A big part of modern agriculture depends on artificial fertiliser supplying nitrogen to increase yield. However, artificial fertiliser is both expensive and can potentially harm the environment. Legumes do not need nitrogen fertiliser, and now researchers have identified how many other plants can circumvent the problem.
Legumes have characteristics that agriculture would like to be able to copy – a very special symbiotic relationship with bacteria that produce large quantities of nitrogen on which the legumes can thrive in return for carbohydrate.
This symbiotic relationship with nitrogen-fixing bacteria means that legumes can grow in soil without significant nitrogen, which other crops cannot do.
Crops without this relationship to nitrogen-fixing bacteria need considerable nitrogen to grow, which requires that farmers buy expensive artificial fertiliser and apply it to their fields. This affects both farm profitability and the environment, which is harmed when fertiliser runoff reaches watercourses and lakes.
However, good news may be on the way, since researchers have recently identified how legumes create an optimal molecular biological environment for nitrogen-fixing bacteria in their roots. This discovery may eventually open up the possibility that other crops, such as cereals, can mimic this.
“If we want to help agriculture to become more sustainable, we need to determine how to reduce the use of artificial fertiliser. It is interesting that legumes do not need nitrogen to the same degree and can therefore thrive with much less fertiliser,” explains a researcher behind the study, Henriette Rübsam, Postdoctoral Fellow, Department of Molecular Biology, Aarhus University.
The research has been published in Science.
Symbiosis benefits all
Researchers have long been aware of the symbiosis between nitrogen-fixing bacteria and legumes, but some gaps in their knowledge still exist. The plants can distinguish between beneficial and harmful bacteria and develop small root nodules in which the bacteria can thrive.
Nitrogen-fixing bacteria produce signal molecules that two Nod factor receptors in the legumes can recognise. When the plant perceives the signal molecules, it forms small tubes in the root hairs that the bacteria can penetrate down to the root nodules, where they can thrive comfortably with ample access to sugar as long as they draw nitrogen out of the atmosphere and make it available to the plants.
“We already knew about that part, but we did not know how the Nod factor receptors interact to initiate the symbiotic programme between plants and bacteria,” says Henriette Rübsam.
Fused relevant receptors
The researchers carried out the most thorough investigation so far of the interaction between Nod factor receptors NFR1 and NFR5 by fusing a green fluorescent protein to the NFR5 receptor and the LaG16 nanobody that binds green fluorescent protein to NFR1.
The coupling of the two receptors with green fluorescent protein and a nanobody enabled the researchers to bring NFR1 and NFR5 into close proximity and thus observe how this affected the plants.
“We did not know how these two receptors interact, because observing this in living cells is very difficult. Many researchers had ideas about what happens, but no one knew for sure. However, our experiments showed how this interaction occurs,” explains Henriette Rübsam.
Barley has similar receptors
The results showed that the interaction between NFR1 and NFR5 is needed to activate the root nodule symbiosis in legumes. Although the researchers did not add bacteria to their experimental set-up, the plant roots began making root nodules as if they were there.
“This is our main finding. All it takes to start the symbiotic process is for these two receptors to interact. In nature, this happens when the receptors recognise signal molecules from the nitrogen-fixing bacteria,” says Henriette Rübsam.
In another part of the study, the researchers examined barley plants for similar receptors and found two that are structurally similar to NFR1 and NFR5. These two receptors are not capable of binding signal molecules from nitrogen-fixing bacteria, but when the researchers genetically engineered them into legumes and then made them interact using the green fluorescent protein and antibody method, the barley receptors also initiated root nodule symbiosis.
More sustainable food production in the future
Henriette Rübsam sees great potential in the discovery. The long-term goal is to use the new knowledge to engineer crops other than legumes so that they also become less dependent on large quantities of artificial fertilisers.
The researchers showed that receptors in barley can already initiate symbiosis and make root nodules in legumes. Now they aim to do this in barley, and they want to determine whether other cereals have similar receptors with the same potential.
“Many other crops that require substantial artificial fertiliser can potentially be engineered to activate symbiosis with nitrogen-fixing bacteria, which will contribute enormously to more sustainable food production in the future,” concludes Henriette Rübsam.
