Interaction between Alternaria alternata and monoterpenoids caused by fungal self-protection

Alternaria alternata is a critical plant pathogen, and the black spot caused by this fungus is the main cause of crop loss. In this study, four monoterpenes commonly found in plant essential oils, β -lonone, citral, γ -terpinene and myrcene, were found to possess a two-way interaction relationship with A. alternata . Four monoterpenes exhibited moderate inhibitory activities against A. alternata with MIC values of 128, 256, 512 and 256 μg/mL, respectively. However, when the concentration of monoterpenes in the culture medium was reduced to 1/4 MIC, A. alternata could facilitate the biotransformation of the aforementioned four monoterpenes by hydroxylation, dihydroxylation or carboxylation. These biotransformations were verified to be promoted by the P450 enzyme. Considering that none of the converted products showed inhibitory activity against A. alternata , these transformations were hypothesized to be self-protective mechanism of A. alternata . Expectedly, the combined use of P450 enzyme inhibitors and monoterpenes exhibited stronger antifungal activities, and could significantly inhibited black spot development in tomatoes. This work provides a reference for the development of effective antifungal agents.