Abstract
Against the potential risk in oxygenic photosynthesis, that is, the generation of reactive oxygen species, photosynthetic electron transport needs to be regulated in response to environmental fluctuations. One of the most important regulations is keeping the reaction center chlorophyll (P700) of photosystem I in its oxidized form in excess light conditions. The oxidation of P700 is supported by dissipating excess electrons safely to O2, and we previously found that the molecular mechanism of the alternative electron sink is changed from flavodiiron proteins (FLV) to photorespiration in the evolutionary history from cyanobacteria to plants. However, the overall picture of the regulation of photosynthetic electron transport is still not clear in bryophytes, the evolutionary intermediates. Here, we investigated the physiological roles of FLV and photorespiration for P700 oxidation in the liverwort Marchantia polymorpha by using the mutants deficient in FLV (flv1) at different O2 partial pressures. The effective quantum yield of photosystem II significantly decreased at 2kPa O2 in flv1, indicating that photorespiration functions as the electron sink. Nevertheless, it was clear from the phenotype of flv1 that FLV was dominant for P700 oxidation in M. polymorpha. These data suggested that photorespiration has yet not replaced FLV in functioning for P700 oxidation in the basal land plant probably because of the lower contribution to lumen acidification, compared with FLV, as reflected in the results of electrochromic shift analysis.
Highlights
To survive natural environmental fluctuations, oxygenic phototrophs have developed a variety of regulatory mechanisms for photosynthetic electron transport
Since Flavodiiron proteins (FLV) can mask the effect of photorespiration on P700 oxidation, here, we used the mutant deficient in FLV that was generated in our previous study in comparison with the wildtype Tak-1 (Shimakawa et al, 2017)
We investigated the roles of FLV and photorespiration in regulating photosynthetic electron transport in the basal land plant M. polymorpha using the mutants deficient in FLV1 and Proton gradient regulation 5 (PGR5)
Summary
To survive natural environmental fluctuations, oxygenic phototrophs have developed a variety of regulatory mechanisms for photosynthetic electron transport. The suppression of electron transport at the cytochrome (Cyt) b6f complex contributes to P700 oxidation The mechanism for this is either through a difference in the proton concentration across thylakoid membrane (ΔpH)dependent mechanism, by the so-called photosynthetic control (Foyer et al, 1990; Kramer et al, 2003), or through the proposed mechanism dependent on reduced plastoquinone (PQ) pool, termed as RISE (Shaku et al, 2016). Rubisco catalyzes the oxygenation reaction of RuBP to generate PGA and 2-phosphoglycolate The latter one is converted to PGA with reduced Fd and ATP in the so-called photorespiration. Flavodiiron proteins (FLV) function as an alternative electron sink at PSI by transferring electrons from reduced Fd to O2 in excess light conditions (Helman et al, 2003; Sétif et al, 2020). Proton gradient regulation 5 (PGR5) and PGR5like 1 (PGRL1) proteins are required for P700 oxidation but their physiological roles are still elusive (Munekage et al, 2002; DalCorso et al, 2008; Mosebach et al, 2017; Rantala et al, 2020)
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