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1 Department of Biology, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
Abscisic acid (ABA) is a plant hormone involved in growth, development, and stress adaptation. It acts via multiple pathways including rapid closure of stomatal pores by ion efflux from guard cells (thereby decreasing water loss) and by slower changes in gene expression. The Wilty2, Wilty3, and Wilty-2445 mutants are nonallelic members of a class of dominant mutants whose top leaves wilt when plants are subjected to drought conditions. We investigated the ABA responses of the Wi2 mutant by analysis of leaf transpiration rates and RAB17 (Responsive to ABA) gene expression. Wi2 leaves transpired less than those of wild-type siblings, but the slopes of Wi2 and wild-type ABA dose-leaf transpiration curves were identical, suggesting that Wi2 guard cell sensitivity to ABA is normal. Based on total RNA blot analysis, RAB17 transcripts in unstressed and drought-stressed Wi2 leaves were elevated relative to wild-type tissue. Wi2 ABA concentrations were also elevated relative to wild type in both unstressed and drought-stressed leaves. Similar to the Wi2 phenotype, Wilty3 and Wi-2445 mutants transpired less than their wild-type siblings and had normal ABA and ABA-conjugate levels, as measured by gas chromatography-mass spectrometry. Despite lower leaf transpiration rates, Wi2 mutants lost a greater percentage of fresh mass over time compared to the wild type. The previously characterized recessive mutant wilty1, which has a defect in vascular element development, also had reduced transpiration rates. It is concluded that the dominant Wi2, Wi3, and Wi-2445 mutants are not impaired in ABA metabolism or signaling. It is hypothesized, based on preliminary data, that the dominant mutants described here are impaired in vascular element development, analogous to the wilty1 mutant.
Key Words: ABA • abscisic acid • drought • gene expression • plant vascular development • transpiration
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