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Regulation of photosynthetic light use efficiency by the LHCSR1 protein.

Pinnola A., Ballottari M., Dall'Osto L., Cerullo G., Schlau-Cohen G., Bassi R.
  Giovedì 14/09   09:00 - 11:30   Aula A209   V - Biofisica e fisica medica
The productivity of photosynthetic organisms is severely limited by photoinhibition: the oxidative damage caused by the reaction of chlorophyll-excited states with molecular oxygen. Photon harvesting for oxygenic photosynthesis is finely regulated to prevent the formation of these harmful photoproducts by safely dissipating chlorophyll-excited states in excess into heat. Photosynthetic electron transport from water to $CO_{2}$ is coupled to proton transport through the chloroplast's thylakoid membrane and lumen acidification. Protons return to the stroma compartment through the ATPase which catalyzes the ADP+Pi $\rightarrow $ ATP synthesis. Light in excess with respect to the maximal capacity for $CO_2$ fixation depletes ADP and Pi and limits ATPase and proton back transport to chloroplast stroma, leading to lumen over-acidification. This initiates two photoprotective reactions: 1) de-epoxidation of violaxanthin to zeaxanthin by the $p$H-dependent enzyme violaxanthin de-epoxidase (Pinnola et al. 2013, Alboresi et al. 2010). Zeaxanthin, in turn, binds to the LHCSR1 protein, which becomes sensitive to acidification (Ballottari et al. 2016) thus undergoing dramatic changes of its chlorophyll-excited state dynamic leading to massive heat dissipation. With the aim to uncover the photoprotective functional states responsible for dissipation in green algae and mosses, we compared the fluorescence dynamic properties of the light-harvesting complex stress-related (LHCSR1) protein, to the major LHCII antenna protein which mainly fulfills light harvesting function. Both LHCII and LHCSR1 had a chlorophyll fluorescence yield and lifetime strongly dependent on detergent concentration and yet the transition from long- to short-living states was far more complete and rapid in the latter. Also, binding of zeaxanthin and low $pH$ enhanced the relative abundance of short-living states, characterized by the presence of a 80 ps decay component with a red-shifted spectrum. This fast lifetime was the only decay component in conditions with Zea and low $pH$ as it synergically occurs in the chloroplast membrane under excess light conditions. We suggest that energy dissipation occurs in the chloroplast by the activation of 80 ps quenching sites in the thylakoid membranes which spill over excitons from the photosystem II antenna system whose lifetime falls in the 1--2 ns range. The same proteins were analyzed by single molecule lifetime spectroscopy which revealed the existence of two states in the lifetime/fluorescence yield space (Kondo et al. 2017). Zea and acidification enhanced the probability of transition from unquenched to quenched states leading to overall increase of the quenching population and decrease of lifetime in the photosystem-II antenna system. Experimental manipulation of the genes encoding LHCSR proteins in unicellular algae reduced energy dissipation and increased light use efficiency for biomass production (Berteotti et al. 2016), Cazzaniga et al.2014) suggesting that biomass for food and fuels can be significantly enhanced provided extreme environmental conditions are avoided such as in agricultural practice and algal growth in photobioreactors.