Comparative anatomy and histochemistry of the association of Puccinia poarum with its alternate hosts.

The relationship of the macrocyclic rust fungus PUccinia poarum with
its pycnial-aecial host, Tussilago fapfaPa, and its uredial-telial
host, Poa ppatensis, has been investigated, using light microscopy, electron microscopy and micro-autoradiography. Aspects of the morp- hology and ontogeny of spores and sari, which were previously disputed,
have been clarified.
Monokaryotic hyphae grow more densely in the intercellular spaces of
Tussilago leaves than the dikaryotic intercellular hyphae on Poa.
Although ultrastructurally sbnilar, monokaryotic hyphae differ from
dikaryotic hyphae in their interaction with host cell walls, often
growing embedded in wall material which may project into the host
cells.
The frequency of penetration of Poa mesophyll cells by haustoria of
the dikaryon is greater than that of Tussilago cells by the relatively
undifferentiated intracellular hyphae of the monokaryon. Intracellular
hyphae differ from haustoria in their irregular growth, septation, lack
of a neck-band or markedly constricted neck, the deposition of host
wall-like material in the external matrix bounded by the invaginated host plasmalemma and in the association of callose reactions \vith
intracellular hyphae and adjacent parts of host walls. Monokaryotic
and dikaryotic infections differ also in the changes induced in the
organization and ultrastructure of their respective host cells.
Intracellular hyphae in bundle sheath, xylem parenchyma, transfer cells
of phloem parenchyma and companion cells, give the monokaryon of
P. poaPUm direct access to nutrients translocated in vascular tissue
of Tussilago. Bundle sheath cells of Poa contain unusually long
haustoria but· there is no penetration of the endodermis and vascular
tissue by the dikaryon.
After uptake of tritiated glycerol by infected tissue, microautoradiographic
investigation shows radioactivity to be concentrated in sporulation
structures, haustoria and, in the case of Poa, in host nuclei.
Cells of uninfected tissues differ from those of infected tissue in
accumulation of label in chloroplasts. These studies contribute to
an understanding of the physiological interaction of P. poaPUm with
its alternate hosts.