Volume 6, Issue 47 October 2009
Shopping the Biological Bazaar
Ellen Simms also served as Director of the UC Botanical Garden from 1998 to 2003. Image credit: Thomas Colton
To fast-moving humans, plants can appear as passive as the soil they grow in. But such judgments do plants a grave disservice. To survive in the wild, says Ellen Simms, a Berkeley professor of integrative biology, plants amass allies, adjust their behavior and maneuver for advantage like all other organisms.
"They're involved in the same kinds of negotiations and give and take with their partners in nature as humans are," Simms says.
Simms studies mutualisms—where individuals of different species help one another so that both benefit—among plants and their ecological communities. Her goal is to examine how mutualisms influence evolution in natural populations.
Scientists have identified many potential plant mutualisms in nature. For example, ants colonize plants that provide nutritional rewards called food bodies. Without ants, these plants get devoured by herbivores. However, the relationship might not be so equal: the ant colony might be just as well fed and successful living elsewhere.
Bradyrhizobium bacteria, which live both free in soil and in the roots of legumes, can take nitrogen in air and transform it into a chemical form that plants can use as fertilizer. Image credit: Ellen Simms
"Early in this work, I realized there were a lot of 'just so' stories about mutualistic interactions, but people rarely tested if the assumptions were valid," Simms says.
Simms avoids those pitfalls by investigating partnerships where the benefits can be clearly measured for both parties. Her current research involves plants called legumes and nitrogen-fixing bacteria. Peas, alfalfa and other legumes grow root nodules to house these microbial helpers. In exchange for sugars and a place to live, the bacteria transform the nitrogen in air into fertilizer. Air is made up of several elements besides oxygen; nitrogen is among them. Legumes regulate the amount of air reaching their nodules and nitrogen-fixing bacteria.
When a legume seed sprouts, it must recruit its colonies of nitrogen-fixing bacteria from scratch. "Right away that's a recipe for disaster for the plant. The plant encounters lots of different genotypes of bacteria in the soil, so it has to figure out which are the good ones," Simms says. She compares these dealings to traders in a market, where everyone seeks the best deal for herself.
Plants in the legume family harbor colonies of nitrogen-fixing bacteria in root nodules such as these from Lupinus arboreus. Image credit: Ellen Simms
To determine how legumes select appropriate strains of bacteria, Simms marries old-fashioned, get-your-hands-dirty greenhouse work with cutting edge genetic analysis. She is connecting the genes that bacteria carry with their phenotypes, or observable traits, especially their influence on plants.
One way in which legumes find useful microbes is by employing molecules on their roots that fit receptors on rhizobia like a lock and key. "But the genes involved in those mechanisms are not the ones involved in nitrogen fixation. So there's the possibility a bacterium with a bad nitrogen fixation gene with the right key can get into a plant's roots, and then the plant might be stuck," Simms says.
Lupines and the much smaller native legume trefoil (Lotus strigosus) engage in complex interactions with nitrogen fixing bacteria at Bodega Marine Reserve in Sonoma County. Among the plants Simms studies are these yellow bush lupine, Lupinus arboreus, overlooking Salmon Creek Beach at the reserve. Image credit: Ellen Simms
Legumes, Simms finds, use several techniques to discourage such cheaters. When she inoculated plants with both industrious nitrogen fixers and lazier microbes, the plants grew bigger nodules for the harder workers. And larger nodules, Simms has shown, contain more bacteria, a direct benefit. "The plant has a way of evaluating how much nitrogen it's getting from a nodule and rewards each accordingly," Simms concludes.
Simms is now investigating legume-microbe relationships in the wild. The dunes at Bodega Marine Reserve near Mendocino are populated by two native legumes, lupines and trefoils. Both interact with nitrogen-fixing Bradyrhizobium bacteria.
With a combination of genetic sequencing data and good old-fashioned greenhouse work, Simms found that the bacteria in trefoil roots can also benefit lupines. By contrast, lupine bacteria can nodulate trefoils but don't provide any nitrogen.
Bacteria aren't the only organisms that have close relationships with plants. Bumblebees such as this one visiting a varicolored lupine, Lupinus variicolor, are among dozens of insects and other species found to interact with lupines. Image credit: Ellen Simms
Since the lotus is at a disadvantage, Simms wondered, how are they able to persist? She then realized that almost all of the trefoils were found within a zone of shifting, sandy dunes, while lupines were more likely to grow amid older, more stable dunes containing more nutrients. "It turns out that what we thought of as lotus bacteria are really new dune bacteria, and the others are old dune bacteria," she says.
"We're starting to widen our perspective and realize that because there are other partners out there engaging in their own interactions, what is beneficial to one host is not necessarily beneficial to the other," Simms says. In other words, biological partners don't just interact in pairs as described in textbooks: they exist within interconnected communities, where one organism's relationships affect the presence and success of others, and all are overlaid by the demands of the environment.