Changes in Unsaturated Levels of Fatty Acids in Thylakoid PSII Membrane Lipids During Chilling-induced Resistance in Rice

Temperature is one of the abiotic factors limiting growth and productivity of plants. In the present work, the effect of low non-freezing temperature, as an inducer of ＂chilling resistance＂, was studied in three cultivars of rice （Oryza sativa L.）, japonica cv. 9516 （j-9516）, the two parental lines of superhigh-yield hybrid rice between subspecies, Peiai/E32 （ji-PE）, and the traditional indica hybrid rice Shanyou 63 （i-SY63）. Leaves of chill-treated rice showed chilling-induced resistance, as an increase of their low-temperature tolerance was measured using chlorophyll fluorescence measurements, revealing a change in photosystem II （PSII） efficiency. After 5 d of exposure to 11 ~C under low light （100 pmol·m^-2·s^-1）, levels of unsaturated fatty acids in PSII thylakoid membrane lipids decreased during the initial 1-2 d, then increased slowly and reached 99.2%, 95.3% and 90.1% of the initial value （0 d） in j-9516, ji-PE and i-SY63, respectively, on the fifth day. However, under medium light （600 μmol·m^-2·s^-1）, all cultivars experienced similar substantial photoinhibition, which approached steady state levels after a decline in levels of unsaturated fatty acids in PSII thylakoid membrane lipids to about 57.1%, 53.8% and 44.5% of the initial values （0 d） in j-9516, ji-PE and i-SY63 on the fifth day. Under either chilling-induced resistance （the former） or low temperature photoinhibition （the latter） conditions, the changes of other physiological parameters such as D1 protein contents, electron transport activities of PSII （ETA）, Fv/Fm, xanthophyl cycle activities expressed by DES （deepoxide state） were consistent with that of levels of unsaturated fatty acids in PSII thylakoid membrane lipids. So there were negative correlations between saturated levels of fatty acids （16：1（3t）, 16：0, 18：0）, especially the 16：1（3t） fatty acid on thylakoid membrane and other physiological parameters, such as D1 protein contents, ETA and （A＋Z）/（A＋V＋Z）. A specific role of desaturation of fatty acids and the photoprotective pigments of the xanthophyl cycle, leading to an acclimation response in thylakoid membrane lipids may be involved. We conclude that chilling-induced resistance is accelerated by the unsaturation of thylakoid membranes, and the ability of rice plants to cold-harden can be enhanced by genetic engineering.

Relationships between Phosphatidylglycerol Molecular Species of Thylakoid Membrane Lipids and Sensitivities to Chilling-induced Photoinhibition in Rice

In an attempt to explore the relationships between phosphatidylglycerol （PG） molecular species of thylakoid membrane lipids and sensitivities to chilling-induced photoinhibition, PG molecular species, D1 protein, electron transport activities of thylakoid membrane and the potential quantum yield （FvlFm） in rice treated under middle and low photon flux density （PFD） at 11℃ were analyzed by high performance liquid chromatography, enzyme hydrolysis, gas phase chromatography （GC） and so on. Results showed that the major molecular species of PGs in rice thylakoid membrane were 18：3/16：0, 18：3/16：1（3t）, 18：2/16：0, 18：2/16：1（3t）, 18：1/16：0, 18：1/16：1（3t）, 16：0/16：0, 16：0/16：1（3t）. There were large differences in the contents of unsaturated PG molecular species such as 18：1-3/16：0-16：1（3t） and saturated PG molecular species like 16：0/16：0-16：1（3t） among japonica cv 9516 0-9516）, japonica-indica hybrid F1 j-9516/i-SY63 （ji-95SY） and indica cv Shanyou 63 （i-SY63）. J-9516 containing higher contents of unsaturated PG molecular species was manifest in stable D1 protein contents under chill and tolerant to chill-induced photoinhibition. In contrast to j-9516, i-SY63 with lower contents of unsaturated PG molecular species, exhibited unstable D1 protein contents under chill and was sensitive to chill-induced photoinhibition, ji-95SY containing middle contents of unsaturated PG molecular species between those of j-9516 and i-SY63, exhibited mid extent of sensitivity to chill-induced photoinhibition. The losses in D1 protein also account for the inhibition in electron transport activity of thylakoid membrane and the observed decline in FvlFm. The PG molecular species that is efficient in raising chilling-resistant capacity were those containing unsaturated fatty acids, namely, unsaturated PG molecular species. These results implied that the substrate selectivity of the glycerol-3-phosphate acyltransferase in chloroplasts towards 16：0 or 18：1 displayed greatly the difference between japonica and indica rice. Itwas possible to enhance the capacity of resistance to chilling-induced photoinhibition by improving or modifying the GPAT gene.