Aquaporin-regulated response of grapevine roots to salinity
Soil salinization is an emerging problem in California vineyards. Research is needed to more fully understand the physiological response of grapevine roots to salt stress in order to develop cultural strategies that improve in-field management and to facilitate breeding of tolerance. Upon exposure to salinity, roots often exhibit a rapid decrease of water uptake capacity caused by inhibition of water-channel proteins called aquaporins. Aquaporins are found throughout fine root cellular membranes and can control the efficiency of water extraction from the soil. Prevention and/or alleviation of salinity-induced aquaporin inhibition have been demonstrated for some plants using calcium supplements in experimental conditions. Such a mechanism may contribute to the success of gypsum (i.e. calcium sulfate) applications used to lessen the detrimental effects of vineyard salinity. In our grant, we proposed to address the following short-intermediate term goals: 1) to quantify aquaporin response to salinity and the ameliorative effects of calcium in a suite of grapevine rootstocks using both hydraulic physiology and molecular probes under hydroponic and soil growth conditions; and 2) to investigate the role that aquaporins play in grapevine rootstock physiological responses to other abiotic factors (i.e. drought, anoxia, nutrient status) and their contribution to vine vigor. We are on target to achieve these goals over a two year funding period (the current Research Support Agreement between USDA-ARS and UCDavis expires 31 September 2010). Our results from 2008 and 2009 indicate that aquaporins play an integral role in water uptake across numerous Vitis rootstocks and exhibit a strong and rapid response to salinity stress. In 2008, we documented significantly higher inherent aquaporin expression in high vigor and drought resistant rootstocks (1103P and 110R) compared to those with low vigor and drought intolerance ratings (420A and 101-14). In additional experiments conducted during 2009, we continued to find this pattern regardless of the growing conditions (i.e. soil or hydroponics). These inherent differences may contribute to the known variation in vigor and drought tolerance among these rootstocks, and represent potential target genes for breeding similar traits. In 2009, we continued to assess the response of roots to salinity stress and the ameliorative affects of calcium. Aquaporin gene expression in numerous rootstocks (Ramsey, Riparia, French Colombard, Thompson seedless, 420A, 101-14, 110R, 039-16, 5BB, 1103P) was highly responsive to salt stress. In most of the rootstocks studied, expression increased significantly under salinity stress across all PIP1 and PIP2 aquaporin genes. In a subsequent hydroponic study, we tracked the hydraulic conductivity of fine roots under control, NaCl, or NaCl plus calcium treatments. We found no ameliorative effects of calcium, but did find that conductivity under salinity was maintained similarly to controls for some of the rootstocks. An up-regulation of aquaporin gene expression under salinity conditions likely plays a role in this response (gene expression for these roots will be completed by Spring 2010). We have begun additional experiments to determine if long term growth in calcium enables tolerance to future exposure to salt stress.