Preliminary characterization of fast growing rhizobial strains isolated from soyabean nodules in Brazil

A survey of soyabean rhizobia was carried out with six Asian and one modern soyabean genotypes as trap hosts. Soyabean seedlings were inoculated with soils from 22 Brazilian field sites, including undisturbed areas or areas traditionally cropped with this legume. A total of 30 fast growing strains, able to establish an effective symbiosis with both types of genotypes, were isolated from 12 of the 22 soils, representing 17% of the population in cropped areas and 24% in undisturbed soils. The bacterial mean generation time varied from 85 to 225 min and after 4 days of growth in YM medium the final pH ranged from 3.7 to 6.9. Although isolated from acid soils, only 37% of the strains were able to grow in TY or YM media at pH 4.0, while 60% were alkaline tolerant (pH 9.5). Most strains produced abundant extracellular polysaccharides (73%), were tolerant to 0.5 M NaCl (60%) and a temperature of 40°C (77%), grew in LB medium (67%) and synthesized melanin (53%). The strains differed in the use of C compounds supplied as sole C sources. The majority of the strains showed an intrinsic resistance to the antibiotics (μg ml−1) chloramphenicol (10), erythromycin (50), gentamicin (20), kanamicin (30), rifampicin (20) and tetracycline (10) and to the heavy metals cobalt chloride (0.5 mM) and potassium chromate (0.25 mM). A cluster analysis with 81 morphological and physiological parameters placed the strains from undisturbed soils in the central part of the dendrogram, even when isolated from distant areas, indicating that they had a common background. The relatedness decreased with the use of the soils for agriculture. Most of the strains isolated from cropped soils under conventional tillage were characterised by a high tolerance to stressful conditions and the ability to grow with several C sources, contrary to most of the strains isolated from soils under no-tillage management system. The strains differed from S. fredii in several characteristics and the protein and lipopolysaccharide profiles showed that each strain was unique. Therefore, although soyabean is an exotic plant in Brazil, several indigenous rhizobial strains may also establish an effective symbiosis with this legume.