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Diversity and Integration in Mycorrhizas
Geoderma 2006 (2003) 1 – 3 www.elsevier.com/locate/geoderma
Book review Diversity and Integration in Mycorrhizas By S.E. Smith and F.A. Smith (Eds.). Kluwer Academic Publishers, 2002. Hardbound, 335 pp. ISBN 1-4020-0269-6. Price o85 The book contains keynote lectures and symposium presentations of the Third International Conference on Mycorrhizas (ICOM-3) held in Adelaide, Australia, in 2001. Mycorrhizal research is a rapidly advancing multidisciplinary field, where both reductionist and holistic approaches are used in the study of the interface between soils, mutualistic fungi and plants. Reductionism is evident in the ever-finer studies of molecular biology when it comes to molecular communication between plants and fungi, or the fine tuning between gene expression by plants and fungi. At the other end, there is a need to upscale our study of mycorrhizal functioning to global scales. In terms of precision, the reductionist approach offers rapid rewards as can be evidenced by the seven papers in section B: Development—from genes to structure. As a consequence, the mycorrhizal community tends to lean to the molecular side, and issues of global relevance may not always find the (financial) support and representation at conferences that they merit. In that regard, the book is probably no exception. The word soil is in the title of only 1 of the 31 chapters—a chapter that treats soil microorganisms other than mycorrhizal fungi. In that chapter, J.I. Prosser sketches modern developments in microbial ecology and the impact that molecular methods have in leaving behind the inherent biases and limited views that the conventional, cultivation-based methods had exerted. While such methods have tremendously increased our understanding of species and functional diversity of soil bacteria, application of these methods in (mycorrhizal) fungal ecology has lagged behind. The chapter suggests way for further improvement, and it is pleasing to note that the number of papers in which such methods are applied to mycorrhizal communities are rapidly increasing. Soil scientists will find more chapters that are relevant for their research. In the introductory chapter, B. So¨derstro¨m sets the scene, pointing out that 20 –30% of the C that is fixed by plants may be consumed by the fungus. A substantial part of that C is immediately oxidized, but a significant part could be sequestered in the external mycelium, ending up in soil carbon. A major challenge to mycorrhizal researchers is to better understand the factors that determine C flows in mycorrhizas and the ways in which global change (increased atmospheric CO2, higher temperatures, increased N deposition) would affect these flows. P.L. Staddon et al. address the question of how global change would affect the plants and their mycorrhizal fungi. On the basis of the available evidence, which is mainly based on pot studies (not field experiments), they conclude that community-level interactions between plants and fungi are the key mechanisms in
doi:10.1016/S0016-7061(03)00112-5
2
Book review
response to global change. It is, therefore, important to explicitly address the feed backs between plant and fungal communities—a theme further treated by J.D. Bever and D.C. Hartnett and G.W.T. Wilson who show that even simple feed back models can generate complex dynamics in which mycorrhizas can both increase and decrease plant species diversity. A.F.S. Taylor addresses the issue of sampling in the study of ectomycorrhizal communities, mainly in boreal forests. Sampling has been superficial in the strict sense of the word because there is more knowledge of the fungi that are found in the organic part of the soil profile than of the fungi in the mineral part, and even less on differences between E and B horizons in a podzol. Common in the various papers of the section on molecular diversity is that the diversity of fungi that form mycorrhizal associations is larger than expected, that multiple colonization of roots is the norm and that several fungi can be the fungal partner in more than one type of association: Sebacina-like fungi have been found in orchid, ericoid and ectomycorrhizal associations, and the ectomycorrhizal fungus of Piceirhiza bicolorata is likely the same as the fungus Hymenoscyphus ericae, a common ericoid mycorrhizal symbiont. Could it be the case that where above-ground plant ecologists see different plant formations, a below-ground perspective would be one of essentially a continuity? D. Redecker emphasizes the old age of the arbuscular mycorrhizal fungi, with which more than 200,000 different plant species associate. Fossil evidence