The genes ABI1 and ABI2 are involved in abscisic acid- and drought-inducible expression of the Daucus carota L. Dc3 promoter in guard cells of transgenic Arabidopsis thaliana (L.) Heynh.

The ABA INSENSITIVE1 (AB11) and AB12 genes encode homologous type-2C protei n phosphatases with redundant yet distinct functions in abscisic acid (ABA) responses. Results from Northern blot analysis showed that ABA- and mannit ol-inducible expression of the COR47 and COR78/LT178/RD29A (COR78) genes wa s more impaired in the abi2 mutant of Arabidopsis thaliana (L.) Heynh than in the abi1 mutant. Furthermore, ABA-plus-mannitol treatments were additive towards COR47 gene expression; however, the ABA-deficient aba1 mutant show ed reduced COR expression relative to the wild type in response to mannitol and ABA-plus-mannitol treatments. These results support the notion that dr ought- and ABA-signalling pathways are separate yet overlapping. To facilit ate quantitative analysis of the genetic control of tissue-specific ABA- an d desiccation-response pathways, we analyzed ABA- and mannitol-inducible ex pression of a carrot (Daucus carota L.) Dc3 promoter:uidA (beta-glucuronida se; GUS) chimaeric reporter (Dc3-GUS) in transgenic wild-type, ABA-deficien t aba1, and ABA-insensitive abi1 and abi2 mutants. The Dc3 promoter directe d ABA- and mannitol-inducible GUS expression in Arabidopsis guard cells and the two treatments were additive. The aba1, abi1, and abi2 mutant genotype s had reduced GUS expression in guard cells of cotyledons in response to ma nnitol, whereas abi1 and abi2 mutants were reduced in ABA-inducible GUS exp ression, consistent with overlapping ABA- and drought-response pathways. Qu antitative fluorometric GUS assays of leaf extracts showed that abi2 mutant s responded less to exogenous ABA than did abi1 mutants, and abi2 mutants r esponded more to mannitol than did abi1 mutants. We conclude that Dc3-GUS A rabidopsis is a tractable system in which to study tissue-specific ABA and drought signalling and suggest that AB12 functions predominantly over AB11 in COR78 and COR47 gene expression and guard-cell Dc3-GUS expression.