Oomycetes: the case of non-pathogenic Pythium species, modes of action and biological control uses

While Pythium species are largely known as plant pathogens, some species do not cause any damage to crops. This review focuses on these non-pathogenic Pythium species, particularly Pythium oligandrum , which has a wide range of strategies to destroy pathogens and control pathogenic attacks on plants. Comparative genomic studies have revealed differences in secreted protein repertoires and effector family composition between pathogenic and non-pathogenic Pythium species. One of the main direct modes of action of non-pathogenic Pythium spp. is mycoparasitism, which is also the most extensively documented; they directly attack pathogens through various strategies, including hyphal penetration, coiling, lysis, and cytoplasmic coagulation. Host-dependent hydrolytic enzyme production (chitinases, β-glucanases, cellulases, proteases) facilitates pathogen cell wall degradation. Antibiosis represents a secondary direct mechanism, with volatile organic compounds from P. oligandrum effectively inhibiting some pathogenic fungi and oomycetes. Indirect protection mechanisms primarily involve the induction of local and systemic plant resistance through microbe-associated molecular patterns (MAMPs), including oligandrin and cell wall proteins. These elicitors activate jasmonic acid/ethylene-mediated signaling pathways, enhancing plant immunity. Additionally, P. oligandrum promotes plant growth via auxin precursor production and volatile organic compounds. Commercially available formulations demonstrate efficacy against a broad range of pathogens. However, effectiveness depends on environmental conditions, plant cultivars, application timing, and inoculum quantity. P. oligandrum also has a minimal impact on native microbial communities in the rhizosphere. The use of P. oligandrum , as well as other non-pathogenic Pythium species to protect plants has therefore to be considered in modern agriculture.