Partial or entire: Distinct responses of two types of chloroplast autophagy

Abstract

Autophagy carries out intracellular degradation of cytoplasmic components, which is important for the removal of dysfunctional organelles and for efficient nutrient recycling in eukaryotic cells. Most proteins in plant green tissues are found in chloroplasts, mainly as photosynthetic proteins that constantly accumulate damage caused by sunlight. Our recent study investigated the involvement of autophagy in the turnover of damaged chloroplasts and found that entire photodamaged chloroplasts are transported into the vacuole for degradation via an autophagy process termed chlorophagy. Our previous studies also established that autophagy can also degrade chloroplast components piecemeal: chloroplast stroma is transported for degradation via autophagy vesicles termed Rubisco-containing bodies (RCB). During sugar starvation-induced senescence in darkened leaves, the RCB pathway is preferentially active. By contrast, we observed active chlorophagy without prior induction of RCB production in photodamaged leaves. These distinct responses between the RCB pathway and chlorophagy support the notion that the induction of the partial-type and entire-organelle-type chloroplast autophagy are differentially regulated by individual upstream molecules. This finding further suggests that the two types of autophagy are coordinated to achieve the controlled chloroplast turnover in response to specific conditions.

Keywords: Arabidopsis; Rubisco-containing body; autophagy; chlorophagy; chloroplasts; photodamage; senescence; sugar starvation; ultraviolet-B.

Figure 1.

Distinct responses of two types of chloroplast-related autophagy to environmental or developmental conditions. During starvation or senescence in leaves, partial transport of chloroplast stroma to the vacuole via Rubisco-containing bodies (RCBs) is preferentially activated to facilitate nutrient recycling. Under photodamage conditions due to the exposure of ultraviolet-B or strong visible light, vacuolar transport of entire chloroplasts by chlorophagy is induced without prior activation of the RCB pathway, which could allow the removal of collapsed chloroplasts produced by photodamage.