The symbiosis between arbuscular mycorrhizal fungi (AMF) and plants is frequently referred to as a nutritional mutualism, but the importance of alternate benefits such as pathogen protection is becoming more recognized.  Apart from abiotic factors, early evidence indicates that both plant root architecture and AMF identity can determine which AMF benefit-nutrient uptake or pathogen protection-is more important to a particular plant. It is still unclear whether there is an additive effect of root architecture and AMF species on the degree of plant pathogen protection and what is their relative contribution to plant productivity.  In this study, we tested the effect of plant root architecture and AMF species identity on the protection of plants from infection and growth depression by a common pathogen, Fusarium oxysporum.  We paired two plants: Setaria glauca, a plant with a finely branched root system and Allium cepa, which has a simple root system with either no AMF or one of six different AMF species from either the Glomaceae or Gigasporaceae.  We then challenged each plant with F. oxysporum and assessed the extent of F. oxysporum colonization, AMF colonization and the net result on plant biomass. 
Results/Conclusions

Our results show that arbuscular mycorrhizal pathogen protection is determined by the interaction between both plant root architecture and AMF identity.  F. oxysporum infection was limited equally well by having either a simple root system or by associating with species of the Glomaceae.  The fine-rooted, Setaria glauca benefited most from associating with the Glomaceae due to its high susceptibility to F. oxysporum.  Allium cepa’s simple root system made it less susceptible and these plants showed the greatest biomass when associating with species from the Gigasporaceae.  This result agrees with previous work showing increased dependence of simple-rooted plants on AMF for nutrient uptake and the greater ability of the Gigasporaceae to facilitate nutrient uptake.   Our data indicate that this specific mycorrhizal function is determined both by plant need and an AMF’s ability to provide that function.  Alternative AM functions like pathogen protection could be a primary function to plants in the field if negative interactions with soil microbes are a stronger influence on plants than nutrient limitation.  Determining where particular AMF fall on the continuum of benefit will require an understanding of both plant stressors and the ability of particular AMF species within a community to help alleviate that stress.