Researchers at the University of Freiburg, together with an international team, have unlocked how indispensable for beneficial bacterial infections of legumes (such as peas, beans, and clover) a so far little characterized protein is. More so, they have succeeded in activating this protein in the non-legume tomato plant. Using a combination of imaging-based, molecular biology and genetic approaches, they discovered a novel role for SYFO2, a protein in legume roots, that is crucial to establish the partnership between nitrogen-fixing bacteria and legumes. Prof. Dr. Thomas Ott, a plant biologist at the University of Freiburg and member of the Cluster of Excellence CIBSS – Centre for Integrative Biological Signalling Studies and his team are looking at signalling processes that allow bacteria to enter the plants’ roots to fix nitrogen for the plant. Their collaborative study published in Science improves our understanding of how the tomato’s own symbiosis-related genes can be controlled. It lays the groundwork for future efforts to enhance beneficial plant–microbe interactions and to transfer nitrogen-fixing abilities to crops with the long-term aim of reducing the need for fertilizer.
A molecular 'gatekeeper' in plants opens the door for beneficial microbes
CIBSS researchers at the University of Freiburg are part of an international team that sheds light on a crucial step in the initiation of the symbiosis between plants and soil bacteria.
Prof. Dr. Thomas Ott. Photo: CIBSS / University of Freiburg
Establishing symbiosis between plant roots and soil bacteria
Most plants allow fungal microorganisms to enter their root cells with the benefit of better receiving nutrients and water in exchange for providing carbohydrates. Only legumes form an additional mutually beneficial symbiotic relationship with soil-born bacteria called rhizobia. In this evolutionary ancient process, the bacteria infect root cells and trigger the formation of root nodules, specialized root structures which they colonize. Within these nodules, rhizobia convert atmospheric nitrogen into ammonia – a natural fertilizer for the plant.
The early phase of establishing a rhizobia infection is tightly regulated. First, a root hair that detects the bacterium curls around it, trapping the microbe. The membrane lining the root hair then reorganizes to create an infection thread, a tunnel-like structure that allows the bacterium to travel deeper into the root cell. Finally, this interaction triggers the plant’s genetic program for nodule formation. How exactly the root hair membrane is remodelled and which molecular pathways are at play are the subject of current research. To build the infection thread, the root hair cell relies on its cytoskeleton, the dynamic scaffold made up of protein fibres such as actin.
A crucial step in opening the door to nitrogen-fixating bacteria
In their work, Ott and his collaborators characterize the function of the formin-type protein SYFO2 in the early steps of rhizobia infection of a model legume (Medicago truncatula). Once the bacteria are entrapped by the root hair, SYFO2 is required for the reorganization of the actin cytoskeleton, a key step to start intracellular infection. “Most legumes have developed sophisticated mechanisms to allow cellular entry of symbiotic bacteria”, says Ott. “In this study we have identified the molecular basis for a key process, where the plant switches from ‘catching the bacteria' to 'opening the door' for them.” The work was further supported by CIBSS researcher Prof. Dr. Robert Grosse at the medical faculty of the University of Freiburg, who contributed with his expertise on actin filament dynamics in mammalian cells.
“Most legumes have developed sophisticated mechanisms to allow cellular entry of symbiotic bacteria. In this study we have identified the molecular basis for a key process, where the plant switches from ‘catching the bacteria' to 'opening the door' for them.”
An ancient gatekeeper of plant symbioses
The SYFO2 protein has stayed almost unchanged throughout evolution and is found in many different plant species – also in those that do not enter symbioses with nitrogen-fixating bacteria. In some of those plants, the researchers could show that SYFO2 was required for the initiation of the most common and evolutionary older type of symbiosis: the mycorrhizal symbiosis between plants and fungi. An important aim is to enable nitrogen fixation in non-legumes – a goal shared by Ott and other researchers as part of the international research project ENSA (Enabling Nutrient Symbioses in Agriculture), funded by Gates Agricultural Innovations. In tomato, a member of the nightshades (Solanaceous plants), the research team succeeded in activating the tomato’s own SYFO2 version by bringing in a regulatory factor of the root nodule symbiosis with nitrogen-fixating bacteria. “This result is especially interesting because it shows that genes normally involved in mycorrhizal symbiosis can be redirected to help engineer bacterial nitrogen-fixing symbiosis in plants”, concludes Ott. He adds: “Within ENSA, we are working together on fundamental research questions like this to advance beneficial symbioses between crops and the soil microbiome in the hope of strengthening sustainable agriculture.”
Laying the groundwork for sustainable farming
Transferring the ability to ‘self-fertilize’ to crops that normally cannot host nitrogen-fixating bacteria is a promising strategy for reducing the amount of fertilizer that farmers need, while maintaining crop yields – both of which are crucial for the global challenge of ensuring food security. Innovation with the potential to reduce fertilizer use could significantly benefit environmental and human health, as well as reduce costs for global farmers, leading to more sustainable and equitable farming.
About ENSA
Enabling Nutrient Symbioses in Agriculture (ENSA) is an international scientific project researching beneficial interactions between crops and the soil microbiome. ENSA’s scientific discoveries aim to support the delivery of equitable and environmentally sustainable crop innovations that boost the yields and livelihoods of global farmers.
About Gates Ag One
Gates Agricultural Innovations (Gates Ag One) is a non-profit organization that accelerates breakthrough agricultural research to meet the urgent and neglected needs of smallholder farmers in sub-Saharan Africa and South Asia. Out of the conviction that all lives have equal value, Gates Ag One serves the interests of smallholder farmers, who are most exposed to climate shocks yet lack the access that others have to the latest agricultural innovations. Gates Ag One works to level the playing field and empower smallholder farmers to transform their agricultural productivity, nutrition security and climate resilience. Learn more at gatesagone.org.