Differential activation of defense genes and enzymes in maize genotypes with contrasting levels of resistance to Fusarium verticillioides

Fusarium ear rot is one of the most important diseases of maize, that is of concern because Fusarium verticillioides produces the mycotoxins known as fumonisins. F. verticillioides can be transmitted either through infected silks or seed-to-kernel. In order to better understand the virulence of F. verticillioides, the effect of the fungus on the defense systems was investigated both in immature kernels and in seedlings. The molecular mechanisms involved in compatible and incompatible responses were also studied. Gene expression data were obtained from microarray hybridizations, comparing healthy and infected kernels of resistant and susceptible maize inbreds 48 h after infection with a fumonisin-producing strain of F. verticillioides. A total of 739 transcripts were differentially expressed between the two inbred lines at one time point after infection. Among all the differentially regulated genes, 7.3% of encoded proteins play a role in cell rescue and defense. The qRT-PCR analysis confirmed that most of the defense genes had already been transcribed before infection occurred in the maize-resistant line. The study was extended to the analysis of enzymes involved in removing reactive oxygen species, namely ascorbate peroxidase, catalase, total peroxidase and superoxide dismutase. In resistant seedlings, before infection, ascorbate peroxidase and superoxide dismutase enzyme activities were higher than in the susceptible ones and, 5 days after treatment, they remained unchanged. On the other hand, in the susceptible seedlings, except for superoxide dismutase, all enzymes assayed were activated after pathogen attack. 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.