Phytophthora infestans RXLR effector SFI5 requires association with calmodulin for PTI/MTI suppressing activity

Summary Pathogens secrete effector proteins to interfere with plant innate immunity, in which Ca 2+ /calmodulin (CaM) signalling plays key roles. Thus far, few effectors have been identified that directly interact with CaM for defence suppression. Here, we report that SFI 5, an RXLR effector from Phytophthora infestans , suppresses microbe‐associated molecular pattern ( MAMP )‐triggered immunity ( MTI ) by interacting with host CaMs. We predicted the CaM‐binding site in SFI 5 using in silico analysis. The interaction between SFI 5 and CaM was tested by both in vitro and in vivo assays. MTI suppression by SFI 5 and truncated variants were performed in a tomato protoplast system. We found that both the predicted CaM‐binding site and the full‐length SFI 5 protein interact with CaM in the presence of Ca 2+ . MTI responses, such as FRK 1 upregulation, reactive oxygen species accumulation, and mitogen‐activated protein kinase activation were suppressed by truncated SFI 5 proteins containing the C‐terminal CaM‐binding site but not by those without it. The plasma membrane localization of SFI 5 and its ability to enhance infection were also perturbed by loss of the CaM‐binding site. We conclude that CaM‐binding is required for localization and activity of SFI 5. We propose that SFI 5 suppresses plant immunity by interfering with immune signalling components after activation by CaMs.