First step to a fertilizer-free future
Lead investigator B.W. (Joe) Poovaiah, a professor in the Washington State University Molecular Plant Sciences program, Department of Horticulture, and Center for Integrated Biotechnology, said his research team’s work (published in Nature, June 29, 2006) opens up the possibility of producing non-leguminous plants that can form symbiotic relationships with nitrogen-fixing bacteria just as legumes do.
Giving non-leguminous crops such as wheat and corn the ability to fix atmospheric nitrogen into a form the plants can use would reduce the need to treat them with nitrogen-rich fertilizers—a boon for farmers, who would spend less on increasingly expensive fertilizers, and for the environment, which would receive less pollution as a result.
While more work is needed before farm-ready applications of his work are available, Poovaiah is optimistic that it could happen within the next decade.
"We're a research university," Poovaiah said. "Our job is to open new doors, and this we have done."
Research and discovery
The team, which includes collaborators at the John Innes Centre in the United Kingdom, has isolated a gene—present in most plants but active only in legumes, such as beans, peas, and alfalfa—that prompts a plant to host billions of nitrogen-fixing bacteria in tiny nodules along their roots. The gene, CCaMK, which Poovaiah’s team first cloned in 1995, codes for a protein that influences many activities within the plant.
In the current study, Poovaiah's team found that in biochemical tests, CCaMK from lily performs much like its counterpart in Medicago truncatula (a relative of alfalfa), even though lily cannot fix nitrogen. They then determined whether the lily gene could function the same way in a real-life situation by removing the gene from Medicago and replacing it with the lily gene. They found that the replacement gene substituted for the missing one. When grown in the presence of nitrogen-fixing bacteria, the experimental plants developed nodules on their roots just like the nodules seen on normal plants.
The researchers did a further experiment in which they removed parts of the CCaMK gene before inserting it into Medicago. When a certain region of the gene was deleted, the Medicago roots made nodules even if no bacteria were present. They did not, however, fix nitrogen; bacteria were still needed to do that. This result indicates that the segment of the CCaMK gene that was removed normally inhibits nodule formation.
Poovaiah said that in a normal legume, bacteria send a chemical signal (known as the “Nod factor”) that tells the root cells to allow CCaMK to become active so that nodules can form. It's still a mystery why lilies and other non-legumes don't make nodules or host nitrogen-fixing bacteria. Since they have a normal CCaMK gene, they are probably missing some other part of the pathway that leads to a legume-type symbiosis.
Opening the door
The current discovery—understanding the genetic basis of nitrogen fixation in plants—is a vital first step. The next step is elucidating the rest of the pathway; finding out what prompts plants to respond to the Nod factor and allow CCaMK to become active is needed to enable the conversion of non-nodule-forming plants into plants that are able to host nitrogen-fixing bacteria.
This would be a tremendous boon to farmers, for whom nitrogen fertilizers are a major expense. Such fertilizers are a petroleum byproduct, so their price has skyrocketed along with the price of crude oil. And reducing the need for nitrogen fertilizers would be a positive step in reducing the world’s dependence on finite oil reserves.
There are also huge environmental costs to using nitrogen fertilizers. Nitrogen leaches from farm fields into groundwater and streams, at times reaching levels high enough to kill nearly everything in the water. According to The Nature Conservancy, thousands of square miles of the Gulf of Mexico are now a "dead zone" due to the 1.5 million metric tons of nitrogen fertilizer that are washed from mid-American farms into the Mississippi River every year.
More information on Dr. Poovaiah’s research
Dr. Poovaiah’s lab:
http://molecularplants.wsu.edu/calcium/
Articles about Dr. Poovaiah’s work and plant sciences research at WSU:
http://researchnews.wsu.edu/health/86.html
http://washington-state-magazine.wsu.edu/stories/2006/May/NoShrinkingViolet.html
Publication in Nature:
Cynthia Gleason, Shubho Chaudhuri, Tianbao Yang, Alfonso Muñoz, B. W. Poovaiah and Giles E.D. Oldroyd. “Nodulation independent of rhizobia induced by a calcium-activated kinase lacking autoinhibition.” Nature 441, 1149-1152 (29 June 2006).
This article was compiled from WSU News Bureau releases and an article in WSU Today, "Deciphering the Code: Huge Discovery Might Change
Farming, Protect Environment" (June 28, 2006), by Cherie Winner.
Nodules form along the fine roots of a Medicago truncatula plant that has been grown in Dr. Poovaiah's laboratory. Each nodule contains billions of bacteria that convert nitrogen into a form the plant can use. In return, the plant supplies the bacteria with energy in the form of carbon-containing compounds.