Molecular Characterization of a Pepper C2 Domain-containing SRC2 Protein Implicated in Resistance Against Host and Non-host Pathogens and Abiotic Stresses

Plants guard themselves against pathogen attack using multi-layered defense mechanism. Calcium represents an important secondary messenger during such defense responses. Upon examination of a pepper cDNA library, we observed that the gene CaSRC2-1 (Capsicum annum SRC2-1) was upregulated significantly in response to infection with the type II non-host pathogen Xanthomonas axonopodis pv. glycines 8 ra, which elicits a hypersensitive response. CaSRC2-1 encodes a protein that contains a C2 domain and it exhibits a high degree of homology to the protein Soybean genes regulated by cold 2 (SRC2). However, little is known about how SRC2 expression is elicited by biotic stresses such as pathogen challenge. Further sequence analysis indicated that the CaSRC2-1 C2 domain is unique and contain certain amino acids that are conserved within the C2 domains of other plants and animals. CaSRC2-1 transcription was up-regulated under both biotic and abiotic stress conditions, including bacterial and viral pathogen infection, CaCl(2) and cold treatment, but unaffected by treatment with plant defense-related chemicals such as salicylic acid, methyl jasmonic acid, ethephone, and abscisic acid. Intriguingly, under steady state conditions, CaSRC2-1 was expressed only in the root system. A CaSRC2-1-GFP fusion protein was used to determine localization to the plasma membrane. A fusion protein lacking the C2 domain failed to target the membrane but remained in the cytoplasm, indicating that the C2 domain plays a critical role in localization. Thus, CaSRC2-1 encodes a novel C2 domain-containing protein that targets the plasma membrane and plays a critical role in the abiotic stress and defense responses of pepper plants.