Identification of MAMP-triggered immunity (MTI)-suppressing RXLR effectors from Phytophthora infestans and functional characterization of the calmodulin-binding effector SFI5

Abstract

An important branch of the plant immune system is based on the sensing of potential pathogens by the recognition of highly conserved microbe-associated molecular patterns (MAMPs), such as the peptide epitope flg22 from bacterial flagellin, and the activation of complex defense signaling events yielding a generic anti-microbial response, which is called MAMP-triggered immunity (MTI). The successful establishment of infection relies on the pathogen’s capability to deliver effectors that subvert plant immunity. Although some effectors from eukaryotic filamentous pathogens have been identified as MTI-compromising factors, our general understanding of the effector-target biology and the molecular mechanisms underlying the mode of action of these effectors is still in its infancy. A large repertoire of candidate effector genes, including hundreds of putative host-targeting RXLR effectors, is present in the genome of Phytophthora infestans, the causal agent of potato and tomato late blight. In this thesis, we used protoplast-based high-throughput assays to identify and characterize RXLR effectors interfering with the early stages of MAMP-induced immune signaling responses e.g. calcium and oxidative burst, post-translational MAP kinase activation and transcriptional up-regulation of MAMP-inducible genes. Among 33 RXLR effectors tested, eight were identified as Suppressor of early Flg22-induced Immune responses (SFI effectors) in tomato protoplasts. Epistatic analysis showed that three RXLR effectors (SFI5-SFI7) disturb flg22-mediated signaling at- or upstream of the MAP kinase cascade, concomitant with their localization at the host plasma membrane. The remaining five RXLR effectors (SFI1-4 and SFI8) act downstream of the MAP kinase cascade, four of them are localized in the host nucleus. Furthermore, we provide evidence that all but one SFI effectors enhance host susceptibility to P. infestans infection. We have identified the calcium sensor calmodulin (CaM) as an interacting plant protein of SFI5 using bioinformatics, proteomics and biochemical approaches. Structure-function analyses with SFI deletion and point mutants showed that the CaM-binding motif in the C-terminal part of SFI5 is crucial for the plasma membrane (PM) localization, MTI-suppressing activity and virulence function of SFI5. In addition, a predicted ATP/GTP-binding site motif (P-loop) at the N-terminus of SFI5 was demonstrated to be necessary for the effector activity but has no influence on CaM binding and PM localization. Our current model predicts a two-step activation mechanism of SFI5 with CaM serving as a co-factor and regulating SFI5 to target potential MTI components at the PM. Altogether, we have shown that P. infestans contains functionally redundant effectors to inhibit MAMP-dependent early signal transduction during host infection. Our results present a conceptual advance in the understanding of the biology of effectors originated from eukaryotic plant pathogens and show parallels with the strategies developed by prokaryotic pathogens.