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A genetic tweak in crops could reduce excess fertilizer use

Researchers discover a gene that, when turned off, could enable plants to absorb more phosphorus from the soil
June 26, 2020

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In some countries, it’s estimated that up to 70% of the phosphorus in fertilizer that gets showered over farmland is wasted, because it’s left unabsorbed by crops. Instead, lying dormant in the soil, it’s eventually washed away by rain—polluting streams, rivers, and the sea. 

But now, researchers writing in the Journal of Experimental Botany have identified a plant gene that could change this, enabling crops to absorb more phosphorus, and so leave less behind in the soil to pollute the environment.

The discovery rests on the fascinating symbiotic relationship between plants and microbes called mycorrhizal fungi, which live in plants’ root systems. These two share a kind of evolutionary agreement: The plants provide fungi with lodging in their roots, as well as a steady supply of sugars and fats. In return, the fungi use their hyphae—extremely long and fine root-like structures—to expand the reach of the plant’s own roots into the soil. Acting almost like a prosthetic, the hyphae enlarge the root network, fanning out into the soil, and so enabling the plant to increase its uptake of nutrients and minerals like phosphorus, which it needs in order to grow.

But this symbiotic relationship isn’t permanently switched on: In fact, there are genetic factors that control it—and therefore the amount of phosphorus that the plant absorbs. But until now, these have been quite poorly understood. 

To explore this, the researchers on the new paper studied the genetics of an experimental plant model, a type of legume called Medicago truncatula. Through a series of experiments, they discovered that in this plant, one particular gene called CLE53 plays a central role in controlling the symbiotic relationship between plants and their mycorrhizal fungal root communities. 

In fact, they discovered that CLE53’s role is actually to suppress the plant’s relationship with the mycorrhizal fungi, which it does by promoting a signaling pathway in the plant that blocks the symbiosis. This means that when the gene is active, the activity of the mycorrhizal fungi is reduced, and the plant reduces its uptake of phosphorus from the soil. 

This discovery gave the researchers an idea about how they might exploit this feature to instead increase the uptake of phosphorus: They hypothesize that by turning off CLE53, they could alter the relationship with root fungi, enabling them to do their nutrient-seeking work unhindered. This way, the plant could absorb more phosphorus from the surrounding soil—instead of leaving it unused, and vulnerable to being washed out into the environment.  

That suggests that tweaking CLE53 could become a way to directly control how much phosphorus a plant takes up. Crucially, CLE53 has homologs in other plants, meaning that the discovery of this gene’s central role in phosphorus uptake could be applied widely to other species, too. “One idea would be to breed plants lacking the gene homolog to CLE53,” explains Thomas Christian de Bang, a plant and soil scientist at the University of Copenhagen, and lead author on the study. 

As well as this, a surprising 90% of all plants engage in symbiotic relationships with mycorrhizal fungi, so this finding could, in the future, have a huge impact on the agricultural landscape—helping plants make better use of the available fertilizer and increasing their yields, while simultaneously reducing the amount of phosphorus that farmers need to apply to their land.

It might be a while before these findings play out in agricultural crops: Regulations in many countries currently limit the necessary gene editing techniques, like CRISPR, that would be required to make it a reality. But the researchers are hopeful that their findings will one day find a foothold in the field: “I would explore the opportunities that CRISPR offers, and test it under lab conditions first, and then move to real world agriculture,” says de Bang. “If it works in crops under lab conditions, I would test it in any country that would have an interest in it.”

With both the excessive application of phosphorus, and its dwindling supply being issues of global concern, agricultural interventions like these are likely to become more and more relevant in years to come.

Source: De Bang. “The CLE53-SUNN genetic pathway negatively regulates arbuscular mycorrhiza root colonization in Medicago truncatula.” Journal of Experimental Botany. 2020
Image: Pixabay

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