Electron microscopy of urediospore formation in Puccinia coronata avenae and P. graminis avenae

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Abstract
Electron microscopy revealed that the sporogenous cells in urediosori of Puccinia coronata avenae and P. graminis avenae were enlarged at one end, and the cytoplasm was dense, vacuolated, and showed an early accumulation of lipid droplets. Urediospore formation was initiated by the outgrowth of a spore bud from the enlarged end of a sporogenous cell. A nuclear division occurred in the spore bud, the spore bud then grew to form the urediospore initial, and a septum formed to delineate the urediospore initial from the sporogenous cell. A further nuclear division occurred in the urediospore initial followed by septation to form the pedicel and spore cell. The urediospores rapidly grew to full size and further differentiation was marked by increased density of the cytoplasm, disappearance of vacuoles, increased lipid droplet accumulation, thickening of the spore wall, and spine formation. Nucleoli were not found in nuclei of mature urediospores. Possible paraphysis cells were intermixed with urediospores near the margins of urediosori of P. coronata. These cells were characterized by small nuclei which contained densely staining patches, a fine membranous network throughout the cytoplasm, and numerous cytoplasmic inclusions of variable morphology.