Bucking convention: Breeding plants that grow and defend themselves
MSU plant biochemist Gregg Howe, is working to help plants overcome the duality of growth and defense.
February 14, 2017 - Author: James Dau
As with anything living, plants in nature, whether in the wild or on a farm, are constrained by limited resources. They must allocate and prioritize available resources — be it water, energy from photosynthesis or nutrients from the soil — to meet their needs and respond to environmental threats.
Scientists around the world addressing questions in both agriculture and ecology have long been interested in understanding how a plant adjusts its growth in the face of threats such as drought, pests and disease. If a plant grows rapidly, researchers have observed diminished ability to defend against pathogens and insects. Conversely, small plants that grow slowly typically have higher resiliency than those that grow quickly.
Michigan State University (MSU) AgBioResearch plant biochemist Gregg Howe, with support from the National Institutes for Health and the U.S. Department of Energy (DOE), is working to help plants overcome this duality.
“It’s been interpreted that there’s this fixed resource pool, and if plants put resources into growth, they necessarily deplete them for defense,” said Howe, MSU Foundation professor in the MSU-DOE Plant Research Laboratory. “It’s a well-studied phenomenon in plant science, but we had an idea — unlikely as it was at the time — that we could produce a plant using genetic technology that could both grow and defend itself at the same time.”
Howe’s lab focuses on the study of plant defense hormones. Chief among them is jasmonate, also called jasmonic acid, a hormone that serves as a defense signal. Produced rapidly when a plant comes under threat, such as from insect feeding, jasmonate triggers changes in the plant’s gene expression that activate the plant’s immune system. At the same time, jasmonate functions as a potent growth inhibitor, slowing that process until the threat abates and the plant no longer produces the hormone.
The team started with a variant of Arabidopsis thaliana, a small flowering plant commonly used as a model in laboratory settings for its genetic simplicity and rapid life cycle. The plant is welladapted for defense but, predictably, is small and slow-growing. Howe began breeding one plant with other varieties in an effort to reintroduce the capacity for rapid growth without sacrificing defensive strengths. After five years and thousands of progeny, his team discovered what they were looking for.
“I remember that I was somewhat skeptical they would succeed, but my students found it,” Howe recalls. “One of the progeny from the experiment had regained its capacity for rapid growth.”
The new variant was able to grow significantly larger than its progenitors, but the real test of the experiment came in probing its defenses. If it hadn’t retained the high resistance to attack, the team would have merely found a way to reverse the plant’s typical growth/ defense balance rather than transcend it.
The gold standard for determining defense is how the plant stands up to exposure to leaf-eating caterpillars. Howe’s students left caterpillars on the plant for a week, allowing them to feed on it unrestricted, and weighed them at the end of that period. They found that the caterpillars had gained very little weight during that time, indicating that they hadn’t fed much on the plant.
Upon closer examination, the new variant, in addition to being able to efficiently produce large quantities of jasmonate, had developed a second genetic change, in a photoreceptor called phytochrome that is used by plants to sense light. By producing reduced levels of the phytochrome receptor, the plant can grow faster while using less energy.
The team had produced a plant capable of both high levels of growth and defense, but their work was far from finished. Howe and his then-graduate student Marcelo Campos published their findings in September. Now Howe is beginning to investigate whether this new capability can be translated to agricultural crops, starting with tomatoes.
“We think there’s a good chance of success because the genetic pathways we modified in Arabidopsis are highly conserved across plant species,” Howe explained. “Jasmonate and phytochrome are present in nearly all plant life, so we think we can take this same approach and apply it in food crops.”
Crops planted in high densities in the field often tend to shade one another. This generally encourages them to grow rapidly to compete for sunlight. And this compromises their immune systems. Howe said his discovery might enable the plants to grow faster without losing their defensive edge, whereby bolstering food security.
“This whole discovery was a surprise, but it’s the surprises that get you up in the morning,” Howe said. “Growth and defense don’t have to be coupled antagonistically, and being able to overcome that is going to pave the way for a more secure food future.”