The oomycete Pythium 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 studied less, and we investigated the contribution of P. oligandrum-produced volatile organic compounds (VOCs) to parasitism. The growth-inhibiting activity of P. oligandrum VOCs was tested on Pythium myriotylum-a host or prey of P. oligandrum-coupled with electron microscopy, and biochemical and transcriptomic analyses. The P. oligandrum-produced VOCs reduced P. myriotylum growth by 80% and zoospore levels by 60%. Gas chromatography-mass spectrometry (GC-MS) identified 23 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 P. oligandrum VOCs led to shrinkage of P. myriotylum hyphae and lysis of the cellular membranes and organelles. Transcriptomics of P. myriotylum exposed to the P. oligandrum VOCs at increasing levels of growth inhibition initially showed a strong upregulation of putative detoxification-related genes that was not maintained later. The inhibition of P. myriotylum growth continued immediately after the exposure to the VOCs was discontinued and led to the reduced infection of its plant hosts. The VOCs produced by P. oligandrum could be another factor alongside hydrolytic enzymes contributing to its ecological role as a microbial parasite in particular ecological niches such as in soil, and may also contribute to the biocontrol of diseases using P. oligandrum commercial preparations. IMPORTANCE Microbe-microbe interactions in nature are multifaceted, with multiple mechanisms of action, and are crucial to how plants interact with microbes. Volatile organic compounds (VOCs) have diverse functions, including contributing to parasitism in ecological interactions and potential applications in biocontrol. The microbial parasite P. oligandrum is well known for using hydrolytic enzymes as part of its parasitism. We found that P. oligandrum VOCs reduced the growth of, and caused major damage to, the hyphae of P. myriotylum (a host or prey of P. oligandrum). Transcriptomic analyses of P. myriotylum exposed to the VOCs revealed the upregulation of genes potentially involved in an attempt to detoxify the VOCs. The inhibitory effects of the VOCs had a knock-on effect by reducing the virulence of P. myriotylum toward its plant hosts. The P. oligandrum VOCs could contribute to its ecological role as a microbial parasite. The VOCs analyzed here may also contribute to the biocontrol of diseases using P. oligandrum commercial preparations.
Keywords: GC-MS; P. myriotylum; P. oligandrum; RNAseq; microbe-microbe interaction; volatile organic compounds.