Chapter 1: What unique role do soybean plants play in sustainable agriculture?
You're listening to TED Talks Daily, where we bring you new ideas to spark your curiosity every day. I'm your host, Elise Hu. We're about to get a little geeky about soybean plants. That's right. In his 2024 talk, plant doctor Giles Oldroyd lays out the rather magical way soybean plants work and the important role they can play in sustainable agriculture. Enjoy it.
So I believe this soybean plant is a prototype for sustainable food production on this planet. So on the roots of this soybean plant are nodules. And these nodules do an amazing thing. They harbor millions of bacteria inside the cells of the nodules. And those bacteria are able to capture nitrogen out of the atmosphere and confede it to this soybean plant.
Now, all plants require a source of nitrogen. They need it so they can make DNA, RNA and proteins. But plants can't access the most prevalent form of nitrogen on the planet, the 78 percent of the air that you're currently breathing, that is molecular dinitrogen.
only bacteria that possess the enzyme nitrogenase can convert this very inert form of nitrogen and convert it into ammonia, a reactive form of nitrogen that bacteria and plants can use to make their DNA, RNA and proteins. So the bacteria inside the nodules of this soybean plant are fixing nitrogen out of the air, converting it into ammonia, and then feeding that ammonia to this soybean plant.
In return, the soybean plant is feeding the bacteria with a source of carbon in the form of sugars derived from photosynthesis in the leaves. This is what we call a mutualistic symbiosis. It's beneficial to the soybean plant, but it's also beneficial to the bacteria inside those nodules.
Now, the roots of the soybean plant are doing a second amazing thing, and to see that, we have to look under a microscope. The roots are heavily infested with a beneficial fungus called mycorrhizal fungi, and these fungi are heavily colonizing the soil and make a much greater contact with the soil surface than the plant root alone is able to achieve.
In so doing, they create a much more efficient platform for the uptake of nutrients, nutrients such as phosphates, nitrates, potassium and water. The fungus isn't only out there in the soil, it's also colonizing the roots of this soybean plant, where it makes these highly branched fungal intrusions into the cells of the root that we call our buscules.
So the fungus is out there in the soil, capturing nutrients from the soil, and it feeds those nutrients to the soybean plant through these arbuscular intrusions. In return, the soybean is feeding the fungus with carbon from photosynthesis. Again, it's a mutualistic symbiosis.
So this soybean plant can get almost all of its phosphate and the totality of its nitrogen needs met through these beneficial microbial associations. And that provides a free and sustainable means to support its crop production. And out in nature, most plants are engaging with one or more of these beneficial microorganisms to help them capture these limiting nutrients from the environment.
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Chapter 2: How do soybean nodules contribute to nitrogen fixation?
Now, the fungal symbiosis is not limited to legumes like that soybean. It's actually pretty widespread within the plant kingdom, and it's already present in our cereal crops. However, when we fertilize our fields, the crop doesn't engage with the fungus. Why pager the fungus with carbon if the nutrients are not limiting?
So while soils in natural ecosystems are packed full of a complex network of these mycorrhizal fungi fed by their host plants, our agricultural soils are greatly depleted for these beneficial fungi.
If we want to really maximize the utilization of this fungal symbiosis in agriculture, then we need to get our crop plants to gauge with the fungus much more proactively, and even when we fertilize our fields. If we can do that, then we can reduce the levels of fertilizers we use, and we'll lose less of those nutrients out into the environment.
So to achieve that, we set about identifying the genetic regulators that control when the plant engages with these beneficial fungi. and we discovered that these proteinaceous regulators are only present when the plant is starved for nutrients. And we were able to rewire that system so that now the plant engages with the fungus much more proactively, and even when the plant is fertilized.
In our field trials, we find that these rewired barley plants get 10 times as much fungus inside their roots. That's a lot more fungus in the crop, but it's also a lot more fungus out there in the field. So now we can control when the plant engages with these beneficial fungi. The next step for us is to test, does that mean we can lower the fertilizer levels and still maintain good production?
I believe this is a first step to really getting that fungal association working for us much more proactively in agriculture. And that's going to be really important, especially for how much phosphates we have to apply to our fields. But if we're going to really cure our addiction to inorganic fertilizers, we also need the nitrogen-fixing bacterial symbiosis.
Now, unfortunately, the nitrogen-fixing symbiosis is limited to legumes like that soybean and their relatives. So we are working on transferring that nitrogen-fixing symbiosis from legumes to our cereal crops.
Myself and my colleagues have spent the last 30 years undertaking genetic dissection to try to identify all the genes that in soybean allows it to engage with those nitrogen-fixing bacteria. During that time, we've identified many genes that are involved in that process. But surprisingly, we haven't yet identified a single gene that is novel to that soybean plant.
In fact, the genes are already present, most of them are already present in our cereal crops. Let me give you an example, the symbiosis signaling pathway. This is a set of proteins that are expressed on the cells on the surface of that soybean root that allow the soybean plant to recognize the nitrogen-fixing bacteria out in the soil.
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