| Summary: | To elucidate defense mechanisms of <i>Piper nigrum</i> against fusariosis, an experiment based on co-inoculation of arbuscular mycorrhizal fungi (AMF) and <i>Fusarium solani</i> f. sp. <i>piperis</i> was performed. Variations in secondary metabolism in plants infected with <i>F. solani</i> f. sp. <i>piperis</i> (FUS) and co-inoculated with AMFs and <i>F. solani</i> (AMF + FUS) were monitored at 7- and 21-days post inoculations (dpi). The pathogen induced a decrease in oxygenated sesquiterpenes (82.0–77.4%), and changes in the concentrations of the main compounds, α-muurolene, α-muurolol, and 2<i>E</i>-hexenal in the leaves. It was observed that the concentration of 2<i>E</i>-hexenal decreased at 7 dpi, α-muurolene decreased at 21 dpi, and α-muurolol increased at 21 dpi. There was a prevalence of sesquiterpene and monoterpene hydrocarbons in the roots, such as β-caryophyllene, δ-elemene, and limonene. The infection and co-inoculation induced greater production of phenolics in the roots at 7 dpi. The enzymatic activity of phenylalanine ammonia-lyase decreased in the leaves at 21 dpi and in the roots on both days, while the lipoxygenase activity decreased only in the roots at 21 dpi. The results demonstrated that co-inoculation with AMFs and <i>F. solani</i> induces changes in the defense metabolism of <i>P. nigrum</i>, but it is not efficient in the biocontrol of fusariosis during the evaluated period.
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