Plant Defense Hormones Build Healthy Microbiomes for Roots

An intricately structured soil bacterium, less than a micron in size, makes its home on the root surface of an Arabidopsis plant. Alice Dohnalkova/Pacific Northwest National Laboratory.
Janet Fang 22 Jul 2015, 22:43

Defensive plant hormones that protect leaves up in the air also help to build a community of helpful microbes for the roots below the ground, a new Science study shows. 

The immune defenses of plants include a mechanism that allows non-pathogenic bacteria to colonize the roots. There, the microorganisms help with the plant’s productivity by improving access to nutrients or protecting it from environmental stressors like acidity. The bacteria might be getting the sugar that’s exuded from the plants in return. But who are the bouncers checking these microbial IDs and keeping out unwanted parasites? 

"Plants could select from among the complex community for strains that help them. But it is a crowded and complex ecosystem. And there are microbes that take plant nutrients and damage the plants," Jeffery Dangl from the University of North Carolina, Chapel Hill, says in a statement. When pathogens are detected in the shoots and leaves, the plant ramps up its production of hormones to activate a defensive response. Little is known about if and how this system interacts with the roots' microbiome. 

Dangl and colleagues conducted a series of experiments with the small flowering plant Arabidopsis thaliana to study how defensive hormones – salicylic acid, jasmonic acid, and ethylene – affect the microbes living in and on the roots. They discovered that salicylic acid shapes plant health both above the ground and below in the dirt.

In Arabidopsis mutants without salicylic acid synthesis or signaling, microbes in the roots and in the surrounding soil layer were different: Based on DNA sequencing, the abundance of some bacterial families increased, some decreased, and some simply vanished. "In some of our mutants we saw what we call a loss of control. Microbes that shouldn’t have been there were there," co-first author Sarah Lebeis from the University of Tennessee, Knoxville, tells The Scientist. "Salicylic acid and the ability to produce it is one of the cornerstone features that plants require to control the [bacterial] community."

By increasing the production of antimicrobial agents, salicylic acid assembles a nice, beneficial mix. But not only do they keep some out, they also actively recruit others. It's a slightly different role from the one they play up in the leaves. "This level of salicylic acid gates potential bad guys out, but it is also required as [a] positive signal to attract bacteria," Dangl adds. "It's not just defense."

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