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Reactive oxygen species production in response to Ophiosphaerella spp. colonization of bermudagrass roots

Abstract:

Bermudagrass (Cynodon dactylon, C. transvaalensis and C. dactylon × C. transvaalensis) is a widely used sports turfgrass worldwide. One of the most damaging diseases of bermudagrass is spring dead spot (SDS), caused by Ophiosphaerella herpotricha, O. korrae, and O. narmari. These fungi are characterized as necrotrophic, but the mechanism by which they render host cells necrotic remains unknown. Necrotrophs can produce toxins and are able to exploit the host hypersensitive response (HR) to colonize and infect host cells. The objective of these studies was to investigate the formation of reactive oxygen species (ROS) during fungal infection of plant cells, which is a common feature of the HR. Roots of three bermudagrass cultivars, 'Tifway', 'Midlawn' and 'U3', were each inoculated with tdTOM-transformed isolate (red fluorescence) of O. herpotricha or O. korrae. The roots were stained with 2',7'-dichlorofluorescein diacetate at 4 to 72 h after inoculation. Roots were examined using bright-field and fluorescence microscopy. Based on positive staining, root cells were considered to have formed ROS in response to fungal infection. In C. dactylon cultivar U3, which is tolerant of SDS, higher levels of ROS were more likely to have been produced than in the interspecific hybrid cultivars Midlawn and Tifway. Formation of ROS was observed consistently at the hyphal tips of colonizing fungi in all cultivar/fungi combinations tested, with O herpotricha producing ROS more frequently than O. korrae. These studies suggest that root necrosis caused by SDS fungi is independent of HR associated with plant-generated ROS. In addition, the increased production of ROS by SDS fungi at the time of root penetration suggests a role of fungal-produced ROS during SDS-fungal attack of bermudagrass.