Genes Involved in Microbe-Plant Interactions

Interdependence between species is a law of nature. The degree of this interdependence is vividly evident in the plant-microbial world. Indeed, there is no axenic plant in nature and one finds various forms of interac­ tions between these two kingdoms ranging from completely innocuous to obligate parasitic. Most of these interactions are poorly understood at the molecular and physiological levels. Only those few cases for which a molecular picture is emerging are discussed in this volume. With the advent of recombinant DNA technology and the realization that some of these interactions are very beneficial to the host plant, a spate of activity to understand and manipulate these processes is occurring. Microbes interact with plants for nutrition. In spite of the large number of plant-microbe interactions, those microbes that cause harm to the plants (i. e. , cause disease) are very few. It is thus obvious that plants have evolved various defense mechanisms to deal with the microbial world. The mecha­ nisms for protection are highly diverse and poorly understood. Some pathogens have developed very sophisticated mechanisms to parasitize plants, an excellent example for this being crown gall caused by a soil bac­ terium, Agrobacterium tumefaciens. A remarkable ingenuity is exhibited by this bacterium to manipulate its host to provide nitrogenous compounds which only this bacterium can catabolize. This is carried out by a direct gene transfer mechanism from bacteria to plants.

Section I. Recognition.- 1 Host Specificity in Rhizobium-Legume Interactions.- I. Introduction.- II. The Infection Process.- III. The Lectin Recognition Hypothesis.- IV. Bacterial Attachment to Legume Root Hairs as an Early Recognition Step.- V. Role of Legume Lectins in Attachment of Rhizobia.- VI. There Are Multiple Lectin Receptors on Rhizobium.- VII. Regulation of Lectin-Rhizobium Interactions.- VIII. Rhizobial Attachment Is Only a Piece of the Puzzle in the Infection Process.- IX. Nodulation and Host Specificity Genes Are Plasmid Encoded.- X. Concluding Remarks.- XI. References.- 2 Interaction of Agrobacterium tumefaciens with the Plant Cell Surface.- I. Introduction.- II. Evidence that Bacterial Attachment Is Required for Tumor Formation.- III. Methods Used to Assay Attachment of Agrobacterium to Plant Cells.- A. Indirect Methods.- B. Direct Assays.- IV. General Requirements for Attachment of A. tumefaciens.- V. Plant Receptors for the Attachment of A. tumefaciens.- A. The Nature of the Receptor.- B. Plant Host Range for Attachment of A. tumefaciens.- VI. Binding Sites of Agrobacterium tumefaciens for Attachment to Plant Cells.- A. The Role of the Bacterium in the Attachment Interaction.- B. The Nature of the Bacterial Binding Site.- C. The Role of Bacterial Cellulose Fibrils in the Attachment of A. tumefaciens.- D. The Genetics of Attachment of A. tumefaciens.- VII. A Model for the Attachment of A. tumefaciens to Plant Cells.- VIII. References.- Section II. Symbiosis.- 3 Legume-Rhizobium-Symbiosis: Host’s Point of View.- I. Introduction.- II. Origin of Symbiotic Nitrogen-Fixing Association.- A. Rhizobium as an Organelle.- B. Why Primarily Legumes?.- III. Role of the Host Plant in Symbiosis.- A. Early Responses of Host.- B. Controlled Invasion as a Means for Regulating Host Defense Response.- C. Phytohormones.- i) Action at Distance.- ii) Phytohormones Produced by Rhizobium.- D. Cell and Tissue Level Organization.- E. Nutritional Role of the Plant Host.- i) Carbohydrate Supply.- ii) Nitrogen Metabolism.- iii) Exchange of Other Nutrients.- IV. Plant Genes Involved in Symbiosis.- A. Genetics.- B. Molecular Studies.- i) Leghemoglobin.- ii) Discovery and Function of Nodulins.- V. Summary and Perspectives.- VI. References.- 4 Rhizobium-Leguminosae Symbiosis: The Bacterial Point of View.- I. Introduction.- II. Strategies of Infection.- III. Localization of Infectible Root Cells.- IV. Genetic Approach to the Analysis of Symbiosis.- V. Isolation and Cloning of Symbiotic Mutants.- VI. Rhizobium Sym(biosis) Plasmids.- VII. Chromosomal Location of Nitrogenase and Nodulation Genes.- VIII. Analysis of Host Specificity.- IX. Role of the Bacterial Cell Surface.