Plant Science 151 (2000) 103 – 104
Book review
www.elsevier.com/locate/plantsci
Mycorrhiza Structure, Function, Molecular Biology and Biotechnology, 2nd ed. Edited by Varma, A., Hock, B., Springer-Verlag, Berlin ISBN 3-540-63981-0 (DM. 398,00).
The things one hears about mycorrhizal fungi often evoke disbelief. Is it possible that plants are metabolically interconnected through the hyphae of a soil fungus that can only exist in symbiosis with plants? Do these fungi really absorb minerals from the soil and transport them to the plant in exchange for carbon produced from photosynthe-sis? Are most plant species truly dependent on these fungi for normal growth? If all of this is true, why are these mycorrhizal fungi not the subject of intense research and engineering?
In ‘Mycorrhiza’ we have over 700 pages that provide substance to these rumors. The editors, in this second edition, have organized a balanced coverage of the subject, and the 55 contributors have produced review papers that are generally clear and well-written. The editors provide a func-tional index, which in many ways gives the reader the easiest way of finding points of interest with-out reading the book cover to cover.
Mycorrhiza is organized into seven sections. I: ‘State of the Art’ provides an introduction and overview of the subject. II: ‘Genetics and Molecu-lar Biology’ contains seven chapters covering de-velopmental biology, genetic variation, somatic incompatibility, genetic transformation, genetic approaches to functional studies, and immuno-chemical properties. III: ‘Structure’ provides a chapter on electron microscopy and another on changes associated with symbiosis. IV: ‘Function’ contains six articles dealing with proteins, metabolism, phosphorus and carbon transport, ni-trogen assimilation, hydrolytic enzymes and sapro-phytic growth. V: ‘Biotechnology’ includes in vitro culture systems, aeroponic culture, applications to orchard and ornamental plants, and correlations
with actinorhizal systems. VI: ‘Ecophysiology’ consists of five papers dealing with population biology, function in arid soils, interactions with other fungi, etc. VII: ‘Systematics’ contains three chapters concerned with diversity, anatomical characteristics and symbiosis with terrestrial orchids.
Answers to the fundamental questions are still lacking. The picture that emerges is of course consistent with prior interpretations. The name of the game for the microorganism is ‘get the car-bon.’ For the plant, it is ‘get the minerals.’ The partners in this mutual back-scratching, a result of co-evolution, provide each other not just with nutrients, but probably with diverse ecologically significant advantages, such as protection against parasites. The relationships are mutualistic, benefiting both partners, but what is the form of the plant carbon that is used by the fungus? Can the fungus pump minerals out of the plant as well? What keeps the association in balance? The possi-bility of a metabolic bridge among plants in a community, including plants of different species, is still largely speculative. Could this physical bridge allow gene exchange? The question becomes even more interesting when one realizes that these fungi are generalists, forming associations with plants of diverse species.
Molecular approaches to mycorrhiza are just beginning to have some effects. They are most effective in taxonomic studies using 16S RNA sequences. The fundamental block to progress seems to be that getting nucleic acids out of myc-orrhizal fungi is at best difficult, entailing dissect-ing and cleandissect-ing spores produced in non-axenic conditions. It is assumed that the extracted DNA comes from the spore and not bacterial contaminants.
Bacterial contaminants are also a grey area in functional studies, since bacteria exist as endo-phytes within the fungal hyphae. To what extent
Book re6iew 104
do these bacteria contribute to the ability of the fungus to promote plant growth?
The book is a valiant attempt at cleaning up a difficult subject. Hopefully, it will stimulate more effort, but until a biotechnological payoff is demonstrated, this will sadly remain a relatively obscure research topic.
D. Tepfer Laboratoire de Biologie de la Rhizosphe`re, INRA, Route de Saint-Cyr, 78026 Versailles Cedex, France
