Biological Control of a Root-Knot Nematode <i>Meloidogyne incognita</i> Infection of Tomato (<i>Solanum lycopersicum</i> L.) by the Oomycete Biocontrol Agent <i>Pythium oligandrum</i>

The biocontrol agent <i>Pythium oligandrum</i>, which is a member of the phylum Oomycota, can control diseases caused by a taxonomically wide range of plant pathogens, including fungi, bacteria, and oomycetes. However, whether <i>P. oligandrum</i> could control diseases caused by plant root-knot nematodes (RKNs) was unknown. We investigated a recently isolated <i>P. oligandrum</i> strain GAQ1, and the <i>P. oligandrum</i> strain CBS530.74, for the control of an RKN <i>Meloidogyne incognita</i> infection of tomato (<i>Solanum lycopersicum</i> L.). Initially, <i>P. oligandrum</i> culture filtrates were found to be lethal to <i>M. incognita</i> second-stage juveniles (J2s) with up to 84% mortality 24 h after treatment compared to 14% in the control group. Consistent with the lethality to <i>M. incognita</i> J2s, tomato roots treated with <i>P. oligandrum</i> culture filtrates reduced their attraction of nematodes, and the number of nematodes penetrating the roots was reduced by up to 78%. In a greenhouse pot trial, the <i>P. oligandrum</i> GAQ1 inoculation of tomato plants significantly reduced the gall number by 58% in plants infected with <i>M. incognita</i>. Notably, the <i>P. oligandrum</i> GAQ1 mycelial treatment significantly increased tomato plant height (by 36%), weight (by 27%), and root weight (by 48%). A transcriptome analysis of tomato seedling roots inoculated with the <i>P. oligandrum</i> GAQ1 strain identified ~2500 differentially expressed genes. The enriched GO terms and annotations in the up-regulated genes suggested a modulation of the plant hormone-signaling and defense-related pathways in response to <i>P. oligandrum</i>. In conclusion, our results support that <i>P. oligandrum</i> GAQ1 can serve as a potential biocontrol agent for <i>M. incognita</i> control in tomato. Multiple mechanisms appear to contribute to the biocontrol effect, including the direct inhibition of <i>M. incognita</i>, the potential priming of tomato plant defenses, and plant growth promotion.