Differential Roles of Carotenes and Xanthophylls in Photosystem I Photoprotection.
Carotenes and their oxygenated derivatives, xanthophylls, are structural elements of the photosynthetic apparatus and contribute to increasing both the light-harvesting and photoprotective capacity of the photosystems. β-Carotene is present in both the core complexes and light-harvesting system (LHCI) of Photosystem (PS) I, while xanthophylls lutein and violaxanthin bind exclusively to its antenna moiety; another xanthophyll, zeaxanthin, which protects chloroplasts against photooxidative damage, binds to the LHCI complexes under conditions of excess light. We functionally dissected various components of the xanthophyll- and carotene-dependent photoprotection mechanism of PSI by analyzing two Arabidopsis mutants: szl1 plants, with a carotene content lower than that of the wild type, and npq1, with suppressed zeaxanthin formation. When exposed to excess light, the szl1 genotype displayed PSI photoinhibition stronger than that of wild-type plants, while removing zeaxanthin had no such effect. The PSI-LHCI complex purified from szl1 was more photosensitive than the corresponding wild-type and npq1 complexes, as is evident from its faster photobleaching and increased rate of singlet oxygen release, suggesting that β-carotene is crucial in controlling chlorophyll triplet formation. Accordingly, fluorescence-detected magnetic resonance analysis showed an increase in the amplitude of signals assigned to chlorophyll triplets in β-carotene-depleted complexes. When PSI was fractioned into its functional moieties, it was revealed that the boost in the rate of singlet oxygen release caused by β-carotene depletion was greater in LHCI than in the core complex. We conclude that PSI-LHCI complex-bound β-carotene elicits a protective response, consisting of a reduction in the yield of harmful triplet excited states, while accumulation of zeaxanthin plays a minor role in restoring phototolerance.
Dipartimento di Biotecnologie, Università di Verona , Strada Le Grazie 15, 37134 Verona, Italy.
Stefano
S
Cazzaniga
Dipartimento di Biotecnologie, Università di Verona , Strada Le Grazie 15, 37134 Verona, Italy.
Mauro
M
Bressan
Dipartimento di Scienze Chimiche, Università di Padova , via Marzolo 1, 35100 Padova, Italy.
Donatella
D
Carbonera
Dipartimento di Scienze Chimiche, Università di Padova , via Marzolo 1, 35100 Padova, Italy.
Alessandro
A
Agostini
Dipartimento di Biotecnologie, Università di Verona , Strada Le Grazie 15, 37134 Verona, Italy.
Luca
L
Dall'Osto
D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism* / Q000528:radiation effects; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D002338:Carotenoids / Q000378:metabolism*; D002734:Chlorophyll / Q000378:metabolism / Q000528:radiation effects; D002736:Chloroplasts / Q000378:metabolism / Q000528:radiation effects; D005453:Fluorescence; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D010788:Photosynthesis / Q000528:radiation effects; D045331:Photosystem I Protein Complex / Q000235:genetics / Q000378:metabolism*; D026082:Singlet Oxygen; D024341:Xanthophylls / Q000378:metabolism*
D029681:Arabidopsis Proteins; D045342:Light-Harvesting Protein Complexes; D045331:Photosystem I Protein Complex; D024341:Xanthophylls; D002734:Chlorophyll; D026082:Singlet Oxygen; D002338:Carotenoids