THE INTERPLAY BETWEEN AUTOPHAGY AND CHLOROPLAST VESICULATION PATHWAYS UNDER DARK-INDUCED SENESCENCE

In cellular circumstances in which carbohydrates are scarce, plants use alternative substrates for cell energetic maintenance. The major plant protein reserve is present in chloroplast, which represents up to 70% of total leaf proteins. The degradation of chloroplasts is a hallmark of natural and stress-induced senescence, and autophagy plays a key role in this process. Remarkably, an autophagic-independent route of chloroplast degradation, the Chloroplast vesiculation (CV) pathway, was recently demonstrated. During energy starvation, CV is highly induced, contributing to the early senescence phenotype of autophagy-deficient mutants. Although the association between autophagy and CV has been suggested, the extent to which these pathways interact to provide energetic substrates remains unclear.

To investigate the importance of CV under extended darkness, Arabidopsis thaliana mutants amir-CV1 and amir-CV2, with low expression of CV gene, were characterized. Following extended darkness, amir-CV mutants did not show differences in photochemical efficiency of PSII (Fv/Fm) and chlorophyll content. The metabolite analysis revealed minor changes in amino acids of amir-CV mutants. Lipid profiling showed slightly higher levels of the phospholipids, phosphatidylcholine and phosphatidylethanolamine, as well minor changes in thylakoid lipids in amir-CV mutants in prolonged darkness. Interestingly, transcript levels of autophagy-related genes were gradually up-regulated during darkness, indicating compensatory effects between these pathways. Collectively, our results suggest that the CV pathway plays a relatively minor role in substrates release under energetic depletion conditions when autophagy is activated. Further characterization of double mutants plants is required to address the compensatory mechanisms of autophagy and CV pathways.