RXLR effector SFI5 of Phytophthora infestans suppress MAMP-triggered immunity via inhibition of NbPHB1 in Nicotiana benthamiana

Abstract

Plants are continuously challenged by pathogenic threats, including fungi, bacteria, and viruses, across all stages of their growth and development. To combat these, plants have evolved a sophisticated innate immune system, with MAMP-triggered immunity (MTI) serving as an important early defense mechanism. The causative agent of late blight, Phytophthora infestans, secretes effector proteins such as SFI5 that suppress host immune responses, resulting in severe damage. This study explores the role of SFI5 in modulating MTI in host plants. Bioinformatics research revealed a conserved ATP/GTP-binding motif in the N-terminal domain of SFI5, with Lys82 being critical for effector function. Lys82 mutation lowered SFI5′s capacity to suppress flg22- triggered reactive oxygen species (ROS) and calcium bursts in tomato protoplasts, but it had no effect on its interaction with calmodulin-binding proteins or kinase activity. In vitro assays confirmed that SFI5 exhibits GTPase activity, unaffected by the Lys82 mutation or the presence of calmodulin. Pull-down assays combined with protein spectrum analysis revealed NbPHB1, a positive regulator of MTI, as a potential binding partner of SFI5. Functional assay demonstrated that NbPHB1 promotes MTI marker gene expression and decreases lesion size in Nicotiana benthamiana, whereas virus-induced gene silencing (VIGS) of NbPHB1 weakens MTI and increases lesion size. SFI5 suppresses NbPHB1 expression, inhibiting MTI and exacerbating lesion development during infection. In conclusion, SFI5 targets the MTI regulator NbPHB1 to inhibit immune responses, with its ATP/GTP-binding motif playing a key role in effector function. These findings provide insights into P. infestans pathogenicity and suggest potential targets for developing resistant crops.