Chloroplast signaling and quality control.

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.