boron

Boron: Exposed

April 22, 2020 By Lauren Hines in Research Tags: boron, plant roots

Assessment of a 18F-Phenylboronic Acid Radiotracer for Imaging Boron in Maize — Figure B is a colorized radiographic image that shows the path of boron in a five-day-old maize seedling. | photo contributed by Alexandra Housh, Michaela Matthes, Amber Gerheart, Stacy Wilder, Kun-Eek Kil, Michael Schueller, James Guthrie, Paula McSteen, and Richard Ferrieri.

By Lauren Hines | Bond LSC

The element Boron, while extremely low in levels, leaves a trail of green and blue radioactive decay as it travels through the veins of plants.

Due to radiotracer technology, this picture of the element’s movement provides this unique insight to what’s going on inside the leaves, stems and roots of plants for the first time ever.

A new collaboration between Bond LSC’s McSteen lab and the MU Research Reactor’s Ferrieri lab led the two labs to present [18 F]-4-FluoropPhenylboronic Acid (FPBA), a radiotracer they designed that can track and visually report on the movement of boron in plants, in a February journal article in the International Journal of Molecular Sciences. The visual tracking was done inside corn.

“It’s the first time anyone has been able to see boron in plants, which is very exciting,” said Paula McSteen, associate professor and researcher at Bond LSC.

The Fifth Element

Boron is an essential micronutrient for plants, but unfortunately, is deficient in soils worldwide. That leads to defects in the roots and shoots of plants, therefore leading to a reduction in crop yields.

“Up to now, there was no way of imaging boron in the plant, which makes it hard because if you could see it somewhere else, like in the cell wall, you would know it has a role there, right?” said Michaela Matthes, postdoctoral scholar in the McSteen lab. “But no one can see it.”

Now, with the equivalent of a PET Scan (Positron Emission Topography) some might receive in a hospital, researchers are able to image and track the movement of boron in live plants to learn where in the plant it’s important and, eventually, how to manipulate it to result in higher crop yields.

You might not think boron would be a controversial topic in the life sciences community, but a longstanding dispute about its importance has been debated for years. The argument is not whether or not the micronutrient is critical to plant development and growth, but where and how it functions. Since there’s such low levels of boron, it’s undetectable. Imaging boron and being able to pick it out will allow a pathway to more direct evidence of its role.

The research study found that the tracer accumulated in the root tip, root elongation zone, lateral root initiation sites and leaf edges in maize.

“So, the accumulation of the tracer basically means there was a signal of the tracer at the root tip, and at the leaf edges, which means boron is there,” Matthes said.

This presence is exactly the kind of direct, visual evidence that supports the argument of the role boron plays in maize growth.

“The other methodologies we looked at would have measured the natural level of boron in the plant, and because that is too low, it is below detection,” Matthes said. “That is why the radiotracer is superior because we are actually adding something to the plant that we then image.”

Micha Matthes — In the McSteen lab, Matthes fills falcon tubes that contain seeds with water, so they can germinate with exposed roots. | photo by Lauren Hines, Bond LSC

Tracking Boron

“When we came here, we were keenly aware of Dr. McSteen’s interest in boron and the challenges of imaging boron, so we started a discussion between our groups about what we could bring to the table with our radiotracer technology,” said Richard Ferrieri, research professor at the MU Research Reactor who came to MU from Brookhaven National Laboratory.

A radiotracer is a chemical compound where a radioactive element is added to whatever a researcher wants to track, which acts as a tag as it moves through the body or plant. Scientists take pictures of its radioactive decay, showing where the element travels and ends up in higher levels.

In this case radioactive fluorine was added to phenylboronic acid (PBA), a compound the plant can easily absorb, which turns PBA into FPBA. FPBA moves exactly like boron in the plant, so it provides an accurate representation of where boron goes and possibly its role.

The Ferrieri lab grew maize hydroponically, in a solution of plant nutrients, under normal lighting conditions. Once they grew to a certain size, they were moved into glass beakers where the roots were submerged in water. A formulation of the FPBA tracer was then added so that the roots could natural absorb the radioactivity.

Using another imaging technique similar to getting an X-ray called, autoradiography, the Ferrieri lab was able to obtain snapshots of the radioactivity inside the plant to see where it accumulated.

“The imaging that we do is giving us information on where the radioactive tracer mimicking boron goes. Having that visual feedback gives us insight about boron’s role in plant growth and development,” Ferrieri said. “I would say in the world of plant biology, this is the first example of chemists sitting down and designing a custom molecular probe to answer some hard, biological questions in regard to boron uptake in plants,” Ferrieri said.

Micha Matthes and Paula McSteen — McSteen and Matthes work together on discovering the roles of Boron in Maize. | photo by Lauren Hines, Bond LSC

What There’s Left to Discover

“Very little is known so far about boron,” Matthes said. “The very basic research question has not been fully answered, and I think it’s fascinating to contribute to that knowledge.”

Researchers were limited in how they could investigate boron’s role without being able to see where boron was in the plant. Matthes said the biggest gap in knowledge in the field of studying boron is whether or not there is an additional role of boron in the plant cell beyond the cell wall.

While there is so little known about boron, researchers aren’t helping themselves due to the lack of method standardization, according to a review written by Matthes, McSteen and Janlo Robil, a Ph.D. candidate in the McSteen lab. When procedures or measurements aren’t consistent across different studies, it makes it hard for other researchers to duplicate experiments and build on existing knowledge.

They also went into evaluating different methodologies investigating boron and how each of them is limited.

“What I think is the best [methodology] is probably a combination of different approaches depending on the question you are asking,” Matthes said.

However, the radiotracer approach can directly locate boron without destroying the plant.

