Responses of hybrid aspen over-expressing a PIP2;5 aquaporin to low root temperature

Aquaporins mediate the movement of water across cell membranes. Plasma membrane intrinsic protein 2;5 from Populus trichocarpa × deltoides (PtdPIP2;5) was previously demonstrated to be a functionally important water conducting aquaporin. To study the relevance of aquaporin-mediated root water transport at low temperatures, we generated transgenic Populus tremula × alba over-expressing PtdPIP2;5 under control of the maize ubiquitin promoter, and compared the physiological responses and water transport properties of the PtdPIP2;5 over-expressing lines (PtdPIP2;5ox) with wild-type plants. We hypothesized that over-expression of PtdPIP2;5 would reduce temperature sensitivity of root water transport and gas exchange. Decreasing root temperatures to 10 and 5 °C significantly decreased hydraulic conductivities (Lp) in wild-type plants, but had no significant effect on Lp in PtdPIP2;5ox plants. Recovery of Lp in the transgenic lines returned to 20 °C from 5 °C was faster than in the wild-type plants. Low root temperature did not induce major changes in transcript levels for other PIPs. When roots were exposed to 5 °C in solution culture and shoots were exposed to 20 °C, wild-type plants had significantly lower net photosynthetic and transpiration rates compared to PtdPIP2;5ox plants. Taken together, our results demonstrate that over-expression of PtdPIP2;5 in P. tremula × alba was effective in alleviating the effects of low root temperature on Lp and gas exchange.