Legume nodules have two types of hemoglobins: symbiotic or leghemoglobins (Lbs) and nonsymbiotic or phytoglobins (Glbs). The latter are categorized into three phylogenetic classes differing in heme coordination and O₂ affinity. This review is focused on the roles of Lbs and Glbs in the symbiosis of rhizobia with crop legumes and the model legumes for indeterminate (Medicago truncatula) and determinate (Lotus japonicus) nodulation. Only two hemoglobin functions are well established in nodules: Lbs deliver O₂ to the bacteroids and act as O₂ buffers, preventing nitrogenase inactivation; and Glb1‐1 modulates nitric oxide concentration during symbiosis, from the early stage, avoiding the plant’s defense response, to nodule senescence. Here, we critically examine early and recent results, update and correct the information on Lbs and Glbs with the latest genome versions, provide novel expression data and identify targets for future research. Crucial unresolved questions include the expression of multiple Lbs in nodules, their presence in the nuclei and in uninfected nodule cells, and, intriguingly, their expression in nonsymbiotic tissues. RNA‐sequencing data analysis shows that Lbs are expressed as early as a few hours after inoculation and that their mRNAs are also detectable in roots and pods, which clearly suggests that these heme proteins play additional roles unrelated to nitrogen fixation. Likewise, issues awaiting investigation are the functions of other Glbs in nodules, the spatiotemporal expression profiles of Lbs and Glbs at the mRNA and protein levels, and the molecular mechanisms underlying their regulation during nodule development and in response to stress and hormones.

中文翻译：

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豆科植物根瘤菌共生中的血红蛋白。

豆类结核有两种血红蛋白：共生或豆球蛋白（Lbs）和非共生或植物血红蛋白（Glbs）。后者分为血红素配位和O ₂亲和力不同的三个系统发育类别。这篇综述着重于Lbs和Glbs在根瘤菌与农作物豆科植物的共生中的作用，以及不确定的（Medicago truncatula）和确定的（Lotus japonicus）结瘤的模型豆类。结节中只有两种血红蛋白功能可以很好地建立：Lb将O ₂传递至类细菌并充当O ₂缓冲液，防止固氮酶失活；Glb1-1调节共生过程中的一氧化氮浓度，从早期开始，避免植物的防御反应，直至结节衰老。在这里，我们严格审查早期和近期的结果，使用最新的基因组版本更新和更正有关Lbs和Glbs的信息，提供新颖的表达数据并确定未来研究的目标。尚未解决的关键问题包括多个Lb在结节中的表达，它们在细胞核中以及在未感染的结节细胞中的存在，以及有趣的是，它们在非共生组织中的表达。RNA测序数据分析表明，Lbs最早在接种后几小时就表达出来，并且在根和豆荚中也可以检测到它们的mRNA，这清楚地表明这些血红素蛋白还发挥了与固氮无关的其他作用。同样，等待调查的问题是结节中其他Glbs的功能，Lbs和Glbs在mRNA和蛋白质水平上的时空表达谱，以及它们在结节形成过程中以及对压力和激素的响应中调控的分子机制。