Differential morphogenesis of the extraradical mycelium of an arbuscular mycorrhizal fungus grown monoxenically on spatially heterogeneous culture media.

A new in vitro experimental system was developed to study the morphogenesis of discrete regions of a single extraradical mycelium of the arbuscular mycorrhizal (AM) fungus Glomus intraradices, growing simultaneously in six different agar-based media. The media were (i) unamended water agar (WA), (ii) WA + PO43- (PO43-), (iii) WA+NO3- (NO3-), (iv) WA + NH4+ (NH4+), (v) WA + NH4+ + MES (NH4+ + MES) and (vi) minimal medium (M, complete nutrients). Each medium was amended with the pH indicator bromocresol purple. The extraradical mycelium of the fungus showed between-treatment differences in morphogenesis, architecture, formation of branched absorbing structures (BAS) and sporulation. Extraradical hyphae that developed in WA or PO43- compartments exhibited an economic development pattern, in which runner hyphae radially extended the external colony. Extraradical hyphal growth in the NO3- compartments was characterized by increased formation of runner hyphae, BAS and spores and an alkalinization of the medium. In the two NH4+-amended media (NH4+, NH4+ + MES), sporulation was suppressed and considerable morphological changes were noted. These results show the plasticity of G. intraradices that lets it efficiently exploit an heterogeneous substrate.