Abstract

The tolerance of photosynthesis to strong light increases in photosynthetic organisms during acclimation to strong light. We investigated the role of carotenoids in the protection of photosystem II (PSII) from photoinhibition after acclimation to strong light in the cyanobacterium Synechocystis sp. PCC 6803. In cells that had been grown under strong light at 1,000 μmol photons m−2 s−1 (SL), specific carotenoids, namely, zeaxanthin, echinenone, and myxoxanthophyll, accumulated at high levels, and the photoinhibition of PSII was less marked than in cells that had been grown under standard growth light at 70 μmol photons m−2 s−1 (GL). The rate of photodamage to PSII, as monitored in the presence of lincomycin, did not differ between cells grown under SL and GL, suggesting that the mitigation of photoinhibition after acclimation to SL might be attributable to the enhanced ability to repair PSII. When cells grown under GL were transferred to SL, the mitigation of photoinhibition of PSII occurred in two distinct stages: a first stage that lasted 4 h and the second stage that occurred after 8 h. During the second stage, the accumulation of specific carotenoids was detected, together with enhanced synthesis de novo of proteins that are required for the repair of PSII, such as the D1 protein, and suppression of the production of singlet oxygen (1O2). In the ΔcrtRΔcrtO mutant of Synechocystis, which lacks zeaxanthin, echinenone, and myxoxanthophyll, the mitigation of photoinhibition of PSII, the enhancement of protein synthesis, and the suppression of production of 1O2 were significantly impaired during the second stage of acclimation. Thus, elevated levels of the specific carotenoids during acclimation to strong light appeared to protect protein synthesis from 1O2, with the resultant mitigation of photoinhibition of PSII.

Highlights

  • Light is necessary for photosynthesis but excess light impairs photosynthesis

  • The second stage of mitigation was associated with the accumulation of zeaxanthin, echinenone, and myxoxanthophyll, which contributed to the enhanced repair of Photosystem II (PSII) via suppression of the production of 1O2 and acceleration of the synthesis de novo of proteins that are required for the repair of PSII, such as the D1 protein

  • We grew cells of the wild-type strain of Synechocystis and the derivative DcrtRDcrtO strain, which is deficient in zeaxanthin, echinenone, and myxoxanthophyll (Kusama et al, 2015), under standard growth light (GL; 70 μmol photons m−2 s−1), moderately strong light (ML; 200 μmol photons m−2 s−1), and strong light (SL; 1,000 μmol photons m−2 s−1)

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Summary

Introduction

Light is necessary for photosynthesis but excess light impairs photosynthesis. Photosystem II (PSII), which is a proteinpigment complex that converts light energy to chemical energy, is known to be sensitive to strong light. Photodamage to PSII depends on the intensity of incident light (Allakhverdiev and Murata, 2004), whereas the repair of PSII is adversely affected by various types of environmental stress and, in particular, by oxidative stress due to reactive oxygen species (ROS), such as the superoxide anion radical, hydrogen peroxide, the hydroxyl radical, and singlet oxygen (1O2: Nishiyama et al, 2001; Nishiyama et al, 2004; Allakhverdiev and Murata, 2004) These ROS are produced in abundance in the photosynthetic machinery under strong light, as a result of the transport of electrons and the transfer of excitation energy (Asada, 1999).

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