Prospects and challenges for practical application of rhizobacteria-mediated induced systemic resistance.

Selected strains of plant growth-promoting rhizobacteria are able to induce a systemic resistance (ISR) in plants, which is phenotypically similar to pathogen-induced systemic acquired resistance (SAR). The generally non-specific character of induced resistance constitutes an increase in the level of basal resistance to several pathogens simultaneously, which is of benefit under natural conditions where multiple pathogens may be present. ISR has been shown to be effective in radish and cucumber under field conditions. However, when induced plants are infected, disease development or severity are reduced but not prevented. Resulting economic losses to farmers make induced resistance alone insufficiently attractive for commercial application in modern intensive agriculture. For practical applications, durable strategies may be devised in which the growth-stimulating properties of resistance-inducing rhizobacteria are combined with other bacterial mechanisms of disease suppression through mixtures of rhizobacterial strains or by combinations with biocontrol fungi, low doses of chemical crop protectants, chemical SAR inducers or partial resistance. In Arabidopsis ISR and SAR are effective against a different, though largely overlapping spectrum of pathogens, depending on the signaling pathways involved in basal resistance. Combination of ISR and SAR can increase protection against pathogens that are resisted through both pathways, as well as extend protection to a broader spectrum of pathogens than ISR or SAR alone.