Concurrent common fungal networks formed by different guilds of fungi

Abstract

Introduction

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.

Fig. 1

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Conceptual diagram for the concurrent common fungal networks formed between two plants, adopting a view of the mycelia of fungi, in which the portion forming connections is in the spotlight. This does not mean that the mycelia not partaking in the plant connection should be neglected. (a, b) Network links formed by fungi can be either direct or indirect, with direct links requiring hyphal continuity of the fungal network between plants. (c) Such common fungal networks can be formed by different guilds of fungi, including mycorrhizal fungi, endophytic fungi, parasitic fungi and saprobic fungi. As saprobic fungi generally do not colonize plants, direct network connections with hyphal continuity are likely less relevant. However, saprobic fungi can also be present inside roots, so this possibility does exist. (d) The concept of common fungal networks provides several new research opportunities, such as studying the interaction of fungal networks in the soil and inside of roots. Many methods can be imported from the study of common mycorrhizal networks, like linkage treatments involving mesh barriers. Target variables for exploring the functional significance of these common fungal networks could be plant performance and effects on soil processes and soil biodiversity.

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.