New insights into roles of arbuscular mycorrhizas in P nutrition of crops reveal the need for conceptual changes

Abstract

Many soils are phosphorus (P) deficient. The cost of P fertiliser is increasing and the reserves of phosphate rock from which it is manufactured are diminishing rapidly. There is accordingly a major need to identify and understand traits that maximise the efficiency of P uptake by plants. Most crops, including cereals, form arbuscular mycorrhizas (AM) in the field; their symbiotic root systems are therefore the normal nutrient absorbing organs. Despite this, cereals often do not show marked positive responses to AM inoculation in the vegetative stages of pot experiments and this has led to a reduced emphasis in recent years on applied research into the roles of AM in cereal P nutrition. However, new research has provided signifi cant insights into the ways that plant roots and AM fungi are integrated, structurally and functionally. The most important finding is that the contribution of AM fungi to plant nutrition may be large but remains hidden unless special techniques are used. We have shown large contributions of AM fungi to P uptake in both wheat and barley, even when growth and total nutrient uptake are less than in non-mycorrhizal (NM) control treatments. Furthermore, such growth depressions in AM plants cannot be explained simply by carbon (C) drain to the fungal symbionts. Future research should be directed to understanding the way in which nutrient uptake pathways through roots and AM fungi are integrated and controlled, so that new crops can be developed that capture benefi ts of both direct and AM uptake of P and make maximal use of nutrient resources in soils and fertilisers.