Infectious enthusiasm

How plants recognize the one invader that will feed them--and the world

Young roots of the legume Medicago show nodules where nitrogen-fixing bacteria have infected.

Photo and home page image by Carrie Thompson

By Deane Morrison

March 25, 2008

Deep in the soils of our planet, a battle rages between hordes of bacteria and the tender roots of plants 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. How they do it is a question that intrigues University plant biologist Kate VandenBosch, who, with her colleagues, has just 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.

The study is published online in the Proceedings of the National Academy of Sciences.

Nitrogen, nitrogen everywhere

Since the dawn of agriculture, farmers have rotated legumes with crops like wheat, corn, and rice to enrich the soil. In cultures around the world, legumes are eaten in combination with grains, such as corn and beans in the New World, chickpeas (hummus) and wheat in the Middle East, and soybeans and rice in the Far East.

Legumes owe their ubiquity to more than their taste: They rescue us from a predicament common to all land plants and animals. Like the Ancient Mariner, 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 takes a lot of energy to make nitrogen into 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. The bacteria have enzymes to fix nitrogen into a usable form."

"I've thought of this as one of biggest ironies in plant biology. Nitrogen is so stable and abundant, but plants can't use it.

But first, the bacteria must find shelter in a legume. Working with MIT colleagues Kay Jones and Graham Walker and others, 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 and kept the door shut, much as they do when greeting a pathogen.

The sites of infection develop into swellings called nodules, and it's inside them that the bacteria go to work fixing nitrogen. Besides supplying the bacteria with a home in its roots, the plant pays them wages in the form of nourishment. It also provides the bacteria with a low-oxygen environment, which the enzyme that converts nitrogen to ammonium needs in order to work.

Protein-hungry world

Today, with global fish stocks declining and dietary protein at a premium, contributions of legumes take on added importance.

In a typical field planted with legumes, nodules churn out between about 67 and 134 pounds of nitrogen per acre, says University and USDA researcher Carroll Vance. That's close to the 150 to 300 pounds per acre a corn crop can consume. A lot of people rely directly on legumes, too.

"In the developed world, between about a quarter and a third of people meet their protein requirements with legumes," says Vance. (This includes vegetarians.) "In the developing world, it's upwards of 60 percent."

Therefore, 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," says VandenBosch. "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."