Abstract
Natural sunlight exceeds the demand of photosynthesis such that it can cause plants to produce reactive oxygen species (ROS), which subsequently cause photo‐oxidative damage. Because photosystem I (PSI) is a major source of ROS, plants actively maintain the reaction center chlorophyll of PSI(P700) oxidized under excessive light conditions to alleviate the ROS production. P700 oxidation is universally recognized in photosynthetic organisms as a physiological response to excessive light. However, it is still poorly understood how P700 oxidation is induced in response to fluctuating light with a variety of frequencies. Here, we investigated the relationships of photosynthetic parameters with P700 oxidation in Arabidopsis thaliana under a sine fluctuating light with different frequencies. As the photon flux density of the light increased, P700 was oxidized concurrently with the chlorophyll fluorescence parameter qL unless the electron acceptor side of PSI was limited. Conversely, we did not observe a proportional relationship of non‐photochemical quenching with P700 oxidation. The mutant crr‐2, which lacks chloroplast NADPH dehydrogenase, was impaired in P700 oxidation during light fluctuation at high, but not low frequency, unlike the pgrl1 mutant deficient in PGR5 and PGRL1 proteins, which could not oxidize P700 during light fluctuation at both high and low frequencies. Taken together, our findings suggested that the changing frequency of fluctuating light reveals the tracking performance of molecular mechanisms underlying P700 oxidation.
Highlights
Plants are vulnerable to photo‐oxidative damage by reactive oxygen species (ROS) that originate from the photosynthetic electron transport system
We sought to investigate the impact of changing frequency of fluctuating light on a variety of photosynthetic parameters in Col‐0, pgrl1, and crr-2 during slow and fast Umibozu and verify the tracking performance of the molecular mechanisms for P700 oxidation in the C3 plant A. thaliana
In Col‐0, P700 oxidation is rapidly induced with the support of donor side mechanisms of photosystem I (PSI) during both slow and fast Umibozu as shown in the strong correlation between qL and Y(ND) except for the moments just after AL was turned on in fast Umibozu, i.e., an early phase of photosynthesis induction (Figure 3a)
Summary
Plants are vulnerable to photo‐oxidative damage by reactive oxygen species (ROS) that originate from the photosynthetic electron transport system. Previous and recent studies have shown that the lack of PGR5 and PGRL1 causes electron acceptor side limitation in PSI and the inability to oxidize P700, resulting in PSI photoinhibition under excessive light conditions in C3 plants Recent studies suggested that the induction of P700 oxidation is impaired in crr mutants of A. thaliana and Oryza sativa, owing to the severe limitation of electron transport on the acceptor side of PSI, under a rectangular fluctuating light (Kono & Terashima, 2016; Yamori et al, 2016). Based on these findings of these studies, we used pgrl and crr-2 and subjected them to acceptor side limitation in PSI in A. thaliana. Crr-2 could oxidize P700 following light fluctuation only when the frequency of the fluctuating light was low, indicating that this mutant lacks the ability for the rapid initiation of P700 oxidation under fluctuating light in A. thaliana
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