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Induced systemic resistance (ISR) can be activated by biotic agents, including root-associated beneficial bacteria to inhibit pathogen infection. We investigated priming-mediated ISR in cucumber induced by Pseudomonas azotoformans GC-B19 and Paenibacillus elgii MM-B22 against Colletotrichum orbiculare (causal fungus of anthracnose). In addition, we examined whether this ISR expression was bacterial density-dependent by assessing peroxidase activity in the presence and absence of the pathogen. As a result, root treatment with the ISR-eliciting strains GC-B19 and MM-B22 or the chemical inducer DL-β-amino-n-butyric acid (positive control) significantly inhibited fungal infection process (conidial germination and appressorium formation) and disease severity compared with the non-ISR-eliciting strain, Pseudomonas aeruginosa PK-B09 (negative control), and MgSO4 solution (untreated control). These treatments effectively induced rapid elicitation of hypersensitive reaction-like cell death with H2O2 generations, and accumulation of defense-related enzymes (β-1,3-glucanase, chitinase, and peroxidase) in cucumber leaves in the "primed" state against C. orbiculare. In addition, ISR expression was dependent on the bacterial cell density in the rhizosphere. This ISR expression was derived from the presence of sustained bacterial populations ranging from 10(4) to 10(6) cells/g of potting mix over a period of time after introduction of bacteria (10(6) to 10(10) cells/g of potting mix) into the rhizosphere. Taken together, these results suggest that priming-mediated ISR against C. orbiculare in cucumber can be induced in a bacterial density-dependent manner by Pseudomonas azotoformans GC-B19 and Paenibacillus elgii MM-B22.

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