Salt stress-induced volatile organic compounds enhance the resistance of maize to Meloidogyne incognita

As a major soil constraint, salt stress exerts severe negative physiological effects on crops and promotes the occurrence of soil biological constraints, specifically diseases caused by Meloidogyne incognita. However, the mechanism by which maize regulates its immunity under salt stress to defend against M. incognita invasion has rarely been studied. The emission of plant volatile organic compounds (VOCs) is regulated by environmental factors, mediates various interactions between plants and other organisms in ecosystems, and has powerful ecological functions. This study focuses on the roles of five VOCs — (Z)-3-hexenyl acetate (HAC), (Z)-3-hexenal (HAL), (Z)-3-hexen-1-ol (HOL), β-farnesene, and indole — in maize’s defense against the M. incognita under salt stress. Maize plants subjected to mild neutral salt stress (100 mmol∙L⁻¹ NaCl) were first analyzed using Gas Chromatography-Mass Spectrometry (GC-MS) to quantify the emission levels of the target VOCs. Subsequently, a slow-release device was used to expose the maize plants to the individual VOC. The effects of each VOC treatment were systematically evaluated based on key parameters: the chemotaxis of M. incognita towards maize roots, its subsequent infectivity and root-galling ability, and the overall growth of maize plants. GC-MS analysis confirmed that salt stress significantly upregulated the emission of all five VOCs (P<0.05, P<0.001). Functional exposure assays revealed that four VOCs — HAC, HAL, HOL, and β-farnesene — played a direct defensive role. Treatments with HAC (P<0.001), HAL (P<0.05), HOL (P<0.001), and β-farnesene (P<0.05) significantly repelled M. incognita, reducing its chemotaxis towards maize roots. Notably, HAC and HOL exposure resulted in the most profound repellent effects, reducing the number of nematodes around maize roots by 55.67% and 71.33%, respectively, compared to the control. VOCs significantly suppressed nematode infectivity. The inhibition rates of HAC, HAL, HOL, and β-farnesene were 69.84% (P<0.01), 51.85% (P<0.05), 29.72% (P<0.05), and 67.70% (P<0.05), respectively. The capacity of the nematodes to form root galls was markedly reduced. Galling capacity decreased by 69.84% (P<0.001) under HAC, 24.96% (P<0.05) under HAL, 16.33% (P<0.05) under HOL, and 9.52% (P<0.05) under β-farnesene exposure. These defensive benefits were achieved without any adverse effects on maize growth. In contrast, indole exposure did not significantly increase maize resistance across any of the tested parameters. These findings demonstrate that salt stress triggers the emission of specific defence-related VOCs in maize, which in turn enhances resistance against M. incognita. Therefore, the strategic induction of these VOCs represents a promising strategy for mitigating nematode diseases in maize under saline conditions. This study provides novel insights into the ecological functions of stress-induced plant VOCs and expands our understanding of plant immune responses to combined abiotic and biotic stress. The results hold considerable potential for application in “biological early warning” systems and could inform the development of optimized cultivation practices aimed at reducing agricultural chemical inputs.