Three-dimensional ultrastructural change of chloroplasts in rice mesophyll cells responding to salt stress

Abstract

Background and Aims Excess salinity inhibits the metabolism of various systems and induces structural changes, especially in chloroplasts. Although the chloroplast body seems to swell under salinity stress as observed by conventional transmission electron microscopy, previous studies are limited to 2-D data and lack quantitative comparisons because specimens need to be sliced into ultrathin sections. This study shows three-dimensionally the structural changes in a whole mesophyll cell responding to salinity stress by serial sectioning with a focused ion beam scanning electron microscope (FIB-SEM) and compares the differences in chloroplast structures based on reconstructed models possessing accurate numerical voxel values. Methods Leaf blades of rice plants treated with 100 mm NaCl or without (control) for 4 d were fixed chemically and embedded in resin. The specimen blocks were sectioned and observed using the FIB-SEM, and then the sliced image stacks were reconstructed into 3-D models by image processing software. Key Results On the transverse sections of rice mesophyll cells, the chloroplasts in the control leaves appeared to be elongated meniscus lens shaped, while those in the salt-treated leaves appear to be expanded oval shaped. The 3-D models based on serial sectioning images showed that the chloroplasts in the control cells spread like sheets fitted to the shape of the cell wall and in close contact with the adjacent chloroplasts. In contrast, those in the salt-stressed cells curled up into a ball and fitted to cell protuberances without being in close contact with adjacent chloroplasts. Although the shapes of chloroplasts were clearly different between the two treatments, their volumes did not differ. Conclusions The 3-D reconstructed models of whole rice mesophyll cells indicated that chloroplasts under salt stress conditions were not swollen but became spherical without increasing their volume. This is in contrast to findings of previous studies based on 2-D images.

Photosynthetic and Ultrastructural Properties of Eggplant (Solanum melongena) under Salinity Stress

“…Moreover, the number of plastoglobuli increased, suggesting a function storage of thylakoid components such as lipids, plastohydroquinone, and tocopherols [51]. However, Oi et al (2020) reported that the 3D reconstructed models of whole rice chloroplasts under salt-stress conditions were not swollen but became spherical without increasing their volume as compared to controls [52].…”

Effects of Salinity Stress on Chloroplast Structure and Function

“…Swelling of thylakoids under salt stress (~200 mM NaCl) was reported in rice [50]. However, recent 3D analysis confirmed that rice chloroplasts became spherical under salt stress without any changes in the overall chloroplast volume [51]. Contrasting observations regarding chloroplast volume have been reported among different species.…”

Ion homeostasis for salinity tolerance in plants: a molecular approach

“…Eventually, there will be a disruption of the cellular membranes and cell homeostasis is eventually derailed. Under salinity stress, plants demonstrate various anatomical changes as seen in chloroplasts such as oval and expanded mesophyll cells compared to the elongated shape in unstressed leaf tissue (Oi et al 2020). Therefore, chloroplasts no longer remain swollen during salt toxicity which affects water turgor pressure.…”