The Unsaturation Of Phosphatidylglycerol In Thylakoid Membrane Alleviates PSⅡ Photoinhibition Under Chilling Stress

Plants are incapable of escaping from unfavorable environment, such as low temperature. The membrane is the primary location that was damaged under chilling stress. Chilling stress can damage the structure and function of thylakoid membrane, causing the decrease of photosynthesis, but the increase of unsaturated fatty acid of thylakoid membrane can enhance the photosynthesis ability of plants. GPAT from chilling-resistant plants prefers oleoyl-ACP (18:1-ACP) to palmitoyl-ACP (16:0-ACP) as a substrate. Thus, the proportion of oleic acid (18:1) changes at the sn-1 position of PG in chilling resistant plants. So, it is significant to study the relationship between the proportion of unsaturated fatty acids and chilling tolerance by using GPAT transgenic tomato plants.In this study, wild type (WT), sense transgenic lines [T₂-19(+), T₂-5(+)] and antisense transgenic lines [T₂-16(-),T₂-2(-)] of tomato were used to determine the fatty acid composition of PG in thylakoid membrane, oxygen evolving activity, the maximal photochemical efficiency of PSII (Fv/Fm), the production of chloroplastic reactive oxygen species (ROS) and activities of antioxidase in chloroplasts, and the repair of D1 protein under chilling stress. The results were showed as follows:(1) The results showed that the content of fatty acids of PG changed markedly. Compared with WT, a higher content of unsaturated fatty acids in PG was detected in sense transgenic plants. In contrast, the decreased unsaturation of PG in antisense transgenic plants was observed.(2) Under long time chilling stress, the growth of WT and transgenic lines were all decreased. But the fresh weight of the sense transgenic plants was higher than that of WT and antisense plants. Under chilling stress, the O2 evolution rates and Fv/Fm in WT and transgenic plants obviously decreased. Those decreases were more obvious in WT than those in transgenic plants. These results indicated that decrease of the fresh weight was owing to inhibition of the photosynthesis. The relative electronic conductance of leaves increased in both WT and transgenic plants under chilling stress for 4 days. But it increased more quickly in antisense plants than that in WT and sense transgenic plants. These results indicated that higher unsaturation ratio of PG reduced membrane damage under chilling stress.(3) Compared with WT, the sense transgenic plants could maintain higher activities of SOD and APX, and produce lower content of O₂(?) and H₂O₂ under chilling stress. In contrast, activities of SOD and APX were lower and the content of ROS was higher in the antisense transgenic plants. Relative to other lines, the higher activities of SOD and APX can scavenge more ROS, mitigate the extent of lipid peroxidation caused by the accumulation of ROS, and maintain the integrity of the cytomembrane.(4) We analysed the influence of chilling stress on the repair of D1 protein. During chilling stress for 8 h, the total content of D1 protein decreased among five lines. Compared with WT, there was a higher level of D1 protein in sense transgenic plants and lower level of D1 in antisense ones. After the treatment of streptomycin (SM) and low temperature, the content of D1 was lower in sense trangenic plants than in antisense ones.We concluded that the increase of unsaturation in PG can enhance the ability of repair of D1 protein, alleviate the injury of membrane, thus improve the activities of antioxidase in chloroplasts.