Aspergillus flavus that transgenically expressed the green fluorescent protein was used to follow infection in ears of maize hybrids resistant and susceptible to the fungus. Developing ears were needle-inoculated with GFP- transformed A. flavus 20 days after silk emergence, and GFP fluorescence in the pith was evaluated at 1, 2, 3, 4, 5, 10, and 20 days after inoculation. Fluorescence levels in the pith of susceptible lines were significantly higher (P < 0.0001) than in resistant lines at all time points. Pith sections apical to the inoculation point displayed higher fluorescence levels compared to other sections of the ear, suggesting fungal spread via the water/nutrient transport system. Fluorescence levels in resistant lines did not change significantly over time, implying spread of the fungus but not growth. Fluorescence in susceptible ears was highest at early time points, suggesting that conditions were more conducive to spread than at the later time points. These results suggest that the rachis could retard the spread and/or growth of the fungus inside the developing maize ear. Although fluorescence was observed in kernels from resistant ears, it occurred at a much higher frequency in those from susceptible hybrids. Together, these results suggest that the rachis is used by maize as a defense structure similar to other preformed types of resistance
