Aquaporins (AQP) are membrane bound proteins that facilitate the movement of water and other small neutral solutes across cellular membranes. Plant aquaporins belong to a large family of highly conserved proteins called the Membrane Intrinsic Protein (MIP) superfamily. In many plant species the expression of aquaporin genes and their regulation has been linked to water stress. Grapevines respond to water stress with a variety of physiological mechanisms, including the susceptibility to xylem embolism. The formation of embolised vessels can lead to a reduction in hydraulic conductivity of the xylem. Recently, it has been hypothesised that aquaporins may contribute to the water movement required for embolism recovery of xylem vessels thus restoring the hydraulic pathway. Molecular and physiological techniques have been combined to study the putative role of plasma membrane and tonoplast membrane aquaporins in response to water stress induced xylem embolism in two cultivars of grapevine (Vitis vinifera cv. Chardonnay and Grenache). Water-stress induced cavitation was measured in the stems and petioles of pot grown grapevines of a drought tolerant (Grenache) and a drought sensitive variety (Chardonnay) by the detection of ultrasonic acoustic emissions (UAEs) over both a drying and diurnal cycle. Vulnerability curves were generated by correlating the UAEs with the leaf water potential (ψL). Varietal differences in cavitation vulnerability and hydraulic properties were observed. Grenache was more susceptible to water-stress induced xylem embolism than Chardonnay, and displayed a higher hydraulic capacity (measured by maximum hydraulic conductivity). This is most likely due to anatomical differences of the xylem vessels. Chardonnay displayed vulnerability segmentation, with cavitation occurring first in the petiole and later in the stem, before developing into “runaway” cavitation under severe water stress. Vulnerability segmentation was not observed in Grenache, with both petioles and stems equally vulnerable to the formation of xylem embolism. Under severe water stress, Grenache did not develop runaway cavitation indicating that they must have some mechanism to prevent the onset of runaway cavitation. To determine the role of aquaporins, candidate genes were identified, by screening a Vitis vinifera cv. Cabernet Sauvignon cDNA library, for aquaporin cDNAs encoding members of the Plasma membrane Intrinsic Protein (PIP) and Tonoplast Intrinsic Protein (TIP) subfamilies. The screen resulted in the identification of 11 full-length and two partial aquaporin cDNAs. Sequence analyses of these cDNAs reveal five are homologous to PIP2 aquaporins, six to PIP1 and two to the TIP aquaporins. Functional expression of the fulllength AQP cDNAs in Xenopus oocytes showed PIP2 members have significantly higher water permeability compared to PIP1 aquaporins. VvPIP2;1 showed very high water permeability which was reduced by acidic cytosolic pH, as has been reported for other members of the PIP2 family. Transcript analysis of some of these aquaporin genes provides preliminary evidence that aquaporins may contribute to differences in the hydraulic response of these two grapevine varieties to conditions of water stress. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1313316 / Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2008

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