Possible contribution of blast spores to the oxidative burst in the infection droplet on rice leaf

The infection-induced overproduction of reactive oxygen species (ROS) in resistant plants is usually ascribed to the host. Here we tested the possible contribution of the parasite, the rice blast fungus to ROS production. Droplets of spore suspensions or water were kept on rice leaves or on a plastic. After one day, superoxide radical and hydrogen peroxide were chemically assayed in drop diffusates. Similar measurements were done on diffusates of rice calli submerged in spore suspension or water. Negligible amounts of ROS were found in diffusates of plant tissues treated with water. In contrast, diffusates from tissues treated with spore suspensions had appreciable levels of ROS, usually higher in incompatible combinations than in compatible ones. However, diffusates of spores incubated on plastic produced ROS to an extent comparable to those of infected tissues. In diffusates of spores, O2 - was found after their germination, and H2O2 was found after appressorium formation. Various fungal strains differed in ROS production. The results suggest that spores of the blast inoculum may contribute significantly to ROS production on rice leaves, at least, at early stages of the disease. This might be a factor of incompatibility suppressing a parasite and/or inducing defense responses of a host. © 2007 Akadémiai Kiadó, Budapest.