Comparative Analysis of Mesophyll and Bundle Sheath Chloroplasts from Maize Plants Subjected to Salt Stress

Abstract

The effect of salt stress was studied in mesophyll (M) and bundle sheath (BS) chloroplasts of maize plants treated with NaCl for 5 days. Pigment content, chlorophyll fluorescence at 77 K, activity of photosystems (PS) I and II, polypeptide composition, and ultrastructure of the thylakoid membranes were investigated in plants grown under different salinity conditions (0, 100, 200, and 250 mM NaCl). Salt treatment caused a decrease in the fluorescence, photochemical activity of PSII and PSI, and protein content of the thylakoids. At high salt concentrations, the F735/F686 fluorescence ratio was reduced in M chloroplasts and increased in BS chloroplasts. The photochemical activity of PSII was reduced in both types of chloroplasts, while no statistically significant difference was observed in the activity of PSI compared to the control. Analysis of the thylakoid membrane protein composition in the M and BS chloroplasts revealed that proteins of the PSII core antenna (47 and 43 kDa) and LHCII (28-24 kDa) were present in both types of membranes, but their content in the BS thylakoids was much lower. The synthesis of the PSI core 68-kDa apoprotein was suppressed in the M membranes. No noticeable changes were observed in the membrane system of BS thylakoids. Salt stress had a greater impact on the ultrastructure of M chloroplasts than on BS chloroplasts and caused formation of granal stacks in BS chloroplasts. These results indicate different response of the M and BS chloroplasts to salt stress.