An abscisic acid (ABA) homeostasis regulated by its production, catabolism and transport in peanut leaves in response to drought stress

Abstract

26ABA is an important messenger that acts as the signaling mediator for regulating the adaptive response of 27 plants to drought stress. Two production pathways, de novo biosynthesis and hydrolysis of 28 glucose-conjugated ABA by β-glucosidase (BG), increase cellular ABA levels in plants. ABA catabolism 29 via hydroxylation by 8'-hydroxylase (CYP707A), or conjugation by uridine diphosphate 30 glucosyltransferase (UGT), decreases cellular ABA levels. The transport of ABA through ATP-binding 31 cassette (ABC)-containing transporter proteins, members of ABC transporter G family (ABCG), across 32 plasma membrane (PM) is another important pathway to regulate cellular ABA levels. In this study, 33 based on our previously constructed transcriptome of peanut leaves in response to drought stress, fourteen 34 candidate genes involved in ABA production (including AhZEP, AhNCED1 and AhNCED3, AhABA2, 35 AhAAO1 and AhAAO2, AhABA3, AhBG11 and AhBG24), catabolism (including AhCYP707A3, 36 AhUGT71K1 and AhUGT73B4) and transport (including AhABCG22-1 and AhABCG22-2), were 37 identified homologously and phylogenetically, and further analyzed at the transcriptional level by 38 real-time RT-PCR, simultaneously determining ABA levels in peanut leaves in response to drought. The 39 high sequence identity and very similar subcellular localization of the proteins deduced from 14 identified 40 genes involved in ABA production, catabolism and transport with the reported corresponding enzymes in 41 databases suggest their similar roles in regulating cellular ABA levels. In response to drought stress, ABA 42 accumulation levels in peanut leaves agree very well with the up-regulated expressions of 43 ABA-producing genes (AhZEP, AhNCED1, AhAAO2, AhABA3, AhBG11 and AhBG24) and PM-localized 44 ABA importer genes (AhABCG22-1 and AhABCG22-2), although the expression of ABA catabolic genes 45 (AhCYP707A3 and AhUGT71K1) was also up-regulated. It is likely that drought-responsive induction of 46 catabolic genes helps not only to maintain ABA levels within a permissible range, but also to prepare the 3 47 plant for degradation of ABA after removal of the stress. These results suggest that ABA homeostasis in 48 peanut leaves in response to drought may be coordinated by a master regulatory circuit that involves 49 production, catabolism, and as well as transport. 50 73 stomatal closure, seed dormancy, growth and various abiotic stress responses [1,2]. ABA is mainly 74 produced by the de novo biosynthetic pathway through the oxidative cleavage of carotenoids [3]. In this 75 pathway, zeaxanthin epoxidase (ZEP/ABA1) catalyzes the formation of all transviolaxthin from 76 zeaxanthin [4]. Nine cis-epoxycarotenoid dioxygenase (NCED) cleaves carotenoids to form xanthoxin 77 [5,6]. Xanthoxin is assumed to be transported from the plastids to the cytosol, although the precise 78 mechanism that mediates this transport is not yet known [2]. The short-chain alcohol 79 dehydrogenase/reductase (SDR/ABA2) converts xanthoxin derived from cleavage of carot...