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Zinc could be the surprise solution to excess fertilizer on farms

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Zinc could be the surprise solution to excess fertilizer on farms

Zinc triggers a domino-like effect in the genetic machinery of some crops, prompting them to take up more nitrogen from the soil.
July 5, 2024

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Researchers have made a potentially groundbreaking discovery: zinc can increase the rate at which certain plants absorb fertilizer from the soil. If this is put into practice on farm fields, it could help crops scavenge nitrogen far more efficiently from the earth.

The new paper, published in Nature, shows that zinc triggers a domino-like effect in the genetic machinery of legumes, which changes how they respond to nitrogen levels in the soil. The researchers decided to focus their work on legumes, as these plants have a unique relationship with soil microbes that form tiny nodules attached to legume roots which scavenge nitrogen from the air to feed to the plant, in exchange for a steady supply of carbon. Supplying carbon to voracious root bacteria is an energy-intensive process for the plant, however, so legumes have developed an ability to control how much nitrogen fixation occurs—and that is a mechanism the researchers were keen to understand. 

Their work started with an intricate genetic screen of over 150,000 legume plants. This ultimately helped them to identify a set of genes that are able to sense environmental signals from the surrounding environment, like nitrogen levels, and link those to biological responses in the plant. One gene stood out for its role in controlling the activity of many other genes linked to sensing and fixing nitrogen—a kind of parent gene that the researchers named Fixation Under Nitrate or ‘FUN’. 

Experiments on test plants revealed that when nitrogen levels were high, FUN became activated, and this slowed the activity of the bacterial root nodules, curbing the amount of nitrogen that they fixed from the soil and supplied to the plant. But when nitrogen levels were low, FUN was inactivated—and so with the brakes off, the roots were able to increase the pace of nitrogen fixation from the soil. 

 

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However it still wasn’t clear precisely how FUN was sensing the available nitrogen, and therefore what was actually triggering the activation or deactivation of this gene. To interrogate this further, the researchers pored over this gene, and discovered that its sensors shared a distant ancestry with certain metal-binding proteins. 

The researchers connected this clue to another critical piece of evidence: they found that when soil nitrate levels spiked, levels of zinc—a metal element—in the root nodules declined. Putting two and two together, they realized that this was the signal to FUN to activate: its sensors detected the decreasing zinc levels as a proxy for the spiking nitrogen, bringing it out of dormancy to set off a chain of events in other genes that caused the nitrogen-fixing bacteria to stop their work. 

Meanwhile, when nitrogen is low and zinc levels peak, the researchers found the opposite occurred: FUN was transformed into an inactive protein filament, effectively going dormant until it was needed again, and leaving nitrogen uptake to go on.

Zinc could therefore accelerate a plant’s uptake of nitrogen from the soil, the scientists reason. That could be revolutionary in situations where there is excess nitrogen—for instance where synthetic fertilizer has been applied liberally to the land. Farmers often do this to offset the risk of low yields, but the trade-off is that chunks of that fertilizer go unused, running off and polluting rivers, lakes, and the sea. Perhaps zinc could change this scenario by coaxing plants to scavenge more nitrogen from the earth, the team of scientists say.

They’re working on that possibility right now, with new research into how major legume crops like soybeans and cowpeas respond to nitrogen when the FUN circuit is switched off. Soon, they hope this work will help “increase the delivery of fixed nitrogen to agriculturally important crops.”

Lin et. al. “Zinc mediates control of nitrogen fixation via transcription factor filamentation.” Nature. 2024.

Image: ©Anthropocene Magazine

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