Photosynthetic Regulation Mechanism Of Chloroplast NAD(P)H Dehydrogenase-like Complex In Zostera Marina

To clarify the photosynthetic regulation mechanism of chloroplast NAD(P)H dehydrogenase-like complex in Zostera marina,chlorophyll fluorescence and western blot and transcriptome were employed in response to high light.Furthermore,based on the photo-inactivated OEC,we verified the assumption that NDH-dependent PSI-CEF and chlororespiratory were activated to maintain a fine balance among ?p H,ATP,and reducing power and take part in the recovery of photo-inactivated OEC.The main results showed as following:1.The highly efficient NDH-dependent photosystem I cyclic electron flow pathway in the marine angiosperm Zostera marinaZostera marina,a fully submerged marine angiosperm with a unique evolutionary history associated with its terrestrial origin,has distinct photochemical characteristics caused by its oxygen-evolving complex(OEC)being prone to deactivation in visible light.Based on the present phylogenetic analysis,the chloroplast NAD(P)H dehydrogenase-like(NDH)complex was found to be completed in of Z.marina,unlike other marine plants,suggesting its crucial role.Thus,the responses of electron transport to irradiation were investigated through multiple chlorophyll fluorescence techniques and western blot analysis.Moreover,the respective contribution of the two photosystem I cyclic electron flow(PSI-CEF)pathways to the generation of trans-thylakoid proton gradient(?p H)was also examined using inhibitors.The contributions of the two PSI-CEF pathways to ?p H were similar;furthermore,there was a tradeoff between the two pathways under excess irradiation: the PGR5/L1-dependent PSI-CEF decreased gradually following its activation during the initial illumination,while NDH-dependent PSI-CEF was activated gradually with exposure duration.OEC inactivation was continuously under excess irradiation,which exhibiting a positive linear correlation with the activation of NDH-dependent PSI-CEF.We suggest that PGR5/L1-dependent PSI-CEF was preferentially activated to handle the excess electron caused by the operation of OEC during the initial illumination.Subsequently,the increasing OEC inactivation with exposure duration resulted in a deficit of electrons.Limited electrons from PSI might preferentially synthesize NAD(P)H,which could support the function of NDH-dependent PSI-CEF to generate ?p H and ATP,thereby maintaining OEC stability.2.Chlororespiration serves as photoprotection for photo-Inactivated oxygen-evolving complex in Zostera marina,a marine angiosperm.As an alternative electron sink,chlororespiratory,may play a significant role in sustaining the redox equilibrium between stroma and thylakoid membrane.Here,a distinctive role was observed in Zostera marina,a marine angiosperm,whose oxygen evolving complex(OEC)was prone to photo-inactivation due to the inherent susceptibility to excess irradiation.The strong connectivity between OEC peripheral proteins and chlororespiratory key enzymes demonstrated in interaction network of the differentially expressed genes suggested that the recovery of photo-inactivated OEC was in connection with chlororespiratory.Multiple data of chlorophyll fluorescence,western blot and transcriptome verified a new physiological role of chlororespiration to function as photoprotection to generate proton gradient across thylakoid membrane for the recovery of photo-inactivated OEC.Moreover,the continuous impairment of OEC resulted in deficient electrons in the electron transport chain.For reducing the unnecessary electrons consumption,chlororespiratory was activated only under darkness.At variance with the commonly consuming the redundant reducing power during the phase of light to dark transition,the activation of chlororespiratory was prone to proactivity in Z.marina,as evidenced by further activation during initial darkness phase and subsequent persistently high level of oxidative pentose-phosphate pathway which synthesize NAD(P)H for the demand of chlororespiratory.