CPI-SMEL: A novel antibacterial and nematicidal peptide from Solanum melongena characterized as a carboxypeptidase inhibitor

Solanaceae plants are rich sources of antimicrobial peptides (AMPs), offering considerable potential for the discovery of novel molecules to address sustainable plant pathogen control strategies. Among them, Solanum melongena, commonly known as eggplant, has emerged as a promising candidate, with peptide extracts exhibiting potent antimicrobial properties. This study aims to identify and characterize a potent new AMP isolated from aqueous extracts of Solanum melongena 'Florida Market' leaves. The peptide isolation process commenced with the extraction of eggplant leaf material, followed by selective heating. Subsequently, the extract underwent ammonium sulfate precipitation and ultrafiltration to concentrate the peptide fraction. Chromatographic purification was performed using a combination of ion exchange, size exclusion, and reversed-phase HPLC techniques to achieve high purity level. The obtained fractions were analyzed using SDS-Tricine-PAGE. Peptide identification was accomplished using LC-MS/MS and automated Edman sequencing. Additionally, comprehensive bioinformatics analyses were conducted, including conservation, phylogeny, three-dimensional structure modeling, and molecular dynamics simulations, to provide insights into the peptide structure and function. The purified peptide, named CPI-SMEL, with 4.156 Da, was characterized as a carboxypeptidase inhibitor (CPI), demonstrating significant antimicrobial activity and structural stability. Evaluation against the plant pathogenic bacteria Ralstonia solanacearum and Clavibacter michiganensis subsp. michiganensis, as well as the root-knot nematode Meloidogyne incognita, revealed strong antimicrobial efficacy of two eggplant peptide extracts Pool1 and PC1. The characterization of CPI-SMEL underscores the potential of CPIs in the development of safe and sustainable plant defenses, highlighting eggplant and other Solanaceae species as promising sources of AMPs for effective plant pathogen management.