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.