Preface

I would like to take this opportunity to mention briefly and express my aim about this book.

Rhizobia are composed of specific groups of bacteria that have the ability to induce symbiotic nitrogen-fixing nodules on the roots or stems of leguminous plants. Rhizobia have attracted a great attention for more than 4 decades because of their enormous agricultural and economic values in sustainable agriculture. Up to present time, rhizobia are classified in to diverse taxonomic groups, distributing in 17 genera (118 species) of the alpha (α), beta (β) and gamma (γ) subclasses of Proteobacteria. The classification of rhizobia is becoming increasingly complex and is revised periodically because of the discovery of new rhizobial members in the other genera and species, the proposal of novel rhizobial genera and species as well as the criteria used for classification. An assessment of rhizobial diversity provides pivotal information in understanding the horizontal gene transfer among bacterial genera and species, the bacterial evolution and the symbiotic effectiveness.

The main concepts of this book present the background knowledge of rhizobia, the broad perspective on rhizobial diversity, the information on characteristics specific to each group of rhizobia, the relation between rhizobial groups and genetic factors contributed to rhizobial diversity, the contemporary methods for examination of rhizobial diversity, the plant-growth-promoting traits of rhizobia and the application of rhizobia. In the first chapter, classification and taxonomy of rhizobia are provided with general information of rhizobia, criteria used in classification of rhizobia, symbiotic gene transfer among rhizobial members, non-symbiotic rhizobia and symbiotic promiscuity of rhizobia. A compiled list of rhizobial species with their hosts is also included. The second chapter describes phenotypic diversity of rhizobia based on numerical analysis, enzyme production and serogroups. The third chapter is about symbiotic variation and plant-growth promoting traits of rhizobia including production of phytohormones, siderophores and ammonia, phosphate-solubilizing activity and production of ACC deaminase. The forth to eight chapters contribute to genotypic diversity of rhizobia, separating molecular techniques used for examination. The techniques listed in this book are the most frequently used ones including plasmid profile, PCR fingerprinting, PCR-RFLP, RFLP and sequence analysis. The details of each technique include principle concept, application to rhizobia, advantages and limitations as well as comparison of resolution level and consistency. The ninth chapter focuses on the application of rhizobia. The uses of rhizobia in agriculture and bioremediation, co-inoculations of rhizobia and other plant-growth-promoting rhizobacteria as well as influence of inoculants on genotypic diversity of indigenous rhizobia are involved.

I hope the book can bring the interesting information and ideas to readers world-wide. From background to specific sections, it may facilitate the proper and efficient experiments related to rhizobial and bacterial diversity.