Effect of the fungus Fusarium verticillioides on the defense systems in developing kernels of resistant and susceptible maize genotypes.

Developing kernels of maize with contrasting levels of resistance to Fusarium ear rot were inoculated with Fusarium verticillioides. Selected defense systems were investigated using realtime reverse transcription-polymerase chain reaction to monitor the expression of pathogenesisrelated (PR) genes (PR1, PR5, PRm3, PRm6) and genes protective from oxidative stress (peroxidase, catalase, superoxide dismutase and ascorbate peroxidase) at 72 hours post inoculation. In this respect, the hydrogen peroxide and malondialdehyde contents were studied to evaluate the oxidation level. The study was also extended to the analysis of the ascorbate-glutathione cycle and catalase, superoxide dismutase, and cytosolic and wall peroxidases enzymes. Higher levels of gene expression, enzymatic activities and antioxidant metabolites were observed in uninoculated kernels of resistant lines, conferring a major readiness to the pathogen attack. After inoculation, the level of gene induction was stronger in susceptible lines, although their enzymatic activity was higher in the resistant lines. The susceptible genotypes, therefore, respond specifically to pathogen inoculation, while resistant genotypes have already high levels of defense-related transcripts before infection, which provide a basal defense system. In the susceptible maize, the presence of oxidative stress due to its incapacity to remove ROS might help to explain its susceptibility to F. verticillioides. These results support our previous findings of a basal defense response provided by maize genotypes resistant against F. verticillioides infection, both in kernels and seedlings. Our data provide an important basis for further investigation of defense gene functions in developing kernels in order to improve resistance to fungal pathogens. Maize genotypes with overexpressed resistance traits could be profitably utilized in breeding programs focused on resistance to pathogens and grain safety.