The sweet path: how scientists try to understand sugar movement in plants

Roots play a key role in regulating where sugar ends up, such as in this tomato plant.

Roots play a key role in regulating where sugar ends up in plants like tomato.

Plant scientists are borrowing a tool from medicine to unravel how plants fight off an attack.

The Schultz-Appel Chemical Ecology lab used PET scans to decipher how and when a plant uses resources to fight off a disease or insect.

Positron emission tomography (PET) scans detect radioactive tracers and how they travel over time. In humans the scan tracks blood flow to find cancers, understand brain activity and show uptake of drugs in preclinical trials.

In plants, PET scans shine light on how plants divvy up sugars to protect against attackers.

“Doctors inject radioactive tracers into a patient, and it lights up a tumor on a PET scan because the tumor accumulates a lot of sugar and energy to grow,” said Jack Schultz, director of MU’s Bond Life Sciences Center. “The question in plants is how and where do sugars travel when young leaves are attacked by a pest.”

The sugar trip

Young leaves are of particular interest.

They attract pests because they are full of the nutrients while also being more vulnerable to attack. These youngsters can’t photosynthesize enough sugar to build chemical defenses for themselves, so they get sugars elsewhere through the phloem, part of the plant’s transportation system.

“Think of phloem as a tube with ports for the leaves,” Schultz said. “Older leaves are almost always making sugars, loading them into the phloem where they are transported to various places.”

Abbie Ferrieri, a former doctoral student in Schultz’ lab, wondered if these old leaves were supplying young leaves directly or otherwise. Using Arabidopsis thaliana, she worked with Brookhaven National Laboratory to investigate using PET scans. This mustard plant is commonly used as a model in research because of its relative simplicity and the breadth of knowledge about its inner workings.

This image shows radioactive sugar concentrations 2.5 hours after these tracer molecules were fed to older leaves in the plant. The reds indicate high concentrations in young leaves that draw sugar from the older leaves since they are the growing the most. This PET scan technique is similar to how doctors  detect some fast-growing cancer tumors.

This image shows radioactive sugar concentrations 2.5 hours after tracer molecules were fed to older leaves in the plant. The red color indicates high levels of beta particles as those sugars emit radioactivity. Concentrated in young leaves, the image indicates that these fast-growing young leaves draw the radioactive sugar from the older leaves. This PET scan technique is similar to how doctors detect some fast-growing cancer tumors.

First, she fed radioactive sugar – 2-[18F]fluoro-2-deoxy-d-glucose – to old leaves on the plant. Then she injured young leaves with a mechanical wheel and introduced methyl jasmonate to the wounds. Methyl jasmonate works as a chemical signal, telling other parts of the plant that an attack is happening.

The researchers took PET scans to determine where these sugars traveled. Ferrieri found that most sugars travel to roots and also to leaves in a line above and below them, a connection called orthostichy, in the injured plant.

But, three hours later, PET scans showed that radioactive sugars migrated to the attacked leaves regardless of whether they were in the same row on the stem. Damaged leaves then used those sugars to make phenolic glycosides, compounds that help defend the leaf.

Using the more short-lived radioactive tracers, Ferrieri saw leaves send sugar to the roots within minutes of an attack on nearby leaves. But 24 hours later, damaged leaves started receiving more sugar.

Root of the issue

This change in sugar flow and its control is a continuing curiosity that scientists like Schultz want to decipher.

Schultz’ lab knew from previous research that an insect chewing on a young leaf triggered an enzyme called invertase to break down sugars faster and call for more. But, Ferrieri thought roots might play a bigger role in controlling sugar.

To test her theory, she decided to shut the roots of her plants down using ice water.

“Plants are cold-blooded, essentially, so by submerging their roots in a lake of ice water, she found that the roots altered the rates all these things happened,” Schultz said. “You could say the roots are acting somewhat like a brain, controlling how fast carbon is moving from one place to another at any point in time.”

From human to plant

Plant scientists have been playing with radioactive particles for decades. Researchers like Melvin Calvin used radioactive carbon in the 1940s to figure out how plants capture carbon dioxide for photosynthesis.

But, this field focused more on medical applications until the past decade when plant research received renewed interest.

“The focus has changed a bit to move more into general biological and not just nuclear medicine,” said Silvia Jurrison, an MU professor of chemistry who works to find better uses of the technology.

