One of the greatest threats to crops lives underground and can’t be seen by the naked eye.
Plant parasitic nematodes are microscopic roundworms that range in size from as small as 250 micrometers, or 0.0098 inches, to a maximum of several millimeters in length. It’s a parasite that Melissa Mitchum has devoted her career to fighting. Mitchum, a professor in the Division of Plant Sciences and Bond Life Sciences Center, has 20 years of plant nematology research experience. Mitchum also was recently selected to hold the Missouri Soybean Merchandising Council Endowed Professorship.
Mitchum was introduced to nematodes while working toward her Ph.D. degree at North Carolina State University.
“I always wanted to conduct research on something that had a global and human impact,” Mitchum said. “I wanted to work to solve a disease problem that affects how the world is fed.”
Nematodes are extremely aggressive and adaptive. Some nematodes, like cyst nematodes, are picky about which crops they infect. There are sugar beet cyst nematodes, potato cyst nematodes, cereal cyst nematodes and corn cyst nematodes, among many others. The majority of Mitchum’s research is on soybean cyst nematodes and their interactions with the soybean plant. She has received multiple grants in the area of nematology.
“Cyst nematodes are one of the most economically important groups of plant parasitic nematodes, and for this reason, also one of the best studied,” Mitchum said.
To fight nematodes – and to raise awareness of how they operate – Mitchum follows the research, teaching and extension mission of the University of Missouri College of Agriculture, Food and Natural Resources.
“I believe strongly in an integration of research, teaching and extension,” she said. “My program has always been tightly linked to the farmer. We’re doing research, teaching about nematodes and trying to provide a service that is beneficial and increases the profitability of soybeans for our farmers.”
Soybean cyst nematodes set up a feeding site in the roots of the soybean plant. As the female nematodes go through their lifecycle, they swell and eventually emerge from the root as small lemon-shaped structures visible upon close inspection of the roots. The dead adult female nematode forms a cyst.
“The cyst encases hundreds of eggs,” Mitchum said. “This cyst breaks off into the soil. That is the survival structure, and it’s extremely hard to eradicate, as the cyst protects the eggs in the soil. You can have an empty field for 10 years, then go back in and plant new soybeans, and the eggs within those cysts will be stimulated to hatch into infective juveniles.
“Once an infective juvenile is in the root, it’s in. It’s really unstoppable, unless the soybean plant carries one or more resistance genes that offer protection against the nematode.”
Soybean cyst nematode causes more than $1 billion in soybean crop losses annually in the United States.
Mitchum is attacking the nematode problem from all angles. Her research focuses on the molecular basis of plant-nematode interactions. She mainly works with the interactions between the soybean cyst nematode and the soybean plant. She is doing research in the lab on both the nematode and the plant side of the interaction, taking that research to the field and working with farmers to battle the pest.
One aspect of Mitchum’s research is studying spit, as she looks at the secretions delivered to the host plant by the nematode through a hollow mouth spear. She is coupling this knowledge with modern genome editing tools to come up with a plan to improve soybean resistance.
“Think of it as a toolbox,” Mitchum said. “You have a hammer, a screwdriver and a wrench in your toolbox. If I start taking away your tools, you become less effective. We’re trying to identify what tools nematodes are using to cause disease and how we can take those tools away. Those tools work with specific items, like a screw. If you don’t have a screw that fits your screwdriver, you become less effective as well. Similarly, the tools nematodes use typically work on certain types of plant proteins. If we can alter those plant proteins so that the nematode’s tool no longer fits, we may be able to improve the plant’s resistance.”
The feeding structure developed by nematodes is highly specialized. They’re actively transforming root cells to support their growth and development.
“We’re trying to figure out how the nematodes form the feeding structure in the root,” Mitchum said. “We have identified molecular mimics, which are nematode-secreted peptides, small proteins, that look and act like the peptides that a plant naturally produces to regulate its own growth and development.
“The nematodes attack the root and secrete peptides to trick the plant into doing something that it normally wouldn’t do. We’re talking about highly adaptive, evolved pathogens.”
Mitchum was part of a group that cloned a soybean resistance gene in 2012, one of the first soybean resistance genes identified. During that process, the group found an enzyme involved in key processes in the plant. They are collaborating with computational and structural biologists to figure out the exact properties of this enzyme. The discovery has already improved the process for developing resistant soybean varieties for farmers. It’s also providing insight into how these nematodes adapt to overcome resistance in the plant, currently a widespread problem in farmers’ fields due to the repeated use of a single type of resistance (thescncoalition.com).
“With basic science, it’s hard to see an immediate impact of the results we generate,” Mitchum said. “With a chemical trial or a seed treatment study, you get a result in a short period of time. The farmer knows if it worked or not. Without basic science, though, we wouldn’t be where we are today.
