Volatile organic compounds (VOCs) of Bacillus velezensis TA-1 exhibit toxic effects against Meloidogyne incognita

The use of Bacillus volatiles to manage root-knot nematodes (RKNs) is a topic that gained extensive attention because it is safe and environmentally benign. Bacillus velezensis TA-1, isolated from a continuously cropped soil, exhibited strong nematicidal activity against RKNs in vitro and in field conditions. However, it remains poorly understood whether TA-1 volatile organic compounds (VOCs) could be effective against Meloidogyne incognita. In this study, the results of chemotaxis activity test revealed that B. velezensis TA-1 exhibited strong repellent effects on second-stage juveniles (J2s) of M. incognita. In in vitro assays, TA-1 VOCs in a three-compartment Petri dish assay exhibited a mortality rate of 84.1% at 48 h and 92.8% at 72 h to M. incognita J2s. Further evidence revealed that TA-1 VOCs significantly inhibited the gene expression of mpk-1, flp-18 and ord-1 by 61.0%, 44.8% and 54.5%, respectively. Moreover, TA-1 VOCs increased the content of reactive oxygen species (ROS), resulting in severe oxidative stress and death of nematodes. Results from a double-layered greenhouse experiment indicated that the populations of J2s in the soil and in cucumber roots were notably reduced by TA-1 VOCs compared to the untreated control. Furthermore, eleven VOCs were identified from TA-1 by solid-phase microextraction gas chromatography-mass spectrometry (SPME-GC/MS) analysis, of which benzaldehyde, 2-decanol, and 2-undecanone had strong contact nematicidal activities against M. incognita J2s. In addition, benzaldehyde and 2-dodecanone exhibited fumigation effects on M. incognita J2s. Overall, these results demonstrated that TA-1 VOCs exhibited direct contact nematicidal and fumigation activities against RKNs and could be considered as potential biocontrol agents for the control of RKNs.