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

Wheal spikes infected by Fusarium graminearum result in Fusarium head bligh t, a devastating disease of wheat. The spikes respond to infection by induc ing a set of defense response genes in infected spikelets much as has been shown in other plant-pathogen interactions. To determine whether defense re sponse genes are expressed systemically within F. graminearum-inoculated wh eat spikes, we examined transcript accumulation of four defense response ge nes (peroxidase, PR-1, PR-3 and PR-5) in colonized and uncolonized portions of wheat spikes of resistant and susceptible genotypes. We determined whet her sampled regions of point-inoculated wheat spikes were colonized or not using a Fusarium spp.-specific plate assay and an F. graminearum isolate ex pressing beta -glucuronidase (GUS). The GUS-expressing F. graminearum isola te proved particularly useful for visualizing and following the spread of t he 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 suscepti ble genotypes at 48 h after inoculation. These results demonstrate that dir ect contact with the pathogen is not required for induction of defense resp onse genes and that these genes are activated in both susceptible and resis tant genotypes. (C) 2001 Academic Press.