What if plants could make their own fertilizer—from the air?

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Applying vast quantities of fertilizer to cropland is a major culprit behind the fact that agriculture now contributes one-third of global greenhouse gas emissions to the atmosphere. But, what if we could bypass this problem in its entirety by engineering plants that produce their own fertilizer–directly from the air?

Writing in the journal mBio, scientists from Washington University have provided some of the most convincing evidence yet that one day this could be possible. To do so, they capitalise on the fact that 78% of earth’s atmosphere is made of nitrogen, which is the key growth-promoting ingredient in fertilizer, and a major component required for plant photosynthesis. But getting plants to make use of this abundant source of nitrogen floating around in the atmosphere relies on the ability to solve one quirk of plant biology: The ingredient that fixes nitrogen from the air, an enzyme called ‘nitrogenase’, can’t operate in the presence of oxygen–which of course many living things, like plants, need in order to carry out photosynthesis and survive.

However, there are some unusual species of photosynthesising bacteria on the planet, called Cyanothece, that can photosynthesise in the daytime, but delay nitrogen fixation until night time when most of oxygen generated by photosynthesis has been expelled from the plant–thereby keeping the processes separate. The researchers wanted to test whether they could transfer the traits from these adaptable bacteria into another bacteria species–called Synechocystis–that didn’t have this quality, as a practice run for what might one day be accomplished with plants.

In doing so, they were able to identify 35 genes associated with night-time nitrogen fixation in the Cyanothece bacteria, which they then transferred to the non-nitrogen fixing Synechocystis species. They discovered that these newly-retrofitted bacteria could then accomplish 30% of the nitrogen-fixing activity of their Cyanothece donors. This discovery wasn’t without its caveats, however. Despite their new set of genes, Synechocystis’ ability to fix nitrogen from the air was still hampered by the presence of oxygen. But the researchers found they could tweak the bacterias’ nitrogen fixation rates by changing the genes they transferred. Finding the optimal arrangement of genes will now be the focus of future studies, the scientists note.

In any case, the major advantage of the research so far is that it has proved it might be possible one day to place this same nitrogen-fixing genetic machinery into the cells of crop plants. If researchers can succeed in breeding future plants to carry these traits, crops would be able to transform nitrogen directly from the surrounding air into fuel for their own growth.

This localised production would obviate the need to manufacture such vast quantities of fertilizer, a process that–alongside the application of this ingredient to soil–generates huge amounts of greenhouse gas emissions annually. Nor would farmers necessarily need to shower these nutrients on crops, so alleviating the problem of fertilizer run-off being washed into rivers, lakes, and the sea, and polluting the life there.

This reality may still be far off. But in an ideal future, says lead author Himradi Pakrasi, “Each crop plant will fix its own nitrogen, there will be no need for application of chemical fertilizers, and there will be no runoff.”

Source: Liu et. al. “Engineering Nitrogen Fixation Activity in an Oxygenic Phototroph.” mBio. 2018.
Image: Makeagif

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