Most land plants benefit from endosymbiotic interactions with mycorrhizal fungi, including legumes and some non-legumes that also interact with endosymbiotic nitrogen-fixing bacteria to form nodules. In addition to these helpful interactions, plants are continuously exposed to would-be pathogenic microbes: discriminating between friends and foes is a major determinant of plant survival. Recent breakthroughs have revealed how some key signals from pathogens and symbionts are distinguished. Once this checkpoint has been passed and a compatible symbiont is recognized, the plant coordinates the sequential development of two types of specialized structures in the host. The first serve to mediate infection, and the second, which appear later, serve as sophisticated intracellular nutrient exchange interfaces. The overlap in both the signaling pathways and downstream infection components of these symbioses reflects their evolutionary relatedness and the common requirements of these two interactions. However, the different outputs of the symbioses, phosphate uptake vs. nitrogen fixation, require fundamentally different components and physical environments and necessitated the recruitment of different master regulators, NLPs and PHRs, for nodulation and mycorrhization, respectively.
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