Plants found to use genes to recruit microbial cavalry

In the battle against the legions of lethal soil pathogens that beset crops, plants, apparently, have the ability to summon the microbial cavalry.

Scientists have long known that beneficial soil microorganisms tend to flock to plant roots — along with their detrimental bacterial and fungal counterparts. But they’ve never known how.

Now scientists from UW–Madison, writing in the current issue (April 27) of the Proceedings of the National Academy of Sciences (PNAS), report that the ability to call for help is genetically wired into plants. The finding chips away at a fundamental mystery of symbiotic behavior and suggests that through careful breeding, the battle against devastating soil microbes can be turned.

“We now have genetic evidence that plants contribute significantly” to the activity of beneficial soil microorganisms, said Robert M. Goodman, a UW–Madison professor of plant pathology and a co-author of the PNAS paper. “Genes somehow play a role in terms of what kinds of microbes are recruited.”

The work suggests that the tools are now available to breed plants that are good hosts for beneficial microorganisms.

Soil is host to a zoo of microorganisms, some good and some bad. Soil pathogens — bacteria, fungi and other microbes — infect nearly all cultivated plants, reducing yields and in some instances wiping out entire crops. The most tragic example is the Irish potato famine of the mid-19th century when a Phytophthora fungus rotted the staple food of Irish life, leading to mass starvation and migration.

More recently, chemical pesticides have been used to successfully control such blights. But microbes may develop resistance to the chemical agents, which also pose the threat of pollution to ground water and soil. By finding that plants themselves have the ability to recruit microbes that combat other disease-causing organisms, Wisconsin scientists have opened a new front in a battle that is as old as agriculture.

“This work is a start,” said Goodman. “It is an experimental tool to tease out the genetics” that could enable plant breeders to create strains of plants that act as magnets not only for diseases-fighting microbes, but also for other beneficial microorganisms such as nitrogen-fixing bacteria.

The problem of how microbes of all kinds are drawn to the roots of plants is an old one, said Goodman, but most work has focused on the role of the microbe. “What we are no longer ignoring is the contribution of the plant to these associations,” he said.

The discovery was made using an experimental population of plants derived from a cross between a cultivated tomato and a related wild species to create plants with varying genetic abilities. Those plants were then exposed to a pathogen that causes seed and seedling diseases, and then to a disease-suppressing bacterium known as UW85, a soil microbe discovered in 1985 by UW–Madison plant pathologist Jo Handelsman.The Wisconsin team observed that the combined effects of several tomato genes contributed to the ability of plants to support populations of the disease-thwarting UW85 bacterium.

The catch, said Goodman, is that while the team was able to demonstrate the influence of genes and roughly locate where those genes lie on tomato chromosomes, more work is needed to precisely identify the genes involved and find out exactly what they do to help attract good microbes.

“Each (tomato) line had a characteristic level of biocontrol,” said Goodman, “although we don’t yet know what these genes are.”

But what the work does do is firmly establish that plants play a role in beneficial interactions with one bacterium, and it points to the appealing idea that plants may have active ways of attracting different kinds of soil microbes to the critical root environment. It is likely, Goodman explained, that there are a number of genes at work and that they initiate a chain of biochemical communication that somehow signals microbes and draws them to the plants. Other factors, such as the physical features of plant roots, probably play a role as well, he added.

Goodman and co-authors Kevin P. Smith, now on the faculty at the University of Minnesota, and Jo Handelsman, a UW–Madison professor of plant pathology, are now extending the research to corn. The critical crop plant, cultivated on more than 80 million acres in the midwestern United States, has fairly well-known genes, but it requires significant amounts of chemical fungicide to survive.