Even though much has been accomplished already, Matthes will keep working towards filling in those gaps.

Photos were used from Assessment of a ¹⁸F-Phenylboronic Acid Radiotracer for Imaging Boron in Maize.

Housh, A.B.; Matthes, M.S.; Gerheart, A.; Wilder, S.L.; Kil, K.-E.; Schueller, M.; Guthrie, J.M.; McSteen, P.; Ferrieri, R. Assessment of a ¹⁸F-Phenylboronic Acid Radiotracer for Imaging Boron in Maize. Int. J. Mol. Sci. 2020, 21, 976.

Critical transport: Bond LSC team finds boron vital for plant stem cells, corn reproduction

August 25, 2014 By Roger Meissen in Research Tags: boron, corn

Carbon’s next-door neighbor on the periodic table typically receives little attention, but when it comes to corn reproduction boron fills an important role.

According to University of Missouri scientists, tiny amounts of boron play a key part in the development of ears and tassels on every cornstalk. The July 2014 edition of the journal Plant Cell published this research.

“Boron deficiency was already known to cause plants to stop growing, but we showed a lack of boron actually causes a problem in the meristems, the stem cells of the plant,” said Paula McSteen, a Bond Life Sciences Center researcher. “That was completely unknown before, and for plant scientists that’s an important discovery.”

MU post doc Amanda Durkbak, Bond LSC researcher Paula McSteen, research specialist Sharon Pike and Bond LSC researcher Walter Gassmann — Amanda Durkbak, MU Biological Sciences post doc; Paula McSteen, Bond LSC researcher and associate professor of Biological Sciences; research specialist Sharon Pike; and Walter Gassmann, Bond LSC researcher and professor of Plant Sciences.

Meristems are a big deal to a plant. These pools of stem cells are the growing points for each plant, and every organ comes from them. They are how plants can survive for 500 or 5,000 years, continuously making new organs in the form of leaves, flowers, and seeds throughout its life.

“When you mow your grass, it keeps growing because of the meristems,” said Amanda Durbak, first author on the paper and MU biological sciences post doc. “In corn, there are actually hundreds of meristems at the tips and all sides of ears and tassels.”

But without enough boron, these growing points disintegrate, and, in corn, that means vegetation is stunted, tassels fail to develop properly and kernels don’t set on an ear. This leads to reduced yield. Missouri and the eastern half of the U.S. are typically plagued by boron-deficient soil, an essential micronutrient for crops like corn and soybeans, indicating that farmers need to supplement with boron to maximize yield.

The tassel-less mutant

The team’s discovery started with a stunted, little corn plant that just couldn’t grow tassels, only created a tiny ear and died within a few weeks. These maligned reproductive organs piqued McSteen’s interest and her team of collaborators set out to figure out which gene was affected in this mutant plant. Graduate student Kim Phillips mapped the mutation to a specific gene in the corn genome involved in transporting molecules across the plant membrane.

But, what was this defect preventing the plant from receiving? Two experiments helped find the answer.

They started by looking at similar genes in other plants and animals. Simon Malcomber of California State University-Long Beach compared the gene – named the tassel-less gene after its mutant appearance – to similar genes in other plants and animals. He found that many were known to make a protein that transports boron and a few other elements.

From field to frog

To clinch this hypothesis, McSteen looked to Bond LSC scientist Walter Gassmann and the African clawed frog. Gassmann harvests eggs from these frogs and uses them to “test” the function of genes from both plants and animals.

“What we do is we inject the frog egg with RNA made in a test tube from the corn’s DNA,” Gassmann said. “The egg is a single, living cell that will actually use the message provided by this RNA to make the boron transporting protein and put it in the egg’s membrane.”

Frog eggs don’t naturally have the ability to transport boron, so an uninjected egg in a solution of boron can’t move the element into the cell.

“Corn RNA provides the egg with instructions to make a boron transporter protein, so the boron solution should move from outside to inside the egg,” Durbak said. “The egg should swell, showing this protein moves boron, and, in fact, these eggs swell so much they explode.”

A tank and a bucket guaranteed boron was the culprit. Durbak went back to the cornfield, watering some mutant tassel-less corn with boron fertilizer and other mutant plants with only water.

Only the ones given boron recovered and grew like normal corn plants, showing that the mutant corn has difficulties obtaining enough boron under natural, low boron conditions without this boron transporter. The boron content of the plants were later tested at the MU Research Reactor and the MU Extension Soil and Plant Testing Laboratory, affirming their observations.

A closer look

But, what does boron deficiency look like on a cellular level?

To see, the team collaborated with biochemist Malcolm O’Neill at University of Georgia. He looked at the cell walls in the plant and discovered that the pectin was affected. Pectin stabilizes the plant cell wall, and many home canners know pectin for its help in making jelly and jam solidify. Pectin is strengthened when boron cross-links two carbohydrates together, giving rigidity to the plant cell wall.

“The effect is that it locks in the cellulose, so without it plant cells won’t have nearly the stability,” McSteen said. “What we think is going on is that plant meristems basically disintegrate because they don’t have the support of pectin.”

While McSteen’s team identified the gene that controls the protein for boron transport into a cell, a research team from Rutgers University identified a gene that controls the protein that transports boron out of a cell. See more about both studies in Plant Cell’s “In Brief” section.

The next step in this research is to look more closely at what happens in these boron-deficient cells early on as they develop to understand the mechanism of boron action in stem cells.

A grant from the National Science Foundation supported this research.

In addition to their Bond LSC appointments, Paula McSteen is an associate professor of biological sciences in the MU College of Arts and Sciences and Walter Gassmann is a professor of plant sciences in the MU College of Agriculture, Food and Natural Resources.