PHYSORG. 12 January 2016.  Scientists have wondered for years how legumes such as soybeans, whose roots host nitrogen-fixing bacteria that produce essential plant nutrients out of thin air, are able to recognize these bacteria as both friendly and distinct from their own cells, and how the host plant’s specialized proteins find the bacteria and use the nutritional windfall.

Now a team of molecular biologists led by Dong Wang at the University of Massachusetts Amherst, working with the alfalfa-clover Medicago truncatula, has found how a gene in the host plant encodes a protein that recognizes the cell membrane surrounding the symbiotic bacteria, then directs other proteins to harvest the nutrients. Details appear online in the January edition of Nature Plants.

As Wang explains, plants often recruit microbes to help them satisfy their nutritional needs, offering the products of photosynthesis as a reward. A process used by most land plants depends on a symbiotic relationship with mycorrhizal fungi. These form structures known as arbuscules that help plants capture phosphorus, sulfur, nitrogen and other micronutrients from the soil. This method is akin to scavenging, Wang says, because the amount of nitrogen available in soil is quite limited.

By contrast, the less common process, found mostly in legumes, goes one giant step further: it uses bacteria called rhizobia, which live in root nodules and fix nitrogen from the air and make it into ammonia, a plant fertilizer. Symbiosis with rhizobia means legumes can make ammonia by fixing nitrogen in the air, which at 78 percent of the atmosphere, is “essentially limitless,” the biochemist adds.

Thanks to this feat, legume plants can get as much nitrogen fertilizer as they need, rather than relying on often scarce nitrogen in the soil. This is why beans are so nutritious, Wang notes. “The next time you eat your tasty tofu or edamame, you have those little bacteria, and their ‘marriage’ with legumes to thank.”

“Talk to anyone in our field, and the dream is to make it possible for our crops that can’t fix nitrogen to get that ability,” Wang suggests. “This discovery moves us one step closer. Beans are special, but what our result says is they are not that special because some of the basic infrastructure is already there in plants that use arbuscular mycorrhizal fungi instead of nitrogen-fixing bacteria, which no one understood before.”
Read more at: