Genetic and molecular interactors of ABA insensitive1 gene (abi1) of Arabidopsis thaliana

The plant hormone abscisic acid (ABA) is important in numerous functions ranging from the establishment of seed dormancy and germination to protecting adult plants from a variety of environmental stresses. Previously in Arabidopsis, mutants with reduced sensitivity to applied ABA ( abi) have been described. One ABA insensitive mutation, designated abi1, identifies a gene that encodes a type 2C protein phosphatase suggesting the phosphorylation status of the plant is important for correct ABA action. To further understand the role of ABI1 in ABA signal transduction and to identify new genes involved in ABA action, an abi1 mutant background was used to screen for mutations that further reduce ABA responsiveness in Arabidopsis. This ab i1 e&barbelow;nhancer ( abe) screen resulted in the isolation of mutations in 6 genetic loci. Three complementation groups define the previously identified ABA response loci ABI3, ABI4 and ABI5. Mutations at two loci identify new genes designated ABE1 and ABE2 and the last complementation group was found to be allelic to the CTR1 gene, a known negative regulator of ethylene signaling in Arabidopsis. This last result suggests that ABA and ethylene may have overlapping interactions that define the sensitivity of the plant seeds to these growth factors. Interestingly, in a parallel study it was discovered that a number of known mutants that reduce ethylene sensitivity also show altered ABA responsiveness. Since mutations that confer ethylene insensitivity cause seed ABA hypersensitivity and mutations that confer constitutive ethylene response cause increased seed ABA insensitivity, I proposed that ethylene signaling functions in the negative regulation of seed ABA responses. Furthermore, studies involving the interaction between ABA and ethylene in the root suggest ABA may signal directly through ethylene signal transduction pathway in an ethylene independent manner.