"Unknown Function" Gene HC1 Regulates Growth and Hydraulic Conductivity in Poplar Trees

High hydraulic conductivity (HC) is hypothesized to be required for rapid growth and biomass productivity in 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 mapping population of Poplar trees. Quantitative trait loci (QTL) analysis identified a major locus that regulates vessel diameter, HC and growth. A genetical genomic approach integrating xylem gene expression and trait QTL data identified a previously uncharacterized gene, referred hereafter as HC1, as a potential regulator the traits. Down-regulation of HC1 expression in Poplar using RNAi was lethal. However, Poplar up-regulating HC1 using the 35S promoter accumulate over 30% more biomass than wild-type trees. Analysis of the stem properties of these plants indicate that they have vessels with larger diameter, higher count per area and longer vessel elements than those observed in the wild-type. The measured sapwood specific conductivity is significantly higher in the over-expression lines. However, no significant difference in the vulnerability to cavitation was observed. Thus, other factors, such as inter-vessel pit membrane morphology, may contribute far more to vulnerability to drought-induced cavitation. Currently, Arabidopsis lines are also being analyzed for biomass and vessel properties as well as protein localization using GFP fusion. Functional characterization of this gene will improve annotation of this gene family present in all land species but annotated as unknown function and has potential for cis-genic use for biomass increment.