Chloroplast Damage Induced by the Inhibition of Fatty Acid Synthesis Triggers Autophagy in Chlamydomonas

Abstract

Fatty acids are synthesized in the stroma of plant and algal chloroplasts by the fatty acid synthase complex. Newly synthesized fatty acids are then used to generate plastidial lipids that are essential for chloroplast structure and function. Here, we show that inhibition of fatty acid synthesis in the model alga activates autophagy, a highly conserved catabolic process by which cells degrade intracellular material under adverse conditions to maintain cell homeostasis. Treatment of cells with cerulenin, a specific fatty acid synthase inhibitor, stimulated lipidation of the autophagosome protein ATG8 and enhanced autophagic flux. We found that inhibition of fatty acid synthesis decreased monogalactosyldiacylglycerol abundance, increased lutein content, down-regulated photosynthesis, and increased the production of reactive oxygen species. Electron microscopy revealed a high degree of thylakoid membrane stacking in cerulenin-treated cells. Moreover, global transcriptomic analysis of these cells showed an up-regulation of genes encoding chloroplast proteins involved in protein folding and oxidative stress and the induction of major catabolic processes, including autophagy and proteasome pathways. Thus, our results uncovered a link between lipid metabolism, chloroplast integrity, and autophagy through a mechanism that involves the activation of a chloroplast quality control system.

Linking Autophagy to Abiotic and Biotic Stress Responses

“…This is in line with evidence from Chlamydomonas, in which a deficiency in carotenoid synthesis triggers autophagy in the light, but not in the dark, where the chloroplastic electron transport is inactive [111]. Recently, inhibition of the fatty acid synthase complex in Chlamydomonas was shown to affect chloroplast integrity, particularly by the hyperstacking of thylakoid membranes, which correlated with higher autophagic flux in a ROS-independent way [112]. Moreover, it was recently reported that the hydrotropic response of arabidopsis roots requires autophagic activity, evidenced by the accumulation of autophagosomes, and the presence of H 2 O 2 [113].…”

Co-expressed subunits of dual genetic origin define a conserved supercomplex mediating essential protein import into chloroplasts

“…As shown in Fig. 6E, tic214 repression caused a massive cellular swelling and resulted in a chloroplast with highly disorganized thylakoid ultrastructure and accumulation of starch granules, both typical traits of cells experiencing chloroplast proteotoxic stress (71,73,74). In agreement with this observation, we found that the time-dependent decrease in Tic214 inversely correlates with the induction of Vipp2 (Fig.…”

“…6D), a marker of the chloroplast unfolded protein response (cpUPR) (71,75). The cpUPR is a stress-induced signaling network that responds to impairment of chloroplast proteostasis to maintain organellar health (71,73,(76)(77)(78)(79).…”

Autophagy induced accumulation of lipids inpgrl1andpgr5ofChlamydomonas reinhardtiiunder high light

“…Based on this result, we interpret that lipid accumulation in mutant strains could be explained that PSII or PSI is over reduce, which led to high ROS production. These increased ROS levels act as an autophagy inducer (Heredia-Martinez et al, 2018).…”