suggests an age of at least 460 million years, and these data add to the strong circumstantial evidence that the conquest of the land by primitive, still rootless, plants was made possible through their association with mycorrhizal fungi. The standard view of mycorrhizas is that they enhance uptake of nutrients with low mobility, such as P and Zn, by extending depletion zones. A simple method then is to calculate P content of mycorrhizal and non-mycorrhizal plants, and use the difference as an estimate of the mycorrhizal contribution. Such calculations are likely misleading, as S.H. Burleigh and I.E. Bechmann summarize evidence that the mycorrhizal fungi alter the expression of (down-regulate) the plant nutrient transporters both through nutrient transport and by an unknown mechanism not directly involving nutrient transfer. Due to the importance of mycorrhizas in enhancing P uptake, it has been suggested that mycorrhizal fungi could act as a kind of biofertilizers. M.H. Ryan and J.H. Graham suggest that arbuscular mycorrhizal fungi do not always play a vital role in P nutrition in organic agriculture. However, they may still play a role in the acquisition of micronutrients and also be important in maintaining soil structure. For that reason, mycorrhizal functioning should be considered in agricultural management including responsiveness to mycorrhizas in novel plant breeding. When ecologists study complex systems, there seems to be no end to it. Even a mycorrhizal system with plant and fungus is incomplete. Spores of certain arbuscular mycorrhizal fungi contain endosymbiotic bacteria (Burkholderia), and these bacteria possess genes for associative N fixation. This capacity of a mycorrhizal fungus to fix N through specific bacteria could also be relevant for sustainable agriculture. Overall, the book provides a good overview of the state of the art in mycorrhizal research. The papers are well written and the references are generally up to date. Institutes that subscribe to the journal Plant and Soil will note that the book is a hard cover reprint of
Book review
3
Volume 244, Issues 1– 2. Institutes not subscribing to the journal will find a balanced review of many of the important themes within mycorrhizal research. Thomas W. Kuyper Subdepartment of Soil Quality, Wageningen University, P.O. Box 8005, 6700 EC, Wageningen, The Netherlands E-mail address: [email protected] Tel.: +31-317-482352; fax: +31-317-483766
Book review Diversity and Integration in Mycorrhizas By S.E. Smith and F.A. Smith (Eds.). Kluwer Academic Publishers, 2002. Hardbound, 335 pp. ISBN 1-4020-0269-6. Price o85 The book contains keynote lectures and symposium presentations of the Third International Conference on Mycorrhizas (ICOM-3) held in Adelaide, Australia, in 2001. Mycorrhizal research is a rapidly advancing multidisciplinary field, where both reductionist and holistic approaches are used in the study of the interface between soils, mutualistic fungi and plants. Reductionism is evident in the ever-finer studies of molecular biology when it comes to molecular communication between plants and fungi, or the fine tuning between gene expression by plants and fungi. At the other end, there is a need to upscale our study of mycorrhizal functioning to global scales. In terms of precision, the reductionist approach offers rapid rewards as can be evidenced by the seven papers in section B: Development—from genes to structure. As a consequence, the mycorrhizal community tends to lean to the molecular side, and issues of global relevance may not always find the (financial) support and representation at conferences that they merit. In that regard, the book is probably no exception. The word soil is in the title of only 1 of the 31 chapters—a chapter that treats soil microorganisms other than mycorrhizal fungi. In that chapter, J.I. Prosser sketches modern developments in microbial ecology and the impact that molecular methods have in leaving behind the inherent biases and limited views that the conventional, cultivation-based methods had exerted. While such methods have tremendously increased our understanding of species and functional diversity of soil bacteria, application of these methods in (mycorrhizal) fungal ecology has lagged behind. The chapter suggests way for further improvement, and it is pleasing to note that the number of papers in which such methods are applied to mycorrhizal communities are rapidly increasing. Soil scientists will find more chapters that are relevant for their research. In the introductory chapter, B. So¨derstro¨m sets the scene, pointing out that 20 –30% of the C that is fixed by plants may be consumed by the fungus. A substantial part of that C is immediately oxidized, but a significant part could be sequestered in the external mycelium, ending up in soil carbon. A major challenge to mycorrhizal researchers is to better understand the factors that determine C flows in mycorrhizas and the ways in which global change (increased atmospheric CO2, higher temperatures, increased N deposition) would affect these flows. P.L. Staddon et al. address the question of how global change would affect the plants and their mycorrhizal fungi. On the basis of the available evidence, which is mainly based on pot studies (not field experiments), they conclude that community-level interactions between plants and fungi are the key mechanisms in
