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Priming-Mediated Systemic Resistance in Cucumber Induced by Pseudomonas azotoformans GC-B19 and Paenibacillus elgii MM-B22 Against Colletotrichum orbiculare

First author: Laboratory of Plant Disease and Biocontrol, Division of Biotechnology, Institute of Life Science and Natural Resources, Korea University, Seoul 136-713, Republic of Korea; second, third, fourth, and sixth authors: Laboratory of Plant Disease and Biocontrol, Division of Biotechnology, Korea University, Seoul 136-713, Republic of Korea; and fifth author: Faculty of Bioscience and Industry, College of Applied Life Science, Research Institute for Subtropical Agriculture and Biotechnology, Cheju National University, Jeju 690-756, Republic of Korea.

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 10⁴ to 10⁶ cells/g of potting mix over a period of time after introduction of bacteria (10⁶ to 10¹⁰ 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.

Additional keywords: hydrogen peroxide, plant activator, reactive oxygen species.