Ultrastructure of the pycnial and aecial stages of Puccinia recondita

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Abstract
The haploid stage of Puccinia recondita in Thalictrum speciosissimum was established by basidiospore infection. Haploid hyphae were inter- and intra-cellular. Intracellular structures arose from intercellular hyphae without prior formation of specialized mother cells. They gained entrance into host cells via wide-diameter penetration sites; they were hyphal-like and formed abundantly in both mesophyll and epidermal cells. Commonly, they exited host cells to become intercellular hyphae or intracellular hyphae in adjacent host cells. Numerous, globose pycnia were borne in wart-like humps on the leaf tissue. Paraphyses and flexuous hyphae extended through the ostiole, initially aggregated into a column, but later become radiate. Globose to oblong aecia developed one to three cells below the lower epidermis of humped leaf tissue. Aeciospore initials arose from a stroma of sporogenous tissue at the base of the aecia. Successive divisions of aeciosporophores produced catenulate aeciospores separated by intercalary cells. The first appearance of aeciospore wall ornaments (verrucae) was associated with the division of aeciospore initials into aeciospores and intercalary cells. The electron-lucent verrucae were contiguous with the spore plasmalemma and enlarged to full size (ca. 0.5 × 0.3 μm) in the primary cell wall. During maturation aeciospores underwent a two- to three-fold increase in diameter and the intercalary cells disintegrated. Simultaneously, the primary wall of the aeciospores broke down, exposing the verrucae. The secondary wall formed centripetal to the disintegrating primary wall to provide the major portion of the mature spore wall. The verrucose, mature aeciospores were globose to elliptical (ca. 16.5 × 19 μm). Germ pores were locally thickened areas within the secondary cell wall and contained small electron-lucent interspersions. The peridial cells which delimit the aecia were differentially thickened. The thick outer cell wall contained dagger-shaped, electron-lucent processes and the relatively thin inner wall was ornamented with smooth, knob-like, electron-lucent clavae.