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ABA UDP-glucosyltransferases Play a Crucial Role in ABA Homeostasis in Arabidopsis

ABA UDP-glucosyltransferases Play a Crucial Role in ABA Homeostasis in Arabidopsis
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The phytohormone abscisic acid (ABA) is crucial for plant growth and development as well as adaptive responses to biotic and abiotic stress conditions. The endogenous ABA levels are regulated by balance between its biosynthetic and catabolic activities. The balance may occur at multiple levels including expression of genes involved in these processes. Until now, most of genes in ABA biosynthetic pathways and hydroxylation pathway have been characterized
however, little is known about the conjugation pathway. Moreover, in response to various stress conditions, plants continuously adjust the ABA level to meet physiological needs, but how ABA homeostasis occurs is not fully understood. In particular, the contribution of ABA conjugation in the homeostasis regulation of cellular ABA level has been less clear. This study provides evidence that UGT71B6, an ABA uridine diphosphate glucosyltransferase (UGT), and its two closely related homologs, UGT71B7 and UGT71B8, play crucial roles in ABA homeostasis and in adaptation to dehydration, osmotic stress and high-salinity stresses in Arabidopsis. Ectopic expression of all three UGTs (UGT71B6, UGT71B7, and UGT71B8) in protoplasts suppressed the ABA-induced expression of RD29Ap:LUC. And the expression of UGT71B7 and UGT71B8 was rapidly induced by ABA, NaCl and osmotic stress, as it was reported for UGT71B6. UGT RNAi transgenic plants, which displayed multiple growth defective phenotypes, such as smaller rosette leaves, shorter roots and pale green leaves, showed a 2-fold increase in the cellular ABA levels. Moreover, UGT RNAi transgenic plants showed hypersensitivity to exogenous ABA during germination and post-germination growth, and displayed enhanced resistance to osmotic and dehydration stresses. However, the ectopic expression of UGT71B6 in the atbg1 mutant background aggravated the ABA-deficient phenotype of atbg1 mutant plants. In addition, modulation of the expression of the three UGTs affects the expression of CYP707A1 to CYP707A4, which encode ABA 8′-hydroxylases
four CYP707As were expressed at higher levels in the UGT RNAi plants but at lower levels in the UGT71B6:GFP-overexpressing plants. Based on these data, this study proposes that UGT71B6 and its two homologs play a critical role in ABA homeostasis that depends on intrinsic cellular and environmental conditions in plants.
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