Is induced systemic resistance the mechanism for control of black rot in Brassica oleracea by a Paenibacillus sp.?

• Paenibacillus isolate P16 reduced the severity of black rot in cabbage.
• Temporal/spatial separation of P16 and Xcc suggested ISR in the disease reduction.
• No alteration was detected in expression of the defense genes in response to Xcc.
• P16 induced a primed status in the seedlings in response to physical injury.

Paenibacillus isolate P16 has been identified as a promising biological control agent for black rot (Xanthomonas campestris pv. campestris or Xcc) in cabbage (Brassica oleracea var. capitata). The mode of action of this isolate, in particular its ability to induce systemic resistance (ISR) in cabbage was investigated. Cabbage seedlings (2 and 4-weeks old) grown from seeds treated with isolate P16 were challenged with Xcc at two different concentrations, 1 × 106, and 1 × 109 CFU/ml. A reduction in disease severity was observed in P16-treated seedlings compared to the non-treated control when Xcc was injected into the leaves of 4-week old seedlings. Isolate P16 was not detected in the leaves of seedlings at the time of Xcc challenge. Based on the temporal and spatial separation for the P16 and Xcc applications, ISR appeared to be the mechanism underlying this disease reduction. The expression of seven defense-related genes (CAS, RubisCO, BIK1, COI1, EIN3, WRKY70, and PRB1) was studied in P16-treated and non-treated 4-week old cabbage seedlings injected with Xcc inoculum at 1 × 106 CFU/ml. Despite evidence that biocontrol by P16 is via ISR, no changes were observed in the expression levels of these genes in P16-treated seedlings that had been challenged with Xcc. A differential response to wounding was detected. This study has provided a starting point for further research at the molecular level to better understand the apparent systemic resistance induced by P16 in cabbage in response to Xcc infection.

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