Battling to form nodules

Competition between blue-coloured nodules and magenta-coloured nodules after inoculation.

Photo: George Mwenda

Rhizobium inoculants work well for soyabeans, whereas the response in other legumes such as the common bean is erratic. PhD student George Mwenda investigated why and discovered that some introduced strains of rhizobia are capable of displacing large populations of rhizobia already in the soil.

Rhizobium inoculants ensure that there are lots of rhizobia bacteria around the roots that emerge from the germinating seed so that the legumes can form numerous root nodules and fix large amounts of nitrogen. N2Africa researchers tested ‘off-the-shelf’ rhizobium inoculants with soyabeans in many countries with convincing results.

Rhizobium inoculants work well on the soyabean as it is a specific legume that needs a fairly particular type of rhizobium. Furthermore, soyabeans are often being grown for the first time in the farmers’ fields and so the soils contain very few rhizobia that can form nodules with soyabeans. This is the perfect situation for the test. Soyabean yields were greatly increased and farmers were soon convinced of the benefits of this magic “black powder”, as they called it.

Soyabean nodules up close.

Photo: Ken Giller

The situation in many other grain legumes is more complex. Much less research has been conducted on rhizobial inoculation of the common bean, cowpea, chickpea, faba bean and groundnut so this was a key research priority for N2Africa. Nodule formation in these plants generally involves a wide range of bacteria commonly present in the soil. These are often called ‘promiscuous’ legumes. Introduced rhizobia therefore have to compete against this large population of rhizobia in the soil.

A particular case is the common bean, studied by PhD student George Mwenda. “The common bean is generally seen as being much less able to fix nitrogen than many other grain legumes,” says George, who conducted his studies at the Centre for Rhizobium Studies at Murdoch University in Perth, Australia. “One of the reasons we think that the common bean does not fix nitrogen well is that soils contain a lot of bean rhizobia are only weakly able to fix nitrogen.”

Introduced nodule-forming rhizobia have to compete against rhizobia in the soil

These rhizobia are referred to as “poorly effective” or “ineffective”. When common beans are grown in soil that contains a large population of these ineffective rhizobia, it is difficult for the inoculant strain to compete against them to form nodules.

George Mwenda working in his lab on nodule inoculation. Photo: N2Africa

“In my PhD research, I studied this phenomenon for nodulation of the common bean,” says George. “Using techniques from molecular biology, I introduced gene markers into various strains of rhizobia. I marked the standard inoculant strain for common bean with a gene that turned nodules blue when exposed to a specific chemical. Other strains were marked with a gene that led to magenta-coloured nodules being formed. This let us study the competition for nodulation between different strains of rhizobia. The surprising result was that I found some strains were able to displace a large background population of rhizobia entirely!”

Video: Legume nodulation explained. Produced by Taskscape Associates

Many scientific researchers argue that there is a need for a search for novel strains of rhizobium that are better adapted to local conditions than the strains generally used in commercial inoculants. However, N2Africa’s experience indicated the opposite: off-the-shelf inoculants that contained elite strains produced good results in a wide range of soil types.

According to George Mwenda, his results are key to the development of the next generation of rhizobial inoculant strains that will be both highly effective at fixing nitrogen and highly competitive against a large background population of rhizobia.

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