Modulation of Host Immunity by Beneficial Microbes

Abstract

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.

Microbe-Associated Molecular Patterns-Triggered Root Responses Mediate Beneficial Rhizobacterial Recruitment in Arabidopsis

“…These data suggest that flg22-activated root defense in Arabidopsis is independent of JIN1/MYC2 in JA-signaling, but FB17-mediated suppression of flg22-activated root defense is JAR1/JIN1/MYC2 dependent. These findings are consistent with the general convention that the plant growthpromoting rhizobacterial colonization in host plant roots may require the suppression of MAMPs signaling to protect the beneficial bacteria against MAMPselicited defense in the early phase of root colonization (Zamioudis and Pieterse, 2012).…”

“…In beneficial symbiotic interactions, especially legumerhizobia and mycorrhizal fungi, rhizobium and mycorrhizae have evolved mechanisms to efficiently suppress the host immune systems to establish successful infections (for review, see Zamioudis and Pieterse, 2012). Similarly, the beneficial symbiotic microbes such as PGPR, that often grow endophytically inside roots or on root surfaces, may also minimize stimulation of the host's immune system.…”

Pythium oligandrum: an example of opportunistic success

“…Later on, mutualists cope with the host immune response, allowing them to stay alive in the plant tissues. The fact that the invading hyphae of P. oligandrum degenerate concomitantly with the accumulation of plant defence reactions strongly suggests that P. oligandrum is not able to short-circuit plant defence responses as do most other mutualistic agents through the production of effector-like molecules (Plett et al, 2011;Zamioudis & Pieterse, 2012). Undoubtedly, genome sequencing of P. oligandrum will expand our knowledge of the mechanisms involved in the unusual relationships that this oomycete establishes with the plant.…”

Rhizosphere microbiome assemblage is affected by plant development

“…Accordingly, it was hypothesized that seedlings of roots release sugars as substrates for a wide diversity of microbes at early stages of development but as the plant ages it releases specific substrates and potentially antimicrobial compounds in an effort to select for particular microbial inhabitants of the rhizosphere (Badri et al, 2013a;Chaparro et al, 2013). This potential selection of microbes in the rhizosphere as the plant ages might be associated with the ability of beneficial microbes to suppress pathogenic ones (Mendes et al, 2011), trigger induced systemic tolerance to overcome abiotic stress (Selvakumar et al, 2012), increase the plant's innate immunity (Zamioudis and Pieterse, 2012), help in mineral nutrition (Bolan, 1991;van der Heijden et al, 2008) and in overall plant health (Berendsen et al, 2012;Chaparro et al, 2012). Here, we tested this hypothesis by analyzing the rhizosphere microbial composition of Arabidopsis by 454 pyrosequencing at four distinct physiological stages of development: seedling (four-five leaf stage), vegetative (rosette), bolting and flowering.…”