U of M News Wire
March 27, 2008
Infectious enthusiasm
How plants recognize the one invader that will feed them – and the world
By Deanne Morrison
U of M News Wire
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 Mariners, 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," VandenBosch said. "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."
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, said 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," Vance said. (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," VandenBosch said. "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."
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Carbon count
A U professor's research could help explain large-scale variations in the carbon cycle
By Stephanie Xenos
U of M News Wire
To those with a deep yearning for warm places fueled by long, cold winters, Jennifer Powers' research sounds suspiciously like a vacation. She spends several weeks two or three times a year in the dry tropical forest of Costa Rica's Santa Rosa National Park (Guanacaste). A kilometer-long white sand beach is a mere 20 minutes away. Pineapple, papaya and watermelon are her daily staples. She even brings her family with her on occasion.
But Powers is hardly on holiday. Her packing list? A soil sampler, rubber boots, a laptop, DVDs and plastic bags. "I dig a lot of holes," she explains with a laugh.
Powers works at the crossroads of geography and ecology using tools such as remote sensing and geographic information systems (GIS) to understand how tropical forests differ in terms of their carbon cycle –the amount of carbon stored in the soil and released into the atmosphere – and what that variation means for big-picture processes such as climate change.
"There's a real variable response of ecosystems to land-use change and deforestation [in tropical forests]. Sometimes soil carbon stores increase, sometimes they decrease. Global-scale models average over all this variation," Powers said. "We need to upscale the information we've collected at a plot scale ... to understand the regional and continental drivers and the implications."
Her current project in the province of Guanacaste in the northwest corner of Costa Rica, funded by a three-year NASA New Investigator Award, involves taking soil samples, identifying and measuring trees and logging coordinates for well over 50 plots around Santa Rosa National Park and Palo Verde National Park, another dry tropic forest to the south.
The data will be matched up with satellite imagery in an effort to document changes in carbon storage and biodiversity on a regional scale as dry tropical forests return after years as pasture or following more recent deforestation.
Her research has implications for everything from better understanding the dynamics of climate change and the carbon cycle to public policy and even economics in the form of carbon trading.
The arduous work of collecting detailed field data, down to the contents of the soil, to feed regional-scale analysis of carbon cycling and storage means Powers must make connections between the micro and macro. "My work spans everything from microbial ecology to continental-scale field studies," she notes.
"There are big questions of whether tropical forests are sources or sinks of atmospheric carbon dioxide," Powers adds. "Understanding carbon cycling processes at both field and regional scales fits right into this context."
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Growing Concerns
A parenting question-and-answer column with Dr. Martha Erickson of the University of Minnesota
Question: We were visiting my cousin’s family in another city over the weekend and when we got home we discovered that our five-year-old son had brought home a toy that belongs to their child. Their child has more toys than any kid I’ve ever seen, so he probably won’t even miss it. But should we discipline our son anyway and, if so, how? I’m not sure he even has a concept of stealing at this young age.
Answer: Never mind that your cousin’s son has so many toys or that the toy won’t be missed; it’s the principle that matters. This is an important teachable moment in your child’s life, and there are several steps in driving that lesson home.
• First, tell your son clearly and firmly that it is never okay to take something that does not belong to him. It doesn’t matter how small the item is or how many possessions the other person has. This is stealing, plain and simple.
• Second, engage your son in thinking about how he would feel if someone took something that belonged to him. At age five, children can begin to understand the perspective of others. One-way to help a child perspective-take is to have him imagine himself on the receiving end of the hurtful act.
• Next, let your son know that he needs to apologize to your cousin’s child. Since your cousin and his family live in another city, phone them and have your son explain to the other child and/or the parent what he did, followed by an apology. (It sometimes helps to role-play the phone call with your child first so that he knows what to say.)
• Finally, mail the toy back to the other child and, if possible, deduct the amount of postage from your son’s allowance or money in his piggybank.
When you follow through with these steps, there’s no need for anger or shaming. Just be firm, clear and matter-of-fact: stealing is wrong and this is what happens when you choose to take something that doesn’t belong to you. By the way, the four steps I suggest for dealing with stealing can be applied in many different situations that call for discipline: 1) state the rule or principle you are teaching; 2) focus on the feelings of the person who has been harmed; 3) apologize; and 4) make restitution. It is those small lessons that, over time, add up to life’s big lessons about values and character.
Want to hear more parenting advice?
Dr. Erickson and her daughter can be heard every Sunday, from 2 - 4 pm, on “Good Enough Moms,” on FM107.1 radio in the Twin Cities or via Webcast at www.FM1071.com
