1.
Chloroplast avoidance movement: a novel paradigm of ROS signalling.
Majumdar, A, Kar, RK
Photosynthesis research. 2020;(1):109-121
Abstract
The damaging effects of supra-optimal irradiance on plants, often turning to be lethal, may be circumvented by chloroplast avoidance movement which realigns chloroplasts to the anticlinal surfaces of cells (parallel to the incident light), essentially minimizing photon absorption. In angiosperms and many other groups of plants, chloroplast avoidance movement has been identified to be a strong blue light (BL)-dependent process being mediated by actin filaments wherein phototropins are identified as the photoreceptor involved. Studies through the last few decades have identified key molecular mechanisms involving Chloroplast Unusual Positioning 1 (CHUP1) protein and specific chloroplast-actin (cp-actin) filaments. However, the signal transduction pathway from strong BL absorption down to directional re-localization of chloroplasts by actin filaments is complex and ambiguous. Being the immediate cellular products of high irradiance absorption and having properties of remodelling actin as well as phototropin, reactive oxygen species (ROS) deemed to be more able and prompt than any other signalling agent in mediating chloroplast avoidance movement. Although ROS are presently being identified as fundamental component for regulating different plant processes ranging from growth, development and immunity, its role in avoidance movement have hardly been explored in depth. However, few recent reports have demonstrated the direct stimulatory involvement of ROS, especially H2O2, in chloroplast avoidance movement with Ca2+ playing a pivotal role. With this perspective, the present review discusses the mechanisms of ROS-mediated chloroplast avoidance movement involving ROS-Ca2+-actin communication system and NADPH oxidase (NOX)-plasma membrane (PM) H+-ATPase positive feed-forward loop. A possible working model is proposed.
2.
The role of chloroplasts in plant pathology.
Sowden, RG, Watson, SJ, Jarvis, P
Essays in biochemistry. 2018;(1):21-39
Abstract
Plants have evolved complex tolerance systems to survive abiotic and biotic stresses. Central to these programmes is a sophisticated conversation of signals between the chloroplast and the nucleus. In this review, we examine the antagonism between abiotic stress tolerance (AST) and immunity: we propose that to generate immunogenic signals, plants must disable AST systems, in particular those that manage reactive oxygen species (ROS), while the pathogen seeks to reactivate or enhance those systems to achieve virulence. By boosting host systems of AST, pathogens trick the plant into suppressing chloroplast immunogenic signals and steer the host into making an inappropriate immune response. Pathogens disrupt chloroplast function, both transcriptionally-by secreting effectors that alter host gene expression by interacting with defence-related kinase cascades, with transcription factors, or with promoters themselves-and post-transcriptionally, by delivering effectors that enter the chloroplast or alter the localization of host proteins to change chloroplast activities. These mechanisms reconfigure the chloroplast proteome and chloroplast-originating immunogenic signals in order to promote infection.
3.
Chloroplast signaling and quality control.
Rochaix, JD, Ramundo, S
Essays in biochemistry. 2018;(1):13-20
Abstract
Although chloroplasts contain their own genetic system and are semi-autonomous cell organelles, plastid biogenesis and homeostasis are heavily dependent on the nucleo-cytosolic compartment. These two cellular compartments are closely co-ordinated through a complex signaling network comprising both anterograde and retrograde signaling chains. Developmental changes or any perturbation in the chloroplast system induced by a particular stress resulting from changes in environmental conditions such as excess light, elevated temperature, nutrient limitation, pathogen infection, give rise to specific signals. They migrate out of the chloroplast and are perceived by the nucleus where they elicit changes in expression of particular genes that allow for the maintenance of plastid homeostasis toward environmental cues. These genes mainly include those of photosynthesis-associated proteins, chaperones, proteases, nucleases and immune/defense proteins. Besides this transcriptional response, a chloroplast quality control system exists that is involved in the repair and turnover of damaged plastid proteins. This system degrades aggregated or damaged proteins and it can even remove entire chloroplasts when they have suffered heavy damage. This response comprises several processes such as plastid autophagy and ubiquitin-proteasome mediated proteolysis that occurs on the plastid envelope through the action of the ubiquitin-proteasome system.