A classic view of the evolution of mutualism is that it derives from a pathogenic relationship that attenuated over time to a situation in which both partners can benefit. If this is the case for rhizobia, then one might uncover features of the symbiosis that reflect this earlier pathogenic state. For example, as with plant pathogens, it is now generally assumed that rhizobia actively suppress the host immune response to allow infection and symbiosis establishment. Likewise, the host has retained mechanisms to control the nutrient supply to the symbionts and the number of nodules so that they do not become too burdensome. The open question is whether such events are strictly ancillary to the central symbiotic nodulation factor signaling pathway or are essential for rhizobial host infection. Subsequent to these early infection events, plant immune responses can also be induced inside nodules and likely play a role in, for example, nodule senescence. Thus, a balanced regulation of innate immunity is likely required throughout rhizobial infection, symbiotic establishment, and maintenance. In this review, we discuss the significance of plant immune responses in the regulation of symbiotic associations with rhizobia, as well as rhizobial evasion of the host immune system.

中文翻译：

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植物先天免疫在豆科植物-根瘤菌共生中的作用

共生主义进化的一个经典观点是，它源于一种病原关系，随着时间的推移，这种关系减弱到双方都可以受益的情况。如果根瘤菌就是这种情况，那么人们可能会发现反映这种早期致病状态的共生特征。例如，与植物病原体一样，现在普遍认为根瘤菌会主动抑制宿主免疫反应，以允许感染和共生。同样，宿主保留了控制共生体营养供应和结节数量的机制，以免它们变得过于繁重。悬而未决的问题是，这些事件是否严格附属于中央共生结瘤因子信号通路，还是对根瘤菌宿主感染必不可少。在这些早期感染事件之后，植物免疫反应也可以在根瘤内诱导，并可能在例如根瘤衰老中发挥作用。因此，在根瘤菌感染、共生建立和维持过程中，可能需要平衡调节先天免疫。在这篇综述中，我们讨论了植物免疫反应在调节与根瘤菌共生关联以及根瘤菌逃避宿主免疫系统中的重要性。