Sometimes, figuring something out only deepens the overall mystery.
Take Pseudomonas fluorescens D7, for example.
Ann Kennedy, a USDA-Agricultural Research Service soil microbiologist at Washington State University, has isolated these native bacteria as a perfectly natural way to fight cheatgrass, also known as downy brome, scientific name Bromus tectorum. Recently, she and her colleagues were awarded a large grant to test the effectiveness of Pseudomonas fluorescens D7 for controlling cheatgrass in rangeland.
Cheatgrass, which was introduced in the late 19th century as a forage crop, is an aggressive invader, a grass that has, according to WSU botanist Richard Mack, changed the ecology, if not the landscape, of much of the western United States. Cheatgrass crowds out other plants and changes the fire ecology of a region. It matures in early spring, then dries out and provides a hot-burning fuel for wildfires.
The reason invasive species are so successful is they are out of context, out of their normal environment. Not all introduced plants are necessarily invasive. They may grow in their new environment, but not sufficiently well to crowd out native species. But if a plant does well in its environment and lacks predators or enemies, then it can become aggressively invasive. Other very visible invasives in the Pacific Northwest are Scotch broom and purple loosestrife. The reason they are so visible is they have no natural controls.
So it is with cheatgrass. Originally from Eastern Europe, it is not the problem there that it is here. In its native environment, it has no advantage over competitors and predators.
Kennedy and her colleagues imported soil from Turkey and Kazakhstan and found that 90 percent of the organisms in it were inhibitory to cheatgrass. Only 50 percent of organisms in domestic soil are inhibitory.
Kennedy and colleagues had earlier studied the effect of inhibitory bacteria on wheat. She first came to WSU as a postdoc to work with soil microbiologist Lloyd Elliott. They were looking at poor growth of winter wheat in early spring and found that the wheat roots were colonized by inhibitory bacteria, also called “deleterious rhizobacteria.” Ninety-five percent of the roots in the early spring were covered with the bacteria.
The bacteria do not kill the plant. Rather, by whatever means, they inhibit cell elongation, resulting in stunted root growth, which stunts the whole plant and gives other plants a competitive advantage.
Eventually, Kennedy became interested in whether inhibitory bacteria could be used against weedy grasses. For wild oat, very few inhibitory bacteria could be found. But in jointed goatgrass and cheatgrass, the bacteria were prevalent and effective. Kennedy and her colleagues identified 20 different isolates that inhibited the growth of cheatgrass or jointed goatgrass or both.
In order to reach that point, however, they screened thousands of isolates from the roots of wheat and cheatgrass. About 50 percent of the organisms they found were inhibitory to cheatgrass. However, only one percent inhibited cheatgrass and not winter wheat.
Of that one percent, the champion they finally selected is Pseudomonas fluorescens D7.
A particularly attractive attribute of Pseudomonas is it flourishes in fall and early spring—when cheatgrass is green and growing. It is inactive in the summer, so there’s little worry about it becoming a pest itself.
Even though the bacterium inhibits plant growth, it is not considered a true pathogen, says Kennedy. It occupies the space between root cells and does not have the necessary enzymes to eat through cell walls. It secretes compounds that together have an inhibitory effect, but is still somewhat of a mystery.
Even more of a mystery, though, is the ecological significance.
“If you’re a bacterium, you try to preserve yourself and your offspring,” says Kennedy. “So why would you colonize a plant you’re trying to kill?”
Regardless of evolutionary reason, this fall, with the support of The Nature Conservancy, Kennedy will begin testing the bacteria’s effectiveness in large-scale field restoration to plants other than cheatgrass. As effective as it is in the lab and small field plots, how it will work under a variety of environmental conditions is unknown. Kennedy and her colleagues will test it in several cheatgrass-infested sites in Washington, two in Oregon, and one site each in Nevada, Utah, Idaho, and California.