Collecting nodules for isolation of rhizobia

For the legumes widely used in commerce, rhizobial strains that are well matched to these for nitrogen fixation (i.e. highly effective at N2 fixation) are usually avail¬able from manufacturers or from gene banks. Some of these sources are listed in Table 2.1. However, new acquisitions of rhizobia may be required for many reasons, for example: to overcome poor N2 fixation from an existing symbiotic relationship as described in Case 2, Chapter 1; to select well-adapted strains for a difficult environment (e.g. acid soil); to assist in legume domestication programs; to match with sequenced legumes for genetic studies of N2 fixation (e.g. Terpolilli et al. 2008); or to undertake biodiversity studies. If researchers feel that currently available strains may not satisfy their research program, then a broader range of rhizobium germplasm must be sought. Strains of rhizobia for many legumes have long been collected from their natural envi¬ronments for this purpose. These environments represent in situ repositories of rhizobium genetic resources (Date 1982). After the collection, isolation and eval¬uation of new strains they should be deposited in curated gene banks. The bulk of this chapter covers methods to acquire nodule bacteria from in situ sources.