Abstract
In the light, photosynthetic cells can potentially suffer from oxidative damage derived from reactive oxygen species. Nevertheless, a variety of oxygenic photoautotrophs, including cyanobacteria, algae, and plants, manage their photosynthetic systems successfully. In the present article, we review previous research on how these photoautotrophs safely utilize light energy for photosynthesis without photo-oxidative damage to photosystem I (PSI). The reaction center chlorophyll of PSI, P700, is kept in an oxidized state in response to excess light, under high light and low CO2 conditions, to tune the light utilization and dissipate the excess photo-excitation energy in PSI. Oxidation of P700 is co-operatively regulated by a number of molecular mechanisms on both the electron donor and acceptor sides of PSI. The strategies to keep P700 oxidized are diverse among a variety of photoautotrophs, which are evolutionarily optimized for their ecological niche.
Highlights
Oxygenic photoautotrophs can safely undergo photosynthesis owing to P700 oxidation system
A recent study has reported an inconsistency between photosystem I (PSI) photoinhibition and P700 oxidation in two different shadeestablished tropical tree species (Huang et al, 2015)
The degrees of PSI photoinhibition are diverse among a variety of oxygenic photoautotrophs, regardless of P700 oxidation levels (Takagi et al, 2017b), which probably reflects the different levels of robustness of PSI against reactive oxygen species (ROS) in each species
Summary
Photosynthetic linear electron flow has been recognized as being limited to the oxidation of reduced PQ (i.e., plastoquinol, PQH2) in Cyt b6/f without any specific regulatory mechanisms at moderate lumen pH values (6.5-7.5) This is based on the understanding that the oxidation of PQH2 is the slowest step in the photosynthetic electron transport system, and that the amount of Cyt b6/f is normally smaller than those of PSII and PSI in plant leaves (Stiehl and Witt, 1969; Anderson, 1992; Schöttler and Tóth, 2014), which is supported by a linear relationship between QA reduction and P700 oxidation (Shimakawa and Miyake, 2018a). FLV mediates the electron transport to O2 in PSI (Helman et al, 2003) and FIGURE 3 | Relationship of P700 oxidation with the alleviation of the photo-oxidative damage in PSI during exposure to constant light Both wild types and flavodiiron protein (FLV)-deficient mutants of the three cyanobacteria species grown under high-[CO2] conditions show the different responses of the photosynthetic electron transport system to the suppression of the Calvin-Benson cycle under CO2 limitation: Synechocystis sp. The effect of RISE on P700 oxidation is possibly observed in C3 plants (Takagi et al, 2016a; Shimakawa and Miyake, 2018a), but further research is required
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