Calcium-dependent signaling in plant adaptation to environmental stresses – CASSIS

As sessile organisms, plants cannot escape from adverse environmental conditions. In particular, drought and salinity are the major abiotic constraints that affect crop yield and productivity. Their impact on agriculture is expected to increase in the near future due to global climate changes. Thus, deciphering the molecular mechanisms of plant stress perception and adaptation is essential to improve crop tolerance and meet the increasing human demands. Plants have developed robust and efficient signaling networks to quickly sense stress stimuli and induce the adaptive cellular responses required for their survival, such as metabolic changes and transcriptional reprogramming. Protein kinases, which modify the activity, stability or localization of their targets by reversible phosphorylation, are key players of plant stress signaling. The model plant Arabidopsis thaliana contains more than a thousand of protein kinases that display specific as well as partially overlapping biological functions, to fine tune plant stress responses and ensure the fundamental cellular processes in any environmental condition. Besides the well-known and most studied mitogen-activated protein kinases (MAPKs), the calcium-dependent protein kinases (CDPKs) are emerging as central regulators of both biotic and abiotic stress responses. They exhibit the unique feature of combining calcium sensing and protein kinase activity in a single protein to efficiently transduce calcium signals that are the most widespread mediators of plant signaling. The Arabidopsis genome encodes 34 CDPKs that exhibit some functional redundancy, which has limited their discovery by classical genetic screens. Only few isoforms have been assigned a specific biological role and most of the substrates remain unknown. New strategies are thus required to elucidate CDPK functions in vivo. In the context of basic research, the goals of the present project are to understand and evaluate the roles of CDPKs in abiotic stress responses. We will focus on two key closely related Arabidopsis isoforms, CPK5 and CPK6, which function at the crossroad of biotic and abiotic stress signaling. In a first part, I propose to decipher the molecular bases of the hypersensitive stress phenotype of the cpk5cpk6 double mutant through a multi-level omic approach, combining transcriptomics, metabolomics and phosphoproteomics. In a second part, I propose to use an integrative approach combining phosphoproteomics, biochemistry, cellular and molecular biology to identify in vivo substrates of CPK5 and CPK6. Additional physiological assays combined with genetic approaches will be developed to validate the candidate genes and characterize their roles in abiotic stress responses mediated by CPK5 and CPK6. Thus, the present project will contribute to elucidate the molecular mechanisms of plant stress signaling.