DAMAGE TO FUNCTIONAL COMPONENTS AND PARTIAL DEGRADATION OF PHOTOSYSTEM-II REACTION CENTER PROTEINS UPON CHLOROPLAST EXPOSURE TO ULTRAVIOLET-B RADIATION

Exposure of thylakoid membranes to ultraviolet-B radiation caused inhibition of semiquinone anion formation at Q(A), inhibition of plastoquinone photoreduction, and lower rates of Photosystem II electron-transport to artificial electron acceptors. The amplitude of pheophytin photoreduction was unaffected by the UV-B treatment, suggesting lack of a UV-B adverse effect on the primary charge separation reaction between the photochemical reaction center P680 and pheophytin. Under the experimental conditions employed, approx. 50% inhibition in Q(A) photoreduction and in the variable to maximal fluorescence ratio (F(v)/F(max)) was observed. However, plastoquinone photoreduction was lowered by about 65% and electron-transport measurements from H2O to dichlorophenol indophenol were inhibited by 70-90% in the UV-B treated thylakoids. Rates of electron-transport through PS II could not be restored upon inclusion of artificial donors such as diphenyl carbazide or hydroxylamine. The results suggest a UV-B-induced damage to the primary quinone acceptor Q(A) and impairment in the function of plastoquinone in the thylakoid membrane. SDS-PAGE and immunoblot analysis of UV-B-exposed thylakoids revealed the appearance of small quantities of polypeptide fragments (13, 11 and 5 kDa) from the Photosystem II reaction-center proteins. We suggest multiple independent targets of UV-B irradiance in Photosystem II and point to plastoquinone, in its many different configurations in the thylakoid membrane, as a primary UV-B photosensitizer molecule.