The rise in home-delivered meals in recent years is nothing new. Plants have been onto the idea for millions of years. Instead of relying on an army of overworked motor-scooter riders, however, floral innovators have had their nutritional needs met by bacteria who produce it in the premises. If we can understand the way plants place their orders, we may be able to tweak them to improve crop growth.

All known lifeforms need nitrogen, and most need it fixed into a form they can consume. The richness of terrestrial life depends in large part on the symbiotic relationship between certain plants and the nitrogen-fixing microbes in their roots. However, when soil is sufficiently rich in organic carbon it also contains enough nitrogen left over from previous plant generations that this need goes away.

Professor Caroline Masiello of the appropriately named Rice University found legumes such as beans and peanuts control the quantity of nitrogen fixers in their root systems using molecules called flavonoids. Organic carbon-rich soils impede the transmission of certain flavonoids so the microbes cease to detect them. Without flavonoids, the formation of nitrogen-forming nodules on the legumes' roots falls sharply.

"It's a gorgeous example of evolution: Plants change a couple of (oxygen/hydrogen) groups here and there in the flavonoid, and this allows them to use soil conditions to control which microbes they talk to," Masiello said in a statement. 

Bacteria are no one's slaves of course, so plants have to pay the bacteria, whose currency is sugar, which the plants produce through photosynthesis. “Microbial symbionts can be really expensive subcontractors, sometimes taking a significant fraction of a plant's photosynthate,” Masiello said. It's probably a fairer rate than your delivery person gets. At least there's no giant corporation taking a cut of the transaction.

This, Masiello explains in Science Advances, is why the plants are so keen to reduce sugar payments when they can source nitrogen more cheaply. Meanwhile, release of the modified flavonoids used to communicate non-nitrogen-related messages continues.

Life is fond of using the same molecules for multiple purposes. Besides controlling symbiotic bacteria, flavonoids are used as defenses against more hostile microbes. They also, as their name suggests, trigger our tastebuds. Among the molecules Maisello explored are those that provide the distinctive tastes of grapefruit, broccoli, and kale.

The paper points to an example where legumes' chemical signaling has been put to use, even without a deep understanding of its working. Legumes placed between rows of maize release signals into the soil that prevent parasitic plants from attaching onto the corn and strangling with its growth. How much more could be done through learning to eavesdrop on the plant-bacteria delivery app?