What is the significance of the arbuscular mycorrhizal colonisation of many economically important crop plants?

Arbuscular mycorrhizal (AM) symbioses are widespread in land plants but the extent to which they are functionally important in agriculture remains unclear, despite much previous research. We ask focused questions designed to give new perspectives on AM function, some based on recent research that is overturning past beliefs. We address factors that determine growth responses (from positive to negative) in AM plants, the extent to which AM plants that lack positive responses benefit in terms of nutrient (particularly phosphate: P) uptake, whether or not AM and nonmycorrhizal (NM) plants acquire different forms of soil P, and the cause(s) of AM 'growth depressions'. We consider the relevance of laboratory work to the agricultural context, including effects of high (available) soil P on AM fungal colonisation and whether AM colonisation may be deleterious to crop production due to fungal 'parasitism'. We emphasise the imperative for research that is aimed at increasing benefits of AM symbioses in the field at a time of increasing prices of P-fertiliser, and increasing demands on agriculture to feed the world. In other words, AM symbioses have key roles in providing ecosystem services that are receiving increasing attention worldwide.