Interactions between arbuscular mycorrhizal fungi and a mycorrhiza-defective mutant tomato: does a noninfective fungus alter the ability of an infective fungus to colonise the roots - and vice versa?

We have investigated whether inoculation of a mycorrhiza-defective mutant (rmc) of tomato (Lycopersicon esculentum) with an individual arbuscular mycorrhizal (AM) fungal species affects subsequent colonisation by another AM fungal species that has different infectivity when presented alone. Inoculum was provided by leek (Allium porrum) growing in 'nurse pots' into which rmc seedlings were planted, and between which they were subsequently transferred. Inoculation with Glomus coronatum, an AM fungal species that has low infectivity towards rmc, did not prevent colonisation by Glomus sp. WFVAM 23 (identified as G. versiforme in our previous studies). Also, colonisation by G. sp. WFVAM 23 did not facilitate colonisation by G. coronatum. The results show that the mechanism by which rmc prevents cortical colonisation by most AM fungi is not dependent on, or modified by, prior plant interactions with individual AM fungal species. The two fungi used here clearly perceive the outcome of the mutation differently. Growth responses of rmc after sequential inoculation with the two AM fungi are briefly analysed in terms of competition for limiting soil nutrient resources with the nurse plants.