Pierre Hohmann

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.

Insights to plant-microbe interactions provide opportunities to improve resistance breeding against root diseases in grain legumes: Microbe-supported resistance breeding in legumes

Root and foot diseases severely impede grain legume cultivation worldwide. Breeding lines with resistance against individual pathogens exist, but these resistances are often overcome by the interaction of multiple pathogens in field situations. Novel tools allow to decipher plant-microbiome interactions in unprecedented detail and provide insights into resistance mechanisms that consider both simultaneous attacks of various pathogens and the interplay with beneficial microbes. Although it has become clear that plant-associated microbes play a key role in plant health, a systematic picture of how and to what extent plants can shape their own detrimental or beneficial microbiome remains to be drawn. There is increasing evidence for the existence of genetic variation in the regulation of plant-microbe interactions that can be exploited by plant breeders. We propose to consider the entire plant holobiont in resistance breeding strategies in order to unravel hidden parts of complex defence mechanisms. This review summarizes (a) the current knowledge of resistance against soil-borne pathogens in grain legumes, (b) evidence for genetic variation for rhizosphere-related traits, (c) the role of root exudation in microbe-mediated disease resistance and elaborates (d) how these traits can be incorporated in resistance breeding programmes.