Bioagent-Induced Root Volatile Compounds Mediate Defense Responses against Root-Knot Nematode and Fusarium Wilt in Sunflower

Root-knot nematode (Meloidogyne incognita) and Fusarium wilt caused by Fusarium oxysporum frequently occur as a disease complex that severely constrains sunflower productivity. This study investigated the role of Bacillus subtilis RB62 in modulating root-derived volatile organic compounds (VOCs) and enhancing sunflower defense against the above pathogens. The treatment with B. subtilis RB62 significantly improved plant growth and reduced pathogen pressure under combined infection. The treatment reduced the nematode females, egg masses, and soil juvenile populations by 67.2%, 38.3%, and 74.4%, respectively, and the wilt incidence from 81.0% to 15.0% (81.5% reduction). The GC–MS analysis identified 30 root-derived VOCs, with clear treatment-specific separation revealed by principal component analysis. Pathway enrichment analysis indicated significant modulation of fatty acid metabolism and terpenoid biosynthesis pathways associated with plant defense. The sesquiterpenoid 13hydroxy-12-methoxy-8, 11, 13-podocarpatriene-3-one was enriched in RB62-treated plants and exhibited strong predicted binding affinities toward -tubulin of M. incognita (–10.4 kcal/mol) and chitin synthase of F. oxysporum (–14.0 kcal/mol) in molecular docking analyses. These findings suggest that RB62-mediated alteration of root VOC profiles may contribute to defense-related metabolic responses in sunflower.