Abstract
Autophagy is an intracellular process facilitating the vacuolar degradation of cytoplasmic components and is important for nutrient recycling during starvation. We previously demonstrated that chloroplasts can be partially mobilized to the vacuole by autophagy via spherical bodies named Rubisco-containing bodies (RCBs). Although chloroplasts contain approximately 80% of total leaf nitrogen and represent a major carbon and nitrogen source for new growth, the relationship between leaf nutrient status and RCB production remains unclear. We examined the effects of nutrient factors on the appearance of RCBs in leaves of transgenic Arabidopsis (Arabidopsis thaliana) expressing stroma-targeted fluorescent proteins. In excised leaves, the appearance of RCBs was suppressed by the presence of metabolic sugars, which were added externally or were produced during photosynthesis in the light. The light-mediated suppression was relieved by the inhibition of photosynthesis. During a diurnal cycle, RCB production was suppressed in leaves excised at the end of the day with high starch content. Starchless mutants phosphoglucomutase and ADP-Glc pyrophosphorylase1 produced a large number of RCBs, while starch-excess mutants starch-excess1 and maltose-excess1 produced fewer RCBs. In nitrogen-limited plants, as leaf carbohydrates were accumulated, RCB production was suppressed. We propose that there exists a close relationship between the degradation of chloroplast proteins via RCBs and leaf carbon but not nitrogen status in autophagy. We also found that the appearance of non-RCB-type autophagic bodies was not suppressed in the light and somewhat responded to nitrogen in excised leaves, unlike RCBs. These results imply that the degradation of chloroplast proteins via RCBs is specifically controlled in autophagy.
Highlights
Autophagy is an intracellular process facilitating the vacuolar degradation of cytoplasmic components and is important for nutrient recycling during starvation
Rubisco-containing bodies (RCBs) were rarely observed even if excised leaves were incubated in darkness without concanamycin A for 20 h, suggesting that they are rapidly degraded in the vacuolar lumen under natural cell conditions (Ishida et al, 2008)
When excised leaves were incubated in incubation buffer, RCBs were detected more in mature and early senescent leaves than in young leaves, but RCBs were rarely seen by the presence of Murashige and Skoog (MS) medium containing Suc, even in early senescent leaves (Ishida et al, 2008)
Summary
Autophagy is an intracellular process facilitating the vacuolar degradation of cytoplasmic components and is important for nutrient recycling during starvation. We aimed to demonstrate the relationship between the nutrient status of Arabidopsis leaves and chloroplast degradation via RCBs. We examined the effects of nutrient conditions during leaf incubation, leaf carbohydrate contents over a diurnal cycle, mutations affecting starch metabolism, and nitrogen limitation on the appearance of RCBs. All analyses showed that carbon status is a major factor controlling the production of RCBs, while nitrogen status is less important in the nutrient response of autophagy in leaves. We found that the production response of RCBs and non-RCB-type autophagic bodies containing cytoplasmic components other than chloroplasts to nutrient conditions was not always the same in excised leaves This suggests that there is a mechanism controlling RCB production in plant autophagy
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