Electron flow to oxygen in higher plants and algae: rates and control of direct photoreduction (Mehler reaction) and rubisco oxygenase
Electron flow to oxygen in higher plants and algae: rates and control of direct photoreduction (Mehler reaction) and rubisco oxygenase
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DOI：
10.1098/rstb.2000.0704
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发表时间：
2000-10-29
影响因子：
6.3
通讯作者：
Nakano, H
中科院分区：
文献类型：
--
作者：
Bader, MR;von Caemmerer, S;Nakano, H
叶绿体中的线性电子传递产生许多与光系统I （PS I）相关的还原组分，这些组分随后可能参与还原O-2的反应。被广泛研究的两个主要反应是：第一，通过与PS I相关的还原供体将O-2直接还原为超氧化物（Mehler反应），第二，rubisco加氧酶（核酮糖1,5-二磷酸羧化酶EC 4.1.1.39）反应以及光呼吸途径相关的过氧化物酶体和线粒体反应。本文综述了最近和过去对高等植物、藻类和蓝藻的一些研究，这些研究试图量化各种条件下的O-2通量及其对包括光子能量耗散在内的许多作用的贡献。在C-3和天冬氨酸代谢（CAM）植物中，梅勒O-2摄取反应不太可能支持显著的电子传递流（可能少于10%）。此外，如果它存在，它似乎与光合碳氧化循环（PCO）和光合碳还原循环（PCR）活性成比例。在低温、高温和强光条件下对rubisco减少的反义烟草植物的研究，以及对马铃薯、葡萄和油菜在水分胁迫下的研究，都支持了这一点。在这些植物中缺乏显著的Mehler直接证明了在PCR和PCO循环中缺乏ATP消耗的情况下，对Mehler反应有很强的控制。C-3和C-4植物之间的区别主要在于C-4植物依赖光的O-2吸收水平通常要低得多，并且对外部CO2浓度相对不敏感。这种主要差异很容易归因于C-4 CO2浓缩机制的运行。藻类显示出一系列依赖光的O-2吸收速率，类似于C-4植物。与C-4植物一样，即使在缺乏二氧化碳浓缩机制的物种和明显限制无机碳供应的条件下，O-2的吸收似乎在很大程度上对CO2不敏感。对此的部分解释可能是，许多藻类rubsicos具有相当不同的加氧酶动力学特性，在空气中表现出远较低的加氧酶活性。这将导致这样的结论：与C-3植物相比，观察到的O-2摄取的更大比例可能是由于梅勒反应，而较少是由于rubisco。与藻类和高等植物相比，蓝藻似乎具有较高的梅勒O-2吸收能力，这似乎不受ATP消耗的很好耦合或限制。在所有具有发达的非光化学猝灭机制的高等植物和藻类中，非辐射能量耗散可能是在胁迫条件下吸收光复合物吸收的多余光子的主要耗散机制。然而，对于蓝藻来说，缺乏显著的非光化学猝灭，情况可能会有所不同。
Linear electron transport in chloroplasts produces a number of reduced components associated with photosystem I (PS I) that may subsequently participate in reactions that reduce O-2. The two primary reactions that have been extensively studied are: first, the direct reduction of O-2 to superoxide by reduced donors associated with PS I (the Mehler reaction), and second, the rubisco oxygenase (ribulose 1,5-bisphosphate carboxylase oxygenase EC 4.1.1.39) reaction and associated peroxisomal and mitochondrial reactions of the photorespiratory pathway. This paper reviews a number of recent and past studies with higher plants, algae and cyanobacteria that have attempted to quantify O-2 fluxes under various conditions and their contributions to a number of roles, including photon energy dissipation. In C-3 and Crassulacean acid metabolism (CAM) plants, a Mehler O-2 uptake reaction is unlikely to support a significant flow of electron transport (probably less than 10%). In addition, if it were present it would appear to scale with photosynthetic carbon oxidation cycle (PCO) and photosynthetic carbon reduction cycle (PCR) activity. This is supported by studies with antisense tobacco plants with reduced rubisco at low and high temperatures and high light, as well as studies with potatoes, grapes and madrone during water stress. The lack of significant Mehler in these plants directly argues for a strong control of Mehler reaction in the absence of ATP consumption by the PCR and PCO cycles. The difference between C-3 and C-4 plants is primarily that the level of light-dependent O-2 uptake is generally much lower in C-4 plants and is relatively insensitive to the external CO2 concentration. Such a major difference is readily attributed to the operation of the C-4 CO2 concentrating mechanism. Algae show a range of light-dependent O-2 uptake rates, similar to C-4 plants. As in C-4 plants, the O-2 uptake appears to be largely insensitive to CO2, even in species that lack a CO2 concentrating mechanism and under conditions that are clearly limiting with respect to inorganic carbon supply A part explanation for this could be that many algal rubsicos have considerably different oxygenase kinetic properties and exhibit far less oxygenase activity in air. This would lead to the conclusion that perhaps a greater proportion of the observed O-2 uptake may be due to a Mehler reaction and less to rubisco, compared with C-3 plants. In contrast to algae and higher plants, cyanobacteria appear to have a high capacity for Mehler O-2 uptake, which appears to be not well coupled or limited by ATP consumption. It is likely that in all higher plants and algae, which have a well-developed non-photochemical quenching mechanism, non-radiative energy dissipation is the major mechanism for dissipating excess photons absorbed by the light-harvesting complexes under stressful conditions. However, for cyanobacteria, with a lack of significant non-photochemical quenching, the situation may well be different.