Systemic expression of defense response genes in wheat spikes as a response to Fusarium graminearum infection

Abstract

Wheat spikes infected by Fusarium graminearum result in Fusarium head blight, a devastating disease of wheat. The spikes respond to infection by inducing a set of defense response genes in infected spikelets much as has been shown in other plant-pathogen interactions. To determine whether defense response genes are expressed systemically within F. graminearum -inoculated wheat spikes, we examined transcript accumulation of four defense response genes (peroxidase, PR-1, PR-3 and PR-5) in colonized and uncolonized portions of wheat spikes of resistant and susceptible genotypes. We determined whether sampled regions of point-inoculated wheat spikes were colonized or not using a Fusarium spp.-specific plate assay and an F. graminearum isolate expressing β-glucuronidase (GUS). The GUS-expressing F. graminearum isolate proved particularly useful for visualizing and following the spread of the fungus and for measuring the amount of fungal biomass in spike tissues. We found that transcripts of the four defense response genes accumulated in both colonized and uncolonized regions of spikes of resistant and susceptible genotypes at 48h after inoculation. These results demonstrate that direct contact with the pathogen is not required for induction of defense response genes and that these genes are activated in both susceptible and resistant genotypes.