Shimakawa Ginga, Hanawa Hitomi, Wada Shinya, Hanke Guy T, Matsuda Yusuke, Miyake Chikahiro
Graduate School of Agricultural Science, Kobe University, Kobe, Japan.
Research Center for Solar Energy Chemistry, Osaka University, Suita, Japan.
Front Plant Sci. 2021 Aug 19;12:668805. doi: 10.3389/fpls.2021.668805. eCollection 2021.
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 O, 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 by using the mutants deficient in FLV () at different O partial pressures. The effective quantum yield of photosystem II significantly decreased at 2kPa O in , indicating that photorespiration functions as the electron sink. Nevertheless, it was clear from the phenotype of that FLV was dominant for P700 oxidation in . 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.
针对有氧光合作用中的潜在风险，即活性氧的产生，光合电子传递需要根据环境波动进行调节。最重要的调节之一是在光照过强的条件下，使光系统I的反应中心叶绿素（P700）保持氧化状态。P700的氧化通过将多余的电子安全地耗散到O₂来支持，我们之前发现，在从蓝细菌到植物的进化过程中，替代电子汇的分子机制从黄素二铁蛋白（FLV）转变为光呼吸。然而，在进化中间产物苔藓植物中，光合电子传递调节的整体情况仍不清楚。在这里，我们通过使用在不同O₂分压下缺乏FLV（）的突变体，研究了地钱中FLV和光呼吸对P700氧化的生理作用。在2kPa O₂条件下，光系统II的有效量子产率在中显著降低，表明光呼吸起到了电子汇的作用。然而，从的表型可以清楚地看出，FLV在地钱中对P700氧化起主导作用。这些数据表明，在地钱这种基部陆地植物中，光呼吸在P700氧化功能方面尚未取代FLV，这可能是因为与FLV相比，其对类囊体腔酸化的贡献较低，这在电致变色 shift 分析结果中有所体现。