Bond LSC labs – including those of Gary Stacey, Paula McSteen and Jack Schultz – collaborate with Jurrison to use radiotracers to explore plant processes. Jurrison hopes new compounds will make that easier.

“We’re working to develop other fluorinated compounds, such as sucrose, which could be more relevant and useful in studying plants since they use sucrose more readily.”

While research in Schultz’ lab works to explain basic plant processes, it one day could lead to new advancements.

It could help sequester carbon to slow global warming or be used to better protect future crops from pests.

“Plant defense’ effectiveness depends not only on what they are made of, but also how fast and how strongly they can respond. Anything you can understand about how a plant defends itself against pests is potentially useful,” Schultz said.

The Department of Energy’s Biological and Environmental Research division funded this research. Ferrieri now works at the Max Planck Institute for Chemical Ecology in Germany.

Read more about some of this research in the journal Plant Physiology. 

Schultz atrium web 082013-2514

Jack Schultz is director of the Bond Life Sciences Center.

Searching for the gene: MU scientist works to find link to nutrient content of seeds

David Mendoza-Cozatl uses Arabidopsis plants like these as a model to understand how plants transport nutrients from soil to seeds and leaves.

David Mendoza-Cozatl uses Arabidopsis plants like these as a model to understand how plants transport nutrients from soil to seeds and leaves.
Courtesy Randy Mertens/CAFNR

Forget fruits and vegetables, seeds provide a critical part of the average person’s diet. From beans to cereal grains, understanding how genes and soil types impact nutrition could one day help produce more nutritious food.

One University of Missouri researcher wants to know which genes control the elements in these nutrient-rich packages.

“Iron and zinc deficiencies are considered two major nutritional disorders in the world, so there’s a lot of interest in developing plants with enhanced amounts of these micronutrients,” said David Mendoza-Cozatl, a Bond Life Sciences Center plant scientist. “The question for labs like mine is how can you convince a plant to accumulate more of these metals even though high concentrations can be toxic to plants?”

In a five-year collaboration with researchers at the University of Nevada and UC San Diego, the group measured the amounts of 14 elements in both plant seeds and leaves of mutant Arabidopsis thaliana plants planted in different soil types (salty, alkali, heavy metal and normal).

These mutants were special. Each plant had a different gene disabled, allowing researchers to tell if the disabled gene  affected uptake of minerals into the seeds or leaves.

The teams found that 11 percent of genes influence proteins relevant to the nutritional content in seeds. Soil types also played a role in the significance each gene’s impact.

The approach could be likened to understanding how a car works.

“What we are doing here is we have a car with different parts missing, and the question we are asking is what happens to the car without each of these parts,” Mendoza-Cozatl said. “In plants we ask how more or less elements or nutrients can accumulate without each of these parts, these genes. That’s how we are trying to assign the function of a gene to nutrient homeostasis.”

Mendoza-Cozatl’s work with the group focused on soil laced with non-essential heavy metals (e.g. cadmium and arsenic). They grew mutant plants from seeds and compared nutrient content of those plants’ leaves and seeds to a baseline. To do this, researchers “digested” leaves and seeds, separately, then quantified the amounts of elements.

“We turn everything into basic elements by putting the seeds or leaves in a tube, adding nitric acid and boiling them,” Mendoza Cozatl said. “We found that the mineral makeup of the seed and leaves can be different.”

This surprised him since nutrients in seeds come through the leaves via the phloem. The phloem is living tissue that transports nutrients throughout a plant.

Nutrition isn’t the only area that could benefit from knowing what controls the transport of minerals in plants.

A newly engineered plant could be made to use less fertilizer or move particular types of minerals, like toxic heavy metals.

“Many former industrial areas contain fields contaminated with heavy metals like cadmium and arsenic,” Mendoza-Cozatl said. “Understanding genes important to nutrient transport could help both with bioremediation in soil and bio fortification in food.”

Genes identified through this study will lead to new research in the Mendoza lab as well as other labs involved in this large project.

“The mechanism underlying these changes in nutrient seed composition are not known, so we still need to find how these genes are affecting the seed composition,” Mendoza said. “That’s where the advance will be more significant, and we’re not there quite yet.”

A grant from the National Science Foundation funded this research. Mendoza-Cozatl is an assistant professor of plant sciences in MU’s College of Agriculture, Food and Natural Resources and a member of MU’s Interdisciplinary Plant Group.

Read details about the research in the science journal PLOS ONE.