Exogenously Applied Trehalose Protects The Structure And Function Of Photosystem Ⅱ(PSII) Under Heat Stress

Wheat (Triticum aestivum L.) is the most important food crop in temperate zones and it suffers from heat stress more frequently with the global warming, which causes a serious threat to the wheat yield. Heat stress can lead to the destruction of the photosynthetic apparatus, thereby inhibiting photosynthesis and reducing yields of crops. It has been known that photosystem Ⅱ (PSII) is the most susceptible part damaged by environmental stresses in the photosynthetic system of higher plants. Plants can accumulate trehalose in order to improve stress tolerance and maintain its relatively normal growth under stress conditions. However, the influences of trehalose on PSII and whether it is associated with improved thermal resistance under heat stress have not been reported. In this paper, chlorophyll fluorescence parameters, fast chlorophyll fluorescence induction curve, the content of malondialdehyde (MDA), D1 protein and PSII pellets, photosynthetic oxygen evolution rate and protein secondary structure of PSII of wheat seedlings were measured under heat stress and subsequent recovery to investigate the effects of exogenously applied trehalose on PSII structure and function of wheat seedlings, and to provide a theoretical basis for protection mechanism of trehalose in the photosynthetic apparatus and coping strategies in the field. The main results are listed as follows:1. Exogenously applied trehalose pretreatment reduced the damage of lipid peroxidation under heat stressMDA content increased significantly in control group under heat stress, reflecting heat stress caused damage to the proteins and enzymes on cell membrane. Under the room temperature and heat stress conditions, exogenously applied trehalose significantly decreased MDA content and membrane lipid damage caused by lipid peroxidation, thereby protecting the stability of the cell membrane.2. Exogenously applied trehalose pretreatment protected D1 protein and protein secondary structures under heat stressD1 protein content obviously declined by 44.19% in control group under heat stress, reflecting the serious losses of D1 protein. D1 protein content of trehalose pretreatment group under heat stress was 60.94% higher than the control, indicating that exogenous trehalose alleviated the damage caused by heat stress and played a role in the protection of D1 protein. After recovery at room temperature from heat stress, D1 protein contents of all treatment groups had a certain recovery. In addition, heat stress resulted in a decline of a-helix content while an increase of β-sheet and β-turn in the control group, causing the degeneration of PSII complexes. The content of ordered a-helix structure in exogenous trehalose treatment group was higher than that in control and other secondary structures changed little under heat stress, suggesting exogenous trehalose can effectively protect the protein secondary structure of PSII reaction center complexes especially a-helix.3. Exogenously applied trehalose alleviated the reduction of photochemical efficiency under heat stressHeat stress induced a decrease of Fv/Fm, Y (Ⅱ), ETR (Ⅱ), qP, and an increase of Y (NPQ), indicating that the high temperature caused a decline of maximal photochemical efficiency of PSII, actual photochemical efficiency of PSII and electron transfer rate, simultaneously decreased the energy for photochemical electron transfer while increasing heat dissipation of energy. Higher Fv/Fm, Y (Ⅱ), ETR (Ⅱ), qP and lower Y (NPQ) of exogenous trehalose treatment group demonstrated that exogenous trehalose reduced the energy for non-photochemical quenching while increasing the proportion of the energy for photochemical reactions, improving the photochemical efficiency; Under heat stress the initial slope of rapid chlorophyll fluorescence induction curve decreased, and J. I. P peak had a significant decline. Meanwhile the inflection point between J and P disappeared, and the differences among J, I, P fluorescence value diminished, as a result the curve tended to be smooth, which reflected the inhibition of electron transfer of donor side and acceptor side of PSII and inactivation of PSII reaction centers. Trehalose pretreatment group remained a basic shape of OJIP curve under heat stress. At the same time, the slope of initial fluorescence, and the peaks of J and P decreased slightly but they were much higher than those of the control, indicating that trehalose can protect the donor side and acceptor side of PSII as well as the reaction centers, reducing the damage caused by heat stress. The reduction in the photosynthetic oxygen evolution rate of the control reflected the inhibition of photo synthetic ability. However, photosynthetic oxygen evolution rate of exogenous trehalose pretreatment group under heat stress and recovery conditions were higher than the control, suggesting that trehalose can protect the photosynthetic capacity under heat stress and speed up the rate of recovery from the stress.4. Exogenously applied trehalose alleviated the loss of PSII pellets under heat stressThe content of PSII pellets decreased significantly by 51.7% under heat stress, while exogenous trehalose pretreatment maintained a much higher content than the control, and alleviated the damage of PSII pellets.In summary, the exogenously applied trehalose can reduce the damage of lipid peroxidation, and protect Dl protein and protein secondary structure of PSII reaction center complexes, thus alleviating the inhibition of PSII donor side, acceptor side and reaction center, and improving the photochemical efficiency and photosynthetic capacity. All of this helps to protect PSII structure and function to improve the heat resistance of wheat seedlings.