Arbuscular mycorrhizal fungi enhance alfalfa production by changing root morphology and physiology

Soil phosphorus (P) deficiency can severely limit crop and forage productivity. With limited P resources, breeding programs to select high-P efficiency (HPE) genotypes have been developed, but the role of arbuscular mycorrhizal fungi (AMF) in altering root morphology and physiology to increase P use efficiency and production remains poorly understood. In this study, we compared mycorrhizal responsiveness, and plasticity of root morphological and physiological traits between two low-P efficiency (LPE) and two HPE alfalfa genotypes under low and high P treatments. Plants were grown either in soil with naturally occurring AMF or in sterilized soil added with AMF-free bacteria. The results indicated that the AMF symbiosis significantly increased alfalfa productivity and physiological P use efficiency by enhancing total root length and root surface area while reducing carboxylate release. Under low P conditions, HPE genotypes with AMF symbiosis showed higher shoot dry weight, greater mycorrhizal responsiveness, thicker and more robust roots, as well as increased carboxylate release compared to LPE genotypes. We conclude that exploitation of the dominant species in indigenous AMF populations and breeding of crop genotypes with high mycorrhizal responsiveness show promising avenues with which to improve forage productivity and alleviate P-limitation in modern agricultural ecosystems.