Maize ribosome-inactivating protein inhibits normal development of Aspergillus nidulans and Aspergillus flavus

The abundant maize kernel ribosome-inactivating protein 1 (RIP1) was tested for antifungal activity against Aspergillus nidulans and Aspergillus flavu s. A microculture assay was developed to monitor fungal growth and developm ent after treatment of conidia with RIP1 or control proteins. A striking de crease in hyphal proliferation was observed when conidia of A. nidulans, a genetically well-characterized nonpathogenic species, were treated with RIP 1 protein. Treatment with a RIP1 mutant protein that lacked enzymatic ribos ome-inactivating activity caused no observable effects. RIP1 treatment of c onidia from the maize pathogen A. flavus resulted in increased hyphal branc hing. Examination of the branched hyphae after Congo red staining revealed only one growing hyphal tip per conidium, These results indicate that both fungi were affected by RIP1 treatment, but the lysis seen with treatment of A. nidulans was apparently avoided by A. flavus. A developmental time cour se revealed that both fungal species were affected by RIP1 at the postdivis ional growth stage. The inhibitory activity of RIP1 against normal fungal g rowth is consistent with a biological function to protect the seed from fun gal invasion.