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Abstract
A strain of Fusarium graminearum, the predominant causal agent of fusarium head blight (FHB) in North America, which was transformed to constitutively express the green fluorescent protein (GFP) from jellyfish, was used to perform a comparative study of the early events of colonization in wheat between FHB-susceptible ('Roblin') and FHB-resistant ('Sumai 3') cultivars. Greenhouse grown plants from both cultivars were point inoculated at anthesis with a spore suspension of GFP-Fusarium, then maintained under intermittent misting conditions until harvest. Heads were harvested for microscopy at several points in timeduring the first 24 h after inoculation and, subsequently, at longer intervals, up to 21 days after inoculation. Infected material was visualized as whole mounts; in addition, tissue was fixed and cryosectioned to examine the progress of the fungus within the wheat tissues. In both 'Roblin' and 'Sumai 3', hyphae were visible inside the floret at the point of inoculation within a few hours of the inoculation. The fungus displayed a particular affinity for the pollen and anthers of both wheat cultivars. Fungal growth was most prolific on the ovaries and stigmas of both cultivars throughout the course of the infection. In general, only after the soft tissues of the ovaries and stigmas were thoroughly colonized, did the fungus begin to move into the rachis and to the exterior of the floret. The fungus was able to spread along the spike both internally, through the rachis, and across the external surfaces of the rachis and florets. The major difference in infection pattern between susceptible 'Roblin' and resistant 'Sumai 3' was that the spread of the fungus through the parenchyma and vascular tissues of the rachis was slower and less extensive in the latter.