Jason Terpolilli

The use of rhizobiumRhizobium inoculants for improvement in nitrogen-fixationNitrogen and productivity of grainGrainlegumesLegume has been well established in developed countries. However, the practice is still under-utilized in NigeriaNigeria. NitrogenNitrogen (N) is the most frequently deficient nutrient for crop production, while nitrogenNitrogenfertilizersFertilizers are costly, inadequate, and may not be timely in supply. These make rhizobia inoculants a cheaper, easier and safer option to improve the N2-fixation and productivity of grainGrainlegumesLegume. Inoculant use in NigeriaNigeria was initiated in the 1970s, but still remains very limited. Studies conducted on inoculant use were initially on “US type” SoybeanSoybean (Glycine max (L.) Merrill), which has been found to require specific inoculation with Bradyrhizobim japonicum for optimum productivity. Studies were also conducted on inoculation of cowpeaCowpea (Vigna unguiculata (L.) Walp), but rarely on bambara groundnutGroundnut (Vigna subterranea (L.) Verdc.) and groundnutGroundnut or peanutPeanut (Arachis hypogaea L.). In the 1980s, the International Institute of TropicalTropicalAgricultureAgriculture (IITA) Ibadan, NigeriaNigeria, introduced promiscuous soybeanSoybeancultivarsCultivars; TropicalTropical Glycine Cross (TGx). These genotypesGenes nodulate freely with the indigenous rhizobiumRhizobium population, fix large amount of atmosphericAtmospherenitrogenNitrogen and produce higher grainGrainyieldsYields than the localLocal genotypes. However, some experiments indicated up to 40–45% increases in yieldYields by some of the genotypesGenes on inoculation. Hence, the ultimate solution remains the development of inoculants using highly effective indigenous rhizobia strains for particular crops. The recent efforts of the project “Putting NitrogenNitrogenfixationNitrogen fixation to work for smallholder farmersFarmers in Africa (N2Africa)Africa” towards the promotion of inoculants technology are highly welcomed in the country.

After strains of rhizobia have been isolated from nodules (Chapter 3), and (ide¬ally) before long-term preservation (Chapter 4), the strains should be examined to ensure they retain the essential features of nodule bacteria. The first step in this process is termed ‘authentication’, which examines the ability of the strain to in¬fect a legume to form a nodule. Following this, strains may be evaluated for their ability to fix nitrogen. This latter characteristic is sometimes termed ‘effectiveness’; it is an assessment of the genetic compatibility between the host plant and the rhizobium strain for nitrogen fixation. If a strain can nodulate a legume and fix N2 effectively in the glasshouse environment, the researcher may wish to proceed further, to assessment in the field. However, if the strain is to be released to the field, then ‘duty of care’ requires that we have an understanding of its host-range (Section 5.2). This is because releasing strains into the general environment that might be detrimental to existing legumes either agricultural or natural would be negligent. The techniques described in this chapter allow a researcher to compare strain symbiotic performance across a spectrum of plant genotypes to fulfil this duty of care.

The Medicago genus is of global importance to agriculture, with the perennial M. sativa being the most widely cultivated and studied member. After many years of studying this plant along with its microsymbiont Sinorhizobium meliloti, it became clear that another host was required to allow simultaneous study of the genetic determinants of both symbiotic partners.M. sativawas unsuited to this role as it is autotetraploid, allogamous and shows strong in-breeding depression, making the analysis of recessive mutations no easy task. Researchers identified the annual medic M. truncatulaas a viable alternative as this host is diploid, autogamous and possess a rapid generation time, among other traits. Consequently, this organism was chosen for sequencing.