CHAMPAIGN, Ill. -- In humans, the heart is the mechanical pump that circulates the blood of life. In plants, according to new research, it is sucrose accumulation that energizes the heart's equivalent -- an osmotically driven pressure-gradient -- in a vascular system that regulates and directs where life-giving resources go.
At the heart of the findings, published in the April 14 issue of the Proceedings of the National Academy of Sciences, is improved understanding of how plants regulate the distribution of organic nutrients synthesized during photosynthesis, says a University of Illinois scientist.
"We could have diagrammed the mechanics of the plant's vascular system 10 years ago, but what we didn't have at that time were biochemical and molecular understanding of the proteins that mediate the key steps," said Daniel R. Bush, a professor of plant biology and scientist with the Photosynthesis Research Unit of the U.S. Department of Agriculture-Agricultural Research Service.
Bush previously had used a biochemical assay to describe a proton-sucrose transporter in plant cells. Sucrose is the primary end product of photosynthesis. When it departs from leaves, it is loaded into the elongated phloem cells of the vascular tissue by the transporter, which boosts sucrose concentration 50 to 100 times higher than it is in all surrounding cells. As water moves in, Bush said, positive hydrostatic pressure builds, forcing the sucrose through the continuously linked phloem cells and to the non-photosynthetic sink tissues that need organic nutrients.
These nutrient import-dependent tissues include every harvested product in agriculture.
Bush and postdoctoral researcher Tzyy-Jen Chiou, the co-author on the PNAS paper, discovered a regulatory system that controls the long-distance transport of sucrose.
"This paper provides the first clue to a system that can regulate resource allocation," Bush said. "The vascular tissue, I think, is going to turn out to be a very dynamic tissue in a plant, because it's here where cells move organic material around, not unlike the human vascular system. This is not only a pathway for materials, but also for information."
All plants respond to changing environmental conditions by redirecting organic nutrients from the leaves to different organs as needed. "In a period of drought, you may see leaves stunted, in a holding pattern, but the roots are proliferating," Bush said. "How does the plant allocate resources?
"This finding is an important first step in understanding resource allocation," he said. "I believe there are other regulators here. My suspicion is that there are other control pathways that are integrating with this system to communicate information such as water content or nitrogen availability."
Understanding resource distribution in plants, Bush said, may lead to the ability to manipulate the distribution of carbon to maximize crop yields or manipulate nutritional value.