A new imaging technique reveals a strategy that weedy rice repeatedly uses to get the upper hand: roots that minimize below-ground contact with other plants.
Weedy rice is neither wild rice nor crop rice, but rice gone rogue that has shed some traits important to people. It’s also an incredibly aggressive, potentially detrimental weed that pops up almost everywhere rice is grown, and it can reduce crop yields by more than 80 percent if it invades a field.
The hidden half
“Weedy rice may have evolved a go-it-alone ‘cheater’ root growth strategy that could allow it to exploit the nutrient-sharing soil environment of rice fields,” says Kenneth M. Olsen, professor of biology in Arts & Sciences at Washington University in St. Louis and senior author of a new paper in New Phytologist.
“We tend to think of competition occurring above ground because that’s the part of the plant we see. But that’s only half the plant,” Olsen says.
“It’s the ‘hidden half’—i.e., the root system—that plays a critical role in some of the most important aspects of plant growth and survival, including water uptake and competition for essential nutrients like nitrogen and phosphorous.”
Some estimates suggest root growth is actually a more important determinant of competitive success than above-ground growth, Olsen says.
“This appears to be particularly true for agricultural weeds such as weedy rice, which compete against crop varieties in agricultural fields.”
Scientists are only just beginning to get their first realistic glimpses of how root growth and below-ground root interactions affect a plant’s ability to compete for resources.
In the past, to get a decent look they either had to dig the plant up—which inevitably damages it—or grow plants in artificial conditions, such as sandwiched between two glass plates.
Root growth
For this study, researchers compared the roots of two independently evolved types of weedy rice that occur in the same rice fields in the southern United States.
Using new imaging techniques, including a semi-automated optical tomography approach that Christopher Topp at the Donald Danforth Plant Science Center developed, the researchers took more than 70 photographs of the root systems of each of 671 different weedy rice plants. Then they modeled the pictures in 3D to create 360-degree digital maps of their roots.
The researchers used a variety of algorithms to analyze 98 physical traits—including root depth, root system width, certain exploratory traits, and root-soil angles. They also conducted genetic analyses to track the weeds’ separate paths from their domesticated pasts to their persona-non-grata status in the rice fields of today.
“Natural selection says that they (the two types of weedy rice) should respond to this environment by evolving similar traits,” says first author Marshall J. Wedger, a PhD candidate in Olsen’s laboratory. “They did evolve similar traits in response to similar environmental pressures, but they did so using very different genetic mechanisms.”
The new study shows how two independently evolved weedy rice strains have convergently arrived at a similar pattern of root growth that may play a role in their ability to out compete cultivated rice for soil nutrients.
“By looking at the genetic basis of weedy rice evolution, we can see whether—at the genetic level—there’s more than one way to evolve a weed,” Olsen says. “What we find, both for above ground traits and now with this study for below ground traits, is that the answer is a definitive yes.
“In other words, it’s disconcertingly easy to evolve a weed from a domesticated crop. This can occur multiple times independently from different crop varieties.”
