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Ultrastructure of Urediniospore Development in Coleosporium ipomoeae as Revealed by High‐Pressure Freezing Followed by Freeze Substitution

Transmission electron microscopy of high‐pressure frozen/freeze‐substituted samples was used to study urediniospore development in Coleosporium ipomoeae in leaves of Ipomoea coccinea. The urediniospores of this species are morphologically identical to aeciospores. Our study showed that these spores developed in the same fashion as aeciospores. Spores formed in a basipetal fashion along with intercalary cells from a layer of binucleate sporogenous cells lining the base of each uredinium. High‐pressure freezing/freeze substitution yielded excellent preservation of ultrastructural details of the spore formation process until the point of secondary wall formation in maturing spores. When viewed with scanning electron microscopy, mature urediniospores were verrucose in appearance, except for a slightly reticulate area at the base of each spore. Both the reticulations and the small wartlike ornaments present on spore surfaces arose from tiny electron‐transparent deposits that formed on the outer surface of the spore plasma membrane beneath the extremely electron‐dense primary wall of each spore. Flattened cytoplasmic cisternae were closely associated with developing warts/reticulations. As developing warts/reticulations enlarged, they protruded into the primary wall. A layer of secondary wall material not nearly as electron dense as the primary wall then formed beneath the warts/reticulations and displaced them from the plasma membrane. Warts/reticulations eventually became exposed on the spore surface following the dissolution of the primary wall.