Salt stress-induced volatiles enhance the resistance of maize to Meloidogyne incognita

As a major soil constraint, salt stress exerts severe negative physiological effects on crops and simultaneously promotes the occurrence of soil biological constraints – specifically, the disease caused by the Meloidogyne incognita. However, how corn regulates its own immunity under salt stress to defend against Meloidogyne incognita invasion is rarely studied. The emission of plant volatile organic compounds (VOCs) is regulated by environmental factors and mediates various interactions between plants and other organisms in the ecosystem, possessing powerful ecological functions. This study focuses on the roles of five volatiles — (Z)-3-hexenyl acetate (HAC), (Z)-3-hexenal (HAL), (Z)-3-hexen-1-ol (HOL), β-farnesene, and Indole — in corn’s defense against the Meloidogyne 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 employed to expose maize plants to these individual volatiles. The effects of each VOC treatment were then 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 the maize plants. The GC-MS analysis confirmed that salt stress significantly upregulated the emission of all five VOCs (P<0.05). Functional exposure assays revealed that four of the volatiles—HAC, HAL, HOL, and β-farnesene — played a direct defensive role. Chemotaxis: Treatments with HAC, HAL, HOL, and β-farnesene significantly repelled M. incognita, reducing its chemotaxis towards maize roots (P<0.05). Notably, HAC and HOL exposures resulted in the most profound repellent effects, reducing the number of nematodes around maize roots by 55.67% and 71.33%, respectively (P<0.001), compared to the control. Infectivity: These same VOCs significantly suppressed nematode infectivity. The inhibition rates for 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. Root Galling: The capacity of nematodes to form root galls was also 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. Plant Growth: Crucially, these defensive benefits were achieved without any adverse effects on maize growth. In contrast, exposure to indole did not confer any significant increase in maize resistance across the tested parameters. The findings demonstrate that salt stress triggers the emission of specific defense-related VOCs in maize, which in turn enhance resistance against the M. incognita. Therefore, the strategic induction of these volatile compounds represents a promising strategy for mitigating nematode disease in maize under saline conditions. This study provides novel insights into the ecological functions of stress-induced plant volatiles, expanding our understanding of plant immune responses under 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.