Volatile organic compounds from the mycoparasite Pythium oligandrum contribute to microbial antagonism and have wide-ranging adverse effects on host survival

The oomycete P. oligandrum is a soil-inhabiting parasite and predator of both fungi and oomycetes and uses hydrolytic enzymes extensively to penetrate and hydrolyze its host or prey. Other mechanisms have been overlooked, and we investigated whether P. oligandrum-produced volatile organic compounds (VOCs) could also be contributing to antagonism. VOCs have diverse functions, including contributing to antagonism in ecological interactions and potential applications in biocontrol. The growth-inhibiting activity of P. oligandrum VOCs was tested on P. myriotylum – a host or prey of P. oligandrum – coupled with electron microscopy, biochemical assays and transcriptomic analysis. The total VOCs produced by P. oligandrum reduced P. myriotylum growth by 80% and zoospore levels by 60%. GC-MS identified twenty-three VOCs, and methyl heptenone, D-limonene, 2-undecanone and 1-octanal were potent inhibitors of P. myriotylum growth and led to increased production of reactive oxygen species at a concentration that did not inhibit P. oligandrum growth. Exposure to the total VOCs of P. oligandrum led to shrinkage of P. myriotylum hyphae, and lysis of the cellular membranes and organelles. Transcriptomic analysis of P. myriotylum exposed to the P. oligandrum VOCs at increasing levels of growth inhibition showed initially a strong upregulation of putative detoxification related genes that was not maintained later during exposure to the VOCs. The inhibition of P. myriotylum growth continued after the exposure to the VOCs was discontinued and led to reduced leaf lesion size during P. myriotylum infection of its plant host. The VOCs produced by P. oligandrum could be another factor alongside hydrolytic enzymes contributing to its ecological role as a microbial antagonist where the concentration of the VOCs may be inhibitory in particular ecological niches such as in soil. The VOCs analyzed here may also be contributing to the biocontrol of diseases using P. oligandrum commercial preparations. Compare gene expression profiles of Pythium myriotylum with and without exposure to volatile compounds produced by Pythium oligandrum at three time-points with triplicate biological replicate samples.