Nutrient uptake: The arbuscular mycorrhiza fungal symbiosis as a plant nutrient acquisition strategy

Symbiotic association of roots with arbuscular mycorrhizal (AM) fungi is a very widespread strategy by which plants facilitate their acquisition of mineral elements from the soil. Studies employing sophisticated methodology in the fields of in vitro culture of AM colonized roots, microscopy, isotope labeling and molecular biology have shed light into the physiology of AM fungal nutrient uptake, transport and delivery to the host plant. In addition to the direct contribution to element uptake via the symbiotic pathway, AM mycelia have also been shown to affect root morphology and functioning, as well as mycorrhizosphere soil properties. This may lead to indirect effects of the AM association on plant nutrient availability and uptake. With their thin diameter, AM hyphae might be able to access smaller soil pores, and better compete with soil microbes for nutrient resources, compared with plant roots. Alone or in collaboration with associated hyphosphere bacteria, AM mycelia might also promote chemical mobilization of nutritional elements from sparingly plant available resources. Similar with plant root systems, AM mycelia appear to differ considerably in their architecture and physiological activities depending on their genotype. Whether such inherent traits represent different strategies in nutrient acquisition in collaboration with functionally compatible host roots, still remains speculative. Not much is known about how the AM fungal symbiosis is integrated into particular plant nutrient acquisition strategies, but it can be assumed that individual symbiotic strategies are highly diverse. The AM mycelium might assist the roots in spatial and/or chemical soil nutrient resource exploitation in a complementary and/or synergistic way. Knowledge about what factors determine the extent of functional compatibility between individual plant nutrient acquisition strategies and certain AM fungal traits would contribute to our understanding of ecosystem functioning, and might assist further development of mycorrhiza technology for plant production.