Multiple host targets ofPseudomonaseffector protein HopM1 form a protein complex regulating apoplastic immunity and water homeostasis

Abstract

Bacterial type III effector proteins injected into the host cell play a critical role in mediating bacterial interactions with plant and animal hosts. Notably, some bacterial effectors are reported to target sequence-unrelated host proteins with unknown functional relationships. The Pseudomonas syringae effector HopM1 is such an example; it interacts with and/or degrades several HopM1-interacting (MIN) Arabidopsis proteins, including HopM1-interacting protein 2 (MIN2/RAD23), HopM1-interacting protein 7 (MIN7/BIG5), HopM1-interacting protein 10 (MIN10/14-3-3ĸ), and HopM1-interacting protein 13 (MIN13/BIG2). In this study, we purified the MIN7 complex formed in planta and found that it contains MIN7, MIN10, MIN13, as well as a tetratricopeptide repeat protein named HLB1. Mutational analysis showed that, like MIN7, HLB1 is required for pathogen-associated molecular pattern (PAMP)-, effector-, and benzothiadiazole (BTH)-triggered immunity. HLB1 is recruited to the trans-Golgi network (TGN)/early endosome (EE) in a MIN7-dependent manner. Both min7 and hlb1 mutant leaves contained elevated water content in the leaf apoplast and artificial water infiltration into the leaf apoplast was sufficient to phenocopy immune-suppressing phenotype of HopM1. These results suggest that multiple HopM1-targeted MIN proteins form a protein complex with a dual role in modulating water level and immunity in the apoplast, which provides an explanation for the dual phenotypes of HopM1 during bacterial pathogenesis.

Logistics of defense: The contribution of endomembranes to plant innate immunity

“…Internalized proteins are sorted at the TGN and MVB. In addition to the TGN-associated MIN7, a recent study implicates another TGN-localized protein, hypersensitive to Latrunculin B 1 (HLB1), in PTI ( Nomura et al, 2023 , Preprint ). While our understanding of the mechanisms for PRR activation is extensive, how activated PRRs are internalized is limited.…”

“…HLB1 is associated with MIN7-decorated TGN and with actin filaments, and it has been hypothesized to regulate endocytic and exocytic traffic ( Sparks et al, 2016 ). A recent study has implicated HLB1 in immunity ( Nomura et al, 2023 , Preprint ). More studies aimed at identifying COBPs involved in immunity will help us expand our knowledge of the plant protein repertoire enabling a robust defense response.…”

“…Several studies have demonstrated that pathogen effectors are useful tools to identify intracellular trafficking candidates. Using the established Arabidopsis–Pseudomonas pathosystem, it has been possible to identify some direct host targets of pathogen effectors as evidenced by recent studies revealing immune-related functions of TGNap1 and HLB1 ( Bhandari et al, 2023 ; Nomura et al, 2023 , Preprint ); however, it is likely that other host targets exist. Future studies incorporating a tiered approach of combining chemical perturbation of host processes and pathogen effector mutants are likely to discover new targets and immune-related trafficking components (see Box 3 ).…”