Modulation of host immunity by beneficial microbes

In nature, plants abundantly form beneficial associations
with soilborne microbes that are important for plant survival and, as such, affect plant biodiversity and ecosystem
functioning. Classical examples of symbiotic microbes are
mycorrhizal fungi that aid in the uptake of water and minerals, and Rhizobium bacteria that fix atmospheric nitrogen for the plant. Several other types of beneficial soilborne
microbes, such as plant-growth-promoting rhizobacteria
and fungi with biological control activity, can stimulate plant
growth by directly suppressing deleterious soilborne pathogens or by priming aboveground plant parts for enhanced
defense against foliar pathogens or insect herbivores. The
establishment of beneficial associations requires mutual recognition and substantial coordination of plant and microbial
responses. A growing body of evidence suggests that beneficial microbes are initially recognized as potential invaders,
after which an immune response is triggered, whereas, at
later stages of the interaction, mutualists are able to shortcircuit plant defense responses to enable successful colonization of host roots. Here, we review our current understanding of how symbiotic and nonsymbiotic beneficial soil
microbes modulate the plant immune system and discuss
the role of local and systemic defense responses in establishing the delicate balance between the two partners.