Characterization of the smallest chitosan oligomer that is maximally antifungal to Fusarium solani and elicits pisatin formation in Pisum sativum

Abstract

Chitosan (β-1,4-linked glucosamine polymer) oligomers were characterized by their antifungal effect on Fusarium solani f. sp. pisi and F. solani f. sp. phaseoli and their ability to elicit pisatin formation in immature pea pods. The antifungal and pisatin-inducing abilities were shown to increase as the polymer size increased. Monomer and dimer units showed no antifungal activity and induced little pisatin. Trimer through pentamer units were antifungal at high concentrations and were moderate in ability to induce pisatin formation. A sharp increase in antifungal activity and pisatin formation was noted for the hexamer unit, while the heptamer unit was maximal in both antifungal activity and formation of pisatin. Chitosan and its derivatives showed maximal activities in both antifungal action and pisatin induction, while chitin and chitin derivatives showed no antifungal activity and only weak pisatin formation activity. These findings show that high-molecular-weight chitosan fragments are more active in both antifungal and pisatin formation activity than are the intermediate and low-molecular-weight fragments. The chitosan oligomers which most effectively induce pisatin and inhibit the F. solani formae speciales are the non-acetylated heptamers.