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

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.”

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. 

Article originally published on Decoding Science.