MOLECULAR-IDENTIFICATION OF ZEAXANTHIN EPOXIDASE OF NICOTIANA-PLUMBAGINIFOLIA, A GENE INVOLVED IN ABSCISIC-ACID BIOSYNTHESIS AND CORRESPONDING TO THE ABA LOCUS OF ARABIDOPSIS-THALIANA

Abscisic acid (ABA) is a plant hormone which plays an important role i n seed development and dormancy and in plant response to environmental stresses. An ABA-deficient mutant of Nicotiana plumbaginifolia, aba2, was isolated by transposon tagging using the maize Activator transpos on. The aba2 mutant exhibits precocious seed germination and a severe wilty phenotype, The mutant is impaired in the first step of the ABA b iosynthesis pathway, the zeaxanthin epoxidation reaction, ABA2 cDNA is able to complement N.plumbaginifolia aba2 and Arabidopsis thaliana ab a mutations indicating that these mutants are homologous. ABA2 cDNA en codes a chloroplast-imported protein of 72.5 kDa, sharing similarities with different mono-oxigenases and oxidases of bacterial origin and h aving an ADP-binding fold and an FAD-binding domain, ABA2 protein, pro duced in Escherichia coil, exhibits in vitro zeaxanthin epoxidase acti vity. This is the first report of the isolation of a gene of the ABA b iosynthetic pathway, The molecular identification of ABA2 opens the po ssibility to study the regulation of ABA biosynthesis and its cellular location.