Legumes, such as clover and alfalfa, have long been known for their nitrogen-fixing properties, and a group of University of Kentucky (UK) scientists have discovered a more efficient way for these plants to fix nitrogen.

Through a symbiotic relationship with soil-borne rhizobia bacteria, legumes can fulfill their own nitrogen needs and produce and leave enough in the soil for other plants to use. This reduces the need for topical nitrogen fertilizers, which are costly and can cause environmental pollution.

But not all legumes are the same when it comes to their nitrogen fixation efficiencies. In fact, different environments and bacterial strains can result in legumes fixing little to no nitrogen at all.

Recently, Hongyan Zhu, PhD, a professor in the UK College of Agriculture, Food and Environment, and his team of researchers found two antimicrobial peptides in the model legume Medicago truncatula (barrelclover or barrel medic) that kill certain rhizobial bacteria as the nitrogen fixation process begins. This model legume is closely related to the forage legume alfalfa.

“This finding offers scientists a strategy to improve nitrogen fixation in legumes by selecting or manipulating these genes to accept more bacteria,” Zhu said. “This could potentially allow legumes to fix more nitrogen.”

Zhu believes the original function of these antibacterial genes was to kill bacteria as they entered the plant, but they have evolved to manipulate certain bacteria to start the nitrogen fixation process. Bacteria that do not tolerate the peptides die almost immediately.

In addition to the UK researchers, scientists from Brigham Young University, University of Massachusetts, and Cornell University, as well as collaborators from Hungary and the Netherlands, contributed to the study.

Zhu’s research findings were published in the Proceedings of the National Academy of Sciences and can be viewed online at pnas.org/content/early/2017/06/06/1700715114.abstract and pnas.org/content/early/2017/06/07/1700460114.abstract.

He received funding from the USDA National Institute of Food and Agriculture and Kentucky Science and Engineering Foundation for this study. He will continue to study more efficient ways legumes can fix nitrogen.

Katie Pratt is an agricultural communications specialist at UK.

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