Isolation, identification, and characterization of novel, aluminium tolerant rhizobia strains from Kenyan soil

The agricultural sector is the cornerstone of Kenya’s economy. It employs about 40% of Kenya’s population, more than 70% in rural areas, and contributes to about 33% of Kenya’s gross domestic product (GDP). Kenya’s vision 2030 identifies agriculture, including common bean cultivation, as one of the main pillars that will play a role in achieving sustainable 10% annual GDP. However, the main impediment to realizing this goal is the high cost of fertilizer which is not affordable for most farmers. An alternative to nitrogenous fertilizers is the application of rhizobia for common bean production, which converts atmospheric dinitrogen to reduced ammonium, suitable for plant use. Inoculant formulations must include local elite rhizobia because they can compete with the indigenous rhizobia in the soil and tolerate the harsh conditions in Kenyan soils. I isolated three nitrogen-fixing rhizobia (B3, S2, and S3) from these soils. In greenhouse experiments, plants infected with these rhizobia produced more biomass and accumulated more nitrogen than uninoculated plants and plants inoculated with a commercial strain CIAT899 which Kenyan farmers widely use. The biomass of the plants infected by either of the three isolates was comparable to that of plants supplied with nitrogenous fertilizer. Field experiments in Kenya demonstrated that the seed dry weight of plants inoculated with S3 was significantly higher than that of all other plants. S3 conferred more benefits to the plants than S2 and B2 and the commercial strain CIAT899. Kenyan soil faces detrimental abiotic stresses, particularly low pH and the toxic metallic ion Al. The three isolates were better adapted to low pH and Al toxicity than CIAT899. Isolate B3 grew in media with pH 4.8, a pH that was detrimental to the commercial isolate CIAT899. Short-term viability assays and long-term recovery experiments demonstrated that B3 performs better under Al stress than CIAT899. Al did not only bind to the rhizobia membran