Abstract
Common bean is one of the primary protein sources in third-world countries. They form nodules with nitrogen-fixing rhizobia, which have to be adapted to the local soils. Commercial rhizobial strains such as Rhizobium tropici CIAT899 are often used in agriculture. However, this strain failed to significantly increase the common bean yield in many places, including Kenya, due to the local soils’ low pH. We isolated two indigenous rhizobial strains from the nodules of common bean from two fields in Western Kenya that have never been exposed to commercial inocula. We then determined their ability to fix nitrogen in common beans, solubilize phosphorus, and produce indole acetic acid. In greenhouse experiments, common bean plants inoculated with two isolates, B3 and S2 in sterile vermiculite, performed better than those inoculated with CIAT899 or plants grown with nitrogen fertilizer alone. In contrast to CIAT899, both isolates grew in the media with pH 4.8. Furthermore, isolate B3 had higher phosphate solubilization ability and produced more indole acetic acid than the other two rhizobia. Genome analyses revealed that B3 and S2 are different strains of Rhizobium phaseoli. We recommend fieldwork studies in Kenyan soils to test the efficacy of the two isolates in the natural environment in an effort to produce inoculants specific for these soils.
Highlights
Common bean (Phaseolus vulgaris L.) is an essential source of proteins, carbohydrates, vitamin B complex, and vital minerals in human nutrition
Reinoculating the bacteria in the common bean identified 10 isolates that generated nodules, which confirmed them as rhizobia
Inoculation with these two rhizobia allowed the common bean to grow without nitrogen supply, improved plant health (Figure 1A) and yield in terms of total dry weight (Figure 1B) and tissue nitrogen content (Figure 1C)
Summary
Common bean (Phaseolus vulgaris L.) is an essential source of proteins, carbohydrates, vitamin B complex (riboflavin, thiamine, niacin, and folic acid), and vital minerals in human nutrition. Flavonoids and isoflavonoids, mainly produced as defense compounds against phytopathogens, and antioxidants as protectors against UV radiations act as anti-cancer agents that inhibit the tyrosine kinase, cyclooxygenase, protein kinase C, and lipoxygenase enzyme activities (RomeroArenas et al, 2013). Common bean is a source of essential nutrients and has medicinal features. In many parts of the world, the production of common bean is restricted by poor soil fertility, nitrogen limitations (Simon et al, 2014). The farmers cannot compensate for this shortage by applying fertilizers, mainly because of economic reasons (cf Khonje, 1989; Onim, 1993; Savci, 2012).
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