Susan Breen’s research while affiliated with The University of Warwick and other places

Successful recognition of pathogen effectors by plant disease resistance proteins (effector triggered immunity, ETI) contains the invading pathogen through a localized hypersensitive response (HR). In addition, ETI activates long-range signalling cascades that establish broad spectrum systemic acquired resistance (SAR). Using a novel and sensitive reporter we have been able to image the spatio- temporal dynamics of SAR. We demonstrate that local ETI triggered SAR signal generation, followed by rapid propagation and establishment in systemic responding leaves, is dependent on both jasmonate biosynthesis and perception. Further, ETI initiates calcium- and jasmonate-dependent systemic surface electrical potentials, reminiscent of those activated by herbivory but with slower propagation kinetics. Thus, jasmonate signalling is crucial to the initiation and establishment of systemic defence responses against a diverse range of phytopathogens.

Microbe associated molecular pattern (MAMP) triggered immunity research has traditionally centred around signal transduction pathways originating from activated membrane localised pattern recognition receptors (PRRs), culminating in nuclear transcription and post translational modifications. More recently, chloroplasts have emerged as key immune signalling hubs, playing a central role in integrating environmental signals. Notably, MAMP recognition induces chloroplastic ROS (cROS) which is suppressed by pathogen effectors, which also modify the balance of chloroplast‐synthesised precursors of the defence hormones, jasmonic acid (JA), salicylic acid (SA) and abscisic acid (ABA). This study focuses on how well characterised PRRs and co‐receptors modulate chloroplast physiology, examining whether diverse signalling pathways converge to similarly modulate chloroplast function. Pre‐treatment of receptor mutant plants with MAMP and D(Damage)AMP peptides usually protect against effector modulation of chlorophyll fluorescence and prevent Pseudomonas syringae effector mediated quenching of cROS and suppression of maximum dark‐adapted quantum efficiency (Fv/Fm). The MAMP‐triggered immunity (MTI) co‐receptor double mutant, bak1‐5/bkk1‐1, exhibits a remarkable decrease in Fv/Fm compared to control plants during infection, underlining the importance of MTI mediated signalling in chloroplast immunity. Further probing the role of the chloroplast in immunity we unexpectedly found that even moderate changes in light intensity can uncouple plant immune signalling. This article is protected by copyright. All rights reserved.

Microbe associated molecular pattern (MAMP) triggered immunity research has traditionally centred around signal transduction pathways originating from activated membrane localised pattern recognition receptors (PRRs), culminating in nuclear transcription and post translational modifications. More recently, chloroplasts have emerged as key immune signalling hubs. Chloroplasts play a central role in integrating environmental signals. Notably MAMP recognition induces chloroplastic ROS (cROS) which is suppressed by pathogens effectors, which also modify the balance of defence hormone precursors, jasmonic acid (JA), salicylic acid (SA) and abscisic acid (ABA), whose precursors are chloroplast synthesised. This study focuses on how well characterised PRRs and co-receptors modulate chloroplast physiology, examining whether diverse signalling pathways converge to similarly modulate chloroplast function. Pre-treatment of receptor mutant plants with MAMP and D(Damage)AMP peptides usually protect against effector modulation of chlorophyll fluorescence and prevent Pseudomonas syringae effector mediated quenching of cROS and suppression of F/F . The MAMP-triggered immunity (MTI) co-receptor double mutant, bak1-5/bkk1-1, exhibits a remarkable decrease in F/F compared to control plants during infection, underlining the importance of MTI mediated signalling in chloroplast immunity. Further probing the role of the chloroplast in immunity we unexpectedly found that high light uncouples plant immune signalling.

The chloroplast has recently emerged as pivotal to co‐ordinating plant defence responses and as a target of plant pathogens. Beyond its central position in oxygenic photosynthesis and primary metabolism – key targets in the complex virulence strategies of diverse pathogens – the chloroplast integrates, decodes and responds to environmental signals. The capacity of chloroplasts to synthesize phytohormones and a diverse range of secondary metabolites, combined with retrograde and reactive oxygen signalling, provides exquisite flexibility to both perceive and respond to biotic stresses. These processes also represent a plethora of opportunities for pathogens to evolve strategies to directly or indirectly target ‘chloroplast immunity’. This review covers the contribution of the chloroplast to pathogen associated molecular pattern and effector triggered immunity as well as systemic acquired immunity. We address phytohormone modulation of immunity and surmise how chloroplast‐derived reactive oxygen species underpin chloroplast immunity through indirect evidence inferred from genetic modification of core chloroplast components and direct pathogen targeting of the chloroplast. We assess the impact of transcriptional reprogramming of nuclear‐encoded chloroplast genes during disease and defence and look at future research challenges.