RESUMEN
Biochemical and physiological acclimation to different light environments is crucial for plant growth and survival. In high light (HL), feedback de-excitation (qE) is a well-known photoprotective mechanism that dissipates excess excitation energy in the light-harvesting antenna of photosystem II (PSII) and relieves excitation pressure in the photosynthetic electron transport chain. The xanthophylls zeaxanthin (Z) and lutein (L) function in qE, but also have roles as antioxidants. Although several studies have shown that qE is important during short-term fluctuations in light intensity, here we show that it is not required for the growth of Arabidopsis thaliana in prolonged HL conditions in the laboratory. Mutants that are deficient in qE alone, qE and Z synthesis, or in qE, Z synthesis and also L synthesis were able to grow at 1800 micromol photons m(-2) s(-1) and exhibited no major symptoms of photooxidative stress. The mutants (and wild type) acclimated to HL by increasing photosynthetic capacity and decreasing light harvesting, which together rendered qE less important for photoprotection. At a metabolite level, the HL-grown mutants appeared to compensate for their remaining qE deficit with increased alpha-tocopherol and ascorbate levels compared to the wild type. The specificity of this response provides insight into the relationship between qE and the antioxidant network in plants.
Asunto(s)
Adaptación Fisiológica , Antioxidantes/metabolismo , Arabidopsis/fisiología , Luz , Mutación , Fotosíntesis , Arabidopsis/genética , Arabidopsis/metabolismo , Peroxidación de Lípido , Espectrometría de FluorescenciaRESUMEN
The biochemical, biophysical, and physiological properties of the PsbS protein were studied in relation to mutations of two symmetry-related, lumen-exposed glutamate residues, Glu-122 and Glu-226. These two glutamates are targets for protonation during lumen acidification in excess light. Mutation of PsbS did not affect xanthophyll cycle pigment conversion or pool size. Plants containing PsbS mutations of both glutamates did not have any rapidly inducible nonphotochemical quenching (qE) and had similar chlorophyll fluorescence lifetime components as npq4-1, a psbS deletion mutant. The double mutant also lacked a characteristic leaf absorbance change at 535 nm (DeltaA535), and PsbS from these plants did not bind dicyclohexylcarbodiimide (DCCD), a known inhibitor of qE. Mutation of only one of the glutamates had intermediate effects on qE, chlorophyll fluorescence lifetime component amplitudes, DCCD binding, and DeltaA535. Little if any differences were observed comparing the two single mutants, suggesting that the glutamates are chemically and functionally equivalent. Based on these results a bifacial model for the functional interaction of PsbS with photosystem II is proposed. Furthermore, based on the extent of qE inhibition in the mutants, photochemical and nonphotochemical quenching processes of photosystem II were associated with distinct chlorophyll fluorescence life-time distribution components.
Asunto(s)
Proteínas de Arabidopsis , Proteínas del Complejo del Centro de Reacción Fotosintética/metabolismo , Complejo de Proteína del Fotosistema II/metabolismo , Proteínas de Plantas , Secuencia de Aminoácidos , Arabidopsis/metabolismo , Clorofila/metabolismo , Concentración de Iones de Hidrógeno , Luz , Complejos de Proteína Captadores de Luz , Modelos Moleculares , Datos de Secuencia Molecular , Mutación , Proteínas del Complejo del Centro de Reacción Fotosintética/genética , Complejo de Proteína del Fotosistema II/genéticaRESUMEN
Acclimation to changing environments, such as increases in light intensity, is necessary, especially for the survival of sedentary organisms like plants. To learn more about the importance of ascorbate in the acclimation of plants to high light (HL), vtc2, an ascorbate-deficient mutant of Arabidopsis, and the double mutants vtc2npq4 and vtc2npq1 were tested for growth in low light and HL and compared with the wild type. The vtc2 mutant has only 10% to 30% of wild-type levels of ascorbate, vtc2npq4 has lower ascorbate levels and lacks non-photochemical quenching of chlorophyll fluorescence (NPQ) because of the absence of the photosystem II protein PsbS, and vtc2npq1 is NPQ deficient and also lacks zeaxanthin in HL but has PsbS. All three genotypes were able to grow in HL and had wild-type levels of Lhcb1, cytochrome f, PsaF, and 2-cysteine peroxiredoxin. However, the mutants had lower electron transport and oxygen evolution rates and lower quantum efficiency of PSII compared with the wild type, implying that they experienced chronic photooxidative stress. The mutants lacking NPQ in addition to ascorbate were only slightly more affected than vtc2. All three mutants had higher glutathione levels than the wild type in HL, suggesting a possible compensation for the lower ascorbate content. These results demonstrate the importance of ascorbate for the long-term acclimation of plants to HL.