| Summary: | <i>Pythium oligandrum</i>, strain M1, is a soil oomycete successfully used as a biological control agent (BCA), protecting plants against fungal, yeast, and oomycete pathogens through mycoparasitism and elicitor-dependent plant priming. The not yet described <i>Pythium</i> strains, X42 and 00X48, have shown potential as BCAs given the high activity of their secreted proteases, endoglycosidases, and tryptamine. Here, <i>Solanum lycopersicum</i> L. cv. Micro-Tom seeds were coated with <i>Pythium</i> strains, and seedlings were exposed to fungal pathogens, either <i>Alternaria brassicicola</i> or <i>Verticillium albo-atrum</i>. The effects of both infection and seed-coating on plant metabolism were assessed by determining the activity and isoforms of antioxidant enzymes and endoglycosidases and the content of tryptamine, amino acids, and heat shock proteins. Dual culture competition testing and microscopy analysis confirmed mycoparasitism in all three <i>Pythium</i> strains. In turn, seed treatment significantly increased the total free amino acid content, changing their abundance in both non-infected and infected plants. In response to pathogens, plant Hsp70 and Hsp90 isoform levels also varied among <i>Pythium</i> strains, most likely as a strategy for priming the plant against infection. Overall, our results show in vitro mycoparasitism between <i>Pythium</i> strains and fungal pathogens and <i>in planta</i> involvement of heat shock proteins in priming.
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