How plants recognize the one invader that will feed them and the world
Legumes use friendly bacteria to “fix” nitrogen.Deep in the soils of our planet, a battle rages between hordes of bacteria and the tender plant roots they are trying to infect. The roots muster their defenses and fight off the invaders—all except for one, which they invite in.
The plants are legumes—like beans, soybeans, peas, chickpeas, and peanuts—and the infecting bacteria promptly set up shop inside their roots. There, they “fix,” or convert, nitrogen from the air into ammonium, a form of nitrogen that legumes and other plants can use to make the protein they need. Grazing animals obtain protein from plants, and so the whole food web is made possible.
But it would all collapse if the legume roots couldn’t tell bacterial friend from foe. The question of how they do it intrigues University plant biologist Kate VandenBosch, who, with colleagues from the University and MIT, has discovered a key step in the process. Understanding the special relationship between legumes and nitrogen-fixing bacteria may yield knowledge to reduce both hunger and the fossil fuel-intensive production of artificial fertilizer.
Since the dawn of agriculture, farmers the world over have rotated such grain crops as wheat, corn and rice with legumes, whose ability to fix nitrogen rescues us from a predicament. Like the Ancient Mariner surrounded by salt water, we’re surrounded by an atmosphere rich in nitrogen, the key element of protein, but there’s not a morsel to metabolize.
“I’ve thought of this as one of biggest ironies in plant biology,” says VandenBosch. “Nitrogen is so stable and abundant, but plants can’t use it. It would take far too much energy to convert nitrogen to ammonium and ammonium into nitrate, another usable form of nitrogen. Legumes and some close relatives have found a way to get around this by partnering with bacteria that have enzymes to fix nitrogen into a usable form.”
But first, the bacteria must find shelter in a legume. VandenBosch found that a chemical signal works for nitrogen-fixing bacteria the same way “open, sesame” worked for Ali Baba. The chemical, called succinoglycan, is a slimy carbohydrate molecule attached to the outer surfaces of the bacteria. When the bacteria touch cells of the legume’s root hairs, the cells detect the succinoglycan and throw down the welcome mat. Like a sock turning itself inside out, the root hair cells grow inward, forming an “infection tube” that paves the way for the bacteria to enter the root. But if bacteria lacked succinoglycan, the team found that the plants raised their defenses as if the bacteria were pathogens.
The infection sites develop into swellings called nodules; in these tiny workshops, the bacteria fix nitrogen. In turn, the plant furnishes them a home in its roots and pays wages in the form of nourishment.
Helping legumes protect themselves from disease promises far-reaching benefits and is one goal of VandenBosch’s research. “It’s important to know how a plant can recognize friend from foe, often simultaneously,” she says. “Then we can enhance plant defenses against deleterious organisms. There’s a lot of interest in how defense mechanisms are regulated and how exceptions are made.” —Deane Morrison
