Plant roots in natural ecosystems are typically colonized by a wide ra
nge of fungi. Some of these are pathogenic, others appear to be opport
unistic and have no apparent impact, while mycorrhizal fungi are gener
ally regarded as mutualistic. Of the various types of mycorrhizal fung
i, the arbuscular mycorrhizal (AM) association is by far the most abun
dant and widespread. While the most widely accepted model of AM functi
on depends upon plants benefiting from the facilitation of phosphorus
uptake, recent data from field-based studies in temperate ecosystems i
ndicate that only plant species with poorly branched root systems bene
fit from AM fungi in this way: species with highly branched root syste
ms may benefit in other ways, such as by being protected against root
pathogenic fungi. These two responses apparently represent extremes al
ong a continuum of AM benefit determined by root system architecture.