Identification and role of organic acids in watermelon root exudates for recruiting Paenibacillus polymyxa SQR-21 in the rhizosphere

Paenibacillus polymyxa can serve as a biocontrol agent with a broad host range, but knowledge of the possible contribution of root exudates to its colonization of the rhizosphere remains limited. In this experiment, we identified several organic acids in the root exudates of watermelon. Chemotaxis and swarming assays were performed to investigate the ability of these organic acids to induce the motility of P. polymyxa SQR-21. Oxalic acid, malic acid and citric acid were present in the root exudates but only the intermediate products in tricarboxylic acid cycle, i.e., malic acid and citric acid, could significantly induce motility in P. polymyxa SQR-21. The maximal inducing ability was obtained with malic acid. Values for malic acid were 3.9 and 1.5 times higher than the control in the chemotaxis assay and the swarming assay, respectively. An in vitro experiment further confirmed that these intermediate products in tricarboxylic acid cycle could promote recruitment to P. polymyxa SQR-21, thereby increasing the population in the rhizosphere. In conclusion, some of the organic acids secreted by roots could play an important role in root colonization of SQR-21. This finding contributes to our understanding of the interactions of bacteria and plants under natural conditions.