A Changing Paradigm

"Thus, the different mycorrhizal types do occur in soils that current dogma regards as atypical."
--The Ecology of Mycorrhizae, M.F. Allen (1991)

    The field of mycorrhizal research is relatively young, and the "modern" age of mycorrhizal research is often said to have started with the 1953 work of B. Mosse.  Mosse built on two of the few correct early attempts at classification of mycorrhizae, and showed that mycorrhizal strawberry plants were indeed colonized by a species of Endogone, which would later be transferred to Glomus (Peyronel 1923 and Butler 1939 in Smith and Read 1997).  The field grew rapidly, but the early literature focused almost entirely on terrestrial plants.  While there was overwhelming evidence for VA mycorrhizae in terrestrial plants all over the world (Allen 1991, Hendrix et al. 1995 in Mukerji and Mandeep 1998, Smith and Read 1997), there was a great deal of evidence for the absence of such infection in aquatic plants.  Asai (1934) and Maeda (1954) reported the absence of mycorrhizae in flooded soil plants, and evidence that non-mycorrhizal species could become mycorrhizal when placed in drier soils (in Mukerji and Mandeep 1998).  Khan (1974 in Mukerji and Mandeep 1998) reported the absence of mycorrhizae in any of the plants investigated in swamps and muds of Pakistan.  Read et al. (1976 in Søndergaard and Laegaard 1977) found a lack of infection in marsh plants.  Studies showing a lack of mycorrhizal infection in aquatic plants were generally accepted and rarely refuted, or tested.  One of the early books about mycorrhizae, J.L. Harley's The Biology of Mycorrhizae (1969), stated the commonly accepted view that aquatic plants were not infected by VA mycorrhizae.
    Yet there was evidence to show that mycorrhizal and hydrophyte were not mutually exclusive descriptions of plants.  Even as early as 1927, Rayner had found VA mycorrhizal fungi in peat bogs.  Dowding found VA mycorrhizal spores on external hyphae in wet soils (1959 in Mukerji and Mandeep 1998).  This early evidence was most likely overshadowed by the much more abundant evidence for the lack of mycorrhizal infection in aquatic plants.  In 1977, a letter to Nature by Morten Søndergaard and Simon Laegaard detailed a study of aquatic plants from lakes in Denmark.  They reported VA mycorrhizal infection levels of up to 96% in plants covered by 0.3 to 0.8 m of water.  (Even these researchers were looking for something else, root hair development, when they discovered that their aquatic plant roots were mycorrhizal.)  This short letter shifted the paradigm in which mycorrhizasts operated, and within several years the evidence for mycorrhizal infection in aquatic plants, and preliminary evidence for a genetic role in the dependence of some aquatic species on mycorrhizae, had appeared throughout the mycorrhizal journals (Keeley 1980, Clayton and Bagyaraj 1984 in Cantelmos and Ehrenfeld 1999).  By the time Harley and Smith (1983) (yes, the same J.L. Harley) published their first edition of Mycorrhizal Symbiosis, they wrote that "some water plants are typically mycorrhizal."  In the past two decades, evidence for and explanations of mycorrhizal infection in aquatic plants have grown at an amazing pace (see Cantelmos and Ehrenfeld 1999 for an excellent bibliography).  One of the currently accepted theories of the origin of VA mycorrhizae suggests that these fungi were influential in the colonization of land by aquatic plants, based on fossil evidence from the Rhynie Chert (Nicolson 1975 in Smith and Read 1997).
    Although aquatic plants are now known to be mycorrhizal under some circumstances, VA mycorrhizae are less commonly found in aquatic plants than in terrestrial plants.  Basic questions such as the ecological impact of VA mycorrhizal infection on aquatic plants, the ability of VA mycorrhizal fungi to colonize and survive in anaerobic soils, the persistence of their spores in flooded soils, and even which species of VA mycorrhizae are most commonly found in aquatic systems remain to be answered.  In many ways, the literature regarding aquatic mycorrhizal infection is far behind the rest of the field.  Current interest in wetland restoration and the physiological adaptations of plants to aquatic environments should and is generating more research in this area.  For more information on current research on aquatic plants and mycorrhizae, see Where do we go from here?