Constituents from the periderm and outer cortex of Ipomoea batatas with antifungal activity against Rhizopus stolonifer

Abstract

Rhizopus stolonifer invades sweetpotato roots through injuries and infected roots are rapidly consumed by a soft rot. However, not all injuries are equally susceptible to infection; shallow injuries (1–2 mm deep) are less prone to infection than deeper injuries (>5 mm deep). The presence of antifungal compounds in external tissues may partially explain the resistance of shallow injuries to infection. To test this hypothesis, we developed a quantitative bioassay for measuring the growth of R. stolonifer utilizing the vital stain 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT). Acetone extracts of the fresh interior flesh of four cultivars had no antifungal activity, but extracts of the exterior 2 mm of the root were inhibitory. We used the R. stolonifer bioassay to guide the purification of the active components. Two active fractions were isolated. One active fraction contained predominately caffeic acid, but this compound was determined not to be the most biologically active component. The second active fraction contained 3,5-dicaffeoylquinic acid (3,5-DCQA), and this compound was found to be active, with an EC 50 of 2.2 g l −1 . The presence of antifungal compounds in the external tissues helps explain why shallow injuries are resistant to infection. Additionally, we demonstrated that interior flesh tissues accumulate antifungal compounds when elicited and incubated under curing conditions (30°C and 90–95% RH) for 24 h.