Molecular biology and regulation of abscisic acid biosynthesis in plants

The phytohormone abscisic acid (ABA) is involved in seed dormancy and the r esponse to various environmental stresses. Our understanding of the ABA bio synthetic pathway has been increased recently through the use of plant muta nts and the cloning of many of the genes encoding for the enzymes involved. C-40 Xanthophylls are precursors of ABA and are now known to be derived fr om isopentenyl phosphate (IPP) synthesized in plastids via a mevalonate-ind ependent pathway. Enzyme reactions downstream of zeaxanthin have recently b een reported to be important for the precise regulation of ABA levels. Zeax anthin epoxidase (ZEP) catalyses the conversion of zeaxanthin to violaxanth in. Changes in ZEP gene expression appear to regulate ABA biosynthesis in s eeds and roots, but not in leaves which might be expected considering the i mportant role of epoxy-carotenoids in photosynthesis and photoprotection. T he isomerization of the resulting all-trans-violaxanthin to 9-cis-epoxy-car otenoids awaits elucidation. Although 9-cis-epoxy-carotenoid dioxygenase (N CED), which subsequently cleaves the resulting carotenoids could use the 9- cis isomers of both violaxanthin and neoxanthin as substrates in vitro, the in vivo substrates remain to be determined. NCEDs are apparently encoded b y multigene families and identification of the various members is required to determine their relative contribution to the regulation of ABA levels. S tudies on those already available indicate that their up-regulation upon wa ter stress is compatible with a key role in the modulation of ABA levels. T he genes encoding for the enzymes that convert the cleavage product xanthox in to ABA are not yet known, although recently cloned aldehyde oxidases may act on ABA-aldehyde. (C) Elsevier, Paris.