Abstract
On the screening of 200 nonhost plants, an Amaranthus gangeticus extract was found to attract and subsequently inhibit the motility of Aphanomyces cochlioides zoospores. The attractant was identified as a rare N-trans-feruloyl-4-O-methyldopamine (1) and the motility inhibitor as nicotinamide (2) using bioassay-guided fractionation and spectroscopic methods. The attractant had no inhibitory effect on zoospore motility whereas 2 immediately halted the motility and caused encystment in a dose-dependent manner (MIC, 5×10−8 M). Interestingly, the cystospores produced by 2 regenerated zoospores instead of germinating. Concomitant application of 1 and 2 produced cystospores that germinated to give hyphae. Preliminary TLC examinations revealed that Am. gangeticus seedling exuded sufficient amounts of nicotinamide from the roots. These results suggest that exudation of nicotinamide from Am. gangeticus might be involved in its resistance against the soilborne oomycete phytopathogen A. cochlioides.
