Features of cues and processes during chloroplast-mediated retrograde signaling in the alga Chlamydomonas

Authors: REA, GIUSEPPINA - National Research Council - Italy; ANTONACCI, AMINA - National Research Council - Italy; LAMBREVA, MAYA - National Research Council - Italy; Mattoo, Autar

Submitted to: Plant Science
Publication Type: Review Article
Publication Acceptance Date: 4/25/2018
Publication Date: 5/2/2018
Citation: Rea, G., Antonacci, A., Lambreva, M.D., Mattoo, A.K. 2018. Features of cues and processes during chloroplast-mediated retrograde signaling in the alga Chlamydomonas. Plant Science. 272:193-206. https://doi.org/10.1016/j.plantsci.2018.04.020.
DOI: https://doi.org/10.1016/j.plantsci.2018.04.020

Interpretive Summary: Plants being sessile have developed intricate mechanisms for perception of a variety of external stimuli generated by a constantly changing environment. How environmental stresses are managed by plants is critical to our understanding and for our development of genotypes that can withstand stresses such as drought, high temperatures and high light without compromising on growth, yield and nutrition. Plastids (chloroplasts) are photosynthesizing organelles that generate quick responses to many environmental cues, and have been labelled as a subcellular environmental sensor. Little is known about how stress signals are perceived, managed and transmitted intra-cellularly to manage stress. Knowledge on these processes is important and could be integrated to modulate and sustain growth and development during plant defense against biotic and abiotic stresses. It has recently become apparent that chloroplasts communicate with the nucleus and influence nuclear-encoded cytosolic protein synthesis. Such an intracellular communication has come to be known as retrograde signalling. A simpler model for studying the components and transduction of retrograde signalling is a unicellular alga, Chlamydomonas, which has only a single chloroplast. Here we have highlighted and reviewed recent research that has unearthed signalling components relevant to chloroplasts such as redox signals, functional tetrapyrrols biosynthesis intermediates, and Ca2+-signalling that modulate nuclear gene expression. There is a practical relevance of the new knowledge being unravelled in these areas, being useful to the industrial biotechnology sector for novel and sustainable alternatives to petrochemical products, and developing biological cell factories that can generate different reagents and sustainable bio-economy. In addition, the proof-of-principle of the unique carbon dioxide concentration mechanisms occurring in Chlamydomonas is useful for sequestration of carbon dioxide in the implementation of the circular economy concept as well as in the improvement of photosynthetic productivity in important C3 crops. This invited review will be useful to students, researchers, scientists, plant and algal biologists, and industrial biotechnology companies alike.

Technical Abstract: Retrograde signalling is a selective process defined by cues generated in chloroplast/mitochondria which traverse membranes and end up regulating nuclear gene expression and protein synthesis. The coding and encoding of organellar message(s) that alter nuclear gene expression and/or cellular metabolism is central to retrograde signalling. Gene medley and metabolic components involved in this process have been identified in higher plants, while unraveling them in unicellular organisms, for example, algae has been relatively less intense. Chlamydomonas reinhardtii is an oxygenic microalga well known for genetic dissection, containing a single chloroplast amenable for generating mutants, and therefore, it is a good model system for studying retrograde signalling. Here we define and review progress made in various communication networks between the chloroplast and nucleus in this alga.