Alfalfa roots carry pink bumps the size of a pea. Inside them live bacteria that pull nitrogen from the air and turn it into fertilizer for the plant. The plant pays them in sugar. For this deal to work, the partners constantly exchange protein signals, but until recently we could barely hear that conversation.
The neural network AlphaFold3 can guess from protein shapes which ones might lock together like Lego pieces. The researchers checked every possible pair: proteins the bacterium throws outside against alfalfa proteins. Out of hundreds of thousands of options, the network picked the most plausible ones. Two bacterial proteins, SRP86 and SRP485, turned out to be partners of DNF2. Experiments confirmed it: they really do bind to each other in the very gap where the negotiations happen.
Then the scientists removed both proteins from the bacteria at once. The nodules turned white and small, the leaves yellowed, and the plant got little nitrogen. But if you remove just one protein, the plant barely notices the loss. That means DNF2 and the two bacterial proteins work as a team, holding the symbiosis together. Now this contact map is open to everyone, and it can be used to look for ways to make legumes even more self-sufficient in feeding themselves.
The screen of 217,000 protein pairs took more than 10,872 GPU hours, which is over a year of continuous work for a single NVIDIA A100-class graphics card. Meanwhile, the entire rhizobial secretome is active in the nodule: 651 of 653 genes are transcribed simultaneously, as if the bacterium keeps a whole arsenal of tools at the ready.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.