Ectopic expression of a tomato 9-cis-epoxycarotenoid dioxygenase gene causes over-production of abscisic acid

The tomato mutant notabilis has a wilty phenotype as a result of abscisic a cid (ABA) deficiency. The wild-type allele of notabilis, LeNCED1, encodes a putative 9-cis-epoxycarotenoid dioxygenase (NCED) with a potential regulat ory role in ABA biosynthesis. We have created transgenic tobacco plants in which expression of the LeNCED1 coding region is under tetracycline-inducib le control. When leaf explants from these plants were treated with tetracyc line, NCED mRNA was induced and bulk leaf ABA content increased by up to 10 -fold. Transgenic tomato plants were also produced containing the LeNCED1 c oding region under the control of one of two strong constitutive promoters, either the doubly enhanced CaMV 35S promoter or the chimaeric 'Super-Promo ter'. Many of these plants were witty, suggesting co-suppression of endogen ous gene activity; however three transformants displayed a common, heritabl e phenotype that could be due to enhanced ABA biosynthesis, showing increas ed guttation and seed dormancy. Progeny from two of these transformants wer e further characterized, and it was shown that they also exhibited reduced stomatal conductance, increased NCED mRNA and elevated seed ABA content. Pr ogeny of one transformant had significantly higher bulk leaf ABA content co mpared to the wild type. The increased seed dormancy was reversed by additi on of the carotenoid biosynthesis inhibitor norflurazon. These data provide strong evidence that NCED is indeed a key regulatory enzyme in ABA biosynt hesis in leaves, and demonstrate for the first time that plant ABA content can be increased through manipulating NCED.