A comparative analysis of Photosynthetic Light use efficiency Regulation Mechanisms from unicellular algae to higher plants through mosses

Roberto Bassia, Alberta Pinnolab, Stefano Cazzanigab, Matteo Ballottarib, Luca Dall’Ostob
a,bDepartment of Biotechnology, University of Verona. Strada Le Grazie 15, 37134 Verona, Italy

All oxygenic photosynthetic organisms possess mechanisms for fine regulation of light harvesting function. This regulation determines fraction of chlorophyll excited states which are channeled to reaction centers for fueling electron transport vs heat dissipation. All these mechanisms are dependent on carotenoid composition and yet the mechanism by which the heat dissipation is catalyzed is still under debate. Here, we report on two proteins involved in LUE (light use efficiency) regulation, namely LHCSR and PSBS, their chromophore requirement and functional properties. These gene products initiate the signal transduction pathway(s) which detects the absorption of excess energy by the photosynthetic apparatus that cannot be used by downstream metabolic reactions and therefore becomes available for ROS production and photoinhibition. In both PSBS and LHCSR-mediated processes, the initial stimulus is lumen over-acidification. Low pH is detected through protonation of glutamate and aspartate residues exposed to the lumen which are the target of the inhibitor DCCD [1, 2].

Further steps down the signal transduction pathways diverge in algae vs plants: the pH-dependent conformational change in LHCSR activates quenching reactions within the protein itself owing to their capacity of binding chlorophyll a and xanthophylls [5, 6], including zeaxanthin, a power activator of Non Photochemical Quenching [4]. Quenched LHCSR further interacts with component of the antenna system of both PSI and PSII [5, 6] thus reducing their functional antenna size and extending heat dissipation to additional antenna domains. The case of PSBS is different, in that the protein itself does not bind pigments and therefore cannot catalyze quenching reactions [7, 8]. Its interaction with monomeric components of the antenna system, chiefly CP29, induces conformational changes which activates quenching reactions [9, 10]. Additional interactions with LHCII are currently analyzed in order to verify their functional role. We suggest that the modes of regulation evolved with the light environment available in water vs terrestrial environments and with the spectroscopic properties of light harvesting systems.

Acknowledgements

This research was financed by the EEC T&M networks ACCLIPHOT (2012-2016) and S2B (2016-2020).

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