PARENT SESSION
Posters P1B Photo-oxidative stress, photoinhibition. Abstracts (394-443)

Photosynthesis without Lutein: a molecular analysis of the photosynthetic apparatus in the -cyclase mutant of Arabidopsis thaliana. Luca Dall'Osto^*,1, Michel Havaux², Chiara Lico³, Giovanni Giuliano³, Roberto Bassi⁴, ¹ Dipartimento scientifico e tecnologico, Verona, Italy² CEA/Cadarache, DSV, DEVM, Saint-Paul-lez-Durance, France³ ENEA, Casaccia Res Ctr, Rome, Italy⁴

ABSTRACT- Lutein is the most abundant xanthophyll in the photosynthetic apparatus of higher plants, where it binds to thylakoid membrane proteins belonging to the Lhc family involved in light harvesting and photoprotection. Previous analysis of pigment binding to Lhc proteins, using recombinant apoproteins refolded in vitro with purified pigments, showed that lutein binds to site L1 of all Lhc proteins. The L2 occupancy by Lutein is indispensable for protein folding and the quenching of harmful triplet states of chlorophyll. Moreover, lutein substitution by violaxanthin decreased the efficiency of LHCII protoprotection. In order to verify the relevance of in vitro results for the biogenesis of chlorophyll proteins in vivo, we have analysed the composition and organisation of chlorophyll/xanthophyll binding proteins of a lutein-less mutant of A. thaliana and found that the biochemical composition and the spectroscopic properties of Lhc proteins isolated from thylakoid membranes of the lut2 mutant closely features those produced by refolding in vitro. The lut2 plants grow well in light limiting conditions while they are less effective than WT plants in performing light induced energy dissipation (NPQ). Photobleaching rate in isolated LHCII from lut2 plants is enhanced with respect to LHCII from WT plants thus suggesting that substitution of lutein by violaxanthin does not fully restore the photoprotection function in Lhc proteins. In addition, we show that an independent effect of L vs V substitution is the disruption of the major LHCII complex trimeric organisation thus yielding monomeric LHCII proteins. Interestingly, the lut2 mutant exhibited lower tolerance to stressing environmental condition, and lipid peroxidation levels higher than WT plants. However, plants unable to synthesize both lutein and zeaxantin (npq1 x lut2 double mutants) show no NPQ and a strong sensitivity to light stress than lut2 plants. These data are discussed with respect to the role of lutein and zeaxanthin in photoprotection and on the interactions of these pigments with Lhc subunits involved in NPQ. It appears that both zeaxanthin and lutein are involved in photoprotection through different independent mechanisms.

KEY WORDS: photoprotection, lutein, Arabidopsis thaliana