Enhanced Photoprotection by Protein-Bound vs Free Xanthophyll Pools: A Comparative Analysis of Chlorophyll b and Xanthophyll Biosynthesis Mutants

Abstract

When light absorbed by plants exceeds the capacity of photosynthesis, the xanthophyll violaxanthin is reversibly de-epoxidized to zeaxanthin in the so-called xanthophyll cycle. Zeaxanthin plays a key role in the protection of photosynthetic organisms against excess light, by promoting rapidly reversible (qE) and long-term (qI) quenching of excited chlorophylls, and preventing lipid oxidation. The photoprotective role of zeaxanthin, either free or bound to light-harvesting complexes (Lhcs), has been investigated by using mutants lacking Chl b (ch1) and/or specific xanthophyll species (npq, lut2). The ch1 mutation causes (1) the absence of Lhcb proteins; (2) strong reduction of the feedback de-excitation (qE); and (3) accumulation of xanthophylls as free pigments into thylakoids. Ch1 mutants showed extreme sensitivity to photo-oxidative stress in high light, due to higher singlet oxygen (¹O₂) release. The double mutant ch1npq1 was more sensitive to photo-oxidation than ch1, showing that zeaxanthin does protect lipids even when free in the membrane. Nevertheless, lack of zeaxanthin had a much stronger impact on the level of lipid peroxidation in Lhcs-containing plants (WT vs npq1) with respect to Lhc-less plants (ch1 vs ch1npq1), implying that its protective effect is enhanced by interaction with antenna proteins. It is proposed that the antioxidant capacity of zeaxanthin is empowered in the presence of PSII-LHCs-Zea complexes, while its effect on enhancement of qE only provides a minor contribution. Comparison of the sensitivity of WT vs npq1 plants to exogenous ¹O₂ suggests that besides the scavenging of ¹O₂, at least one additional mechanism is involved in chloroplast photoprotection.

On the origin of a slowly reversible fluorescence decay component in the Arabidopsis npq4 mutant

“…Similar results were obtained upon removal of both Lhcb4 and Lhcb6 ( figure 3d and table 2), thus ruling out the possibility that minor antennae modulate qM amplitude and kinetics. These data are consistent with the behaviour of npq4ch1 that lacks Chlb, and is thus devoid of all LHCs [37,38]; the slow phase of NPQ relaxation was found to be independent of LHC composition, indeed qM amplitude was similar in npq4 and npq4ch1 mutants ( figure 3e and table 2). State transitions lead to quenching of LHCII fluorescence by PSI [39] upon phosphorylation of LHCII, by STN7 kinase, driving its migration from PSII to PSI [40]; by using the mutant npq4stn7, we checked the possibility that state transition were involved in qM.…”

Zeaxanthin Binds to Light-Harvesting Complex Stress-Related Protein to Enhance Nonphotochemical Quenching in Physcomitrella patens

“…The same level of deepoxidation upon exposure to excess light and zeaxanthin binding to pigment-proteins, as assessed by Suc gradient fractionation and HPLC analysis, was found in wildtype and lhcsr psbs KO plants, as shown by a deepoxidation index of 0.76 in both cases. We observed that the exchange of violaxanthin versus zeaxanthin is rapid and more complete on LHC binding sites with respect to previous reports on Arabidopsis, which has a deepoxidation index of 0.50 (Dall'Osto et al, 2010). When comparing the zeaxanthin distribution pattern from wild-type excess light-treated samples, we observed an enrichment in fractions 23 to 27 of the Suc gradient ( Figure 6B).…”

“…Previous work on C. reinhardtii (Baroli et al, 2003) and Arabidopsis (Havaux and Niyogi, 1999) showed an enhanced ROS scavenging activity of zeaxanthin with respect to preexisting violaxanthin both in the lipid phase (Havaux et al, 2007) and bound to pigment-proteins (Dall'Osto et al, 2010). Recent work has shown a photoprotective ROS-scavenging activity of zeaxanthin under salt and osmotic stress resistance in P. patens (Azzabi et al, 2012).…”

“…This reaction is catalyzed by violaxanthin deepoxidase (VDE) and together with zeaxanthin reepoxidation by zeaxanthin epoxidase (ZEP) upon return to light-limiting conditions of photosynthesis constitutes the xanthophyll cycle. Previous work focused on the modulation of 3 Chl* yield in leaves (Dall'Osto et al, 2012) and ROS scavenging in green algae (Baroli et al, 2003(Baroli et al, , 2004 and in higher plants (Havaux et al, 2007;Dall'Osto et al, 2010). npq1, the Arabidopsis thaliana mutant unable to synthesize zeaxanthin, shows NPQ reduction (Niyogi et al, 1998;Li et al, 2002), implying that zeaxanthin synthesis enhances the amplitude of both qE and qZ components.…”

Carotenoids in nature: insights from plants and beyond

“…Here they absorb light across a broader range of the spectral region in which the sun irradiates maximally and transfer the energy to chlorophyll, initiating the photochemical events of photosynthesis (Polívka and Frank 2010). In plants, carotenoids are bound in discrete pigment-protein complexes referred to as the LHCII trimeric complex, which typically benefits from lutein, neoxanthin, as well as violaxanthin to transfer energy to chlorophyll (Pogson et al 2005;DemmigAdams and Adams 2006;Dall'Osto et al 2010). A curious marine dinoflagellate, Amphidinium carterae alternatively evolved a peridinin-chlorophyll protein complex that contains more of the bound carotenoid (peridinin) when compared with chlorophyll (Hofmann et al 1996;Polívka and Frank 2010) (Fig.…”