Detection of fungal fimbriae by protein A – gold immunocytochemical labelling in host plants infected with Ustilago heufleri or Peronospora hyoscyami f.sp. tabacina

Abstract

Sections of leaves of Nicotiana tabacum L. infected with Peronospora hyoscyami De Bary f.sp. tabacina (Adam) Skalicky and of Erythronium americanum Ker. infected with Ustilago heufleri Fuckel were treated with an antiserum directed against the fimbriae of U. violacea Fuckel and other fungi. The sections were then treated with protein A – gold complexes to detect the presence and location of fimbrial antigens following transmission or scanning electron microscopy. Control preparations involved sections of uninfected leaves, as well as a range of serological control treatments. The infected leaf sections were heavily labelled with gold particles indicating the presence of fimbrial antigens, whereas the control preparations showed only a low background level of labelling. Gold particles were detected on the sections of hyphae, on haustoria, and on the nearby plant cells. The intensity of labelling was much higher for P. hyoscyami f. sp. tabacina than for U. heufleri and was particularly high in the walls of the former species. Relatively high levels of labelling occurred over the cells of infected hosts, but little or none occurred over the cells of uninfected host tissues, or of the infected host tissues treated with a range of serological controls. This high level of labelling was not associated with specific host structures in P. hyoscyami, but was frequently associated with the chloroplasts in U. heufleri. The antigens detected inside the host plant cells appear to indicate that fungal fimbrial protein, either as polymerized fibrils or as isolated subunits, can penetrate the host plasma membrane and therefore enter the host cytoplasm. An alternative possibility is that these antigens derive from host produced proteins synthesized as a result of infection. These results suggest the possibility that fungal fimbriae may play an important role in the molecular interaction between pathogen and host.