Aquaporins have been documented to be regulated in response to water stress transcriptionally, post-transcriptionally and by post-translational modifications (see above). In the hope of elucidating the functional role of aquaporins in planta, a number of studies have attempted to correlate AQP expression and localization in planta with physiological data obtained in response to environmental stimuli (Sakr et al. 2003, Cochard et al. 2007, Secchi et al. 2007). Many studies have examined the response of aquaporin expression to water stress with responses differing markedly between plant species (Bramley et al. 2007a).
The expression profile of aquaporins in response to water stress in roots and the petiole has been found to differ between two grapevine varieties, Char-donnay and Grenache, in response to both water stress and over a diurnal cycle (Shelden 2007). There were two apparent responses in the transcriptional regu lation of aquaporins in the petioles. In the first instance, a diurnal response was observed under favourable conditions when the vines were well watered. A separate mode of transcriptional regulation was apparent in response to water stress.
Both these responses differed in the genes targeted and between varieties, indicating that the transcriptional regulation of aquaporins in grapevines is quite complex and is unique between varieties that contrast in their degree of isohy-dry. In roots, VvPIP1;1 expression showed very different responses between Grenache and Chardonnay (Vandeleur et al. 2008), where it was up-regulated under drought in Chardonnay, but unchanged in Grenache. Re-hydration caused down-regulation in Chardonnay and up-regulation in Grenache. Different transcriptional responses of aquaporins occur under water stress in different organs in the drought tolerant rootstock Richter 110 (Galmes et al. 2007).
No correlation was observed between hydraulic conductivities and expression levels in this instance, leading the authors to suggest that the aq-uaporins are regulated to achieve homeostasis. The transcriptional regulation of aquaporins may help to provide a balance in maintaining cellular water balance, to optimize photosynthesis and prevent water loss under conditions of water stress. A proteomic analysis comparing differences between Chardonnay and Cabernet Sauvignon in response to water deficit and salinity found that cultivar differences were the main source of variation in protein expression (Vincent et al. 2007).
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