Weeds found to influence gene expression and growth in corn.
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 South Dakota State University plant scientist Sharon Clay.
Using sophisticated genetic mapping techniques, Clay and her research team have been documenting how corn and weeds influence one another.
"Weeds grow 'like weeds' when they grow with corn," Clay said. "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.
To figure out how corn and weeds affect each other's gene response, Clay and a team of two research associates and a soil expert planted plots of velvetleaf alone, corn with velvetleaf and corn kept free of weeds.
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 explained. In addition, specific genes that influence 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 found that genes were differentially expressed compared with non-stressed 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, indicated 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.
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 compared to the corn grown without weeds. However, Clay pointed out, the amount of biomass — the stem and leaves — was not significantly different.
"The genes never recovered," even after the weeds were removed, Clay said. "The impact is long term," which further builds the case for controlling weeds early.
These changes in gene expression can help explain instances when the yield is unaffected but a slight reduction has taken place in the plant that scientists cannot pinpoint.
The researchers also studied 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 phosphorous uptake, Clay explained.
Additionally, she said the circadian rhythm — the internal clock that controls the operation of plant cells — was affected. 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 said.
The researchers now "have a clearer idea of how that stress is affecting the plant," she explained. "We didn't have that ability before we had the genome sequence."
The increasing emergence of herbicide-resistant varieties of weeds has refocused attention on weed control, according to Clay. For the last 20 years, glyphosate has provided an inexpensive yet efficient means of controlling weeds.
"Because we had such an easy control method, no one put money into the research," Clay said, pointing out that the newest herbicides were developed years ago. "My bottom line is to get growers to use what they need, where and when they need it."
That, she explained, means "judiciously using all the tools in our toolboxes."
Like many people in agronomy, Clay considers herself an environmentalist. "I want to help producers do the best job they can with the smallest footprint," she noted.
By understanding how weeds and environmental stressors affect gene expression, she said scientists will have one more piece of the puzzle that will improve weed control and decrease crop damage.