Defense hormones guide plant roots’ mix of microbes

Plants help tend their own gardens. Salicylic acid, a plant hormone used to fight bacterial infections in leaves, also helps plants select which bacteria colonize their roots, researchers report online July 16 in Science.

The finding provides an unexpected piece to an unsolved puzzle in plant biology: why some microbes flock to the roots of certain plants regardless of soil type.

Researchers thought there were two possibilities for how specific collections of microbes get together with roots, says Cara Haney, a microbiologist at Massachusetts General Hospital in Boston. “Either plants are just sticks in the mud that certain bacteria like to eat, or plants play a role in shaping that community,” she says. Haney, who was not involved in the new study, investigates interactions between plants and microbes.

Experiments with a small weed called Arabidopsis thaliana indicate that plants take an active role in pruning some bacteria, while nourishing others. The little plants normally attract Actinobacteria and Firmicutes, but the soil around their roots have less Acidobacteria, Bacteroidetes and Verrucomicrobia than surrounding soil, Sarah Lebeis of the University of Tennessee in Knoxville and Jeffery Dangl of the University of North Carolina at Chapel Hill and colleagues discovered.

Root microbiomes — the collection of microbes including bacteria and fungi that live in and around the root — may help keep plants healthy and spur their growth. In some plants, such as legumes, bacteria that live in root nodules convert nitrogen from the atmosphere into a form plants can use. Learning how plants interact with their microbes may lead to new kinds of fertilizers and pesticides, Haney suggests.

In the study, researchers created mutant Arabidopsis plants that lacked the ability to make one or a combination of three important defense hormones: salicylic acid, jasmonic acid and ethylene. With the defense chemicals missing, “we thought it would be a general loss of control and anything could get in,” to the plants’ roots, Lebeis says. Instead, she and colleagues found that some microbes that normally get into roots were barred from the plants that didn’t make salicylic acid. Some bacteria normally kept at bay invaded the roots in the absence of salicylic acid.

Growing bacteria in the lab revealed that some groups, including Mitsuaria bacteria, grow less well when salicylic acid is around. Other bacteria grow better in the presence of the hormone, including one variety of Streptomyces bacteria that use the hormone as fuel.

Researchers don’t yet know the mechanisms by which the hormone controls root microbes.