Study On The Change Of Photosynthetic Membrane Function In The Senescence Process Of Liangyou Peijiu

This study explored the high-yield hybrid rice Liangyoupeijiu during senescence, focusing on the on flag leaf chloroplast the change of the structure and function. We investigated that light energy absorption, transformation and transfer, physiological and biochemical process of dynamic change, variation of photosynthetic membrane (thylakoid membrane) chlorophyll-protein complex (LHCⅡ, ATPase and RuBisCO), each related subunits content changes, and the dynamic monitoring and analysis of key enzyme subunit gene expression level in order to reveal the essence of leaf senescence in rice and other crops, the results showed that:(1) PSII photochemistry in high-yield rice LYPJ flag leaves during senescence stage was measured by chlorophyll a fluorescence analysis. The parameters deduced from JlP-test showed that the inhibition of the donor side of PSII was greater for hybrids rice LYPJ than that of the acceptor side. The natural senescence process was accompanied by the increased relative measure of the inactivation of OEC (VK/VJ) and decreased the total number of active reaction centers per absorption (10 RC/ABS), indicating that the inhibition of electron transport in hybrid rice LYPJ caused by natural senescence might be by a part of uncoupling of the OEC and/or a part of inactivation of PSII reaction centers. These data (φDo, DIo/RC, DIo/CSM) analyzed in this study suggested that energy dissipation was enhanced in order to protect senescent leaves of hybrid rice LYPJ from photo damage.(2) Photosynthetic activities and thylakoid membrane protein patterns as well as the ultrastructure of chloroplasts in flag leaves were investigated during the senescence processes in high-yield hybrid rice LYPJ under field condition. The earlier decrease of PS I activity than PS II in LYPJ was primarily due to the significant degradation of PS I chlorophyll-protein complex. The degradation rate for each chlorophyll-protein complex was different and the order for the stability of thylakoid membrane complexes during flag leaf senescence in rice LYPJ was:LHCII> OEC> PSII core antenna> PSII core> PSI core> LHCI, which was partly supported by the BN-PAGE gel combined with immunoblot analysis. A decrease in the chlorophyll a/b ratio at the senescence stage was observed to coincide with stability of the LHCII subunits. Ultrastructural investigations revealed that the chloroplasts have large loosen stacking grana without interconnecting stroma thylakoids during the senescence processes. It was hypothesized that the stability of grana thylakoids harboring the major LHCII under high radiation condition in summer might played a key role in the dissipation of excess light energy. This alternative strategy would protect photosynthetic apparatus from photo damage and might be causally related to the high yield of this rice cultivar.(3) In dark reaction enzymes activities analyses, the decreased RCA activity caused Rubisco enzyme rapidly with CO2, Mg2+ combined to form the ECM ternary composites, the generation of reactive oxygen species disturbed effect of photosynthetic mechanism. The degradation of rbc S subunit (15 kd) was earlier than rbc L (55 kd) verified in QRT-PCR. It was observed that a little increment of C4 photosynthetic enzymes activities (PEPC, PEPCK, PPDK, NADP-ME, NADP-MDH) at the beginning of flag leaves senescence might compensate, to some degree, for the larger reductions in Rubisco activity, confirmed by quantitative RT-PCR analysis.(4) The active oxygen content of high-yield hybrid rice Liangyoupeijiu increased during senescence stage. The reason may be that the block of electron transfer, depleting degradation LHCII and core complexes and light carbon imbalances. The maximum antioxidant enzymes activity appeared in 14-21 d, the period is mainly rice filling stage. The results indicated that the change of the antioxidant systems was consistent with the performance of photosynthesis. In addition, in the process of senescence, the CAT A isozymes played bigger role than the CAT B and the CATC.(5) Proteomics results showed that the differences in protein to roughly 7 categories:energy, stress defense, photosynthesis, carbohydrate metabolism, amino acid metabolism, protein metabolism, unknown functions. Subcellular localization found that these proteins were in chloroplast, mitochondria and cytoplasm and ribosomes. Protein associated with photosynthesis, such as CP 23, OEC oxygen evolving complex and NADPH protein synthesis, ATP synthase CF1 beta subunit were difference altered. RuBisCO enzyme, RuBisCO enzyme precursor and RuBisCO activase protein were maximum changed. The increment of R2R3-MYB transcription factors and Organic cation from-like protein indicated that these two kinds of proteins played important role in the senescence process of high-yield rice LYPJ.In summary, combined photosynthetic physiology and biochemistry method with proteomics method, we explained the relationship between degradation of thylakoid membrane protein complexes and changes of photoreaction during senescence. It might have great significance in the research of hybrid rice premature aging.