Arbuscular mycorrhizal fungi (AMF) are of great ecological importance, since arbuscular mycorrhiza (AM) is the most widespread plant symbiosis and often improves plant productivity and resistance to nutrient stress. AMF are essentially obligate biotrophs; their life cycle includes growth and proliferation within the host root and surrounding soil. The mutual recognition and the development of the symbiosis will trigger specific developmental programs in both organisms. Recent studies collectively indicate that there exists a functional diversity in AM, as different combinations of host plant and AMF have different impacts on the morphology, nutritional status, symbiotic efficiency and gene expression patterns in the symbiosis. We suggest that differential expression of symbiosis-associated genes among different AM associations is a phenotypic response to the different fungal and plant genotypes involved and the environment they inhabit; functional diversity is therefore the rule rather than the exception and necessitates carefully replicated experiments that combine close observation of morphology, physiological traits and gene expression. (C) 2009 Elsevier Ltd and The British Mycological Society. All rights reserved.