In land plants, photosystem I (PSI) photoinhibition limits carbon fixation and causes growth defects. In addition, recovery from PSI photoinhibition takes much longer than PSII photoinhibition when the PSI core-complex is degraded by oxidative damage. Accordingly, PSI photoinhibition should be avoided in land plants, and land plants should have evolved mechanisms to prevent PSI photoinhibition. However, such protection mechanisms have not yet been identified, and it remains unclear whether all land plants suffer from PSI photoinhibition in the same way. In the present study, we focused on the susceptibility of PSI to photoinhibition and investigated whether mechanisms of preventing PSI photoinhibition varied among land plant species. To assess the susceptibility of PSI to photoinhibition, we used repetitive short-pulse (rSP) illumination, which specifically induces PSI photoinhibition. Subsequently, we found that land plants possess a wide variety of tolerance mechanisms against PSI photoinhibition. In particular, gymnosperms, ferns and mosses/liverworts exhibited higher tolerance to rSP illumination-induced PSI photoinhibition than angiosperms, and detailed analyses indicated that the tolerance of these groups could be partly attributed to flavodiiron proteins, which protected PSI from photoinhibition by oxidizing the PSI reaction center chlorophyll (P700) as an electron acceptor. Furthermore, we demonstrate, for the first time, that gymnosperms, ferns and mosses/liverworts possess a protection mechanism against photoinhibition of PSI that differs from that of angiosperms.

This study aims to elucidate the molecular mechanism for the transient increase in the O -uptake rate in tobacco (Nicotiana tabacum cv Xanthi) leaves after turning off actinic lights (ALs). The photosynthetic O evolution rate reaches a maximum shortly after the onset of illumination with ALs and then decreases to zero in atmospheric CO /O conditions. After turning off the ALs, tobacco leaves show a transient increase in the O -uptake rate, the post-illumination transient O -uptake, and thereafter, the O -uptake rate decreases to the level of the dark-respiration rate. Photosynthetic linear electron flow, evaluated as the quantum yield of photosystem II [Y(II)], maintained a steady-state value distinct from the photosynthetic O -evolution rate. In high-[CO ] conditions, the photosynthetic O -evolution rate and Y(II) showed a parallel behavior, and the post-illumination transient O -uptake was suppressed. On the other hand, in maize leaves (a C4 plant), even in atmospheric CO /O conditions, Y(II) paralleled the photosynthetic O -evolution rate and the post-illumination transient O -uptake was suppressed. Hypothesizing that the post-illumination transient O -uptake is driven by C3 plant photorespiration in tobacco leaves, we calculated both the ribulose 1,5-bisphosphate carboxylase- and oxygenase-rates (Vc and Vo) from photosynthetic O -evolution and the post-illumination transient O -uptake rates. These values corresponded to those estimated from simultaneous chlorophyll fluorescence/O -exchange analysis. Furthermore, the H -consumption rate for ATP synthesis in both photosynthesis and photorespiration, calculated from both Vc and Vo that were estimated from chlorophyll fluorescence/CO -exchange analysis, showed a positive linear relationship with the dissipation rate of the electrochromic shift signal. Thus, these findings support our hypothesis.

In higher plants, the electron-sink capacity of photorespiration contributes to alleviation of photoinhibition by dissipating excess energy under conditions when photosynthesis is limited. We addressed the question at which point in the evolution of photosynthetic organisms photorespiration began to function as electron sink and replaced the flavodiiron proteins which catalyze the reduction of O at photosystem I in cyanobacteria. Algae do not have a higher activity of photorespiration when CO assimilation is limited, and it can therefore not act as an electron sink. Using land plants (liverworts, ferns, gymnosperms, and angiosperms) we compared photorespiration activity and estimated the electron flux driven by photorespiration to evaluate its electron-sink capacity at CO -compensation point. In vivo photorespiration activity was estimated by the simultaneous measurement of O -exchange rate and chlorophyll fluorescence yield. All C3-plants leaves showed transient O -uptake after actinic light illumination (post-illumination transient O -uptake), which reflects photorespiration activity. Post-illumination transient O -uptake rates increased in the order from liverworts to angiosperms through ferns and gymnosperms. Furthermore, photorespiration-dependent electron flux in photosynthetic linear electron flow was estimated from post-illumination transient O -uptake rate and compared with the electron flux in photosynthetic linear electron flow in order to evaluate the electron-sink capacity of photorespiration. The electron-sink capacity at the CO -compensation point also increased in the above order. In gymnosperms photorespiration was determined to be the main electron-sink. C3-C4 intermediate species of Flaveria plants showed photorespiration activity, which intermediate between that of C3- and C4-flaveria species. These results indicate that in the first land plants, liverworts, photorespiration started to function as electron sink. According to our hypothesis, the dramatic increase in partial pressure of O in the atmosphere about 0.4 billion years ago made it possible to drive photorespiration with higher activity in liverworts.

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.