Root-associated bacteria inducing systemic resistance

We describe the nature of rhizobacterially-induced systemic resistance (ISR) and distinquish ISR from the induced resistance triggered by limited pathogen infection, SAR. The progress made in the elucidation of induction, signaling and expression of ISR is documented for Arabidopsis thaliana, the model plant species and several other plants. The results confirm that, in general, ISR is not dependent on salycilic acid (SA), but indicate that, instead, it has a variable requirement or dependence on jasmonic acid (JA) and ethylene signaling. Evidence emerging from recent gene expression studies have suggested that plants can be in the state of ISR without expressing any changes in gene transcripts of leaves in such induced state. As usual, only upon a challenge inoculation, defense responses get boosted. This phenomenon points to the importance of priming of resistance mechanisms in ISR. Such an enhanced defensive capacity is suggested as the main characteristic of rhizobaterially-induced systemic resistance.