Isolation and characterization of a novel class of plant antimicrobial peptides form Mirabilis jalapa L. seeds.

We have isolated from seeds of Mirabilis jalapa L. two antimicrobial peptides, designated Mj-AMP1 and Mj-AMP2, respectively. These peptides are highly basic and consist of 37 and 36 residues for Mj-AMP1 and Mj-AMP2, respectively. Both peptides contain three disulfide bridges and differ from one another only by 4 amino acids. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the reduced and unreduced peptides suggests that the peptides associate into dimers in their native form. The Mj-AMPs exhibit a broad spectrum of antifungal activity since they are active against all 13 tested plant pathogenic fungi. Concentrations required for 50% inhibition of fungal growth vary from 6 to 300 micrograms/ml for Mj-AMP1 and from 0.5 to 20 micrograms/ml for Mj-AMP2. These peptides were also active on two tested Gram-positive bacteria but were apparently nontoxic for Gram-negative bacteria and cultured human cells. Although the Mj-AMPs show sequence similarity to mu-agatoxins, a class of insecticidal neurotoxic peptides isolated from the venom of spiders, they do not affect pulse transmission in insect nerves.