Interviews with scientists:

Feeding sub-Saharan Africa

Teaching Notes

Introduction and context

In the west, we’ve spent 50 years relying on increasing food yields by adding nitrogen-based fertilisers to the soils. But it’s not an approach that seems to be working for the millions of smallholder farmers across western and central Africa. Local farmers, agronomists and scientists from across the world are working together to find innovative solutions.

In this video interview, Professor Giles Oldroyd of the John Innes centre discusses his research.

Teacher summary

Professor Oldroyd works on Nitrogen fixation in Legumes; a group of plants, mostly peas and beans, that form associations with Nitrogen fixing bacteria in their roots.

Nitrogen fertilisers are used to promote plant growth in agriculture. The earth’s population today is about seven billion. It is estimated that the natural nitrogen cycle can only accommodate about three billion people. That means that about 4 billion people are dependent on the application of inorganic fertiliser.

Smallholder farmers in sub-Saharan Africa have very little access to the nitrogen fertilisers that have driven crop productivity around the world and as a result, their productivities are down at 20 or 30% of their potential productivity, had they used fertilisers. Fertilisers cost three to four times as much in sub-Saharan Africa as they do in the UK. Part of this is because fertilisers are bulky and expensive to transport, so getting them to a small-holder farmer in rural Africa with no road infrastructure is very expensive. The small-holder farmers have very few financial resources available to them and wouldn’t be able to afford to buy the fertilisers.

Inorganic Nitrogen fertilisers are very soluble. A lot of the fertiliser that is applied to a crop is taken up by the crop plants, but a lot also escapes into the environment, usually when it rains. The fertiliser is washed into rivers, streams and the ground water, where it adds nitrates to our drinking water supplies. It is also washed into rivers, streams and shallow seas where it greatly increases the nutrient content of these usually low-nutrient environments, causing massive algal blooms and collapses in the diversity of those systems.

Sub-Saharan smallholders should be benefitting from the technologies that have allowed farmers in other parts of the world to increase their productivity. However, because of the environmental consequences of fertiliser use, it would be preferable if inorganic fertiliser use was reduced across the whole planet. The challenge is to find a way of increasing food production for global food security in a sustainable manner.

Legumes have evolved a beneficial interaction with nitrogen-fixing bacteria. These free-living soil bacteria associate with legume roots. The bacteria can fix atmospheric nitrogen, which is molecular di-nitrogen, using an enzyme called nitrogenase, and convert it into ammonia. Bacteria are the only living organisms on the planet that have the enzyme nitrogenase. No other organism can use molecular di-nitrogen, which is the predominant form of Nitrogen on earth. The ammonia produced by the nitrogen-fixing bacteria can be used inside the legume plants, allowing the legumes to overcome the problem of limited nitrogen availability in soils.

African farmers grow legumes, either co-planted with maize or as a rotation crop, to enrich soil nitrogen. However, if a beneficial symbiosis with nitrogen-fixing bacteria could be developed in cereal crops, maximum productivity of cereals could be achieved. Professor Oldroyd’s aspirational research is attempting to transfer the nitrogen-fixing capabilities from legumes to cereals. He is focusing on the signal-transduction pathway, present in all legumes, that allows the recognition of nitrogen-fixing bacteria. The bacteria release a signalling molecule, called nod-factor, and the plant recognises that signalling molecule, using the signal transduction pathway. Cereals also have this signalling pathway, which they use to form beneficial associations with mycorrhizal soil fungi. Only in legumes does the signalling pathway have bi-functionality, allowing it to facilitate the accommodation of both the nitrogen-fixing bacteria and the mycorrhizal fungi. Professor Oldroyd’s research is attempting to engineer the cereal signalling pathway to be bi-functional, analogous to its role in legumes. It is a first step towards transferring the capability to recognise nitrogen-fixing bacteria into cereal crops.

