SlCML55, a novel Solanum lycopersicum calmodulin-like gene, negatively regulates plant immunity to Phytophthora pathogens

Calmodulin-like (CML) proteins are important decoders for intracellular Ca2+ variation controlling diverse cellular functions by regulating the activity of various target proteins. The functions of many CMLs, especially their roles as negative regulators of plant immunity, have not been clarified. The Solanum lycopersicum gene SlCML55, a novel CML gene with a susceptibility-inducing function in Nicotiana benthamiana, was identified in a tomato cDNA library. Silencing of SlCML55 inhibits Phytophthora capsici infection, indicating that SlCML55 negatively regulates plant immunity in tomato. Confocal laser scanning microscopy showed eGFP:SlCML55 to localize to the cytosol and nucleus. SlCML55 was highly expressed in roots compared to other tissues and downregulated during the early stage of P. capsici infection, especially at 3 hours post-infection. Expression of SlCML55 was regulated by Ca2+ and peaked at 6 h after external administration of CaCl2. Expression of SlCML55 was also largely downregulated under disease resistance-related hormone (SA, JA and ET) treatments. Furthermore, silencing SlCML55 promoted PR1 expression and viral defense, as stimulated via the SA immune pathway. Taken together, our findings indicate that SlCML55 acts as a negative regulator during pathogen infection in tomato in association with the SA signaling pathway.