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

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 compositions and ultrastructure of the thylakoid membranes were determined in plants grown under different salinity conditions (0, 100, 200, and 250 mM NaCl). The salt treatment caused a decrease in fluorescence, photochemical activity of PSII and PSI, as well as the protein content of the thylakoids. At high salt concentrations, the F735/F686 nm fluorescence ratio in M chloroplasts was reduced, while it was stimulated in BS chloroplasts. The photochemical activity of PSII was reduced in both chloroplasts, while there was no statistically significant difference in the activity of PSI compared to the control. According to the analysis of the protein content of the thylakoid membranes of M and BS chloroplasts, polypeptides belonging to the core antenna of PSII (47 kDa and 43 kDa) and LHCII (28-24 kDa) were present in both types of membranes, but their intensity was weak in BS thylakoids. The synthesis of the 68 kDa apoprotein belonging to the core of PSI was inhibited in the M membranes. There was no noticeable change in the membrane system of BS thylakoids. Salt stress had a greater impact on the ultrastructure of M chloroplasts than on BS ones and caused the formation of granal stacking in BS chloroplast. These results may indicate different responses of the two chloroplast types to salt stress.