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The phytohormone abscisic acid (ABA) regulates plant growth and development as well as stress tolerance. The Arabidopsis sad1 (supersensitive to ABA and drought) mutation increases plant sensitivity to drought stress and ABA in seed germination, root growth, and the expression of some stress-responsive genes. sad1 plants are also defective in the positive feedback regulation of ABA biosynthesis genes by ABA and are impaired in drought stress induction of ABA biosynthesis. SAD1 encodes a polypeptide similar to multifunctional Sm-like snRNP proteins that are required for mRNA splicing, export, and degradation. These results suggest a critical role for mRNA metabolism in the control of ABA signaling as well as in the regulation of ABA homeostasis.
The three mutant alleles of the ABA locus of Arabidopsis thaliana result in plants that are deficient in the plant growth regulator abscisic acid (ABA). We have used 1802 to label ABA in water-stressed leaves of mutant and wild-type Arabidopsis. Analysis by selected ion monitoring and tandem mass spectrometry of [180]ABA and its catabolites, phaseic acid and ABA-glucose ester (fi-D-glucopyranosyl abscisate), indicates that the aba genotypes are impaired in ABA biosynthesis and have a small ABA precursor pool of compounds that contain oxygens on the ring, presumably oxygenated carotenoids (xanthophylls). Quantitation of the carotenoids from mutant and wild-type leaves establishes that the aba alleles cause a deficiency of the epoxy-carotenoids violaxanthin and neoxanthin and an accumulation of their biosynthetic precursor, zeaxanthin. These results provide evidence that ABA is synthesized by oxidative cleavage of epoxy-carotenoids (the "indirect pathway"). Furthermore the carotenoid mutant we describe undergoes normal greening. Thus the aba alleles provide an opportunity to study the physiological roles of epoxy-carotenoids in photosynthesis in a higher plant.Abscisic acid (ABA) is a sesquiterpenoid plant growth regulator involved in many physiological and developmental processes such as transpiration, germination, dormancy, and adaptation to environmental stresses (e.g., drought, chilling, and pathogen attack) (ref. 1; for review, see ref.2). Although the structure of ABA (Fig. 1D) has been known for 26 years (3), the biosynthetic pathway in higher plants has not been fully elucidated. The evidence favoring the indirect pathway from xanthophylls, as opposed to the direct pathway from farnesyl pyrophosphate (4), can be summarized as follows. (i) The viviparous mutants of maize vp-2, vp-S, vp-7, and vp-9 are blocked in the early stages of carotenoid biosynthesis and are ABA-deficient (5, 6). (ii) The carotenoid biosynthesis inhibitors fluridone and norflurazon also inhibit ABA biosynthesis (7,8). (iii) 1802-labeling experiments with waterstressed leaves show 180 incorporation into the side-chain carboxyl group of ABA but little incorporation in the oxygen functions on the ring (9-11), indicating that there is a large ABA precursor pool (presumably xanthophylls) that contains oxygens on the ring (Fig. 1). (iv) Xanthoxin, a C15 metabolite of epoxy-carotenoids, is found in plants (12) and is readily converted to ABA in vivo (13,14) and in vitro (15). (v) A 1:1 molar correlation between decreases in trans-violaxanthin and 9'-cis-neoxanthin (Fig. 1) levels and concomitant increases in ABA and its catabolites has been shown for dark-grown water-stressed bean leaves (16,17). In contrast to higher plants, phytopathogenic fungi synthesize ABA by a direct pathway from farnesyl pyrophosphate (18, 19). genotype also included the recessive markers ttg (transparent testa glabra) and yi (yellow inflorescence). Seeds were germinated on 1% agar in Petri dishes for 2 weeks after storage at 40C for 2 days to break dormancy...
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