Healthy soil bacteria for healthy crops

Varieties of beans at a local market in Chiapas, Mexico (credit: Marcela Mendoza)

Varieties of beans at a local market in Chiapas, Mexico (credit: Marcela Mendoza)

I’m originally from Mexico, where legume plants such as beans are an important part of our basic food. For low-income families that do not have access to animal products, they are a major source of protein, minerals, fibre, carbohydrates and vitamins. Legumes are also the main source of protein elsewhere in the world, for example throughout Latin American, in many African countries and in India. Unfortunately, in these places, low-income farmers cannot access chemical fertilisers, which would help increase yields, due to their high cost.

On the other hand, if access to chemical fertilisers is not limited, the overuse of plant nutrients can have negative environmental impacts, as seen for example in the process of eutrophication. This occurs when some of these fertilisers leach into ground water, leading to the overgrowth of plants and algae in rivers, lakes and oceans, and results in oxygen depletion in the water, and ultimately an unhealthy ecosystem.

I wanted to help both farmers that do not have access  to chemical fertilisers, and farmers that overuse chemical fertilisers, by studying the “fertilisers” that exist in nature.

Legumes, which include peas and beans, have evolved the ability to host N2 fixing bacteria, known as rhizobia, in specialized organs called root nodules. In this symbiotic partnership, legumes supply nutrients to rhizobia, and in return, rhizobia fix N2 gas from the atmosphere into reduced forms that are supplied to the legume.

Eutrophication at Christ Church Meadow in Oxford, UK (credit: Marcela Mendoza)

Eutrophication at Christ Church Meadow in Oxford, UK (credit: Marcela Mendoza)

This symbiotic relationship has been used in agriculture for many years, via the use of rhizobial inoculants, both as an alternative to economically expensive chemical fertilisers and in organic crop systems. However, in some cases, commercial inoculants fail to compete against native rhizobial strains with inferior N2 fixing abilities. This agricultural dilemma is termed the “rhizobial competition problem”, and results in unhealthy crops.

Until now, the search for elite strains has been a time-consuming process involving numerous studies in the laboratory, followed by assessment of the strains in greenhouse and field trials. To improve the process of identifying elite rhizobial inoculants, it is crucially important to increase the number of rhizobial strains that can be evaluated simultaneously, in the presence of native rhizobia, alongside the assessment of their N2 fixing ability.

Pea crop field in Oxfordshire (credit: Marcela Mendoza)

Pea crop field in Oxfordshire (credit: Marcela Mendoza)

During my DPhil in Professor Phil Poole’s lab in the Department of Plant Sciences, we used synthetic biology techniques to develop a method in which dozens of rhizobia can be individually tagged, the rates of nitrogen production in single nodules assessed, and the rhizobia living inside these nodules identified. (https://www.pnas.org/content/117/18/9822)

Improving rhizobial inoculants that can be used as an alternative to chemical fertilisers can help the most vulnerable farmers who do not have access to chemical fertilisers, and at the same time it can help to reduce the environmental impact in countries where chemical fertilisers are overused. In summary, out work is an important step towards achieving sustainable agriculture by having healthy crops, improving the economic situation of farmers and reducing environment impacts, which is in synchrony with the aims of the UN Sustainable Development Goals.

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The United Nations General Assembly declared 2020 as the International Year of Plant Health (IYPH). The year is a once in a lifetime opportunity to raise global awareness on how protecting plant health can help end hunger, reduce poverty, protect the environment, and boost economic development. For more information: http://www.fao.org/plant-health-2020/about/en/