Three-dimensional analysis of chloroplast protrusion formed under osmotic stress by polyethylene glycol in rice leaves

Abstract

Chloroplast protrusions (CPs), large stromal areas lacking thylakoid membranes, are present before the formation of Rubisco-containing bodies (RCBs). RCBs are round bodies found within the cytoplasm and transported to the vacuole via the autophagy pathway. In the present study, we observed CPs and RCBs in rice mesophyll cells under polyethylene glycol-induced osmotic stress following 3D reconstruction of transmission electron microscopy images of serial sections. Osmotic stress induced the formation of CPs and RCBs whose features were similar to those observed under salt stress, suggesting that osmotic effect contributes to the formation of CPs and RCBs. The 3D image of CPs revealed that some CPs were formed far from the main chloroplast body, and one of the CP was in physical contact with a mitochondrion and a peroxisome, which were located far from the main chloroplast body. Thus, the CPs may aid the association with mitochondria and peroxisomes far from the main chloroplast body. Additionally, a CP with a connection to the main chloroplast body by a narrow structure was observed three-dimensionally. Since the volume of CPs was similar to that of RCBs, the CP with the narrow structure may be a precursor structure just before the release of an RCB into the cytoplasm. In the present study, the volumes of two CPs among 24 CPs were markedly larger than the volumes of other CPs and RCBs. Thus, the large body of the CP could be an unviable structure that fails to release RCBs.