Peptaibol analogs as new effective fungicides against Botrytis cinerea

In the last decades the search for new effective and sustainable fungicides has gained much importance in the European political agenda. The well-known biocontrol agent Trichoderma spp. produces short hydrophobic non-ribosomal peptides, named peptaibols, with antibiotic properties given by their ability to permeabilize lipid bilayers such as the cell membrane. Specifically, the peptaibol trichogin produced by T. longibrachiatum, was used as a model to synthesize several water-soluble analogs (De Zotti et al., 2020 International Journal of Molecular Sciences, 21, 7521). These analogs present one or more substitutions of the glycine residues, which improve their water-solubility while maintaining their thermal and chemical stability. The aim of this study was to assess the fungicidal activity of trichogin analogs against Botrytis cinerea. With an in vitro screening, four peptides were identified as effective in inhibiting conidia germination at 15 µM concentration and the most effective peptide displayed a Minimal Inhibitory Concentration of 1-5 µM. A microscopy analysis confirmed conidia cell death at 15 µM. This peptide was used in further experiments to assess its efficacy in controlling B. cinerea infection on different plant tissues. On bean leaves, this peptide determined a significant reduction of disease symptoms (higher than 95%) at 50 µM, being effective also at 15 µM (75% symptom reduction). Treatment of grapevine leaves and berries at 50 µM showed a significant reduction of disease symptoms of about 70% and 45%, respectively. Several analogs of this peptide, differing in sequence length or C-terminus to decrease synthesis costs, have also been produced and tested. Both changes did not affect peptide efficacy either in vitro and against infection on bean leaves. These results show the potential given by small structural modifications of natural secondary metabolites. Ongoing trials are now focusing on the combination of peptides and other natural antimicrobial compounds, with the aim to identify any synergistic effect against B. cinerea. Synergy would allow to reduce the fungicide dosage with many benefits, including the reduction of treatment cost, a better toxicity profile and a minimized probability of resistance development.