Resistance induced in plants by non-pathogenic microorganisms: elicitation and defense responses

Plants have evolved a number of inducible defense mechanisms against pathogen attacks. Recognition of certain non-pathogenic rhizobacteria can trigger a systemic resistance reaction that renders the host less susceptible to subsequent infection by a virulent agent. Since this induced systemic resistance (ISR) is long-lasting and not conducive for development of pathogen resistance, disease control strategies based on this phenomenon are promising both for greenhouse cultures and under field conditions. The list of beneficial rhizobacteria reported to induce ISR is growing rapidly. Data compiled here also show that ISR may occur in various dicotyledonous and monocotyledonous plants and can be effective against a wide range of pathogens. This review emphasizes the molecular aspects of this three-step process involving sequentially i) the perception by plant cells of elicitors produced by the inducing agents that initiates the phenomenon, ii) signal transduction that is needed to propagate the induced state systemically through the plant and iii) expression of defense mechanisms sensu stricto that limit or inhibit pathogen penetration into the host tissues. The current state of knowledge about rhizobacteria-stimulated ISR is discussed in parallel with the more wellcharacterized systemic acquired resistance induced by incompatible pathogens.