doi:10.1016/S0016-7061(03)00112-5
2
Book review
response to global change. It is, therefore, important to explicitly address the feed backs between plant and fungal communities—a theme further treated by J.D. Bever and D.C. Hartnett and G.W.T. Wilson who show that even simple feed back models can generate complex dynamics in which mycorrhizas can both increase and decrease plant species diversity. A.F.S. Taylor addresses the issue of sampling in the study of ectomycorrhizal communities, mainly in boreal forests. Sampling has been superficial in the strict sense of the word because there is more knowledge of the fungi that are found in the organic part of the soil profile than of the fungi in the mineral part, and even less on differences between E and B horizons in a podzol. Common in the various papers of the section on molecular diversity is that the diversity of fungi that form mycorrhizal associations is larger than expected, that multiple colonization of roots is the norm and that several fungi can be the fungal partner in more than one type of association: Sebacina-like fungi have been found in orchid, ericoid and ectomycorrhizal associations, and the ectomycorrhizal fungus of Piceirhiza bicolorata is likely the same as the fungus Hymenoscyphus ericae, a common ericoid mycorrhizal symbiont. Could it be the case that where above-ground plant ecologists see different plant formations, a below-ground perspective would be one of essentially a continuity? D. Redecker emphasizes the old age of the arbuscular mycorrhizal fungi, with which more than 200,000 different plant species associate. Fossil evidence suggests an age of at least 460 million years, and these data add to the strong circumstantial evidence that the conquest of the land by primitive, still rootless, plants was made possible through their association with mycorrhizal fungi. The standard view of mycorrhizas is that they enhance uptake of nutrients with low mobility, such as P and Zn, by extending depletion zones. A simple method then is to calculate P content of mycorrhizal and non-mycorrhizal plants, and use the difference as an estimate of the mycorrhizal contribution. Such calculations are likely misleading, as S.H. Burleigh and I.E. Bechmann summarize evidence that the mycorrhizal fungi alter the expression of (down-regulate) the plant nutrient transporters both through nutrient transport and by an unknown mechanism not directly involving nutrient transfer. Due to the importance of mycorrhizas in enhancing P uptake, it has been suggested that mycorrhizal fungi could act as a kind of biofertilizers. M.H. Ryan and J.H. Graham suggest that arbuscular mycorrhizal fungi do not always play a vital role in P nutrition in organic agriculture. However, they may still play a role in the acquisition of micronutrients and also be important in maintaining soil structure. For that reason, mycorrhizal functioning should be considered in agricultural management including responsiveness to mycorrhizas in novel plant breeding. When ecologists study complex systems, there seems to be no end to it. Even a mycorrhizal system with plant and fungus is incomplete. Spores of certain arbuscular mycorrhizal fungi contain endosymbiotic bacteria (Burkholderia), and these bacteria possess genes for associative N fixation. This capacity of a mycorrhizal fungus to fix N through specific bacteria could also be relevant for sustainable agriculture. Overall, the book provides a good overview of the state of the art in mycorrhizal research. The papers are well written and the references are generally up to date. Institutes that subscribe to the journal Plant and Soil will note that the book is a hard cover reprint of
Book review
3
Volume 244, Issues 1– 2. Institutes not subscribing to the journal will find a balanced review of many of the important themes within mycorrhizal research. Thomas W. Kuyper Subdepartment of Soil Quality, Wageningen University, P.O. Box 8005, 6700 EC, Wageningen, The Netherlands E-mail address: [email protected] Tel.: +31-317-482352; fax: +31-317-483766