The maize rachis affects Aspergillus flavus movement during ear development

Authors: MAGBANUA, ZENAIDA - Mississippi State University; Williams, William; LUTHE, DAWN - Pennsylvania State University

Submitted to: Maydica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/2013
Publication Date: N/A
Citation: N/A

Interpretive Summary: Contamination of corn with aflatoxin, which is produced by the fungus Aspergillus flavus, is a serious impediment to profitable corn production in the southern United States. Aflatoxin is toxic to livestock, pets, and wildlife and also causes cancer in humans. Genetic resistance to A. flavus infection and the subsequent accumulation of aflatoxin is widely considered a desirable means of reducing aflatoxin contamination of corn. USDA-ARS scientists at Mississippi State have developed and released corn germplasm lines with resistance to aflatoxin accumulation. This investigation was conducted to compare fungal colonization within ears of resistant and susceptible corn germplasm lines. A. flavus expressing green fluorescent protein (GFP) was used to follow infection in developing corn ears. Developing ears were 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 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 movement via the water/nutrient transport system. Fluorescence levels in resistant lines did not change significantly over time. Fluorescence in susceptible ears was highest at early time points, suggesting that conditions were more conducive to movement than at the later time points. These results suggest that the rachis could retard the movement and/or growth of the fungus inside the developing ear. Although fluorescence was observed in kernels from resistant ears, it occurred at a much lower frequency than in those from susceptible hybrids. Kernel infection appeared to first begin in the endosperm and then progressed to the embryo. These findings provide insight into the interactions of A. flavus and resistant and susceptible corn germplasm lines that will be useful in developing corn hybrids with genetic resistance to aflatoxin accumulation.

Technical Abstract: Aspergillus flavus expressing green fluorescent protein (GFP) 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 movement via the water/nutrient transport system. Fluorescence levels in resistant lines did not change significantly over time, implying movement of the fungus but not growth. Fluorescence in susceptible ears was highest at early time points, suggesting that conditions were more conducive to movement than at the later time points. These results suggest that the rachis could retard the movement 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. Kernel infection appeared to first begin in the endosperm and then progressed to the embryo.