Mycorrhizal Symbiosis

Sally E. Smith,David J. Read

• Publisher: Academic Press
• Year: 2010
• ISBN:
  9780080559346
• Edition: 3
• Topic: Scienze biologiche
• Subtopic: Biologia

The roots of most plants are colonized by symbiotic fungi to form mycorrhiza, which play a critical role in the capture of nutrients from the soil and therefore in plant nutrition. Mycorrhizal Symbiosis is recognized as the definitive work in this area. Since the last edition was published there have been major advances in the field, particularly in the area of molecular biology, and the new edition has been fully revised and updated to incorporate these exciting new developments.

• Over 50% new material
• Includes expanded color plate section
• Covers all aspects of mycorrhiza
• Presents new taxonomy
• Discusses the impact of proteomics and genomics on research in this area

Section 2

Ectomycorrhizas

Outline

Chapter 6: Structure and development of ectomycorrhizal roots

Chapter 7: Ectendo- and arbutoid mycorrhizas

Chapter 8: Growth and carbon allocation of ectomycorrhizal symbionts

Chapter 9: Nitrogen mobilization and nutrition in ectomycorrhizal plants

Chapter 10: Phosphorus and base cation nutrition, heavy metal accumulation and water relations of ectomycorrhizal plants

6

Structure and development of ectomycorrhizal roots

Publisher Summary

This chapter presents an overview of current understanding of the structure and development of ectomycorrhizal (ECM) roots. An ECM root is characterized by the presence of three structural components: a sheath or mantle of fungal tissue which encloses the root, a labyrinthine inward growth of hyphae between the epidermal and cortical cells called the Hartig net, and an outwardly growing system of hyphal elements (the extraradical or external mycelium), which form essential connections both with the soil and with the sporocarps of the fungi forming the ectomycorrhizas. Almost all of the plants upon which ectomycorrhizas develop are woody perennials. The anatomical structures of the mantle and of the emanating mycelia are stable at least at the level of the fungal genus and are increasingly used to facilitate characterization of ectomycorrhizas. Advances have been made towards an understanding of the molecular events associated with the formation of mycorrhizas at the level of the individual root tip. Transcriptional profiling has enabled the identities of some of the genes that are upregulated during formation of the symbiosis to be elucidated. Among others, those involved in modification of cell wall structure, in glycolysis, the TCA cycle and respiratory activity have been identified. Work on the characterization of the extraradical mycelial systems of ECM roots has progressed. New methods enabling quantification of its biomass in the field have been developed, and, using molecular methods that now facilitate the identification of small lengths of ECM hyphae, information on selective niche exploitation is being obtained. Genetic profiling of fungal communities associated with fine roots has further confirmed that there is little correspondence between production of sporocarps and representation of a given fungus as a root symbiont and has identified fungi with hypogeous or resupinate fruit bodies as important components of many mycorrhizal fine root communities.

Introduction

An ectomycorrhizal (ECM) root is characterized by the presence of three structural components: a sheath or mantle of fungal tissue which encloses the root, a labyrinthine inward growth of hyphae between the epidermal and cortical cells called the Hartig net, and an outwardly growing system of hyphal elements (the extraradical or external mycelium) which form essential connections both with the soil and with the sporocarps of the fungi forming the ectomycorrhizas.

Almost all of the plants upon which ectomycorrhizas develop are woody perennials. The anatomical structures of the mantle and of the emanating mycelia, are stable at least at the level of the fungal genus and are increasingly used to facilitate characterization of ectomycorrhizas (Agerer, 1987–2002; Ingleby et al., 1990; Goodman et al., 1998). This type of organ is also clearly distinguishable from all other types of mycorrhiza on the basis of the absence of intracellular penetration by the fungus. In the event of penetration of healthy root cells by an ECM-forming fungus, whether from the Hartig net or from the hyphae of the sheath, the structure is referred to as an ectendomycorrhiza (see below and Chapter 7). The identity of the plant can influence the outcome. Some fungi, for example the ascomycete Wilcoxina mikolae, routinely produce ectendomycorrhizas on young plants of Pinus and Larix in nursery soils while forming ectomycorrhizas on Abies, Picea and Tsuga (Mikola, 1988). However, most ECM fungi are capable of forming intracellular penetrations in senescent parts of the root axis, or when the nutrient balance of the association is disturbed. In these circumstances, the fungus appears to be behaving in a weakly pathogenic manner. Many fungi that produce typical ectomycorrhizas on members of the Pinaceae and Fagaceae form extensive intracellular growths as well as Hartig net and sheath in certain ericaceous hosts. This type of colonization is recognized as being of the distinct ‘arbutoid’ category (see Chapter 7).

While the presence of the three structural elements signifies an ectomycorrhiza, there may be considerable variation in the extent to which Hartig net, mantle and extraradical mycelium develop. Indeed, structures that have only a patchy mantle, as in some Asteraceae (Warcup, 1980), that lack a mantle altogether, as in Pinus spp. colonized by Tricholoma matsutake (Ogawa, 1985; Yamada et al., 2006), or lack a Hartig net, as in the roots of Pisonia grandis (Ashford and Allaway, 1982), have been referred to as ‘ectomycorrhiza’. The danger in broadening the category to this extent is that relationships between structure and function established by study of ‘typical’ forms may break down. It is noteworthy in this context that no evidence for positive growth response to colonization was reported in any of the above studies. Of the many studies in which symb...