New Phytologist 154(2): 275 (May 2002)
Coevolution of roots and mycorrhizas of land plants
Mark C. Brundrett

Here, the coevolution of mycorrhizal fungi and roots is assessed in the light of evidence now available, from palaeobotanical and morphological studies and the analysis of DNA-based phylogenies. The first bryophyte-like land plants, in the early Devonian (400 million years ago), had endophytic associations resembling vesicular–arbuscular mycorrhizas (VAM) even before roots evolved. Mycorrhizal evolution would have progressed from endophytic hyphae towards balanced associations where partners were interdependent due to the exchange of limiting energy and nutrient resources. Most mycorrhizas are mutualistic, but in some cases the trend for increasing plant control of fungi culminates in the exploitative mycorrhizas of achlorophyllous, mycoheterotrophic plants. Ectomycorrhizal, ericoid and orchid mycorrhizas, as well as nonmycorrhizal roots, evolved during the period of rapid angiosperm radiation in the Cretaceous. It is hypothesised that roots gradually evolved from rhizomes to provide more suitable habitats for mycorrhizal fungi and provide plants with complex branching and leaves with water and nutrients. Selection pressures have caused the morphological divergence of roots with different types of mycorrizas. Root cortex thickness and exodermis suberization are greatest in obligately mycorrhizal plants, while nonmycorrhizal plants tend to have fine roots, with more roots hairs and relatively advanced chemical defences. Major coevolutionary trends and the relative success of plants with different root types are discussed.

II. Mycorrhizal fungi

There is only limited fossil evidence of Paleozoic fungi (Taylor & Osborn, 1996), but molecular evidence suggests they diverged from other living organisms deep in the Proterozoic (Wang et al., 1999). Thus, it is probable that the first terrestrial fungi colonised land long before plants did. Soil surface microbial communities containing fungi and algae were probably the first terrestrial associations between fungi and photosynthetic organisms (Gehrig et al., 1996; Evans & Johansen, 1999;  Schü̈ßler & Kluge, 2000). Most Paleozoic fossils of fungi resemble oomycetes, chytrids or zygomycetes, protoctistan organisms that are not directly related to any mycorrhizal fungus lineages (Taylor & Taylor, 1997). These fossils provide examples of putative parasitic plant–fungus associations (similar fungi probably were long established parasites of aquatic algae), but evidence of saprophytism is rare until much later (Taylor & Osborn, 1996). There is scant fossil evidence of lichens in the Paleozoic (Taylor & Osborn, 1996), except for a Devonian fossil that may be a zygomycete lichen (Taylor et al., 1997).

Mycorrhizal Fungi