*Estación Experimental del Zaidín, CSIC, PO Box 419, 18080 Granada, Spain.
Biochem Soc Trans. 2011 Jan;39(1):184-8. doi: 10.1042/BST0390184.
NO (nitric oxide) is a signal molecule involved in diverse physiological processes in cells which can become very toxic under certain conditions determined by its rate of production and diffusion. Several studies have clearly shown the production of NO in early stages of rhizobia-legume symbiosis and in mature nodules. In functioning nodules, it has been demonstrated that NO, which has been reported as a potent inhibitor of nitrogenase activity, can bind Lb (leghaemoglobin) to form LbNOs (nitrosyl-leghaemoglobin complexes). These observations have led to the question of how nodules overcome the toxicity of NO. On the bacterial side, one candidate for NO detoxification in nodules is the respiratory Nor (NO reductase) that catalyses the reduction of NO to nitrous oxide. In addition, rhizobial fHbs (flavohaemoglobins) and single-domain Hbs which dioxygenate NO to form nitrate are candidates to detoxify NO under free-living and symbiotic conditions. On the plant side, sHbs (symbiotic Hbs) (Lb) and nsHbs (non-symbiotic Hbs) have been proposed to play important roles as modulators of NO levels in the rhizobia-legume symbiosis. In the present review, current knowledge of NO detoxification by legume-associated endosymbiotic bacteria is summarized.
一氧化氮（NO）是一种参与细胞内多种生理过程的信号分子，但在其产生和扩散速度等特定条件下，它可能变得极具毒性。多项研究明确表明，在根瘤菌-豆科植物共生的早期阶段和成熟的根瘤中会产生 NO。在正常功能的根瘤中，据报道，作为氮酶活性的强抑制剂的 NO 可以与 Lb（豆血红蛋白）结合形成 LbNOs（亚硝酰基-豆血红蛋白复合物）。这些观察结果引发了一个问题，即根瘤如何克服 NO 的毒性。在细菌方面，根瘤中用于 NO 解毒的一个候选物是呼吸 Nor（NO 还原酶），它可催化将 NO 还原为一氧化二氮。此外，根瘤菌 fHbs（黄素血红蛋白）和单结构域 Hbs 可将 NO 氧化二氮形成硝酸盐，这两种蛋白在自由生活和共生条件下都可能用于 NO 解毒。在植物方面，sHbs（共生 Hbs）（Lb）和 nsHbs（非共生 Hbs）被认为在根瘤菌-豆科植物共生中作为 NO 水平的调节剂发挥着重要作用。在本综述中，总结了与豆科植物相关的内共生细菌进行 NO 解毒的现有知识。
