When you're a newly sprouted corn seedling, all alone in the dirt, you need any advantage you can get. After all, you can't pick up your roots and travel to find resources or avoid pests. That's why corn plants emit toxic chemicals that keep away hungry insects aboveground and harmful microbes below. But to at least one kind of bacteria, this poison is more of a beacon. They follow the toxic trail back to the corn plant, set up camp in its roots, and help the vulnerable seedling grow.
A plant's roots are the center of a miniature ecosystem called the rhizosphere. Local bacteria feed on sugars and proteins that trickle out of the roots, like antelopes at a watering hole. Symbiotic fungi enmesh themselves in the plant's roots. Helpless as it may appear, the plant can even release chemicals that encourage certain microbes to live there and discourage others, or prevent competing plant species from growing nearby.
Researchers in the United Kingdom studied one of the toxic chemicals released by the roots of corn plants. The compound is a benzoxazinoid, mercifully abbreviated as BX. Seedlings of corn and other grasses secrete BX molecules to protect themselves from pests and harmful microbes.
But the team, led by Andrew Neal at Rothamsted Research, suspected that certain bacteria weren't bothered by the toxin at all. Neal says this was a bit of a "leap of faith." Many bacteria that are used to clean pollutants from soil are closely related to bacteria that colonize plant roots. And some of the toxins that plant roots produce are similar to these pollutants. So the team asked whether Pseudomonas putida--"one of the best root colonizers we know of," Neal says--might be resistant to plant toxins.
The researchers first took both the plants and bacteria out of the soil to see what was going on. They found that corn seedlings produce the most poison at one week old, protecting themselves at their most vulnerable stage of growth. Over the next couple of weeks, production drops off.
Testing P. putida bacteria, they saw that the concentration of BX molecules around a seedling's roots didn't hurt the bacteria at all. But another common soil bacterium had serious trouble growing, even at a much lower concentration of the toxin. The chemical also broke down more quickly in the presence of P. putida, suggesting that the bacteria might not only tolerate the poison, but eat it.
Next Neal and his coauthors turned to the genes of P. putida to see which ones are most active when the toxic chemical is around. A few dozen genes popped up. Some of these had to do with "chemotaxis," a trick in which bacteria use their wiggly arms to travel toward a high concentration of a chemical they like. Were P. putida bacteria actively seeking out the toxin and the corn roots that released it?
Further experiments showed that the bacteria do, in fact, travel toward areas with more BX molecules. And in the soil, corn seedlings making the toxin attract more P. putida to their roots. (Genetic mutants that can't make BX molecules attract fewer bacteria.) The effect fades by the time the plant is three weeks old.
This is the first time scientists have seen an otherwise toxic root chemical attracting helpful bacteria. A corn plant that has successfully recruited P. putida has a leg up--or a root up--in its development. These bacteria and other friendly microbes keep harmful bacteria away by crowding them out and producing antibiotics against them. They also help the plant reach nutrients such as iron and phosphorous in the soil. The bacteria, too, have an advantage over other microorganisms in the area because they can tolerate the plant's toxin and may even eat it.
Neal says that through breeding, some crops have lost their ability to generate this chemical. "Modern varieties of cereals such as corn, wheat, barley, etc., now produce widely varying amounts of the benzoxazinones we studied," he writes. "Some produce quite a lot, others produce none." Neal hopes this research has shown why BX production is a helpful trait for plants to have.
Today's breeders, better informed about what goes on beneath the soil than their predecessors, may want to create new crop varieties that can once again make their own toxins. Plants that generate BX molecules can inhibit pests and diseases--and call friendly bacteria to their aid. We might be able to use fewer pesticides and fertilizers if we let our crops' bacterial helpers help us, too.
Neal, A., Ahmad, S., Gordon-Weeks, R., & Ton, J. (2012). Benzoxazinoids in Root Exudates of Maize Attract Pseudomonas putida to the Rhizosphere PLoS ONE, 7 (4) DOI: 10.1371/journal.pone.0035498