BY SIMON FROST
Zinc finger identified as key to unlocking legume plants' full nitrogen-fixing potential.
A genetic “off switch” that stops legume plants converting atmospheric nitrogen into nutrients has been identified and controlled for the first time by a team of international scientists, who have found that zinc plays a crucial role in the fixation process (Nature, DOI: 10.1038/s41586-024-07607-6).
The research was conducted as part of Enabling Nutrient Symbioses in Agriculture (ENSA) – a Gates Foundation-funded international research project aimed at using biological nitrogen fixation to sustainably increase yields for smallholder farmers in Africa.
Like beans, peas and lentils, legumes can interact with soil bacteria to capture atmospheric nitrogen and convert it into a usable form. This fixing of nitrogen is an energy-intensive biological process, so legumes have a built-in “switch” to turn it off when there’s already plenty of nitrogen in the soil – either naturally or through synthetic fertilisers.
Using the legume Lotus japonicus as their model, the team pinpointed the genetic transcriptional regulator responsible for this, which they call fixation under nitrate (FUN). When soil nitrate levels are high, FUN halts the process of nitrogen fixation. This led to a surprising second finding: that zinc levels act as a trigger for FUN.
‘We found that changing soil nitrogen alters the levels of zinc in the plant,’ explains Kasper Andersen, co-author of the study. ‘Zinc had not previously been linked to the regulation of nitrogen fixation, but our study found that a change in zinc levels in turn activates FUN, which then controls a large number of genes that shut down nitrogen fixation.’
It follows, then, that deactivating FUN would create a state where the plant no longer hinders its own nitrogen fixing abilities. The team used a combination of biochemistry, gene expression studies and microscopy to find that FUN forms into long protein filaments when it is inactive. Having identified this behaviour, the researchers were able to disable the FUN regulator in model legumes, allowing the plant to continue fixing nitrogen regardless of the soil conditions.
Jieshun Lin, the study’s first author, explained, ‘It’s truly remarkable to discover zinc’s role as a secondary signal in plants. It is a vital micronutrient, and it has never been considered as a signal before. After screening over 150,000 plants, we finally identified the zinc sensor FUN, shedding light on this fascinating aspect of plant biology.’
By understanding how zinc and FUN regulate nitrogen fixation, the researchers hope to develop strategies to optimise the process in legume crops. The hope is that this could lead to increased nitrogen delivery, improving crop yields and reducing the need for synthetic fertilisers, which have both environmental and economic costs.
The researchers are now investigating how legumes such as soya bean and cowpea perform when they lose FUN activity.
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