HC Regulates Growth and Hydraulic Conductivity In Poplar

Hydraulic conductivity is hypothesized to be a critical factor for rapid growth and biomass productivity in forest trees. According to this hypothesis, trees with inherently low hydraulic conductivity would experience more extreme within-tree water potential gradients and, consequently, more frequent stomatal closure. The low photosynthetic gas exchange would reduce carbon gain and growth rates. We previously quantified genetic and phenotypic variation for stem hydraulic properties and growth in a pseudo pseudo-backcross population of Populus deltoides and P. trichocarpa x P. deltoides. Phenotypes measured included mean hydraulic vessel diameter (D_h) and hydraulic conductivity, as well as plant biomass and growth increment. Quantitative trait loci (QTL) analysis identified a major locus on linkage group 1 that regulates D_h, hydraulic conductivity and growth. A genetical genomic approach that integrated differentiating xylem gene expression and trait QTL data identified a previously uncharacterized gene referred hereafter as Hydraulic Conductivity 1 (HC1) as a potential regulator of both growth, as well as hydraulic conductivity traits. Down-regulation of HC1 expression in the Populus tremula×P. alba hybrid genotype 717 was lethal. However, 717 transgenic lines up-regulating HC1 using the 35S promoter accumulate over 50% more biomass than wild-type trees. Analysis of the stem structural properties of these plants indicate that they have vessels with larger diameter than those observed in the wild-type, suggesting higher hydraulic conductivity as the most likely reason for their superior growth.