Complementation of notabilis, an abscisic acid-deficient mutant of tomato:: Importance of sequence context and utility of partial complementation

The abscisic acid (ABA)-deficient tomato mutant notabilis (not) is believed to be a null mutation in the gene LeNCED1, encoding a 9-cis-epoxycarotenoid dioxygenase involved in ABA biosynthesis. We have sequenced and analysed a 19 kb genomic clone containing LeNCED1 and 5.4 kb of its promoter. This clone was transferred to not homozygotes and several non-wilty transformed plants were obtained. The basal ABA content, water relations, shoot and root growth, adventitious rooting, ethylene evolution and ability to accumulate ABA under water stress are described for two of these lines, not(comp.13) and not(comp.1). Partial complementation was observed for most parameters measured for not(comp.1). Full complementation was observed in not(comp.13) for all parameters measured in whole plants under well-watered and water-stressed conditions. These data provide further evidence that LeNCED1 is the wild-type allele of the not mutant gene. However, not(comp.13) was unable to accumulate the wild-type levels of ABA in rapidly dehydrated leaves, indicating that it too was only partially complemented. Since LeNCED1 is an environmentally regulated gene encoding a rate-limiting enzyme, precise levels and patterns of gene expression may be needed to fully recreate wild-type phenotype. The utility of partially complemented lines to study the role of ABA in plant responses to stress conditions, and in promoter analysis, is discussed.