Factors affecting plant responsiveness to arbuscular mycorrhiza

Arbuscular mycorrhiza (AM) is an ancient, widespread symbiosis between most land plants and fungi of the Glomeromycotina, which receives increasing interest for agricultural application because it can promote plant growth and yield. The ability of plants to react to AM with changes in morphology and/or performance in terms of yield is called 'AM responsiveness'. Its amplitude depends on the plant- fungal genotype combination and the abiotic and biotic environment. A molecular understanding of AM responsiveness is key for enabling rational application of AM in agriculture, for example through targeted breeding of AM-optimised crops. However, the genetic and mechanistic underpinnings of AM responsiveness variation remain still unknown. Here, we review current knowledge on AM responsiveness, with a focus on agricultural crops, and speculate on mechanisms that may contribute to the variation in AM response.