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The purpose of the present study was to improve the antifungal activity against selected
phytopathogenic fungi of the previously identified hexapeptide PAF19. We describe some
properties of a set of novel synthetic hexapeptides whose D-amino acid sequences were obtained
through screening of a synthetic peptide combinatorial library in a positional scanning format. As a
result of the screening, 12 putative bioactive peptides were identified, synthesized, and assayed. The
peptides PAF26 (Ac-rkkwfw-NH(2)), PAF32 (Ac-rkwhfw-NH(2)), and PAF34
(Ac-rkwlfw-NH(2)) showed stronger activity than PAF19 against isolates of Penicillium digitatum,
Penicillium italicum, and Botrytis cinerea. PAF26 and PAF32, but not PAF34, were also active
against Fusarium oxysporum. Penicillium expansum was less susceptible to all four PAF peptides,
and only PAF34 showed weak activity against it. Assays were also conducted on nontarget
organisms, and PAF26 and PAF32 showed much-reduced toxicity to Escherichia coli and
Saccharomyces cerevisiae, demonstrating selectivity towards certain filamentous fungi. Thus, the
data showed distinct activity profiles for peptides differentiated by just one or two residue
substitutions. Our conclusion from this observation is that a specificity factor is involved in the
activity of these short peptides. Furthermore, PAF26 and PAF32 displayed activities against P.
digitatum, P. italicum, and B. cinerea similar to that of the hemolytic 26-amino acid melittin, but they
did not show the high toxicity of melittin towards bacteria and yeasts. The four peptides acted
additively, with no synergistic interactions among them, and PAF26 was shown to have improved
activity over PAF19 in in vivo orange fruit decay experiments. |
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