| Water availability is one of the primary factors affecting plant growth and development. Many common environmental conditions, including drought and high soil solute content, can reduce the availability of water for a plant. Consequently, plants have evolved remarkably sensitive and complex means of regulation that occurs during water stress. Many components of the plant water stress response have been identified and characterized. However, no studies have conclusively established the identity of any plant proteins that sense water status. The Arabidopsis thaliana histidine protein kinase ATHK1 is one possible candidate. To explore this possibility, we have utilized a reverse genetics approach to characterize the function of ATHK1 in Arabidopsis. Evidence from athk1 null mutants, ATHK1-rescued athk1 nulls, and 35S:ATHK1 overexpressors indicates that ATHK1 is essential for survival during water stress, and that the mechanism of ATHK1 action is via an ABA-dependent pathway. Furthermore, we have identified a novel connection between environmental water stress sensing and the naturally occurring process of embryo desiccation during seed maturation. Our data suggests that ATHK1 is involved in sensing or regulating important processes during seed development leading to a viable seed. We have employed transcriptome studies to identify putative targets of ATHK1 regulation during water stress, including genes for sucrose and proline biosynthetic enzymes. We also describe a large scale cluster analysis that highlights global patterns of gene expression common among different stress conditions and gives insight into groups of co-regulated genes. The results of this study suggest that ATHK1 is transcriptionally regulated in a manner similar to several type-A response regulators. Like ATHK1, Arabidopsis response regulators are involved in histidine kinase phosphorelays. Finally, we performed metabolome studies in ATHK1 mutants and were able to identify a mechanism by which 35S:ATHK1 overexpressors survive salt stress. Our evidence suggests that 35S:ATHK1 mutants accumulate more carbohydrates and amino acids, and less sodium during salt stress. We also discuss future work involving additional studies into the role of ATHK1 in seed maturation and on additional members of the ATHK1 phosphorelay. |
