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Immune signaling in plants involves both positive and negative regulators. Maintaining a balance between growth and defense responses is important because there is a fitness cost to the plants if immune responses are left unchecked. Suppression of immune responses in the absence of pathogens as well as after the threat has passed is critical in maintaining such a balance between growth and defense responses. Upon pathogen perception, the positive regulators counter the immune repression to induce defense responses. We investigated the roles of two genes, CBP60a and PCRK1 in the regulation of defense responses against Pseudomonas syringae pathogen in the model system Arabidopsis thaliana . CBP60a is a negative regulator of immune responses. We showed that CBP60a is a CaM binding protein and that CaM binding is important for its function in transducing defense signals. Mutants of CBP60a were more resistant to Pseudomonas syringae infection suggesting that CBP60a was a negative regulator of defense responses. We found that CBP60a functions in repressing immune signaling under conditions where the plants are not challenged by a pathogen. We also investigated the role of a putative kinase, PCRK1, in immune signaling. We showed that pcrk1 mutants are more susceptible to Pseudomonas syringae than wild type plants suggesting that PCRK1 has a positive role in immune responses. We also showed that PCRK1 is important for immunity triggered by some of the conserved Microbe Associated Molecular Patterns (MAMP) as well endogenous signals generated as a result of pathogen activity called Damage Associated Molecular Patterns (DAMP).

This study reports the identification Glycine max varieties that form vascular cavities in their primary roots under warm and flooded conditions. Vascular cavity formation is a PCD-mediated process and this study explores the induction of caspase-like proteases, a hallmark of PCD. Injection of caspase inhibitors have resulted in suppression of vascular cavity formation under hypoxia. Substrate cleavage assays using synthetic caspase-specific substrates also show a marked increase in caspase activity after flooding. Further, the caspase like proteases are inhibited only by specific inhibitors and not by general protease inhibitors. Caspase-like proteases are pH sensitive and are effective under acidic pH conditions. Study of in vivo caspase-like protease activity employing peptide-based fluorogenic substrates show their activity in the cytosol of cells in the vascular cylinder of soybean roots. This study is expected to prompt a deeper understanding of hypoxia-induced PCD and add valuable data to existing knowledge of plant PCD