By boosting nitrogen fixation in soybeans and other legumes, farmers might be able to cut down on the nitrogen fertilizer they apply to other field crops grown in rotation.
Agricultural Research Service scientists in the Plant Physiology Research Unit at Urbana, Illinois, have identified a single gene that regulates hypernodulation in soybean root systems. "We're now attempting to map the gene and produce a soybean cultivar that has this trait and also produces high yields in the field," says plant physiologist James E. Harper.
On their roots, soybeans and other legumes create nodules that are tiny homes for a type of bacteria that takes nitrogen from the air and enzymatically converts it to ammonia that plants can use for growth and seed development.
Scientists speculate that the more nodules a plant produces, the more nitrogen is left in plant tissues to be returned to the soil when the plant dies and decomposes. This extra nitrogen is then available to a following crop.
Conventional soybean cultivars provide about 40 pounds of residual nitrogen per acre. If hypernodulated soybean lines were to double the residual nitrogen returned to the soil, this would provide over half of the nitrogen needs of most corn varieties.
ARS scientists know that the chemical signal to regulate nodule formation in the soybean root comes from above ground—from young leaves—during the first 4 weeks of plant growth. But they don't know what the chemical is or how it signals plant roots to regulate nodule number.
"If we can learn what this chemical is and how it works, we can use the information to regulate hypernodulation in soybean lines for commercial use," says Harper.
Hypernodulated soybean lines can circumvent the natural nodule suppression that exists in commercial cultivars. For example, a typical commercially available soybean cultivar generates 70 to 200 nodules on its root system during the first 4 weeks of growth. A hypernodulated mutant soybean generates up to 1,000 during the same period.
Harper proved by using rooted leaf cuttings that the hypernodulation signal comes from young soybean leaves. His research group also showed that the signal is common between soybean and mung bean plants. They did this by grafting a hypernodulated soybean shoot to a mung bean root, inducing hypernodulation on the mung bean.
If scientists can identify the nodule control signal, they may be able to induce hypernodulation in other legumes. — By Dawn Lyons-Johnson,Agricultural Research Service Information Staff, 1815 North University Street, Peoria, IL 61604, phone (309) 681-6534.
"Hypernodulating Gene Found in Soybeans" was published in the January 1998 issue of Agricultural Research magazine. Click here to see this issue's table of contents.