Analysis of PSII antenna size heterogeneity of Chlamydomonas reinhardtii during state transitions - Colloque annuel de la Société Française de Photosynthèse

PSII antenna size heterogeneity has been extensively studied in the past. Based on in vivo DCMU fluorescence rise kinetics, at least two types of photosystems were described. They differ by their apparent antenna size and connectivity (this last term refers to the transfer of absorbed energy from a closed PSII unit to an open neighboring unit). In this study, we analysed PSII heterogeneity in Chlamydomonas reinhardtii using non-linear linear regression fitting on in vivo DCMU fluorescence rise kinetics, with a focus on changes in PSII heterogeneity associated with state transitions. We found that PSIIα possesses a high degree of connectivity and an antenna about 3 times larger than PSIIβ, as described previously. In contrast with most earlier studies, we found some connectivity for PSIIβ (although it was highly variable). This is in agreement with recent models based on biochemical and structural analysis of PSII after gel filtration separation which describe PSII mega-, super- and core- complexes in Chlamydomonas. According to these studies, the smallest unit of PSII in vivo would be a dimer of two core complexes hence still allowing connectivity. We also showed that strain and medium dependent variations in the half-time of the fluorescence rise, generally taken as an indicator of the average cross-section of PSII, can be explained by variations in the proportions of PSIIα and PSIIβ. When analyzing the state transition process, we showed for the first time in vivo that it induces an inter-conversion of PSIIα and PSIIβ. These findings are discussed with respect to the latest insights on the remodeling of the pigment-protein PSII architecture during this process.