“It’s so important to have that genetic blueprint – and it’s been a team effort. The Missouri soybean farmers have been some of the most visionary and that really enabled us to clone that gene. The research we are doing informs my teaching and extension efforts as well.”
Mitchum is a strong proponent of undergraduate research, as her interest in plant pathogens began with a summer fellowship while she was working toward her biology degree at the University of Puget Sound (Washington). She worked with a professor who had just returned from sabbatical in Australia, who was studying barley powdery mildew, a fungal pathogen. It opened her eyes to how plants have to deal with disease just like humans.
“I was going to be a chemistry major originally, with the goal of going to medical school,” Mitchum said. “Once I began, I found that I was much more interested in biology. I took a plant physiology course, which had a section on pathogens. I had an interest in disease in general – and it was then that I saw that plants get sick too.
“During my undergraduate research I was also able to jump into the plant pathogen literature and knew I wanted to study this topic more in graduate school.”
Mitchum earned her master’s in plant pathology from the University of Nebraska and her Ph.D. in plant pathology with a minor in biotechnology from North Carolina State University.
Mitchum teaches two courses at MU – Biology and Pathogenesis of Plant-Associated Microbes, and Research with Plant Stress Agents. The first course, for undergraduates, walks students through the biology of fungi, nematodes, bacteria and viruses, as well as disease development and possible control strategies. Mitchum teaches the nematology section. She leads a second course, for graduate students, which allows students to conduct independent research projects with participating faculty working in the area of plant stress biology.
“A lot of our students will go out and work in the state with various companies,” Mitchum said. “These courses train them and prepare them for the workforce. Because nematodes are such a big problem agronomically, proper training and understanding is vital as they find jobs in the agriculture sector.”
Mitchum is a fan of hands-on activities and projects. Students will learn to extract soybean cyst nematodes and root-knot nematodes, learn to set up tests to assess nematode virulence and evaluate plant resistance, and make management recommendations to a possible grower. She uses her lab as a classroom.
Mitchum also has mentored 26 undergraduate researchers since joining MU, many of whom have gone on to graduate school.
“The ability to improve the profitability of soybeans will only help feed a growing population,” Mitchum said. “It’s a big challenge, though. We need to do our very best to train the next generation of scientists to continue to meet this challenge.”
All of Mitchum’s research and teaching efforts have a common goal, to help seed and biotech industries, researchers, crop advisors and farmers fight nematodes. To do that, Mitchum launched SCN Diagnostics in 2016, serving as the director. SCN Diagnostics, an internal business operation at Mizzou, is in partnership with CAFNR and MU and provides timely, high-quality plant and nematode screening services to several parties. The focus is on soybean cyst nematode, but soil testing for other plant-parasitic nematodes is conducted as well.
SCN Diagnostics offers plant phenotyping, soil testing and population typing. They work closely with farmers and crop advisors to help them identify if they have nematodes, as well as what type of nematode is present.
“If you are a farmer, you can call us or visit our website,” Mitchum said. “There is a sample submission form that can be downloaded, which also includes instructions on how to take a sample, how to send it to us and where to ship it, as well as other important information.”
The origins date back to research that Mitchum has been involved in since she began her program at MU in 2003. SCN Diagnostics is completely self-supported. All revenues go right back into research.
“Every dollar that a farmer spends on a soil test, it goes right back to research on nematodes,” Mitchum said. “No one is profiting from SCN Diagnostics. All of the money is cycled right back into our research program.”
SCN Diagnostics works with industry partners such as Syngenta, Pioneer and Monsanto to do plant and soil testing, as well as to check if export materials are clean. They have partnered with Elemental Enzymes, a former Mizzou start-up, to test seed treatment products, too. SCN Diagnostics screens plant lines for soybean breeders, such as Pengyin Chen and Andrew Scaboo. They also work closely with several MU Extension specialists, in row crops, turf and viticulture, for example. Currently, the states of Iowa and North Carolina send their samples to SCN Diagnostics for population typing.
“We partner with a lot of groups,” Mitchum said. “A lot of it is referral and word of mouth. We’re working hard to be the go-to place for nematode issues.”
SCN Diagnostics has two research specialists – Amanda Howland and Clinton Meinhardt. Howland is the coordinator of the nematode testing, with Meinhardt coordinating the plant testing. John Chester serves as a part-time laboratory assistant as well.
“All of this works so well because of the integration of research, teaching and extension,” Mitchum said. “One of our goals is help educate farmers. If they have knowledge, they are empowered to make better decisions. We’re constantly conducting surveys to connect with farmers and address the major issues.”