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Carotenoid and abscisic acid synthesis in Arabidopsis thaliana
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International audience. Abscisic acid (ABA) is a key element in seed development and germination as well as adaptive responses to environmental stresses. The tissue-specific modulation of its endogenous levels by fine-tuning of synthesis and catabolism determines its physiological action. In seeds, a major increase in ABA levels that occurs during the maturation phase regulates many developmental and maturation processes, including reserve storage, desiccation tolerance and dormancy. In vegetative tissues, ABA accumulation is triggered by water deficit, and induces stomatal closure to prevent water loss as well as longer-term responses. ABA is derived from the cleavage of C40 carotenoid precursors and to date genes coding enzymes responsible for most steps of the ABA biosynthesis pathway have been identified in Arabidopsis. ABA biosynthesis starts by the generation from -carotene of oxygenated carotenoids termed xanthophylls. Xanthophyll conversions in plastids lead to the formation of cis-isomers of violaxanthin and neoxanthin and their cleavage into a C15 compound, xanthoxin, is the first committed step of ABA biosynthesis. Xanthoxin is then converted to ABA in the cytosol. Mutations in two different loci result in the absence of neoxanthin isomers (North et al. (2007) Plant J 50: 810-824; Neumann et al. (2014) Plant J 78: 80-93), however the function of the encoded proteins in the conversion of violaxanthin into neoxanthin remains obscure. Furthermore, despite similar defects of the two mutants neoxanthin-deficient1 (nxd1) and the ABA-deficient4 (aba4) in neoxanthin synthesis, only aba4 was shown to be defective for ABA accumulation. Progress in the characterization of the respective roles of ABA4 and NXD1 will be presented.
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