Frontier Research Institute for Interdisciplinary Sciences (FRIS), Tohoku University, Sendai, Japan; Center for Sustainable Resource Science (CSRS), RIKEN, Wako, Japan
Exploratory Research Center on Life and Living Systems (ExCELLs), National Institutes of Natural Sciences, Okazaki, Japan; National Institute for Physiological Sciences, National Institutes of Natural Sciences, Okazaki, Japan; The Graduate University for Advanced Studies, SOKENDAI, Okazaki, Japan; Research Institute for Electronic Science, Hokkaido University, Sapporo, Japan; Graduate School of Medicine, Juntendo University, Tokyo, Japan
Exploratory Research Center on Life and Living Systems (ExCELLs), National Institutes of Natural Sciences, Okazaki, Japan; National Institute for Physiological Sciences, National Institutes of Natural Sciences, Okazaki, Japan; The Graduate University for Advanced Studies, SOKENDAI, Okazaki, Japan; Research Institute for Electronic Science, Hokkaido University, Sapporo, Japan
Plants distribute many nutrients to chloroplasts during leaf development and maturation. When leaves senesce or experience sugar starvation, the autophagy machinery degrades chloroplast proteins to facilitate efficient nutrient reuse. Here, we report on the intracellular dynamics of an autophagy pathway responsible for piecemeal degradation of chloroplast components. Through live-cell monitoring of chloroplast morphology, we observed the formation of chloroplast budding structures in sugar-starved leaves. These buds were then released and incorporated into the vacuolar lumen as an autophagic cargo termed a Rubisco-containing body. The budding structures did not accumulate in mutants of core autophagy machinery, suggesting that autophagosome creation is required for forming chloroplast buds. Simultaneous tracking of chloroplast morphology and autophagosome development revealed that the isolation membranes of autophagosomes interact closely with part of the chloroplast surface before forming chloroplast buds. Chloroplasts then protrude at the site associated with the isolation membranes, which divide synchronously with autophagosome maturation. This autophagy-related division does not require DYNAMIN-RELATED PROTEIN 5B, which constitutes the division ring for chloroplast proliferation in growing leaves. An unidentified division machinery may thus fragment chloroplasts for degradation in coordination with the development of the chloroplast-associated isolation membrane.
