Photoinactivation Of The Oxygen-evolving Complex Regulates The Photosynthetic Strategy Of The Seagrass Zostera Marina

Chlorophyll fluorescence,western blot and proteome were employed to identify that photoinhibition was derived from the photoinactivation of oxygen-evolving complex(OEC)in Zostera marina and further complete the photosynthetic regulation in response to OEC photoinactivation.The continuous photoinactivation of OEC,which resulted in electron insufficiency,limited electron consumptions of non-photochemical quenching(NPQ),alternative electron flows,and reactive oxygen species(ROS),allowing Z.marina efficiently use the limited electrons to perform photosynthesis.Simultaneously,we verified the assumption that the As A and PSII-CEF were activated to donate electrons to PSII reaction center.The main results showed as following:1.A photosynthetic regulation in response to oxygen-evolving complex photoinactivation in the marine angiosperm Zostera marinaZostera marina,a widespread marine angiosperm,evolved from freshwater ancestor of terrestrial monocots and successfully transitioned into a completely submerged seagrass.We found that its oxygen-evolving complex(OEC)was preferentially photoinactivated,as evidenced by both the increase of the relative variable fluorescence at the K-step and the downregulation of OEC peripheral proteins Psb O and Psb P.Correspondingly,a photosynthetic regulation was addressed at both proteome and physiology level by in vivo study.The unchanged ?p H sensor Psb S protein content and the dynamics of non-photochemical quenching(NPQ)induction,described by a single exponential function,verified the deficiency of the fast q E component.Contents and activities of chlororespiration,Mehler reaction,malic acid synthesis,and photorespiration key enzymes were not upregulated,suggesting that alternative electron flows remained unactivated.Furthermore,neither significant production of singlet oxygen nor increment of total antioxidative capacity during light exposure indicated that ROS was not produced.Together,these low electron consumptions allow Z.marina efficiently use the limited electrons caused by partial OEC photoinactivation to maintain a normal carbon assimilation level.Furthermore,we clarified that rare PSII-CEF and ascorbate assumed photoprotective roles,which could donate electrons to the PSII reaction center to remit oxidation state of the donor side.Finally,how the photosynthetic apparatus worked in coordination to achieve well carbon assimilation under the background of photoinactivated OEC was answered.2.An underlying mechanism of q E deficiency in marine angiosperm Zostera marinaNon-photochemical quenching(NPQ)of photosystem II(PSII)fluorescence is one of the most important protective mechanisms to survive under high light conditions for phototropic organisms.A low-efficient NPQ,characterized by weak capacity of NPQ induction and low level of protective NPQ,was observed in Zostera marina,which inhabits the shallow water regions.Furthermore,chlorophyll fluorescence and Western blot data verified that the fast-induced component of NPQ,the energy-dependent quenching(q E),was not present in this species.In contrast to lack of PSII antenna quenching sites for q E induction in brown algae,all the antenna proteins are present in Z.marina.There was a novel underlying mechanism that the limited construction of trans-thylakoid proton gradient(?p H)caused by photoinactivation of oxygen evolving complex(OEC)did not induce protonation of Psb S,and therefore inability to form quenching sites for q E induction.Although the ?p H constructed under light exposure activated violaxanthin(V)de-epoxidase enzyme to catalyze conversion of V to zeaxanthin,the quenching capacity of de-epoxidised pigment was weak.We suggest that the low-efficient NPQ was conductive to efficiently utilize the limited electrons to perform photosynthesis,resisting the adverse effect of the photoinactivation of OEC on the photosynthetic rate.