Concurrent common fungal networks formed by different guilds of fungi

Abstract

Networks formed by fungi that link among plants have captured the imagination of scientists and the wider public alike (Selosse et al., 2006; Karst et al., 2023). This work on fungal connections among plant roots has almost exclusively focused on mycorrhizal fungi, with most work focusing on arbuscular mycorrhizal and ectomycorrhizal fungi; other groups of mycorrhiza, such as ericoid mycorrhiza and orchid mycorrhiza have also been studied. Reasons underpinning this focus on common mycorrhizal networks (CMNs) are quite evident: these fungi form well-documented and functionally relevant symbioses with the majority of plants and the fungi grow inside the roots, forming symbiotic exchange interfaces (Smith & Read, 2008).

A recently introduced conceptual framework (Rillig et al., 2025) has proposed a hierarchical set of terms to describe such links: the current definition of common mycorrhizal networks demands the presence of hyphal continuous links that forms an uninterrupted cytoplasmic flow between roots of at least two plants (Karst et al., 2023). This is a special case, in reality, for which several criteria have to be fulfilled (Lehmann & Rillig, 2025) to ensure that it is just the resource transfer via the hyphal link that is responsible for any measured plant responses. In the new framework, this special case is referred to as common mycorrhizal networks with hyphal continuity (CMN-HC). In this conceptual framework, common mycorrhizal networks of any kind – involving direct hyphal connections or not – are referred to as CMNs. In addition, the term common fungal network (CFN) has been introduced, representing the most general case of hyphal linkages among plants: those that are formed by any type of filamentous fungus (not limited to mycorrhizal fungi) and that are either direct or indirect in their mode of linking (i.e. hyphal continuity or not).

A systematic mapping of the field of ‘common mycorrhizal networks’ revealed that c. 33% of the experimental research data is on networks formed not just by the targeted mycorrhizal fungi, but with other filamentous fungi present in addition to mycorrhizal fungi (Lehmann & Rillig, 2025). These are mainly field studies or studies using whole microbial communities as inoculum sources for the network. Thus, effects of CFNs are already implicitly part of our experimental results, but we do not know about their contribution to the studied mycorrhizal networks. We propose here that such CFNs are likely the reality in soils, rather than just CMNs, and that this more complex reality should be captured in future work on fungal networks linking among plants (Fig. 1). In this paper, we build on the recent conceptual development and these systematic mapping results to propose research on various forms of CFNs.

In proposing such research, we here adopt a perspective that arises from the focus on the plants that have such fungal networks in common. This should not be understood to mean that we argue against a mycocentric perspective; a more fungi-focused view would emphasize that likely it is only a small part of the mycelia of fungi participating in such interactions with plants. The focus on the fungal links places these connections between the plants in the spotlight. We believe that much can be gained for fungal biology and ecology by embracing the more comprehensive view of such connections, including several fungal guilds.

Given that the root colonizing habit is the factor that likely led to the development of research on CMNs, it seems reasonable to start the discussion with other root colonizing filamentous fungi. For the purpose of discussion we divide these into functional groups, with the understanding that the differentiation between these can be rather fluid (Aguilar-Trigueros et al., 2014; Selosse et al., 2018). In addition to mycorrhizal fungi, parasitic fungi and the wide range of root endophytic fungi (sensu lato, see our definition below) colonize roots, as well. In addition, the case of saprobic fungi needs to be assessed in these systems, which generally do not primarily associate with roots, but can also be found there. Collectively, we refer to networks among plants formed by such fungal guilds as CFNs, but if research on these topics takes off, it may be useful to designate these more specifically by guild involved: common parasitic fungal networks, common endophytic fungal networks, and common saprobic fungal networks. Following the case of CMNs, it might even be useful to further designate if hyphal continuity is examined/demanded or not (e.g. common parasitic fungal network with hyphal continuity) (Fig. 1).

In the previous section, we have discussed possible candidate guilds that may form CFNs. However, it is quite clear that these groups of fungi co-occur in soil, and that therefore there is no inherent reason why they should not also concurrently form CFNs between any given set of host plants. We propose that this would be the norm, if such networks are indeed formed. What does this mean for the overall functioning of CFNs?

A first priority to develop this research line into CFNs formed by different guilds would be to document the existence of such linkages in the first place. This does not necessarily mean hyphal continuity has to be shown, in parallel to the situation with CMNs, so the bar is not that high in terms of experimental effort. We envisage experiments where plants are linked via blocks of substrate separated by meshes that exclude roots, and the presence of a fungal genet in the rhizosphere of one plant, passing through the meshed compartment, leading to the presence in the second plant rhizosphere would constitute sufficient initial evidence that this genet can link plants. This could be explored by high-throughput sequencing of fungi in a spatially explicit manner within this mesh-compartment setup. The next step will have to be testing for functional effects; this could be accomplished by severing links or keeping them intact, in parallel to established workflows for CMN and CMN-HC.

Given the plant-focused perspective within which research on fungal mycelia connecting among plants typically takes place, the response variables to be included in this work will likely focus on plant performance (including growth, nutrient content and other traits). It will also be important to include survival as a response variable, especially for studies focusing on the seedling stage, as most plant mortality occurs at that stage. In addition, we argue that soilborne processes, such as soil aggregation and soil organic matter processing should be included in any effects of CFNs in addition to plant effects (Fig. 1). Effects on soil microbial biodiversity are equally important, and metagenomics could be an excellent tool to explore fine-grained shifts in microbial groups and gene abundances.

We see the following four research challenges (new experimental design innovations, fungal species richness, microbiome connections, and management implications) and offer some thoughts on how to meet them.

We believe that a research program focused on documenting the existence and testing the functional significance of CFNs is highly promising and stands to move the field of plant science and plant–fungal interactions forward. We propose a largely phytocentric framework for such work here, highlighting promising candidate guilds of fungi, and introducing specific research questions. Many of the experimental designs can be imported from the study of CMNs, but there are also specific challenges that such new work will have to face. The biggest challenge will likely be to convince researchers to invest in this line of research on fungal networks, due to the work on mycorrhizal fungi being well-established, and everything outside of this realm constitutes high risk. We hope that with this paper and its research perspective we have given an impetus to test the new waters.

None declared.

MCR wrote the first draft, and AL, IRM and BMB contributed ideas, reviewed the literature and added to the writing. AL designed the figure. All authors approved the final version of the paper.

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