Different carotenoid conformations have distinct functions in light-harvesting regulation in plants

Abstract

To avoid photodamage plants regulate the amount of excitation energy in the membrane at the level of the light-harvesting complexes (LHCs). It has been proposed that the energy absorbed in excess is dissipated via protein conformational changes of individual LHCs. However, the exact quenching mechanism remains unclear. Here we study the mechanism of quenching in LHCs that bind a single carotenoid species and are constitutively in a dissipative conformation. Via femtosecond spectroscopy we resolve a number of carotenoid dark states, demonstrating that the carotenoid is bound to the complex in different conformations. Some of those states act as excitation energy donors for the chlorophylls, whereas others act as quenchers. Via in silico analysis we show that structural changes of carotenoids are expected in the LHC protein domains exposed to the chloroplast lumen, where acidification triggers photoprotection in vivo. We propose that structural changes of LHCs control the conformation of the carotenoids, thus permitting access to different dark states responsible for either light harvesting or photoprotection.

Synthetic biogenesis of chromoplasts from leaf chloroplasts

“…Overaccumulation of phytoene (or a phytoene derivative) might somehow compete with endogenous carotenoids for their binding to photosynthetic protein complexes and membranes, interfere with their functions, and eventually cause the changes that we observed in photosynthetic competence. Consistent with this hypothesis, engineered accumulation of nonchloroplast carotenoids such as astaxanthin in plants alters the properties of thylakoids and grana and interferes with the photosynthetic machinery at several other levels (Roding et al, 2015;Liguori et al, 2017). Transplastomic tobacco plants producing massive levels of asthaxanthin developed leaf plastids that lost their chloroplast features , similar to our results…”

“…In our synthetic system, phase I was very fast (hours) and required a sufficient amount of phytoene to break chloroplast identity in leaves. In chloroplasts, carotenoids such as lutein, beta-carotene, violaxanthin, and neoxanthin are required to maintain the properties of photosynthetic membranes and pigmentprotein complexes responsible for harvesting sunlight and transferring excitation energy to the photosystems (Domonkos et al, 2013;Liguori et al, 2017;Rodriguez-Concepcion et al, 2018). Phytoene is not normally detected in leaf chloroplasts as it is readily converted into downstream (photosynthesis-related) carotenoids.…”

Non‐endogenous ketocarotenoid accumulation in engineeredSynechocystissp. PCC 6803

“…In transgenic Asx-accumulating tree tobacco plants, ketocarotenoids were shown to be present both in the chloroplast envelope as well as in the thylakoid membranes, predominantly as membrane components and only partially associated with photosynthetic complexes (Mortimer et al 2017). Replacement of endogenous carotenoids with Asx has been shown to occur in the main light-harvesting complex of higher plants (Liguori et al 2017) and, similarly, protein-carotenoids binding redistribution occurred in the natural Asx-producer H. pluvialis (Mascia et al 2017). Therefore, considering that in both S6803WZ and S6803ZW strains a large decrease of -car, which is the main carotenoid bound to both photosystem core complexes, was observed, it is reasonable to expect that at least a fraction of non-endogenous ketocarotenoids will be coordinated by photosynthetic complexes.…”

Molecular dynamics simulations in photosynthesis

“…Importantly, structural flexibility can have a functional role in photosynthesis regulation (Ruban et al 2012;Liguori et al 2019) because it correlates with spectroscopic observables in several photosynthetic pigment-binding complexes (Pascal et al 2005;van Oort et al 2007;Liguori et al 2013Liguori et al , 2017Staleva et al 2015;Gwizdala et al 2016;Kondo et al 2017). As above anticipated, single molecule (Krüger et al 2011;Valkunas et al 2012;Schlau-Cohen et al 2015) and Raman spectroscopy (Pascal et al 2005;Ruban et al 2007) have detected conformational changes in the LHCs, but the specific domains involved in the conformational switches remained unidentified for long.…”

“…If proteins and cofactors are also present, as in the LHCs, additional care must be taken to ensure that, during the time needed to relax and equilibrate the solvent and the membrane, the initial high-resolution structure is not perturbed. A good practice to avoid this is to apply strong position constraints (on the order of 1000 kJ mol −1 nm −2 ) on the atomic positions of the protein backbone (or C alpha carbons) and on the pigments' and cofactors' parts whose structure is most critical to the spectral or functional properties of the complex during the first equilibration period (Ogata et al 2013;Liguori et al 2015Liguori et al , 2017. After the surrounding 1 3 system has reached equilibrium, the constraints on the photosynthetic complex can be removed.…”