Sugars Are Important To Plants But Not Essential For Sap Flow
LON TEGELS
MONTICELLO, ARKANSAS
Discovery that plants have a backup plan to move sap even without the presence of sugar – once thought impossible – is the result of a decade-long study published in the Feb. 21 issue of "Nature Plants”.
The multinational research team behind the discovery includes lead author Benjamin Babst, a scientist from the Arkansas Forest Resources Center, and the College of Forestry, Agriculture and Natural Resources at the University of Arkansas at Monticello.
"Nature," established in 1869, is among the most highly regarded journals in science. In its 126-year history, Nature has grown dozens of high-impact offshoot journals, including "Nature Plants."
"What we found is quite a surprise," said Babst, associate professor of plant ecophysiology, "Sugar loading is not required for phloem sap flow in maize plants."
The research team looked at how plants move the sugars they manufacture through photosynthesis from the leaves to the stem, roots, seeds, and fruits. Those sugars are moved through the phloem, tube- like structures running from leaves to other plant parts.
"Plant biologists have been taught for decades what Ernst Münch first proposed in the 1930s – that sap flow is driven by pumping sugars – typically sucrose – into the phloem," Babst said. "The high concentration of sucrose causes water to move in. Water movement into the phloem generates pressure, and the pressure causes the sap to flow down through the phloem toward the stem and roots.
"Under normal circumstances, that appears to be true," he said. Babst and his colleagues wondered what happens when a plant cannot pump sugars into the phloem, for example, when the plant is under stress from drought, flood or heat or cold.
The team found the plant still has different ways to achieve sap flow by using potassium and other ions to drive osmotic movement.
Divide and conquer
Members of the research team tackled different aspects of the problem, which included finding a key mutation and inventing new ways to measure flow within plant tissues.
Babst worked in partnership with David Braun of the University of Missouri, who isolated a corn mutation that prevents the plant from pumping sucrose into the phloem, but still had sap flowing at normal speeds.
Michael Knoblauch of Washington State University found that phloem pressure was high in the mutant maize, similar to wild-type maize.
Researchers found that "other dissolved molecules, solutes, should cause similar behavior of water," Babst said. "Sugars are usually the most abundant molecule by far in the phloem, but there is also a lot of potassium" found in the mutant corn.
New techniques gave accurate measurements that enabled Kaare Jensen, of the Technical University of Denmark, to model liquid flow. Babst said, "The modeling told us that the increase in potassium was enough to maintain phloem pressure and phloem sap flow at the levels we measured in the mutant plants."
"This paper was the culmination of the right mutant and several new non-destructive techniques being used together," he said. "Phloem is very difficult to study. Because it is very small and buried inside of other tissues, it is easy to damage or disrupt the phloem when trying to do physical measurements. Progress on understanding how phloem works has been slow because of these major technical difficulties"."
Of isotopes and insects
Researchers turned to medicine and entomology for the innovative and non-destructive, and minimally invasive techniques used in this study.
"We used carbon-11 radioisotope to measure phloem sap flow in live plants – it's one of the isotopes used in medical imaging," Babst said.
Knoblauch recently invented a pico gauge device that he used to measure phloem pressure.
Gertrud Lohaus of the University of Wuppertal in Germany measured minute amounts of phloem sap using a technique called aphid stylectomy.
"The technique relies on sap-sucking insects, aphids, to find the phloem with their straw-like mouthpart, the stylet," Babst said.
Babst and another team member, Ryan Tappero at Brookhaven National Laboratory, used X-ray fluorescence to image and measure potassium in cross-sections of phloem. Sönke Scherzer and Rainer Hedrich, both of the University of Würzburg in Germany, used another technique to measure potassium in live phloem by inserting special microelectrodes into individual phloem cells of corn plants.
The knowledge gained from this research could help plant breeders develop crops with greater hardiness under climate extremes. Also, the suite of techniques developed for this research can be applied to other plant species and could aid research in enhancing crop yields or wood production.
"I approach my work with the idea that if we can understand basic plant function, we can improve plants through breeding and develop better management strategies, whether that's for forestry or agriculture," said Babst.
The breadth of quantitative analysis that was performed in one study system and the level of modeling allowed is unique. Babst said, "this is why this research represents a big step forward in understanding how phloem transport works." ∆
LON TEGELS: University of Arkansas
Benjamin Babst, associate professor of plant ecophysiology at UA-Monticello, is lead author on a journal article on the
discovery of a plants' backup plan to move sugars through sap while stressed.
UAM photo by Lon Tegels