| BEN |
| BOTANICAL ELECTRONIC NEWS |
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| ISSN 1188-603X |
| No. 334 August 20, 2004 | aceska@victoria.tc.ca | Victoria, B.C. |
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This is a beautifully produced book from three authors who have made major contributions to mycorrhizal research over many years.
The preface states that plant-fungus associations: "are the most prevalent symbiotic systems on earth." True, if one excepts the multiple symbiosis origin of the eukaryotic cell. But as mycologists know, there would be no forests, and many fewer land plants (possibly none at all) if mycorrhizal relationships had not evolved hundreds of millions of years ago. So mycorrhizas are essential components of the biosphere.
The preface provides a rationale for this book as an antidote to the current prevalence of ecological, physiological and molecular investigations of mycorrhizas, and the corresponding neglect of their morphology and anatomy. I agree with the authors, in that all understanding and investigation of mycological phenomena must surely spring from a preliminary visual analysis. How can we know what questions to ask until we have looked at the structures (and in this case the interfaces) involved? This book presents copious photographic visualizations, most in colour, derived from light microscopy (employing many staining techniques), scanning electron microscopy, transmission electron microscopy, and even X-rays (though the specific technique used in visualization is often, unfortunately, not identified), as well as many interpretive coloured diagrams. Of the over 300 illustrations, 271 are numbered, and another 34 un-numbered pictures are presented in 16 'boxes' scattered throughout the text. There are also 8 whole-page colour photographic collages at the beginning of the chapters, mostly representing various ecosystems, each followed by a full-page interpretive black and white version.
This must surely be the most extensive and exhaustive collection of excellent mycorrhizal images ever to appear in a single publication - all packed into the first 153 of the book's 173 pages, which also contain 7+1/2 pages of references, an incomplete 1+1/2 page glossary, a short methodological appendix, and a rather perfunctory index of less than 2+1/2 pages.
The book has 8 chapters, each dealing with a more or less discrete set of mycorrhizal phenomena: Ectomycorrhizas, Endectomycorrhizas, Arbuscular mycorrhizas (note: not Vesicular-arbuscular mycorrhizas [VAM], because some do not produce vesicles), Ericoid mycorrhizas, Arbutoid mycorrhizas, Monotropoid mycorrhizas, Orchid mycorrhizas, and Dark septate fungal endophytes.. Within each chapter, a logical sequence is followed - definition, plants involved, fungi involved, after which other topics such as specialized plant-fungal structures, interfaces, development and anatomy are explored. The 'boxes' cover questions specific to each category. [Two examples of the 'box essays' are given below.]
Although an early section purports to deal with 'Mycorrhizal categories', there is no table comparing the features of the various kinds of mycorrhiza, nor any set of comparative side- by-side diagrams introducing the 7 kinds of mycorrhiza recognized. Nor is there a table of major plant families/genera involved in each kind of association. These are the kind of reader-friendly features that would have added significantly to the didactic value of the book.
This leads me to the first of my two serious criticisms of the book - it costs $70 plus tax, which will put it beyond the reach of many of the (young) people who could have found it most useful. The high price undoubtedly derives from the extensive use of high quality colour reproduction, and a short print run. Lower book prices can stem only from heavy subsidies or long print runs. This leads me to one inescapable conclusion - the project would have been better executed on CD-ROM. There would then have been effectively no limit to the number of illustrations that could be incorporated, and hot links and other search features would have allowed the user to find any kind of information in the twinkling of an eye (perhaps 50 millisecs?) Best of all, the price could have been much lower.
My second criticism is that none of the illustrations indicate a magnification. It is extremely important that the reader understand the relative magnifications of the illustrations in each plate, which often vary enormously. This deficiency reduces the scientific and didactic value of the book, and although the experienced eye quickly makes appropriate judgments, the uninitiated will probably become confused.
Nevertheless, this is a valuable and instructive compilation, and the student who is willing to dig (pun initially unintended) can find many answers, and many unsolved mysteries, in its pages.
An excellent discussion on the history of reports of liverworts being colonized by fungi can be found in review by Read et al. (2000). A remarkable finding is that some members of leafy liverworts can associate with members of Glomeromycota and form arbuscules and vesicles, structures formed in arbuscular mycorrhizal associations in vascular plants. Other species associate with basidiomycetes, and still others with ascomycetes. In the latter case, using a fluorescent probe specific to this group of fungi (Duckett & Reid 1991), and ultrastructural studies in which simple septa with Woronin bodies were demonstrated in hyphae (Duckett et al. 1991), ascomycetes fungi have been localized in the shaft and swollen tip of rhizoids in several leafy liverwort families.
Duckett & Read (1995) later demonstrated, experimentally, that the ascomycetes fungus Hymenoscyphus ericae, a species that forms typical ericoid mycorrhizas with a range of ericoid plant species, colonized rhizoids of several families of leafy liverworts and induced tip swelling. Swollen rhizoid tips are reported in these liverwort families in nature.
These authors also showed that fungi isolated from colonized rhizoids of field-collected liverwort species, formed typical ericoid mycorrhizas with seedlings of Erica carnea, Calluna vulgarisand Vaccinium corymbosum. The physiological role of liverwort-fungal associations has yet to be determined.
References
The relationship between achlorophyllous orchid species and mycorrhizal fungi is complex. Leake (1994) summarized the plant groups that are dependent on fungi for the acquisition of carbon and suggested the term myco-heterotrophic to describe such plants. All orchid species are myco- heterotrophic during seed germination and early seedling establishment and several non-photosynthetic orchid species are myco-heterotrophic throughout their life cycle. Taylor & Bruns (1999) refer to these later species as 'cheating' orchids. It has been known for some time that fungi isolated from fully myco-heterotrophic orchid species could form ectomycorhizas with tree and shrub species (Warcup 1985; Zelmer & Currah 1995); however, more recently the highly specific nature of the association of some of the orchid species with fungal symbionts has been demonstrated using molecular approaches.
In the western United States, the orchid species Cephalanthera austiniae associates only with ectomycorrhizal fungi belonging to the Thelephoraceae clade whereas Corallorhiza maculata associates with members of the Russulaceae clade (Taylor & Bruns 1997). These and other myco-heterotrophic orchid species appear no longer associate with 'Rhizoctonia' fungi that are able to provide sugars from the breakdown of complex organic compounds in the soil, but rather with ectomycorrhizal fungi that link them to tree host species. Further studies (Taylor & Bruns 1999) showed that, although two species of Corallorhiza (Corallorhiza maculata and C. mertensiana) both associated only with fungal species in the family Russulaceae, they did not share the same fungal species even in populations where they co-occurred. The fungal symbionts in any one specimen did not change with the season. There was, however, geographical variation of the fungal symbionts with both orchid species.
In one study, seeds of Corallorhiza trifida were enclosed in nylon mesh bags and placed in soil either below stands of Salix repens or Betula-Alnus; germinating seed associated only with fungi in the Thelephora-Tomentella complex of the Thelephoraceae (McKebdrick et al. 2000a). In a further study (McKendrick et al. 2000b), Corallorhiza trifida seedlings formed hyphal links with roots of Betula pendula and Salix repens but not with Pinus silvestris. Using 14CO2 carbon transfer occurred from Betula pendula and Salix repens, demonstrating that this orchid species is a 'cheater' by sharing fungal symbionts with tree species.
References