This book is the self-contained sixth volume of a comprehensive series on nitrogen fixation. It presents the state-of-the-art in regards to actinorhizal symbioses. Like legumes, actinorhizal plants form root nodules that host nitrogen-fixing soil bacteria. However, because the macrosymbionts are, with one exception, woody plants rather than crop plants, actinorhizal symbioses are less well-known than legume symbioses to which they are phylogenetically related. Actinorhizal plants come from eight different families. They can grow on marginal soils by virtue of these symbioses and are used extensively in reforestation, soil reclamation, and desert agroforestry. The diversity of the involved host plants poses a variety of challenges to the actinorhizal symbiosis and results in interesting strategies, for example, to cope with the O2 dilemma or nutrient exchange between plant and bacterium. The actinorhizal micro-symbionts are Gram-positive actinomycetes of the genus Frankia. The inability to culture several actinorhizal microsymbionts has led to the development of diverse molecular strategies for strain identification. This volume includes chapters that deal with all these aspects of the symbiosis and both symbionts plus their ecological role and use. Other chapters tackle the global distribution of different actinorhizal plants and their microsymbionts and how this impacts the question of co-evolution of the micro- and macrosymbionts as well as comparing the actinorhizal and leguminous symbioses. No other book provides the up-to-date and in-depth coverage of this volume, which is intended to serve as an indispensable reference work for academic, governmental, and industrial scientists working in this area, to introduce students to the global importance of this association, and to provide science administrators with ready access to vital relevant information.

Zusammenfassung
Nitrogen-fixing Actinorhizal Symbioses This book is part of a seven-volume series that was launched in 2004 and covers all aspects of nitrogen fixation from the biological systems to the industrial processes. Volume 6 covers nitrogen-fixing actinorhizal symbioses, which occur between soil actinomycetes of the genus Frankia and a diverse group of dicotyledonous plants, collectively called actinorhizal plants. These symbioses play vital roles in native ecosystems as well as important components in both forestry and land reclamation. The volume is divided into 11 chapters, all authored by well-known scientists in the field. As in previous volumes of this series, the first chapter presents an historical perspective and describes the development of actinorhizal research with its focus on the period after the first reproducible isolation of the responsible microorganism by John Torrey's group in 1978. Very early on, the initial attempts to characterize the bacterium taxonomically had considered this endosymbiont as an obligate symbiotic bacterium and used its ability to form root nodules and its morphological characteristics within root-nodule cells as discriminative criteria to distinguish it from other actinomycetes. These efforts led to the emendation of the family Frankiaceae with the type genus Frankia and also to the definition of host-specificity groups based on inoculation experiments using crushed nodules. However, after Frankia strains were isolated from nodules and pure cultures became available, many of these early results had to be discarded. Chapter 2 describes the techniques used to obtain phenotypic, genotypic and phylogenetic information on the members of the genus Frankia.

Inhalt

Preface to the Series, Preface, List of Contributors, Dedication.
1. Frankia and Actinorhizal Plants: A Historical Perspective: C. T. Wheeler, A. D. L. Akkermans, and A. M. Berry
1. Introduction

2. The Early Years
3. Two Decades to the New Millennium

4. Perspectives
   References

   2. Polyphasic Taxonomy of the Genus Frankia:
   D. Hahn
   1. Introduction

5. Polyphasic Taxonomy Approach
6. 16S-rRNA Sequence Data
7. DNA-DNA Relatedness
8. 23S-rRNA Sequence Data
9. Genomic Fingerprinting

10. Conclusions
    References

    3. Frankia Ecology:
    M. Valdés
    1. Introduction

11. Frankia as a Soil Microorganism
12. Abiotic Soil Factors and Frankia Populations
13. Biotic Soil Factors: Mycorrhizal Fungi and Frankia

14. Conclusions References

    4. Evolution of Actinorhizal Host Plants and Frankia Endosymbionts:
    S. M. Swensen and D. R. Benson
    1. Introduction

15. Host-Plant Systematics and Phylogeny
16. Frankia Phylogeny and Evolution
17. Host-Frankia Interactions and Evolution

18. Future Directions
    References

    5. Oxygen Responses, Hemoglobin, and the Structure and Function of Vesicles:
    W. B. Silvester, R. H. Berg, C.R. Schwintzer and J. D. Tjepkema
    1. Introduction

19. Vesicle Structure and Oxygen Responses of Frankia in vitro
20. Responses in Symbiosis
21. Hemoglobins in Actinorhizal Nodules and Frankia

22. Concluding Discussion
    References

    6. Early Interactions, Infection and Nodulation in Actinorhizal Symbiosis:
    L. G. Wall and A. M. Berry
    1. Introduction

23. Early Interactions
24. Rhizosphere Colonization
25. Root Infection by Frankia
26. Nodule Development

27. Regulation of Nodulation
    References

    7. Carbon and Nitrogen Metabolism in ActinorhizalNodules:
    C. Valverde and K. Huss-Danell
    1. Introduction

28. The Actinorhizal Nodule
29. Studying C and N Metabolism of Nodules
30. Carbon Supply to Nodules
31. Carbon Uptake and Metabolism by Symbiotic Frankia
32. Nitrogen Metabolism
33. The Regulation of N Assimilation

34. ProspectsReferences

    8. Ecology of Actinorhizal Plants:
    J. O. Dawson
    1. Importance of Actinorhizal Plants

35. Occurrence and Distribution of Actinorhizal Plant Taxa and their microsymbionts
36. Ecological Factors Influencing Infective Frankia Populations

37. Ecological of Actinorhizal Plants
    References

    9. Molecular Biology of Actinorhizal Symbioses:
    L. Laplaze, S. Svistoonoff, C. Santi, F, Auguy, C. Franche and D. Bogusz
    1. Introduction

38. Infection Process
39. Nodule Development
40. Nodule Functioning
41. Evolutionary Origin of Symbiotic Genes
42. Future Trends

43. Conclusions
    References

    10. Comparison between Actinorhizal and Legume Symbiosis:
    K. Pawlowski and J. I. Sprent
    1. Introduction

44. Nodule Structure
45. Nodule-Induction Mechanisms
46. Host Specificity
47. Root Nodules and Other Root Symbioses

48. Evolution of Root-Nodule Symbioses
    References

    11. Prospects for the Study of a Ubiquitous Actinomycete, Frankia, and Its Host Plants:
    P. Normand and B. C. Mullin

49. Introduction
50. Development and Use of Molecular Tools for Genetic Analysis of Symbiosis
51. Prospects for Future Utilization of Actinorhizal Plants
52. Questions Still to be Addressed
53. Prospects for Extending Symbioses beyond Current Host Range

54. Conclusions
    References

    Subject Index