Writer: Kathleen Davis, (409) 845-2872, k-davis3@tamu.edu
Contact: Dr. Doug Cook, (409) 845-8743, drc1653@acs.tamu.edu

COLLEGE STATION -- The pathway in legumes -- such as soybeans and alfalfa -- that controls the formation of nitrogen-packed nodules on roots has been identified by researchers at Texas A&M University.

The finding, reported in today's issue of Science magazine, could help scientists better understand how to manipulate the growth of such unique plant organs which are vital to the Earth's ecological health.

Dr. Doug Cook, Texas Agricultural Experiment Station plant pathologist, and graduate student R. Varma Penmetsa found that a mutated form of the gene SKL1 ignores commands from ethylene, a hydrocarbon that occurs naturally in plants and regulates plant growth such as the formation of nodules. Their work involved Rhizobium meliloti, a bacterium, and Medicago truncatula, a relative of alfalfa.

"We have identified a genetic pathway that can potentially be manipulated to increase nodulation in crop legumes, such as alfalfa," Cook said.

From the tiniest plant to humans, all higher organisms are dependant on reduced forms of nitrogen -- such as ammonia and amino acids -- to live. Most organisms use up and don't replenish nitrogen. Certain plants, called legumes, work with bacteria in the soil to convert nitrogen into useable forms.

"One of the most important sources of useful nitrogen is this symbiotic relationship," Cook said.

As leguminous plants grow and send roots deeper, the bacteria in the soil "infect" the roots. The plant responds by forming a protective nodule where free oxygen is regulated and energy is provided to the bacterium for the conversion of nitrogen.

Normally, the plant limits the number of nodules that grow within a length of the root, limited by the presence of ethylene. Plants only need so much nitrogen to survive; too much nitrogen fixation taps into the plant's energy reserve.

"This gene (SKL1) tells us ethylene is used to regulate nodulation on the plant root," Cook said. "The inability to recognize ethylene leads to a large increase in the number of nodules."

Cook noted that the field of biotechnology has a long held a goal of figuring out how to regulate increased nitrogen fixation. However, according to Cook, altering the process for practical purposes in field cropswon't necessarily be simple.

"Ethylene affects other systems in plants such as insect and disease resistance, so genetic engineering of ethylene recognition may have undesired consequences," the researcher pointed out. "Still, it identifies a target which may help us to engineer increased nodulation, potentially improving the benefit of legumes."

Cook said the effort is important because all ecology benefits from the organic nitrogen of legumes.

"Other plants pull nitrogen out of the soil, but legumes return nitrogen to the soil," Cook said.


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