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Walter Gassmann, the new Interim Director of Bond LSC, has been an important part of the MU science community for more than a decade. He’s a member of the Interdisciplinary Plant Group, a researcher in Bond LSC and a professor in the Division of Plant Sciences within the College of Agriculture, Food and Natural Resources.
His research deals with how plants fight diseases and he specifically investigates the inner workings of plants’ immune systems, which are highly specialized in detecting the presence of foreign and potentially harmful organisms. Apart from figuring out how this detection works, Gassmann is interested in finding mechanisms that plants use to keep their immune system in check. The plant immune response is very potent in stopping pathogen spread, but if left unchecked it has the tendency to harm the surrounding plant tissue as well.
Fundamental plant pathology research, what Gassmann’s work deals with, has contributed to many agricultural gains, and will continue to provide avenues for improved crop yields. It has also led to many new insights for biology in general. For example, the concept of a virus was first developed in the late 1800s with work on tobacco mosaic virus. The tit-for-tat between plants and their pathogens has shaped plant immune systems and pathogen countermeasures for eons, and also affords a fascinating glimpse into the processes that shape the evolution of complex organisms.
In recognition of his outstanding contributions to plant pathology, Gassmann was elected as a Fellow of the American Association for the Advancement of Science in 2016.
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.”
A princess kisses a frog and it turns into a prince, but when a scientist uses a frog to find out more information about a grapevine disease, it turns into the perfect tool narrowing in on the cause of crop loss of Vitis vinifera, the world’s favorite connoisseur wine-producing varietal.
MU researchers recently published a study that uncovered a specific gene in the Vitis vinifera varietal Cabernet Sauvingon, that contributes to its susceptibility to a widespread plant disease, powdery mildew. They studied the biological role of the gene by “incubating” it in unfertilized frog eggs.
The study, funded by USDA National Institute of Food and Agriculture grants, was lead by Walter Gassmann, an investigator at the Bond Life Sciences Center and University of Missouri professor in the division of plant sciences.
The findings show one way that Vitis vinifera is genetically unable to combat the pathogen that causes powdery mildew.
Gassmann said isolating the genes that determine susceptibility could lead to developing immunities for different varietals and other crop plants and contribute to general scientific knowledge of grapevine, which has not been studied on the molecular level to the extent of many other plants.
The grapevine genome is largely unknown.
“Not much is known about the way grapevine supports the growth of the powdery mildew disease, but what we’ve provided is a reasonable hypothesis for what’s going on here and why Cabernet Sauvingon could be susceptible to this pathogen,” Gassmann said.
The research opens the door for discussion on genetically modifying grapevine varietals.
Theoretically, Gassmann said, the grapevine could be modified to prevent susceptibility and would keep the character of the wine intact — a benefit of genetic modification over crossbreeding, which increases immunity over a lengthy process but can diminish character and affect taste of the wine.
Grapevine under attack
Gassmann’s recent research found a link between nitrate transporters and susceptibility through a genetic process going on in grapevine infected with the powdery mildew disease.
Infected grapevine expressed an upregulation of a gene that encodes a nitrate transporter, a protein that regulates the makes it possible for the protein to enter the plant cell.
Once the pathogen is attracted to this varietal of grapevine, it tricks grapevine into providing nutrients, allowing the mildew to grow and devastate the plant.
As leaves mature, they go through a transition where they’re no longer taking a lot of nutrients for themselves. Instead, they become “sources” and send nutrients to new “sink” leaves and tissues. The exchange enables plants to grow.
The powdery mildew pathogen, which requires a living host, tricks the grapevine into using its nutrient transfer against itself. Leaves turn into a “sink” for the pathogens, and nutrients that would have gone to new leaves, go instead, to the pathogen, Gassmann said.
“We think that what this fungus has to do is make this leaf a sink for nitrate so that nitrate goes to the pathogen instead of going to the rest of the plant,” Gassmann said.
According to a report by the USDA, powdery mildew can cause “major yield losses if infection occurs early in the crop cycle and conditions remain favorable for development.”
Powdery mildew appears as white to pale gray “fuzzy” blotches on the upper surfaces of leaves and thrives in “cool, humid and semiarid areas,” according to the report.
Gassmann said powdery mildew affects grapevine leaves, stems and berries and contributes to significant crop loss of the Vitas vinifera, which is cultivated for most commercial wine varietals.
“The leaves that are attacked lose their chlorophyll and they can’t produce much sugar,” Gassmann said. “Plus the grape berries get infected directly, so quality and yield are reduced in multiple ways.”
Pinpointing a cause
Solutions to problems start with finding the reason why something is happening, so Gassmann and his team looked at a list of genes activated by the pathogen to find transporters that allowed compounds like peptides, amino acids, and nitrate to pass.
Genes for nitrate transporters, Gassmann said, pointed to a cause for vulnerability to the mildew pathogen.
Over-fertilization of nitrate increases the severity of mildew in many crop plants, according to previous studies sited in Gassmann’s article in the journal of Plant Cell Physiology.
The testing system for isolating and analyzing the genes began with female frogs.
Gassmann used frog oocytes (unfertilized eggs), to verify the similar functions of nitrate transporters in Arabidopsis thaliana, a plant used as a baseline for comparison.
A nitrate transporter, he hypothesized, would increase the grapevine’s susceptibility to mildew.
“The genes that were upregulated in grapevine showed similarity to genes in Arabidopsis that are known to transport nitrate,” Gassmann said. “We felt the first thing we had to do was verify that what we have in grapevine actually does that.”
The eggs are very large relative to other testing systems and act as “an incubating system” for developing a protein. Gassmann and his team of researchers injected the oocyte with RNA, a messenger molecule that contains the information from a gene to produce a protein. The egg thinks it’s being fertilized and protein reproduces and is studied.
“The oocyte is like a machine to crank out protein,” Gassmann said. “We use that technique to establish what we have is actually a nitrate transporter.”
The system confirmed that the gene isolated from grapevine encodes a nitrate transporter.
“We contributed to the general knowledge of the nitrate transporter family,” Gassmann said. “It turned out to be the first member of one branch of nitrate transporters that, even in Arabidopsis haven’t been characterized before.”
The mounting knowledge of Vitis vinifera genes could make genetically modifying the strain to prevent the susceptibility easier.
“Resistance is determined sometimes by a single gene,” Gassmann said. “Until people are willing to have the conversation of genetic modification, the only way to save your grapevines is to be spraying a lot.”
Sharon Pike, Gassmann, other investigators from the MU Christopher S. Bond Life Sciences Center and post-doctoral student, Min Jung Kim from Daniel Schachtman’s lab at the Donald Danforth Plant Science Center in Saint Louis, Mo. contributed to the report.
The article was accepted November 2013 into the Plant Cell Physiology journal.