Interactions between parasitic fungi and mosses: pegged and swollen-tipped rhizoids in Funaria and Bryum

Two kinds of parasitic fungal infections are described in the rhizoidal systems of both wild and cultured materials of Funaria hygrometrica, Bryum capillare and B. pseudo triquetrum. Protonemal and rhizoidal cells of Funaria produce pegs of host wall material around sites of hyphal penetration. Similar structures appear to be much more widespread in hepatics and in Monoclea have been misinterpreted as rudimentary pegs, the characteristic feature of pegged rhizoids. Swollen side branch initials and the tip cells of major rhizoidal axes contain a single rozellopsidalean (Oomycota formerly Chytridiomycota) zoosporangium in Bryum pseudo triquetrum and up to three similar structures in R capillare. Each zoosporangium produces a single exit tube. Whereas the infected cells die the subapical cells remain alive and, after zoospore discharge, produce side branches whose tip cells also become infected. Restriction of the parasites to side branch initials and rhizoid tip cells is perhaps because these are the only cells that the zoospores are able to penetrate. Nutrients translocated along rhizoids to sustain tip growth are diverted to nurture the fungi in infected plants. Differences between the number of zoosporangia per host cell and the number of exit tubes between different mosses suggest that more than one species of rozellopsidalean fungus may form rhizoid galls in mosses and not just Pleotrachelus wildemani described previously. Rhizoid galls appear to be genuinely uncommon in mosses but, following clarification of the differences between these and rhizoidal tubers, further discoveries may be anticipated.