Identification and characterization of a hexapeptide with activity againstphytopathogenic fungi that cause postharvest decay in fruits

A hexapeptide of amino acid sequence Ac-Arg-Lys-Thr-Trp-Phe-Trp-NH2 was dem onstrated to have antimicrobial activity against selected phytopathogenic f ungi that cause postharvest decay in fruits. The peptide synthesized with e ither all D- or all L-amino acids inhibited the in vitro growth of strains of Penicillium italicum, P. digitatum, and Botrytis cinerea, with MICs of 6 0 to 80 mu M and 50% inhibitory concentration (IC50) Of 30 to 40 mu M. The inhibitory activity of the peptide was both sequence- and fungus-specific s ince (i) sequence-related peptides lacked activity (including one with five residues identical to the active sequence), (ii) other filamentous fungi ( including some that belong to the genus Penicillium) were insensitive to th e peptide's antifungal action, and (iii) the peptide did not inhibit the gr owth of several yeast and bacterial strains assayed. Experiments on P. digi tatum identified conidial germination as particularly sensitive to inhibiti on although mycelial growth was also affected. Our findings suggest that th e inhibitory effect is initially driven by the electrostatic interaction of the peptide with fungal components, The antifungal peptide retarded the bl ue and green mold diseases of citrus fruits and the gray mold of tomato fru its under controlled inoculation conditions, thus providing evidence for th e feasibility of using very short peptides in plant protection. This and pr evious studies with related peptides indicate some degree of peptide amino acid sequence and structure conservation associated with the antimicrobial activity, and suggest a general sequence layout for short antifungal peptid es, consisting of one or two positively charged residues combined with arom atic amino acid residues.