Preliminary considerations

The study of mycorrhizal symbiosis may be said, after 100 years of investigation, to have come of age. We can now see the symbiotic associations of fungi with higher plants and with bryophytes in perspective and understand, broadly, their importance in the living world. It is essential, however, to be frank with ourselves and not to pretend to understanding in areas where there is still confusion and ignorance. This chapter provides only a quick survey of past work, but it also attempts to highlight our ignorance where it exists.

The study started with the work of Kamienski in 1881 on the chlorophyll-less Monotropa and of Frank in 1885 on forest trees. Indeed, it is important to realize that many properties of mycorrhizal t Deceased

METHODS IN MICROBIOLOGY Copyright © 1991 by Academic Press Limited

VOLUME 23 ISBN 0-12-521523-1 All rights of reproduction in any form reserved symbiosis, confirmed by recent work, were adumbrated by the views expressed by the earliest workers. The section on Historical Perspectives which comprises the first 74 pages of the Proceedings of the 6th North American Conference on Mycorrhizae (1985), provides an historical account of the subject, with translations into English of early papers, and their scientific background. The relationship of mycorrhiza, lichens, and nitrogen-fixing symbioses was early recognized. We can now see that symbiotic associations of fungi with higher plants, bryophytes and algae are the most abundant and widespread of all symbioses, but it would perhaps be rash to conclude that they were therefore the most important. The corals, associations of algae and coelenterate animals, both in the physiological effects on their environment and in their reefand island-building activities are, and have been for millions of years, of great global importance. In a different way the symbiotic nitrogen-fixing symbioses of Rhizobium and Frankia with green plants, harnessing as they do a direct supply of photosynthetic products to the microbial process of nitrogen fixation, are again of very great significance in natural ecosystems, as well as in plant production for human and animal use.

Although symbioses between fungi and green plants conform to relatively few different morphological types, they involve more than one quarter of the described species of fungi, including members of almost all large groups, with some 70% or more of the species of angiosperms, most or all species of gymnosperms, a large number of pteridophytes, a significant but yet unknown number of bryophytes and members of about 36 genera of algae and cyanobacteria of which only eight genera are common in lichens.

This summary of the occurrence of fungal symbioses with green plants is enough to convince us that they must be an important biological study, but one of considerable complexity involving many facets, taxonomy, physiology, genetics and ecology of the constituent organisms and their joint structures, together with their possible value in the understanding of plant life and ecology and their importance in the technologies of plant production.

Although in the study of mycorrhiza our aim is to understand the relationships between fungi and green plants which are mutualistic symbioses, we must realize that the results of work with corals, lichens and even damaging biotrophic parasites, like the rust fungi, may enlighten our problems, as has been shown by Smith, D. C. et al. (1979) and Smith, D. C. and Douglas (1987).

The mycorrhiza, composite organs of roots and fungi, formed on most species of angiosperms and gymnosperms and on some pteridophytes, have been classified under relatively few types, as shown in Table I. These types show variation within themselves, and from time to time new variants are described which depart from those previously recognized, but the classification has in general been very satisfactory and has led to the assumption that each category has most of its salient aspects of structure, physiology and biochemistry and functioning in common. In the mycorrhizal host*, the root systems usually consist of mycorrhiza and uncolonized roots. These latter are often represented only by new root apices and mature mother roots. At the same time mycorrhiza, like all subterranean organs, are surrounded by an entourage of other microorganisms—especially fungi and bacteria—which flourish on exudates and sloughed cells and tissues of their host organs. All these organisms, mycorrhizal fungi and other members of the populations of the rhizosphere and root or mycorrhizal surface, have significant effects on the physiological processes of their hosts, especially in terms of absorption, by virtue of their position. They exist in the interface of soil and root or mycorrhiza and so may alter the soil atmosphere and by absorption, release, or change of form or pH, the availability of chemical compounds in the root region. They may also inhibit or perhaps encourage soil-borne pathogens.

In spite of such complexities, the mycorrhizal fungi have dominant importance in the root region by virtue of their close relationship with the cells and tissues of their hosts, on the one hand, and with soil particles and soil solutions on the other.

Although mycorrhizal symbiosis is so common amongst angiosperms, certain genera and even families are usually or often non-mycorrhizal. This absence of mycorrhiza has been considered by Testa et al. (1987). It should be noted that a number of common beliefs in respect of the absence of mycorrhiza are not tenable as generalities. For instances, it is not true that all plants of wet places or that all annual plants are always free of fungi. The problems are much more complicated and may more likely turn on specificity and recognition of the symbionts, one to another.

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