Amplified degradation of photosystem II D1 and D2 proteins under a mixtureof photosynthetically active radiation and UVB radiation: Dependence on redox status of photosystem II

Plants exposed to a mixture of photosynthetically active radiation (PAR) an d UVB radiation exhibit a marked boost in degradation of the D1 and D2 phot osystem II (PS II) reaction center proteins beyond that predicted by the su m of rates in PAR and UVB alone (amplified degradation). Becausee degradati on driven by visible or UVB radiation alone is uncoupled from PS II redox s tatus, it was therefore assumed that the mixed-light-amplified component of degradation would behave similarly. Surprisingly, amplified degradation pr oved to be coupled tightly to the redox status of PS II. We show that inact ivation of the PS II water oxidation by heat shock or oxidation of the plas tosemiquinone (Q(A)(-)) by silicomolybdate nullifies only the amplified com ponent of degradation but not the basic rates of degradation under PAR or W E alone. The data are interpreted to indicate that formation of plastosemiq uinone or an active water-oxidizing Mn-4 cluster, is the UVB chromophore in volved in amplified degradation of the D1 and D2 proteins. Furthermore, acc umulation of Q(A)(-) by 3-(3,4-dichlorophenyl)-1,1-dimethylurea or 2-bromo- 3-methyl-6-isopropyl-4-nitrophenol stimulated the mixed-light-amplified deg radation component. Thus, amplified degradation of the D1 and D2 proteins i n mixed radiance of PAR plus WE (which simulates naturally occurring radian ce) proceeds by a mechanism clearly distinct from that involved in degradat ion under PAR or UVB alone.