To avoid infection with the wrong type of microbes, legume plants need to accurately identify the beneficial symbiotic bacteria that help them to fix nitrogen. A group of scientists working on the ENSA project have discovered the mechanism that enables legumes to do this, offering a major step towards their goal to engineer nitrogen fixation in cereals.
The Engineering Nitrogen Symbiosis for Africa (ENSA) team includes Professor Giles Oldroyd, director of the Crop Science Centre, an alliance between the University of Cambridge and NIAB, that is working to enable sustainable production for everyone.
Nitrogen fixation in cereals
Professor Oldroyd is working at the forefront of research that aims to transfer the ability to fix nitrogen to other types of plants, like wheat or barley. This would increase growth and yield for these crops – particularly in developing countries where farmers have less access to nitrogen fertilisers.
Nitrogen is the most abundant gas in the atmosphere and legumes are able to take nitrogen out of the air and incorporate it into their cells. This is possible because legumes have developed a symbiotic relationship with a particular type of soil bacteria called Rhizobia, that are housed within their roots. These bacteria take up (or ‘fix’) the nitrogen and pass it to the plant in exchange for sugars and other nutrients. This function enables legumes to grow with less nitrogen fertiliser.
The research, published in the journal Science, has found that legumes use small, well-defined motifs in receptor proteins to read molecular signals produced by both pathogenic and symbiotic microbes and this is used to direct the response towards either antimicrobial defence or symbiosis.
These remarkable findings have enabled the researchers to reprogram immune receptors into symbiotic receptors, which is the first milestone for engineering symbiotic nitrogen fixing symbiosis into cereal crops.
All land plants have LysM receptors that ensure detection of various microbial signals, but how a plant decides to mount a symbiotic or an immune response towards an incoming microbe is unknown.
“We started by asking: Can we identify the important elements by using very similar receptors, but with opposing function as background for a systematic analysis?” says Zoltán Bozsoki. “The first crystal structure of a Nod factor receptor was a breakthrough. It gave us a better understanding of these receptors and guided our efforts to engineer them in plants.”
The multidisciplinary team used structural biology and biochemistry with the systematic functional tests in plants. Using this approach, the researchers identified previously unknown motifs in the LysM1 domain of chitin and Nod factor receptors as determinants for immunity and symbiosis.
“There are only very few, but important, residues that separate an immune from a symbiotic receptor and we now demonstrate that it is possible to reprogram LysM receptors by changing these residues” says Kasper Røjkjær Andersen.
The long-term goal is to transfer the unique nitrogen-fixing ability that legume plants have into cereal plants to limit the need for polluting commercial nitrogen fertilisers and to benefit and empower the poorest people on Earth.
To read the paper