Dissertation for the Degree of Doctor of Philosophy in Physiological Botany

Expression of ATHB6 and ATHB7 was transcriptionally activated in plants subjected to water deficit or exogenous treatment with abscisic acid (ABA). Transgenic plants constitutively expressing the ATHB7 gene displayed a delayed elongation growth of the main inflorescence stem after transition to reproductive development. This phenotype is consistent with ATHB7 acting as a negative regulator of growth and development of the elongating stem in response to water availability. Transgenic abi1-1 mutant plants constitutively expressing the ATHB7 gene displayed a reduced wiltiness as compared to monogenic abi1-1 mutants. These data are consistent with the ATHB7 protein having a central role in the drought stress response, regulating the water balance of the plant, and acting downstream to ABI1. Furthermore, the data is consistent with ATHB7 acting as a positive regulator of the drought stress response. The ABA-induced expression of the ATHB7 gene displayed a dependence on the phytochrome system, suggesting an interplay between light and osmotic stress signaling in the regulation of the ATHB7 gene