PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

Effect of xanthophyll composition on higher plants Photosystem I-light harvesting I complex. Tomas Morosinotto^{*,1, 3}, Matteo Ballottari¹, Jean Alric, Luca Dall'Osto¹, Roberto Bassi^{1, 3}, ¹ Dipartimento scientifico e tecnologico, Università di Verona, Verona, Italy³ Université Aix-Marseille II, LGBP- Faculté des Sciences de Luminy, Marseille, France

ABSTRACT- Xanthophyll composition of the chloroplast changes in response to light intensity with violaxanthin being de-epoxidated to zeaxanthin in excess light conditions. Zeaxanthin has been shown to affect Photosystem II function by both enhancing PsbS dependent NPQ component and decreasing ¹Chl^* lifetime in Lhc proteins. It was recently reported that zeaxanthin accumulates not only in PSII but also in PSI and, although Photosystem II is generally believed to be the major target of photoinhibition, PSI also could be affected in stressing conditions. We analyzed the effect of zeaxanthin on PSI-LHCI complex in order to identify a possible physiological effect. To this aim, we analyzed the formation of P700+ as a function of light intensity in PSI-LHCI preparations isolated from WT Arabidopsis thaliana and its npq2 mutant accumulating zeaxanthin. We could not identify any significant effect from zeaxanthin on the amount of P700+ formed at low light intensities. On the contrary, when light intensity increases and approach saturation, the behavior of PSI from npq2 differed with respect to the WT complex. In fact, the first complex containing zeaxanthin needed more intense light to be saturated than the latter, binding violaxanthin. This experiment, therefore, suggest the presence in Photosystem I of an alternative mechanism of energy dissipation competing with charge separation. This alternative way of de-excitation is enhanced by the presence of zeaxanthin and its effect is significant in conditions when charge separation is inhibited. Since zeaxanthin is only bound to LHCI moiety of PSI, we further analyze its effect on isolated LHCI subunits. A recombinant LHCI component, Lhca4, was reconstituted with either violaxanthin or zeaxanthin. Fluorescence yield of isolated Lhca4 was reduced by 50% in zeaxanthin binding samples, confirming that zeaxanthin is effective in enhancing energy dissipation in LHCI. This difference was not due to a decreased efficiency of energy transfer from carotenoid to Chl a since the efficiency of energy transfer was similar in complexes containing violaxanthin or zeaxanthin. Further analysis have being performed in order to understand the relevance of the effect observed in vivo conditions.

KEY WORDS: Zeaxanthin, Charge separation, Photosystem I, Photoinhibition