Plant biologists at Michigan State University and the University of Illinois have pinpointed the area of genomes within nitrogen-fixing bacteria in roots, called rhizobia, that's being altered when the plant they serve is exposed to nitrogen fertilizer.
The study, published in the Royal Society journal "Proceedings of the Royal Society B," deepens the understanding of rhizobia, which are particularly beneficial to legumes – such as clover, beans, peas, soybeans, lentils and others. However, rhizobia become less beneficial for plants when they are exposed to nitrogen fertilizer.
"This is one of the first times we've found at the genetic level the basis of an evolutionary change in mutualism," said Katy Heath, professor of plant biology at Illinois and one of the study's authors, referring to the mutually beneficial relationship between rhizobia and plants that has evolved over millions of years. Rhizobia receive sugar from the plant and in turn provide the plant with nitrogen.
Heath conducted the study with Jennifer Lau, a plant biologist at MSU and Christie Klinger, a researcher at U-I. The researchers also conducted last years' study revealing that human-made nitrogen fertilizer altered the relationship between rhizobia and plants.
"Humans are dumping fertilizer everywhere," Heath said. "And so one thing we were interested in asking is whether long-term nitrogen additions would disrupt this long, many tens of millions years old symbiosis that is pretty important to the ability of legumes to compete in natural ecosystems."
By studying legumes at MSU’s W.K. Kellogg Biological Station, site of a long-term ecological research site created by the National Science Foundation in part to study the effects of nitrogen fertilizer on plants, the researchers determined last year that in fact fertilizer caused rhizobia to become less beneficial to the plants they served. The new study was launched to determine why that was so.
"This new study is extending that work with whole genome sequencing of the bacteria," Heath said. "We sequenced our samples from our control group and from the nitrogen-fertilized group, and we located this key region on the genome that appears to be differentiated between those two groups."
They found the difference in an area called the symbiosis plasmid, which is an area of extra chromosones in rhibozia that enables them to be mutually beneficial with the plants – it's where the gene is located that actually breaks the bond between nitrogen molecules and the air to "fix" it into ammonium that the plant can use.
This research matters because scientists are looking for more sustainable solutions. Rhizobia are really special because they can do it themselves, Heath added.