VANCOUVER, Wash. - Most major crops, such as wheat and corn, require expensive nitrogen fertilizer to flourish. But what if bacteria could help those plants draw nitrogen from the atmosphere, as peas and beans do?
New research from Washington State University takes an important step in that direction, identifying a key cluster of genes that can be moved from rhizobia bacteria that harvest nitrogen into bacteria that don't - raising the possibility that microbes that dwell in cereal crops could eventually be engineered to atmospherically harvest nitrogen as well.
It's a proof-of-concept finding that shines light on how plants and microbes have evolved to work together over tens of millions of years, as well as signaling future potential for reducing fertilizer use at a time of shortages and skyrocketing costs for farmers.
"These challenges emphasize the importance of finding more natural pathways for getting nitrogen to crops," said Stephanie Porter, associate professor of biological sciences at WSU Vancouver. "We developed a new way to successfully move a big cluster of genes that allow the bacteria to harvest nitrogen and colonize host plants into new bacteria that could not do these things at all. We can convert these regular bacteria into ones that are able to harvest nitrogen to fertilize plants in one single step."
These findings, arising from research spanning a decade and published in the journal Cell Biology, focus on a key evolutionary process: endosymbiosis, in which microbes and host cells unite into one organism. Here, the microbes live inside of the plant cells. Such endosymbiotic organisms are crucial to understanding how plants evolved and function within an ecosystem, driving diverse processes such as nitrogen fixation for plants and photosynthesis for corals on reefs.
Porter, who was the senior author of the new publication, studies such "mutualisms" - or cooperation between species - in her lab. Angeliqua Montoya, a postdoctoral scholar in Porter's lab, was the lead author of the paper, and researchers at Brigham Young University were co-authors.
They began by mating nitrogen-harvesting bacteria with those that do not harvest nitrogen. Such mating is rarely successful, but the researchers developed a genetic tool that dramatically improved their success rate.
They then were able to transfer the cluster of genes associated with nitrogen fixation, known as a "symbiosis island," into non-harvesting bacteria, and then pair those with plant cells-millions of such pairings with different bacteria and host cells were performed.
Many separate strains were successfully converted, with bacteria that were more closely related to nitrogen-fixing bacteria showing the most success. Most of the interactions were beneficial to the host organism or not harmful-a finding that surprised the researchers because new symbionts are often thought to start out as harmful intruders that benefit at the host's expense.
Next, the scientists will study which specific genes and variants lead to the most success in transferring the nitrogen-harvesting ability.
"Our study could hopefully motivate people to continue transferring these nitrogen-fixing segments of genes into new strains to understand how they interact with the background genome, and to slowly pick apart what makes it work or not work," Montoya said.
The eventual goal is to transplant nitrogen-fixing bacteria into major crops that now rely on fertilizers.
"Say there are microbes that typically live in corn or soybean or something else we would like to transfer this capability into - this approach could allow us to do that," Porter said. "We can study the kinds of genes and variants that make this transfer successful and get better and better at making these kinds of conversions to help farmers have enough nitrogen for their crops."
