Graham O’Hara, Wayne Reeve, Jason Terpolilli, Ravi Tiwari, John Howieson
Centre for Rhizobium Studies, School of Biological Sciences and Biotechnology,
Murdoch University, Australia

Crop, pasture and forage legumes are important components in many productive agricultural systems. Symbiotic nitrogen-fixation is often essential for the establishment, productivity and sustainability of these legumes. In many situations inoculation with compatible rhizobia is required because soils do not naturally contain rhizobia that form effective nitrogen-fixing symbioses with these legumes. During the past 15 years the Centre for Rhizobium Studies (CRS) has led the National Rhizobium Program in Australia and developed 15 new strains of rhizobia for commercial production as inoculants, and contributed to development of new legumes which, together with their rhizobia, have been sown over 5 million ha. in southern Australia. In addition, new insights have been provided into phenomena affecting symbiotic performance e.g. the role of horizontal gene transfer in development of poorly effective soil populations of rhizobia, and selective nodulation. The CRS culture collection currently houses >6,000 strains of internationally important rhizobia. This collection is being consolidated as one of the premier world repositories of strains of rhizobia with unique agricultural, environmental, ecological and scientific importance.
Selective nodulation describes an active phenomenon that results in the establishment of a symbiosis between a legume and an effective strain of rhizobia in the presence of ineffective strains. Selective nodulation can overcome a numerical disadvantage in the effective microsymbiont when there is a population size window of between 1,000 and 100,000 cells. The existence of selective nodulation in major agricultural legumes such as common bean (Phaseolus), soybean (Glycine) and clover (Trifolium spp.) is being investigated using molecular techniques combined with traditional glasshouse and field-based studies.
Alternative species of herbaceous perennial legumes (e.g. Lebeckia spp., Rynchosia spp.) originating in southern Africa and nodulated by novel Burkholderia spp. are being evaluated for their potential role in providing green feed in autumn and late spring. Target soils do not sustain traditional legumes (e.g. lucerne, white clover, lotus) because of abiotic stresses such as low pH, low clay content, low nutrient status and low rainfall. Glasshouse grown plants nodulated promptly and fixed N when inoculated with high numbers of compatible inoculant strains. However inoculated plants failed to nodulate in the field. Investigation showed poor survival of strains when inoculated onto seed (100% mortality in 4 h). Seed toxicity was not the cause. Further study showed strains varied in survival on seed and selection of a robust strain has enabled field evaluation to proceed.