- X. Future Studies.- XI. References.- 5 Nitrogen Assimilation in the Legume-Rhizobium Symbiosis: A Joint Endeavour.- I. Introduction.- II Rhizobial Metabolism.- A. Nitrogenase.- B. Ammonia Assimilation in Free-living Rhizobia.- C. Regulatory Controls on Ammonia Assimilation.- D. Nitrogen Metabolism in Rhizobia Under Symbiotic Conditions.- E. Summary.- III. Plant Metabolism.- A. Ammonia Assimilation.- B. Synthesis of Nitrogenous Transport Compounds.- C. Plant Genes Involved in Nodule Nitrogen Metabolism.- IV. Conclusions.- V. References.- 6 Hydrogenase and Energy Efficiency in Nitrogen Fixing Symbionts.- I. Hydrogenase — A Suitable Candidate for Genetic Manipulation?.- II. Hydrogen Evolution by Nitrogenase.- III. Hydrogen Loss by Root Nodules.- IV. Occurrence of Uptake Hydrogenase in Rhizobium.- V. Potential Benefits Associated with Uptake Hydrogenase.- VI. Relationships Between Hydrogenase Determinants, Plasmids and Other Symbiotic Genes.- VII. Biochemical Components of the Hydrogenase System.- VIII. Genetic Components of the Hydrogenase System.- IX. Cloning the Hydrogenase Genes.- X. Problems of Gene Stability and Gene Expression for a Cloned hup System.- XI. Conclusions.- XII. References.- 7 Symbiotic Relationships in Actinorhizae.- I. Introduction.- II. The Symbiotic Association.- A. The Host Plant.- B. The Endophyte.- C. Host Plant-Endophyte Specificity.- D. Host Plant-Endophyte Interactions.- III. Nitrogen Fixation.- IV. Conclusions.- V. References.- 8 Host-Fungus Specificity, Recognition and Compatibility in Mycorrhizae.- I. Introduction.- II. Plant-Fungus Specificity.- III. Host-Fungus Interactions.- A. Rhizosphere Environment.- B. Recognition Phenomena.- i) Ectomycorrhizae.- ii) Ectendomycorrhizae.- iii) Endomycorrhizae.- C. Interactions of Mycorrhizal Fungi with Non-Host Plants.- IV. Functional Compatibility.- V. References.- 9 Molecular Biology of Stem Nodulation.- I. Introduction.- II. Occurrence of Stem-Nodulating Legumes.- III. Structure of Stem Nodules.- IV. Host-Specificity and Physiology of Stem Nodulation in Aeschynomene.- V. Identification of Stem and Root Leghemoglobins of Aeschynomene.- VI. Characterization of Stem Rhizobia.- VII. Genetic Manipulation of Stem Rhizobium.- VIII. Future Outlook.- IX. References.- Section III. Plant Tumor Induction.- 10 Induction of Cell Proliferation by Agrobacterium tumefaciens and A. Rhizogenes: A Parasite’s Point of View.- I. Introduction.- II. Crown Gall Tumor Cell Phenotypes.- III. Plasmid Logic and Crown Gall Cell Phenotype.- IV. Mechanism Underlying the Opine Concept.- V. Further Extensions of the Opine Concept.- VI. Conclusion.- VII. References.- 11 Gene Organization of the Ti-Plasmid.- I. Introduction.- A. Crown Gall.- B. Agrobacterium tumefaciens.- C. Ti-Plasmids.- II. Molecular Genetics of Ti-Plasmids.- A. DNA Homology Among Ti-Plasmids.- B. Genetic Map of a Ti-Plasmid.- C. Catabolic Functions and Conjugative Transfer.- D. Replication and Incompatibility.- E. The T-Region.- F. The Vir-Region.- III. General Conclusion.- IV. References.- 12 Phytohormone-Mediated Tumorigenesis by Plant Pathogenic Bacteria.- I. Introduction.- II. Crown Gall.- A. IAA Synthesis in Crown-Gall Tumor Cells.- B. IAA Synthesis by A. tumefaciens.- C. Cytokinin Synthesis in Crown Gall Tumors.- D. Cytokinin Synthesis by A. tumefaciens.- E. A Mechanism for the Role of IAA and Cytokinin in Crown Gall Formation.- III. Olive Knot.- IV. Fasciation and Witches’ Broom.- V. Concluding Remarks.- VI. References.- Section IV. Plant Pathogens and Defence Mechanisms.- 13 Genetic and Biochemical Basis of Virulence in Plant Pathogens.- I. Introduction.- II. Genetic Control of Host-Pathogen Interactions.- A. The Gene-for-Gene Hypothesis for Race/Cultivar Specificity.- B. Genetic Analysis in Other Systems.- III. Toxins as Virulence Factors.- IV. Cutinases and Pectinases.- V. Extracellular Macromolecules Implicated in Vascular Wilts.- VI. Hypersensitive Reaction, Phytoalexins, and Inhibitor Detoxification.- VII. Summary.- VIII. References.- 14 Defense Responses of Plants.- I. Introduction.- II. Phytoalexin Induction.- III. Proteinase Inhibitor Induction.- IV. Molecular Cloning and Characterization of Proteinase Inhibitor Genes.- V. Summary.- VI. References.