Homeodomain protein ATHB6 is a target of the protein phosphatase ABI1 and regulates hormone responses in Arabidopsis

ABI1, a protein phosphatase 2C, is a key component of signal transduction in Arabidopsis. It regulates diverse responses to the phytohormone abscisic acid (ABA) such as stomatal closure, seed dormancy and inhibition of vegetative growth. By analysing proteins capable of interacting with ABI1, we have identified the homeodomain protein ATHB6 as a regulator of the ABA signal pathway. Critical for interaction between ATHB6 and ABI1 is an intact protein phosphatase domain and the N-terminal domain of ATHB6 containing the DNA-binding site. ATHB6 recognizes a cis-element present in its promoter, which encompasses the core motif (CAATTATTA) that mediated ATHB6- and ABA-dependent gene expression in protoplasts. In addition, transgenic plants containing a luciferase gene controlled by the ATHB6 promoter documented a strong ABA-inducible expression of the reporter which was abrogated in the ABA-insensitive abi1 mutant. Arabidopsis plants with constitutive expression of the transcriptional regulator revealed ABA insensitivity in a subset of ABI1-dependent responses. Thus, the homeodomain protein ATHB6 seems to represent a negative regulator of the ABA signal pathway and to act downstream of ABI1.