An Expression Analysis of a Gene Family Encoding Plasma Membrane Aquaporins in Response to Abiotic Stresses in Arabidopsis Thaliana

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Abstract
Aquaporin belongs to a highly conserved group of membrane proteins called major intrinsic proteins that facilitate water transport across biological membranes. The genome of Arabidopsis encodes 35 aquaporin genes with 13 homologs in the plasma membrane intrinsic protein (PIP) subgroup. However, the function of each individual aquaporin isoform and the integrated function of plant aquaporins under various physiological conditions remain unclear. As a step toward understanding the aquaporin function in plants under various environmental stimuli, the expressions of a gene family encoding 13 PIPs in Arabidopsis thalianaunder various abiotic stress conditions including drought, cold, and high salinity, or abscisic acid (ABA) treatment were investigated by a quantitative real-time reverse transcription-PCR analysis. Several PIPgenes were predominantly expressed either in the roots or in the flowers. The expressions of both the highly expressed aquaporins including PIP1;1,PIP1;2,and PIP2;7and the weakly expressed aquaporins such as PIP1;4,PIP2;1,PIP2;4, and PIP2;5were modulated by external stimuli. The analyses of our data revealed that only the PIP2;5was up-regulated by cold treatment, and most of the PIPgenes were down-regulated by cold stress. Marked up- or down-regulation in PIPexpression was observed by drought stress, whereas PIPgenes were less-severely modulated by high salinity. The responsiveness of each aquaporin to ABA were different, implying that the regulation of aquaporin expression involves both ABA-dependent and ABA-independent signaling pathways. Together, our comprehensive expression profile of the 13 members of the PIP gene family provides novel basis to allocate the stress-related biological function to each PIP gene.