Mechanisms of PGPR-induced resistance against pathogens

Plant growth-promoting rhizobacteria can suppress diseases through antagonism between the bacteria and soilborne pathogens, as well as by inducing a systemic resistance in the plant against both root and foliar pathogens. Specific Pseudomonas strains induce systemic resistance in carnation, cucumber, radish, tobacco and Arabidopsis, as evidenced by an enhanced defensive capacity upon challenge inoculation. In carnation, radish and Arabidopsis, the O-antigeni c side chain of the bacterial outer membrane lipopolysaccharide acts as an inducing determinant, but other bacterial traits are also involved. Siderophores have been implicated in the induction of resistance in tobacco and Arabidopsis, and a novel type of siderophore, fluorebactin, may explain induction of resistance associated with salicylic acid (SA) in radish. Although SA induces phenotypically similar systemic acquired resistance, it is not necessary for the systemic resistance induced by most rhizobacterial strains, because this induced resistance is not associated with the accumulation of pathogenesis-related proteins and is fully expressed in Arabidopsis transformants unable to accumulate SA. Although some bacterial strains are equally effective in inducing resistance in different plant species, others show specificity, indicating specific recognition between bacteria and plants at the root surface