Arbuscular mycorrhizal fungi are soil inhabitants that live in permanent symbiosis with plant roots. 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 two decades, 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.We would like to know too which nutrients and at which interfaces are exchanged in this symbiosis.
Arbuscular mycorrhizal (AM) fungi inhabit the soil rhizosphere together with a plethora of microbes many of which also interact with plant roots. AM fungi colonize exclusively the epidermis and root cortex of their hosts in contrast to other root invading biotrophic and hemi-biotrophic fungi that quickly expand invading the vascular system. How are AM fungi perceived by the plant and which signaling cascades their perception elicit is one of our main research topics. To that end, we investigate which plant genes are regulated in response to the approaching and colonizing AM fungus (see Hannahs movie from our paper Kuhn et al., 2010). And analyze how these genes regulate the process of fungal and plant development within the root by silencing or overexpressing those genes (Rech et al., 2013). Thus, we could show that secreted AM fungal signals are able to induce genes that rewire the developmental program of cortical root cells to allow arbuscule formation within roots (Heck et al., 2016).
AM fungal signals perceived by the plant
On the other hand, we analyze the secretion and function of AM fungal signals that might control the development of the symbiosis. It is known, that plants are able to perceive chito-oligosaccharides and lipo-chito-oligosaccharides from AM fungi and induce the symbiotic signaling cascade. We are interested in other signals called effectors (proteins or small RNAs) that are delivered during symbiosis and contribute to control the symbiotic status of the plant (Kloppholz et al., 2011; Voß et al., 2018). Comparing the biology of root colonization of AM fungi and other root invading fungi we expect to gain insights into the mechanisms by which plants give or negate access to microbial root colonization (Heupel et al., 2010).
Nutrient exchange in the symbiosis
The whole complexity of the symbiotic interaction allows to maintain a balanced nutrient exchange between the plant and the AM fungus and avoid a parasitic situation. We would like to know which nutrients and at which interfaces are exchanged in this symbiosis. Currently, is accepted that AM fungi receive carbohydrates and lipids from their host plants, and deliver, in exchange, a series of mineral nutrients, most prominently phosphate. We identified a characterized the first bonafide AM fungal monosaccharide transporter that takes up sugars from the root (see Helber et al., 2011). In that paper, we were also able to establish HIGS (host induce gene silencing) to inactivate fungal genes, as AM fungi are recalcitrant to gene manipulation (Helber et al., 2008). 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 (Manck-Götzenberger and Requena, 2016).
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