Paenibacillus polymyxa SQR-21 systemically affects root exudates of watermelon to decrease the conidial germination of Fusarium oxysporum f.sp. niveum

Paenibacillus polymyxa SQR-21 has been identified as a potential agent for the biocontrol of Fusarium wilt in watermelon, which is caused by the pathogenic fungus Fusarium oxysporum f.sp. niveum (FON). In the present study, the effects of root exudates from watermelon plants inoculated or non-inoculated with either SQR-21 or FON on conidial germination of FON were investigated. Compared to the control, conidial germination was decreased with root exudates from SQR-21-inoculated plants, but conidial germination was enhanced by root exudates from FON-inoculated plants. Maximal germination was found with root exudates from FON-inoculated plants after 30 d, which was 1.35 times more germination than the control. A split-root system was designed to verify that the alterations of the exudation pattern in SQR-21- inoculated or FON-inoculated watermelon roots were not only local, but also systemic. Cinnamic acid was found in the watermelon root exudates. An assay to test the effects of cinnamic acid on conidial germination of FON revealed that the stimulation of conidial germination was observed from cinnamic acid concentrations ranging from 0 to 30 μg/ml. In conclusion, both of SQR-21 and FON systemically affects watermelon root exudates. These results will help to the better understanding of the plant-microbe communication and will guide to improve the biocontrol strategies against Fusarium wilt of watermelon plants.