Exploring Active Peptides with Antimicrobial Activity In Planta against Xylella fastidiosa

SIMPLE SUMMARY: Xylella fastidiosa is one of the most harmful plant bacteria in the world, which could lead to significant economic losses. The lack of direct and efficient therapies to treat plants infected with this bacterium has involved researchers in the search for new approaches and compounds capable of countering this alarming pathogen. Antimicrobial peptides (AMPs) are one of the promising candidates for sustainable and eco-friendly treatment for agricultural applications. Accordingly, nine AMPs against Xf were evaluated, three of which were shown to be promising in limiting Xf growth and its biofilm formation in in vivo and in vitro experiments, and thus, are considered as emerging biocontrol agents against Xf infection. ABSTRACT: Xylella fastidiosa (Xf) is a xylem-limited quarantine plant bacterium and one of the most harmful agricultural pathogens across the world. Despite significant research efforts, neither a direct treatment nor an efficient strategy has yet been developed for combatting Xylella-associated diseases. Antimicrobial peptides (AMPs) have been gaining interest as a promising sustainable tool to control pathogens due to their unique mechanism of action, broad spectrum of activity, and low environmental impact. In this study, we disclose the bioactivity of nine AMPs reported in the literature to be efficient against human and plant pathogen bacteria, i.e., Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa, against Xf, through in vitro and in vivo experiments. Based on viable-quantitative PCR (v-qPCR), fluorescence microscopy (FM), optical density (OD), and transmission electron microscopy (TEM) assays, peptides Ascaphin-8 (GF19), DASamP1 (FF13), and DASamP2 (IL14) demonstrated the highest bactericidal and antibiofilm activities and were more efficient than the peptide PB178 (KL29), reported as one of the most potent AMPs against Xf at present. Furthermore, these AMPs showed low to no toxicity when tested on eukaryotic cells. In in planta tests, no Xf disease symptoms were noticed in Nicotiana tabacum plants treated with the AMPs 40 days post inoculation. This study highlighted the high antagonistic activity of newly tested AMP candidates against Xf, which could lead to the development of promising eco-friendly management of Xf-related diseases.