title: The mechanism of UV-A radiation-induced inhibition of photosystem II electron transport studied by EPR and chlorophyll fluorescence.
abstract: The UV-A (320-400 nm) component of sunlight is a significant damaging factor of plant photosynthesis, which targets the photosystem II complex. Here we performed a detailed characterization of UV-A-induced damage in photosystem II membrane particles using EPR spectroscopy and chlorophyll fluorescence measurements. UV-A irradiation results in the rapid inhibition of oxygen evolution accompanied by the loss of the multiline EPR signal from the S(2) state of the water-oxidizing complex. Gradual decrease of EPR signals arising from the Q(A)(-)Fe(2+) acceptor complex, Tyr-D degrees, and the ferricyanide-induced oxidation of the non-heme Fe(2+) to Fe(3+) is also observed, but at a significantly slower rate than the inhibition of oxygen evolution and of the multiline signal. The amplitude of Signal II(fast), arising from Tyr-Z degrees in the absence of fast electron donation from the Mn cluster, was gradually increased during the course of UV-A treatment. However, the amount of functional Tyr-Z decreased to a similar extent as Tyr-D as shown by the loss of amplitude of Signal II(fast) that could be measured in the UV-A-treated particles after Tris washing. UV-A irradiation also affects the relaxation of flash-induced variable chlorophyll fluorescence. The amplitudes of the fast (600 micros) and slow (2 s) decaying components, assigned to reoxidation of Q(A)(-) by Q(B) and by recombination of (Q(A)Q(B))(-) with donor side components, respectively, decrease in favor of the 15-20 ms component, which reflects PQ binding to the Q(B) site. In the presence of DCMU, the fluorescence relaxation is dominated by a 1 s component due to recombination of Q(A)(-) with the S(2) state. After UV-A radiation, this is partially replaced by a much faster component (30-70 ms) arising from recombination of Q(A)(-) with a stabilized intermediate PSII donor, most likely Tyr-Z degrees. It is concluded that the primary damage site of UV-A irradiation is the catalytic manganese cluster of the water-oxidizing complex, where electron transfer to Tyr-Z degrees and P(680)(+) becomes inhibited. Modification and/or inactivation of the redox-active tyrosines and the Q(A)Fe(2+) acceptor complex are subsequent events. This damaging mechanism is very similar to that induced by the shorter wavelength UV-B (280-320) radiation, but different from that induced by the longer wavelength photosynthetically active light (400-700 nm).
authors.affiliation: Institute of Plant Biology, Biological Research Center, Szeged, Hungary, Department of Chemistry, University of Iraklion, Crete, Greece. imre@everx.szbk.u-szeged.hu
authors.forename: Imre
authors.initials: I
authors.lastname: Vass
authors.forename: Enikö
authors.initials: E
authors.lastname: Turcsányi
authors.forename: Elefterios
authors.initials: E
authors.lastname: Touloupakis
authors.forename: Demetrios
authors.initials: D
authors.lastname: Ghanotakis
authors.forename: Vasili
authors.initials: V
authors.lastname: Petrouleas
mesh_terms: D002734:Chlorophyll / Q000378:metabolism / Q000528:radiation effects*; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport / Q000528:radiation effects; D005296:Ferrous Compounds / Q000378:metabolism / Q000528:radiation effects; D005470:Fluorometry; D045342:Light-Harvesting Protein Complexes; D010782:Photolysis / Q000528:radiation effects; D045322:Photosynthetic Reaction Center Complex Proteins / Q000037:antagonists & inhibitors* / Q000378:metabolism / Q000528:radiation effects*; D045332:Photosystem II Protein Complex; D010971:Plastoquinone / Q000378:metabolism / Q000528:radiation effects; D018724:Spinacia oleracea; D014443:Tyrosine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000528:radiation effects; D014466:Ultraviolet Rays; D014867:Water
chemical_list: D005296:Ferrous Compounds; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; C066447:tyrosyl radical Y(D); C114979:tyrosyl radical YZ; D014867:Water; D002734:Chlorophyll; D014443:Tyrosine; D010971:Plastoquinone