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

Rhiopus stolonifer invades sweetpotato roots through injuries and infected roots are rapidly consumed by a soft rot. However, not all injuries are equ ally susceptible to infection; shallow injuries (1-2 mm. deep) are less pro ne to infection than deeper injuries ( > 5 nim deep). The presence of antif ungal compounds in external tissues may partially explain the resistance of shallow injuries to infection. To test this hypothesis, we developed a qua ntitative bioassay for measuring the growth of R. stolonifer utilizing the vital stain 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (M TT). 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 inh ibitory. We used the R. stolonifer bioassay to guide the purification of th e active components. Two active fractions were isolated. One active fractio n contained predominately caffeic acid, but this compound was determined no t 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 EC50 of 2.2 g 1(-1). The presence of antifungal comp ounds in the external tissues helps explain why shallow injuries are resist ant to infection. Additionally, we demonstrated that interior flesh tissues accumulate antifungal compounds when elicited and incubated under curing c onditions (30 degreesC and 90-95% RH) for 24 h. (C) 2001 Elsevier Science B .V. All rights reserved.