As important players in the ecosystem, fungal saprophytes can also interact with plants 1–3. Thus, the complex interplay between the plant and fungal counterparts is highly diverse. Pathogenic fungi may develop biotrophic or hemibiotrophic lifestyles 4,5. Both interactions negatively affect the plant counterpart, while the fungus benefits from the infected plant nutrients. Fungi and plants benefit from mutualistic interactions, with the advantages to the plants ranging from stress resistance to macro- and micronutrient supply to the maintenance of a specific beneficial microbiome 6–10. In Mycorrhiza (MR), filamentous fungi invade the root system of plants and form a stable, beneficial coexistence 11–13. There are two basic types of mycorrhizae: ecto- and endo-MR, where the latter includes arbuscular (AMR), ericoid, and orchid types. Ecto-MR fungi colonize only the intercellular space, while endo-MR fungi also penetrate individual cells 14. Glomeromycota fungi interact with the vast majority (80%) of land plants and form eponymous structures (arbuscles) in the invaded cells in AMR 15,16. Ericoid 17,18 and orchid MR 19 occur exclusively in plants belonging to Ericaceae and Orchidaceae, respectively. The fungal partner belongs to either asco- or basidiomycota. Fungal growth in plant cortical cells can be observed in both cases, where fungi do not grow through plasma membranes, but rather within the apoplastic space between cell walls and membranes. In temperate trees and shrubs, ecto-MR fungi form a massive fungal mantle surrounding the primary and secondary roots 20. In the cortex of the plants, there is an extensive fungal network called the Hartwig net, but no cell invasion can be observed. Some fungi can also develop an endophytic lifestyle, which does not negatively affect plant development, but as it does not develop MR-characteristic features, it cannot be classified as MR as well 21,22. A successful development and maintenance of an interaction strategy requires communication between both sides, regardless of whether the interaction has a positive or negative impact on the interacting organisms 23.
A novel, stable, and basic system is presented in our research to investigate plant-fungal interactions. The system consists of two well-established model organisms: the ascomycete N. crassa and the grass B. distachyon. Despite extensive research on N. crassa 24–28, little is known about its natural populations and ecology 29–31, and this ascomycete is described as saprophytic. Wild populations have been isolated in parts of India, Pakistan, Middle Africa, South America, and the USA (Louisiana and Texas) from grass, soil, wild sugar cain, cain stubble, and wood 32,33. A single publication also reported the growth of N. crassa in pine trees 34. These results, however, are controversial and have not yet been replicated. B. distachyon is a sweet grass and is closely related to important crop plants like barley and wheat 35–37. There is a wide distribution of this species and it is highly adaptable in terms of its ecological environment 38. In addition to pathogenic interactions, Brachypodium is involved in mutualistic interactions as well.
Many studies examine the properties of highly specialized plant-fungus interactions, involving highly adapted organisms, making investigations and interpretation of data challenging. Due to its simplicity, our study enables us to overcome these difficulties and decipher the general properties of interspecies interactions. As both interacting partners are well-established model organisms, the system provides us with a powerful tool for genetic and cell biological studies. This is the first report showing N. crassa interacting with plant roots in a stable, endophytic manner.