Mycorrhizal fungal networks as plant communication systems

The concept that plants can “communicate” with each other is not a new one (Baldwin and Schultz 1983), but until recently, research evidence pointed to interplant signaling via aerial pathways, which are driven by production of volatile organic compounds (VOCs) produced by plant leaves (Heil and Karban 2009), often in response to mechanical or herbivore-induced damage. In addition, there is now increasing recognition of root-to-root signaling via exudates released into the rhizosphere (Semchenko et al. 2014), and there is even evidence for kin recognition in both aerial (Karban et al. 2013) and root exudate signaling pathways (Lepik et al. 2012). These ›ndings raise intriguing questions concerning the evolutionary drivers of plant-to-plant signaling from the perspective of plants either sending or receiving signals and rede›ne how we consider competitive interactions in plant communities. However, recent exciting work has added a new and fascinating level of complexity to this research arena by providing compelling evidence that fungal mycelium can act as underground conduits for signals transferred from plant to plant in response to pests and pathogens (Figure 37.1; Babikova et al. 2013a,b; Song et al. 2010, 2014, 2015). Thus, the soil fungal community and, speci›cally, mycorrhizal fungi are now known to have a role in plant-to-plant signaling and in mediating plant-plant interactions and broader multitrophic interactions than hitherto thought. Such signaling processes rely on the formation of “common mycorrhizalnetworks” (CMNs), when mycorrhizal mycelia interact and form physical connections between the root systems of two or more host plants (Selosse et al. 2006). Common mycorrhizal networks are likely to be ubiquitous in nature, because most mycorrhizal fungi produce extra-radical mycelium. With the exception of ectomycorrhizal (ECM) fungi that are of the smooth contact type (Agerer 2001), most ECM fungi produce extra-radical mycelium, and some “long-distance” growth forms produce rhizomorphs that can grow many meters through soil, giving them potential to connect many roots (Taylor 2006). In addition, although ericoid mycorrhizal (ERM) fungi are often wrongly considered to produce small amounts of extra-radical mycelia, most ERM hyphae are concentrated in close proximity to host roots (Read 1984), so CMNs formed by ERM fungi are likely to operate only at a very localized spatial scale (Grelet et al. 2010). In contrast, the length of extra-radical mycelia of arbuscular mycorrhizal (AM) and ECM fungi vary considerably among ecosystems, and estimates frequently range from 10 to 100 m hyphae g−1soil or even up to hundreds of meters of hyphae per meter of root length (Leake et al. 2004)