Researchers from the University of Cambridge have made a groundbreaking discovery about the plant hormone gibberellin (GA) and its crucial role in the formation and growth of nitrogen-fixing root nodules in legumes. They have also found that GA can enhance nodule size, shedding light on its precise influence at different stages of nodule development. This research resolves conflicting reports by pinpointing the specific areas where GA is indispensable for the nodulation process.
Cereal crops like wheat, maize, and rice heavily rely on synthetic fertilizers to fulfill their nitrogen requirements. However, synthetic nitrogen fertilizers are energy-intensive, costly for farmers, and detrimental to the environment due to issues such as water pollution.
Unlike cereals, legumes, like peas, beans, and pulses, can obtain their own nitrogen through a natural symbiotic relationship with nitrogen-fixing bacteria, resulting in higher protein content and greater nutritional value. However, legume crops cease nodule production in soil with high nitrogen levels, potentially leading to lower yields.
Researchers worldwide are focused on increasing legume yields and transferring nitrogen-fixing capabilities from legumes to cereals. Achieving this goal requires understanding the intricate genetic and biochemical pathways involved in nodule formation and nitrogen fixation.
Dr. Alexander Jones’ research team at the Sainsbury Laboratory Cambridge University (SLCU) and Professor Giles Oldroyd’s group at the Crop Science Centre have taken a significant stride in achieving this objective by unveiling the dynamics of GA that control the growth, shape, and role of nitrogen-fixing root nodules.
Dr Jones said: “There were some confusing and conflicting reports about the function of GA in nodule symbiosis. Experiments showed that adding GA reduces nodulation, and removing GA increases nodulation in legumes like Medicago truncatula, which suggests GA is antagonistic towards nodulation. But there is also a legume mutant in peas that produces less GA and has fewer nodules, which suggests that GA is somehow required for nodulation.”
“These conflicting results suggest there is probably something going on with spatial-temporal GA patterning. For example, there may be specific places where GA needs to be and some places where it needs to be absent. Or that the precise concentration of GA is important.”
Dr Colleen Drapek utilized the advanced, highly sensitive biosensor nlsGIBBERELLIN PERCEPTION SENSOR 2 (GPS2) developed by the Jones Group to visually observe the precise locations and timings of GA presence and its relative concentrations. In Medicago, who was infected with rhizobium bacteria, she discovered that GA accumulated in the nodule primordium, which is the area in the root cortex where cells initiate division during the early stages of nodule formation.
“Right at the beginning of nodule formation, you start to see an accumulation of GA in the nodule primordia, but there is very little GA anywhere else in the root. As the root nodule further develops, you see GA accumulating at quite high concentrations and remaining at high levels in the mature nodule,” Dr Drapek said.
To further investigate the function of GA, Dr. Drapek employed GA and symbiotic Medicago mutants and manipulated the overexpression of enzymes responsible for breaking down or synthesizing GA. The outcome of the former manipulation was the absence of nodule formation, while the latter resulted in the formation of larger nodules.
“This shows GA is very important for nodules but that its function is specific to zones where the nodule is being initiated and not surrounding areas. We know that low GA is good for the initial rhizobium infection of the roots, but then later, you need GA to be present for the nodulation process to proceed and for nodules to mature.”
In earlier research, it was shown that there is an overlap in the developmental program that plants use to form lateral roots and nitrogen-fixing nodules. Professor Oldroyd said: “These latest findings show that GA accumulation in the root is unique to nodule development and likely, therefore, a critical switch for nodule-specific development. These are essential insights for us in attempting to transfer nitrogen fixation to other crops such as cassava and cereals.”
Journal reference:
- Colleen Drapek, Annalisa Rizza, Nadiatul A Mohd-Radzman, Katharina Schiessl, Fabio Dos Santos Barbosa, Jiangqi Wen, Giles E D Oldroyd, Alexander M Jones. GA dynamics governing nodulation revealed using GIBBERELLIN PERCEPTION SENSOR 2 in Medicago truncatula lateral organs. The Plant Cell, 2024; DOI: 10.1093/plcell/koae201