Abstract
In response to proteotoxic stress, chloroplasts communicate with the nuclear gene expression system through a chloroplast unfolded protein response (cpUPR). We isolated Chlamydomonas reinhardtii mutants that disrupt cpUPR signaling and identified a gene encoding a previously uncharacterized cytoplasmic protein kinase, termed Mars1-for mutant affected in chloroplast-to-nucleus retrograde signaling-as the first known component in cpUPR signal transmission. Lack of cpUPR induction in MARS1 mutant cells impaired their ability to cope with chloroplast stress, including exposure to excessive light. Conversely, transgenic activation of cpUPR signaling conferred an advantage to cells undergoing photooxidative stress. Our results indicate that the cpUPR mitigates chloroplast photodamage and that manipulation of this pathway is a potential avenue for engineering photosynthetic organisms with increased tolerance to chloroplast stress.
Highlights
In photosynthetic eukaryotes chloroplasts fulfill many essential functions such as photosynthetic conversion of light into chemical energy, synthesis of essential amino acids, fatty acids and other secondary metabolites
Immunoblotting confirmed that the reporter strain induced yellow fluorescent protein (YFP) with comparable kinetics to those of the Vipp2 protein induction upon ClpP1 repression (Figure 1C)
We focused on the chloroplast unfolded protein response (cpUPR)-silencing mutants that exhibited YFP levels at least three standard deviations lower-than-average YFP fluorescence of all mutants subjected to ClpP1 repression (Figure 2A)
Summary
In photosynthetic eukaryotes chloroplasts fulfill many essential functions such as photosynthetic conversion of light into chemical energy, synthesis of essential amino acids, fatty acids and other secondary metabolites They act as signaling platforms during plant development and stress adaptation, as they can alter the expression of thousands of nuclear genes and influence many cellular activities that are key to plant performance (Chan et al, 2016). The cpUPR comprises the selective up-regulation of nuclear encoded chloroplast-localized small heat shock proteins, chaperones, proteases, and proteins involved in chloroplast membrane biogenesis Other pathways, such as autophagy and sulfur uptake are activated to mitigate general cellular stress caused by chloroplast metabolic dysfunctions (Ramundo et al, 2014)
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