Synthetic peptide combinatorial libraries: an approach to identifying antimicrobials for the control of plant pathogens

Abstract

Peptides are not only key regulators of physiological processes, but also have the potential of acting as antimicrobials to control the growth of phytopathogenic fungi. A synthetic peptide combinatorial hexapeptide library consisting of 400 different peptide mixtures was synthesized. The library was tested for biological activity against Fusarium oxysporum f.sp. lycopersici and Rhizoctonia solani (AG-1). Initial screening of the 400 peptide mixture library identified biologically active peptide mixtures whose first two amino acid residues were designated. Additional libraries involved in the iterative steps were screened against the above mentioned fungi , as well as, Ceratocystis fagacearum and Pythium ultimum. A fully-defined hexapeptide (66-10)(FRLKFH) and a fully-defined pentapeptide (PPD1)(FRLHF) were developed from the iterative steps. The defined peptides were compared to commercial antifungals for activity against the selected fungi. Hexapeptide, 66-10, and pentapeptide, PPD1, exhibited activity that was somewhat comparable to selected commercial fungicides. Mutagenicity testing using the Ames Salmonella plate incorporation assay, with and without metabolic activation, was performed with 66-10 and PPD1. The peptides did not demonstrate mutagenic or carcinogenic properties. Studies of the mode of action of PPD1 against C. fagacearum, the test isolate, suggested that the potential target of PPD1 may be the cell membrane. The ability of PPD1 to control oak wilt, in vivo, caused by C. fagacearum, was determined. A significant reduction in percent crown loss was observed for trees injected with 30 []/mi PPD1 as compared to the uninjected control trees.