Regulation Mechanism Of Photosynthetic Electron Transport Chain In Zostera Marina L.

Seagrasses form a critical coastal ecosystem:their role in fisheries production,and in sediment accumulation and stabilization,is well documented,but they contribute to the function of marine ecosystems and have direct value to humanity.However,global warming and anthropogenic disturbance resulted in an abnormal condition of high seawater temperature and limited light intensity in seagrass meadows.Many physiological processes have evolved to prevent stress-induced damage and to repair stress-sensitive components and photosynthesis is considered one of the most sensitive physiological indicators in response to environmental stressors.To explore the functional role of photosynthetic electron transport chain played in response to varying stresses,both chlorophyll fluorescence and isobaric tagging for relative and absolute quantitation?iTRAQ?were employed in Zostera marina L..The main results showed as following:1.Photoresponse characteristics of electron transport chain in Zostera marina L.Responses of electron transport to three levels of irradiation(20,200,and 1200?mol photons m^-2 s^-1 PAR;exposures called LL,ML and HL,respectively)were investigated in eelgrass?Zostera marina L.?utilizing the chlorophyll a fluorescence technique.Exposure to ML and HL reduced the maximum quantum yield of photosystem II?PSII??Fv/Fm?and the maximum slope decrease of MR/MR_O(V_PSI),indicating the occurrence of photoinhibition of both PSII and photosystem I?PSI?.A comparatively slow recovery rate of Fv/Fm due to longer half-life recovery time of PSII and 40%lower descending amplitude compared to other higher plants implied the poor resilience of the PSII.Comparatively,PSI demonstrated high resilience and cyclic electron transport?CEF?around PSI maintained high activity.With sustained exposure,the amplitudes of the kinetic components?L₁ and L₂?,the probability of electron transfer from PSII to plastoquinone pool(y_ET2o),and the connectivity among PSII units decreased,accompanied by an enhancement of energy dissipation.Principle component analysis revealed that both V_PSI and Fv/Fm contributed to the same component,which was consistent with high connectivity between PSII and PSI,suggesting close coordination between both photosystems.Such coordination was likely beneficial for the adaption of high light.Exposure to LL significantly increased the activity of both PSI and CEF,which could lead to increased light harvesting.Moreover,smooth electron transport as indicated by the enhancement of L₁,L₂,y_ET2o and the probability of electron transport to the final PSI acceptor sides,could contribute to an increase in light utilization efficiency.2.The interaction of high seawater temperature and light intensity on photosynthetic electron transport of Zostera marina L.The interaction of widely recognized causes of eelgrass decline?high seawater temperature and limited light intensity?on photosynthetic electron transport was investigated via chlorophyll fluorescence technique.High seawater temperature combined light intensity significantly increasing the relative maximum electron transport rate?rETRmax?;at critical temperature of 30oC,the rETRmax increased with the enhancement of light intensity,indicating the elevation of overall photosynthetic performance.Based on the magnitude of effect size??²?,light intensity was the predominant factor affecting the performance index,indicating that photosystem II?PSII?was sensitive to light intensity.Moreover,the donor side was severely damaged as evidenced by the higher decrease amplitude of fast component and its subsequent incomplete recovery.The reaction center exhibited limited flexibility due to the slight decrease amplitude in maximum photochemical quantum yield.In contrast with PSII,photosystem I?PSI?was more sensitive to high seawater temperature,based on the magnitude of?² derived from the maximal decrease in slope.High seawater temperature significantly increased PSI activity,plastoquinol reoxidation capacity,and probability for electron transfer to final PSI electron acceptors.Moreover,it combined elevated light intensity significantly stimulated the activity of cyclic electron flow?CEF?around PSI.Higher activity of both PSI and CEF contributed to balancing the linear electron transport via alleviating the over-reduction of the plastoquinone pool,thus improving the photosynthetic performance at critical temperature.Therefore,limited light intensity decreased the tolerance of eelgrass to critical temperature,which might be a factor contributing factor in the observed decline.3.PS? photoinhibition derived from the direct light-induced inactivation of OEC in Zostera marina L.The photosynthetic characteristics and chloroplast protein profiles were investigated to assess the tolerance of flowering marine plant Zostera marina L.to high light,using chlorophyll fluorescence and iTRAQ technique.Exposed to high light resulted in the increase of the reduction level of primary quinone electron transport acceptor?Q_A?and the rapid enhancement of reoxidation capacity of plastoquinone,thus avoiding the occurrence of acceptor side photoinhibition due to over-reduction of PQ.Along with duration of exposure,the photochemical activity of photosystem II?PSII?exhibited significantly linear decrease tendency.Moreover,the relative fluorescence intensity of K-step gradually elevated and the protein abundance of PsbP and PsbQ significantly decreased,indicating the continuous inactivation of oxygen evolving complexes?OEC?.All these results suggested the occurrence of PSII donor side photoinhibition under visible light.The primary characteristics of this mechanism were:?1?originating from the direct inactivation of Mn cluster,?2?independent of the redox state,?3?avoiding the production of singlet oxygen by the inhibition of elctron transport between the reduced Q_A and the secondary quinone acceptor and the continuous inactivation of OEC,protecting the PSII core protein of D1.Additionally,high light contributed to the coordination of cytochrome b6f complex,photosystem I,and ATP synthetase,maintaining the perfect photosynthetic performance as evidenced by the significant elevation of both the relative maximum electron transport rates and photosynthetic efficiency.