underlying cell wall compositional changes that were caused by tubulin manipulation.