Arbuscular mycorrhizal fungi are soil inhabitants that live in permanent symbiosis with plant roots. They colonize the vast majority of terrestrial plants and are present in all ecosystems. Plants benefit enormously from the association with mycorrhizal fungi because they are provided with nutrients that are scarce or immobile in the soil solution. In this mutualistic symbiosis the fungus also profits from the association with a "green" partner through the provision of carbohydrates downloaded at specialized haustoria, the so-called arbuscules. Much is known about physiology, ecology and agronimical applications of this mycorrhizal symbiosis. However, the precise mechanisms by which plant and fungus recognize each other in the soil, by which they interact with other soil inhabitants, or which nutrients are exchanged in an organized manner - thus preventing a parasitic situation - are still not fully understood. In the last 15 years, major advances have been possible in this research area with the use of molecular biology and genetic approaches.
In our group we are interested in unraveling how AM fungi are perceived by their host plants and discriminated from pathogenic fungi. To investigate these aspects we are using several complementary approaches. On the one hand, we use plant genes induced during early mycorrhization as reporters of fungal presence and activity. By fusing their promoters to fluorescence marker genes like GFP or DsRed and expressing them in planta we can follow the course of the interaction in vivo (see our movie from Kuhn et al., 2010). On the other hand we investigate the secretion of fungal effectors produced by AM fungi that are able to turn on the reporter genes and/or to travel to the plant cell to induce the symbiotic program (see Kloppholz et al., 2011). We recently showed, that secreted signals from the fungus are able to rewire the developmental program of the plant to allow arbuscule formation within roots (see Heck et al., 2016). In addition, we compare the mechanisms of root colonization between mutualistic symbiotic fungi and pathogenic fungi. For instance we use the rice blast fungus Magnaporthe oryzae which besides its well known leaf infection route is also able to colonize the root of rice plants (see Heupel et al., 2010).
We would like to know too which nutrients and at which interfaces are exchanged in this symbiosis. We identified a characterized the first bonafide AM fungal monosaccharide transporter that takes up sugars from the root (see Helber et al., 2011) and showed that the extraradical mycelium is, in contrast to what it was believed, able to take up hexoses. Recently we have also shown that AM fungal colonization significantly alters the root expression and localization of the novel sugar exporters/importers SWEETs, suggesting that they might be partly responsible for the carbohydrate supply to AM fungi (see Manck-Götzenberger and Requena, 2016).
Life cycle of an arbuscular mycorrhiza
a. Spore germination and asymbiotic growth on water-agar.
b. Host recognition and pre-symbiotic growth in the proximity of a host root.
c. Appressoria formation on the root epidermis and colonization of the first root cortex layer.
d. Arbuscules in inner cortical cells.
e. Detail of an intracellular hypha, the so-called coil, in a cell of the root cortex. Observe the big lipid droplets within the fungal hypha.
f. Extraradical mycelium exploring the soil and forming the next spore generation