(Annual Review of Plant Biology)
Yangrong Cao, Morgan K. Halane, Walter Gassmann, Gary Stacey
The nitrogen fixing rhizobium-legume symbiosis is considered mutualistic since both the plant host and bacterial symbiont appear to derive benefit: the plant host in the form of fixed nitrogen, which they can utilize for their metabolism, while the rhizobia gain a steady carbon supply and protected environment. Over time, rhizobial populations in the soil also increase in fields planted with legumes. A classic view of the evolution of mutualism is that it derives from an original pathogenic relationship, which attenuates over time to a situation in which both partners can derive benefit. If this is the case for rhizobia, then one might, for example, uncover features of the symbiosis that reflect this earlier pathogenic state. For example, similar to plant pathogens, it is now generally assumed that rhizobia actively suppress the host immune response, allowing infection and symbiotic establishment. Likewise, the host has retained mechanisms to control nutrient supply to the symbionts and to control the number of nodules formed so that they do not become too much of a burden. Recent results are uncovering the mechanistic details of how the plant innate immune system is induced and circumvented during rhizobial infection. For example, there is now clear evidence that rhizobia are initially recognized as pathogens and transiently activate immune responses. It is also apparent that, as is the case for plant pathogens, rhizobia secrete effector proteins that can actively suppress plant immunity. The open question is whether such events are strictly ancillary to the central symbiotic signaling pathway (e.g., that responding to the symbiotic Nod factor signal) or are in some unexplained way central to the ability of the rhizobia to infect their host. Subsequent to these early infection events, plant immune responses can also be induced inside the nodule, and likely play a role, for example, in 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 avoidance of detection by the host immune system.