Abstract
Photosynthetic organisms protect themselves from high-light stress by dissipating excess absorbed energy as heat in a process called non-photochemical quenching (NPQ). Zeaxanthin is essential for the full development of NPQ, but its role remains debated. The main discussion revolves around two points: where does zeaxanthin bind and does it quench? To answer these questions we have followed the zeaxanthin-dependent quenching from leaves to individual complexes, including supercomplexes. We show that small amounts of zeaxanthin are associated with the complexes, but in contrast to what is generally believed, zeaxanthin binding per se does not cause conformational changes in the complexes and does not induce quenching, not even at low pH. We show that in NPQ conditions zeaxanthin does not exchange for violaxanthin in the internal binding sites of the antennas but is located at the periphery of the complexes. These results together with the observation that the zeaxanthin-dependent quenching is active in isolated membranes, but not in functional supercomplexes, suggests that zeaxanthin is acting in between the complexes, helping to create/participating in a variety of quenching sites. This can explain why none of the antennas appears to be essential for NPQ and the multiple quenching mechanisms that have been observed in plants.
Highlights
The capture and storage of light energy by photosynthetic organisms is the process that sustains virtually all life on earth, but it is a risky business
The largest PSII supercomplex purified from A. thaliana contains two LHCII trimers per core, which are indicated as S (Strongly bound) and M (Moderately bound) forming the C2S2M2 complex that has a molecular weight of 1400 KDa and coordinates around 300 Chls[4]
The complexes were isolated from plants treated with 30 min of high light, which induces the maximum of Zea synthesis[32] and from plants grown in the same conditions without high light treatment, which do not contain Zea
Summary
The capture and storage of light energy by photosynthetic organisms is the process that sustains virtually all life on earth, but it is a risky business. PSI and PSII are multi-protein membrane complexes that coordinate chlorophyll (Chl) and carotenoid molecules. Both complexes are composed of (i) a core, responsible for charge separation and electron transport, and (ii) an outer antenna, which increases the absorption cross section of the core[3]. The outer antenna is composed of members of the light-harvesting complex (Lhc) multigenic family, which together with the cores form supercomplexes. The largest PSII supercomplex purified from A. thaliana contains two LHCII trimers per core, which are indicated as S (Strongly bound) and M (Moderately bound) forming the C2S2M2 complex that has a molecular weight of 1400 KDa and coordinates around 300 Chls[4]. The xanthophylls in the V1 site are loosely bound and they do not participate in excitation energy transfer and triplet quenching[9,10]
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