The chloroplast: A key component of plant defence and target of plant pathogens

Abstract

Plant pathogens secrete a diverse range of effector proteins as part of their virulence strategy that work to supress plant immune responses. These effectors target and manipulate a multitude of key host defence cellular signalling pathways and organelles in the plant cell. The chloroplast represents a prime candidate for such manipulation by phytopathogen effector proteins being not only the site of photosynthesis and many defence hormone biosynthetic pathways in the plant cell, but also for its capabilities of producing the important signalling molecules reactive oxygen species (ROS). This study looked to examine how Pseudomonas syringae effector proteins manipulate the chloroplasts of host Arabidopsis thaliana. Here we show with use of fluorimagery that P. syringae is capable of supressing host Arabidopsis photosynthesis upon infection in an effector protein dependent manner. Furthermore, prior activation of host basal immunity through PAMP pre-treatment of Arabidopsis leaves induces a mechanism of protecting chloroplasts against bacterial suppression of host photosynthesis upon subsequent P. syringae challenge. These fascinating results indicate the chloroplast is a key player in the A. thaliana PTI (PAMP- triggered immunity) response. This study examined this facet of PTI through analysing the bacterial induced suppression of photosynthesis in a variety of PTI signalling mutants and confocal imaging of ROS production post P. syringae inoculation in order to better understand the signalling events linking PTI, phytobacterial virulence strategies and the chloroplast. Knowledge of such host- pathogen interactions will prove crucial for the future engineering of effective sustainable intervention strategies to protect host chloroplasts’ from bacterial effector protein manipulation and enhance plants’ resistance to pathogens.