Breeding for mycorrhizal symbiosis: focus on disease resistance

Modern plant breeding can no longer afford to ignore the interaction between plants and microbial key players. Increasing evidence suggests (i) that the expression of many plant traits (such as nutrient use efficiency or tolerances against biotic and abiotic stresses) is mediated by beneficial microorganisms and (ii) that there is an exploitable genetic base for the regulation of symbiotic relationships. Arbuscular mycorrhizal fungi (AMF) play a key role in many of these trait expressions. While much is known about their ability to mobilise nutrients (especially phosphorus), the complex mechanisms of AMF-mediated disease resistance have only started to become apparent within the past decade. Besides competition for root space and resources, AMF also have the ability to induce plant defence mechanisms. Jasmonic acid (JA) and salicylic acid (SA) appear to be the key phytohormones that regulate relevant signalling pathways. The resulting activation of defence-related compounds can occur locally or systemically, constitutively or primed. Genotype-dependent plant reactions have been demonstrated for mycorrhizal responsiveness (when based on biomass), but not much is known when it comes to genotypic variation for AMF-mediated disease resistance. However, a few studies have provided first valuable insights. It is proposed to (i) include disease resistance as a factor to expand the term mycorrhizal responsiveness and (ii) make use of an indicator called “mycorrhiza use efficiency” as an additional measure to determine an optimum cost-benefit ratio of the mycorrhiza symbiosis. In order to detect differences in the efficiency, genotype selection needs to occur in environments that do not suppress the plant–microbe interaction. Thus, the value of organic breeding programmes is highlighted