Different Modes of Negative Regulation of Plant Immunity by Calmodulin-Related Genes

Plant immune responses activated through the perception of microbe-associated molecular patterns, leading to pattern-triggered immunity, are tightly regulated. This results in low immune responses in the absence of pathogens and a rapid return to the resting state following an activation event. Here, we show that two CALMODULIN-LIKE genes, CML46 and CML47, negatively regulate salicylic acid accumulation and immunity in Arabidopsis (Arabidopsis thaliana). The double mutant cml46 cml47 is highly resistant to the pathogen Pseudomonas syringae pv maculicola (Pma). The effects of cml46 cml47 on Pma growth are genetically additive to that of cbp60a, a known negative regulator in the CALMODULIN-BINDING PROTEIN60 (CBP60) family. Transcriptome profiling revealed the effects of cbp60a and cml46 cml47 on both common and separate sets of genes, with the majorities of these differentially expressed genes being Pma responsive. CBP60g, a positive regulator of immunity in the CBP60 family, was found to be transcriptionally regulated by CBP60a, CML46, and CML47. Analysis of the flg22-induced mRNA levels of CBP60g in cbp60a and cml46 cml47 revealed that cml46 cml47 plants have higher induced expression while cbp60a plants retain elevated levels longer than wild-type plants. Assays for the effect of flg22 treatment on Pma growth showed that the effect is stronger in cml46 cml47 plants and lasts longer in cbp60a plants. Thus, the expression pattern of CBP60g is reflected in flg22-induced resistance to Pma.