The axiom, "growing like a weed," takes on new meaning in light of changes in gene expression that occur when weeds interact with the crops they infest, according to plant scientist Sharon Clay. Using sophisticated genetic-mapping techniques, the South Dakota State University professor and her research team are documenting how corn and weeds influence one another.
"Weeds grow like weeds when they grow with corn," says Clay. "They grow bigger and taller in corn than by themselves." And inversely, "corn grows less among weeds."
Over the last 20 years, Clay has been studying weed management in range and cropping systems, weed physiology and interactions among herbicides, soil and crops. The weed scientist was the first woman to serve as president of the American Society of Agronomy.
She has received two awards from the Weed Science Society of America for outstanding papers published in Weed Science --one in 2007 and another in 2012. Both articles were written in collaboration with David Horvath, a research plant physiologist for the Agricultural Research Service at the U.S. Department of Agriculture in Fargo, N.D.
Growing better among corn
To figure out how corn and weeds affect each other's gene response, Clay and a team of two research associates and a soils expert, planted plots of velvetleaf alone, corn with velvetleaf and corn kept weed-free.
The researchers saw an entirely different response when velvetleaf was grown by itself versus among corn plants. The velvetleaf alone was shorter and stouter, Clay explains. In addition, specific genes that influenced photosynthesis and other important plant responses differed in expression.
Another study compared the corn's growth and yield in response to weeds, lack of nitrogen, or shade. In all cases, Clay and Horvath found that genes were differentially expressed compared with nonstressed plants. However, each stress resulted in very different expression patterns.
Traditionally, weeds have been thought to reduce crop growth and yield due to competition for water, nutrients and light. This study, however, indicates that weed-crop interactions are much more complex than researchers have thought.
When grown with weeds, genes that control the major facets of the corn plant's metabolism were decreased or down-regulated, according to Clay. These included its response to light stimulus, the amount of chlorophyll it produces and its ability to convert raw materials into energy.
In short, these changes in gene expression adversely affect the plant's ability to grow and reproduce.
Having long-term impact
When the researchers started taking weeds out of the corn at early points, such as when the corn had as few as two and four leaves, they still saw differences in gene expression when compared to corn without weeds. However, Clay points out, the amount of biomass--the stem and leaves--was not significantly different.
"The genes never recovered," says Clay, even after the weeds were removed. "The impact is long term," she adds, which further builds the case for controlling weeds early.
These changes in gene expression can help explain instances in which the yield is unaffected, but a slight reduction has taken place in the plant which scientists cannot pinpoint.
Next, the researchers look at the effect of water stress on gene expression using corn planted on high and low ground. The genes of the water-stressed corn on the top of the hill were down-regulated in terms of phosphorus uptake, Clay explains.
In addition, the Circadian rhythm, the internal clock which controls the operation of the plant's cells, "was affected" she adds. This, in turn, affected the plant's wounding response and made it more susceptible to pest injury. Essentially, the water-stressed corn "was getting older, faster," Clay says.
The researchers now "have a clearer idea of how that stress is affecting the plant," she explains. "We didn't have that ability before we had the genome sequence."
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