Growth and sporulation of the arbuscular mycorrhizal fungus Glomus caledonium in dual culture with transformed carrot roots

The arbuscular mycorrhizal fungus Glomus caledonium was established in a du al culture with Ri T-DNA-transformed carrot roots. A modification of the mi nimal M medium buffered at pH 6.50 with 10 mM MES and solidified with 0.4% unpurified gellan gum allowed spore germination and formation of the symbio sis, together with the development of an extensive extramatrical mycelium a nd sporulation. Spore production increased with culture generation and most spores were viable. These spores colonized carrot roots and completed the fungal life cycle. In many cultures, sporulation was accompanied by the for mation of arbuscule-like structures on short and thickened lateral branches of main hyphae. Root colonization was of the Paris-type with hyphae spread ing intracellularly. Most colonized root cells contained coils of thickened hyphae, sometimes surrounded by fine hyphae, but no typical arbuscules wer e observed.