Abstract
Abscisic acid (ABA) is an important phytohormone that plays a critical role in seed development, dormancy, and stress tolerance. 9-cis-Epoxycarotenoid dioxygenase is the key enzyme controlling ABA biosynthesis and stress tolerance. In this study, we investigated the effect of ectopic expression of another ABA biosynthesis gene, ABA2 (or GLUCOSE INSENSITIVE 1 [GIN1]) encoding a short-chain dehydrogenase/reductase in Arabidopsis (Arabidopsis thaliana). We show that ABA2-overexpressing transgenic plants with elevated ABA levels exhibited seed germination delay and more tolerance to salinity than wild type when grown on agar plates and/or in soil. However, the germination delay was abolished in transgenic plants showing ABA levels over 2-fold higher than that of wild type grown on 250 mm NaCl. The data suggest that there are distinct mechanisms underlying ABA-mediated inhibition of seed germination under diverse stress. The ABA-deficient mutant aba2, with a shorter primary root, can be restored to normal root growth by exogenous application of ABA, whereas transgenic plants overexpressing ABA2 showed normal root growth. The data reflect that the basal levels of ABA are essential for maintaining normal primary root elongation. Furthermore, analysis of ABA2 promoter activity with ABA2::beta-glucuronidase transgenic plants revealed that the promoter activity was enhanced by multiple prolonged stresses, such as drought, salinity, cold, and flooding, but not by short-term stress treatments. Coincidently, prolonged drought stress treatment led to the up-regulation of ABA biosynthetic and sugar-related genes. Thus, the data support ABA2 as a late expression gene that might have a fine-tuning function in mediating ABA biosynthesis through primary metabolic changes in response to stress.
Highlights
Plant growth and development are well regulated by the integration of external environmental cues and internal signals
The product xanthoxin is subsequently transported to the cytosol and further converted to abscisic aldehyde by a short-chain dehydrogenase/ reductase 1 encoded by ABA2 in Arabidopsis (LeonKloosterziel et al, 1996; Rook et al, 2001; Cheng et al, 2002; Gonzalez-Guzman et al, 2002)
Further localization of Arabidopsis aldehyde oxidase 3 (AAO3), which catalyzes the last step of Abscisic acid (ABA) biosynthesis and can be considered the ABA production site, shows the protein to be present in vascular bundles in roots, hypocotyls and inflorescence stems, and leaf veins; AAO3 is detectable in guard cells (Koiwai et al, 2004)
Summary
Plant growth and development are well regulated by the integration of external environmental cues and internal signals. ABA has been considered a plant stress hormone because it is highly induced in vegetative tissues under stress conditions; the induction is associated with the up-regulation of ABA biosynthetic genes (such as ZEP, NCED, AAO3, and ABA3) These transcripts or ABA levels are reduced or abolished in most mutant alleles, which reflects the positive feedback regulatory circuit of ABA biosynthesis (for review, see Xiong and Zhu, 2003). A similar result for increased seed dormancy is obtained by the overexpression of ABSCISIC ACID INSENSITIVE (ABI) genes in Arabidopsis (for review, see Finkelstein et al, 2002) These data show an alternative way to generate deeper seed dormancy and stress-tolerant plants through genetic manipulation of ABA biosynthetic and/or responsive gene expression
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