Even small amounts of nitrogen can greatly increase the yields of maize and rice. Relatively modest levels of nitrogen-fixation, much lower than you would see in a fully functional legume nodule, could double yields in small-holder farming in Africa, simply because their crops are so nutrient limited.

Science is hypothesis driven. Professor Oldroyd started with one hypothesis and his research proved this initial hypothesis to be wrong. However, the research has increased the understanding of how the signalling pathway works, allowing new hypotheses to be developed. This is the ongoing process of science. You are continually asking new questions, adapting your questions according to the knowledge you have and trying to design new hypotheses and new experiments that can address those questions.

Questions

  1. Use a diagram to explain what is meant by the ‘natural nitrogen cycle’.
  2. Give two reasons why smallholder farmers in sub-Saharan Africa have little access to inorganic nitrogen fertilisers.

(i) Fertilisers cost three to four times as much in sub-Saharan Africa as they do in the UK. Part of this is because fertilisers are bulky and expensive to transport, so getting them to a small-holder farmer in rural Africa with no road infrastructure is very expensive.

(ii) The small-holder farmers have very few financial resources available to them and wouldn’t be able to afford to buy the fertilisers.

  1. Explain how inorganic nitrogen fertilisers applied to crops can cause pollution of drinking water and eutrophication.

Inorganic Nitrogen fertilisers are very soluble. A lot of the fertiliser that is applied to a crop is taken up by the crop plants, but a lot also escapes into the environment, usually when it rains. The fertiliser is washed into rivers, streams and the ground water, where it adds nitrates to our drinking water supplies. It is also washed into rivers, streams and shallow seas where it greatly increases the nutrient content of these usually low-nutrient environments, causing massive algal blooms and collapses in the diversity of those systems.

  1. Explain how leguminous plants can grow in nitrogen-limited environments.

Legumes have evolved a beneficial interaction with nitrogen-fixing bacteria. These free-living soil bacteria associate with legume roots. The bacteria can fix atmospheric nitrogen, which is molecular di-nitrogen, using an enzyme called nitrogenase, and convert it into ammonia. Bacteria are the only living organisms on the planet that have the enzyme nitrogenase. No other organism can use molecular di-nitrogen, which is the predominant form of Nitrogen on earth. The ammonia produced by the nitrogen-fixing bacteria can be used inside the legume plants, allowing the legumes to overcome the problem of limited nitrogen availability in soils.

  1. What is the aim of professor Oldroyd’s research?

Professor Oldroyd’s research is attempting to transfer the nitrogen-fixing capabilities from legumes to cereals

  1. Why is this research important?

Sub-Saharan smallholders should be benefitting from the technologies that have allowed farmers in other parts of the world to increase their productivity. However, because of the environmental consequences of fertiliser use, it would be preferable if inorganic fertiliser use was reduced across the whole planet. The challenge is to find a way of increasing food production for global food security in a sustainable manner.

Even small amounts of nitrogen can greatly increase the yields of maize and rice. Relatively modest levels of nitrogen-fixation, much lower than you would see in a fully functional legume nodule, could double yields in small-holder farming in Africa, simply because their crops are so nutrient limited.

  1. How do leguminous plants detect the presence of nitrogen-fixing bacteria?

The nitrogen-fixing bacteria release a signalling molecule, called nod-factor, and the plant recognises that signalling molecule, using the signal transduction pathway.

  1. The signalling transduction pathway in leguminous plants is described as bi-functional. Explain why.

The signalling pathway facilitates the accommodation of both the nitrogen-fixing bacteria and the mycorrhizal fungi.

  1. Why is professor Oldroyd’s research attempting to engineer the signalling transduction pathway in cereals to be bifunctional?

It is a first step towards transferring the capability to recognise nitrogen-fixing bacteria into cereal crops.

  1. Outline the ‘ongoing process of science’

Scientists are continually asking new questions, adapting those questions according to the knowledge gained and trying to design new hypotheses and new experiments that can address those questions.

Science & Plants for Schools:

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