The smell of mint and black pepper, orange, cinnamon, thyme. They’re scents one can easily imagine wafting from a kitchen. But scents and other chemical compounds plants produce (on their own or with a help from microorganisms) often exist in nature to help plants defend themselves from herbivores looking for a meal or pathogens searching for a host.
Anyone’s who’s ever been pricked by the thorn of a rose or suffered through a bout of poison ivy knows plants aren’t as passive and defenseless as they appear to be. Like the rest of life on earth, plants have evolved over time. Developing complex methods to fend off attacks has no doubt played an essential role in the evolutionary trajectory of plants. Professor of biology, Dr. Daniel Ballhorn is a chemical ecologist interested in the interactions between plants, insect herbivores, mutualistic and pathogenic microbes and effects of abiotic factors on those interactions. The study of such interactions can lead to discoveries and insights valuable to numerous industries and drive the development of green technologies and sustainable practices.
According to Dr. Ballhorn, plants have developed two methods for defending themselves. The direct method of defense is chemical: the production of secondary metabolites. One such defense employed by thousands of plant species is cyanogenesis, the ability to produce the highly toxic compound hydrogen cyanide. An insect attacks, eating the plant. The plant then releases hydrogen cyanide and kills the insect. In their battle against insect herbivores, plants release volatile organic compounds (VOCs) that can kill, repel, and even cause disruption in the larval development of insects.
In addition to direct defenses, plants also act indirectly. The lima bean, a species Dr. Ballhorn’s research has focused on, along with the direct defense of cyanogenesis, has various indirect defense mechanisms that vary in different populations.
“The lima bean is fascinating in the way it has very different types of defenses. These defenses are more sophisticated in populations growing in nature because they involve the trophic level of the lima bean."
—Dr. Daniel Ballhorn
“Some release VOCs into the air, which are sensed, for example, by wasps," Dr. Ballhorn said. "The wasps are attracted to the damaged plant where they lay their eggs in the insect herbivores. The larva develops within the insect killing it. It’s an indirect defense. Some populations secrete extra-floral nectar that attracts ants. The ants patrol the plant’s surface deterring insect herbivores.”
Dr. Ballhorn’s research focuses, in part, on the relation of and interactions between these two types of defenses. His work has considered what the cost, to the plant, may be for having multiple defense types (see “Trade-offs between direct and indirect defenses of lima bean (Phaselous lunatus).”) Further research includes studies of how rhizobia, a mutualistic soil bacteria that fixes nitrogen and lives inside the roots of legumes, provide lima beans with the nitrogen they need to grow and produce chemicals that ward off insect herbivores like the Mexican bean beetle (see “Dual benefit from a belowground symbiosis: nitrogen fixing rhizobia promote growth and defense against a specialist herbivore in a cyanogenic plant.”)
“These kinds of defenses are very common in the plant kingdom,” Dr. Ballhorn said. “But the combinations of these different types of defenses and their interactions with the environment and abiotic factors aren’t fully understood. Gaining understanding is part of what we do in the lab.”
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Plants have defenses and defenders. Fungal endophytes may be a family of those defenders and they’re another focus of study in the Ballhorn lab.
“This is a huge group of very small, microbial fungi,” Dr. Ballhorn said. They’re everywhere. They’re diversity and general function is totally unknown. You could easily find 500 species of endophytes on a single leaf.”
Like the nitrogen-fixing rhizobia bacteria found in the roots of legumes, endophytes live within the bodies of plants in a symbiotic relationship. Some endophytes benefit their hosts, providing protection against pathogens, drought, and insect herbivores. Chemical compounds produced by endophytes have a wide range of applications known and unknown, including cancer treatment: Taxane agents used in chemotherapy are derived from chemical compounds produced by endophytes that grow on yew trees.
“In the lab we ribotype, cultivate, and study different strands of endophytes. It’s been found that in some kinds of grasses endophytes produce alkaloids that effect insect and even vertebrate behavior. Some endophytic fungi produce colors: red, green, yellow, that might be useful as dyes for clothing. There’s a lot of possibility for applying the compounds endophytes produce to chemicals used in agriculture.
Dr. Ballhorn recently received grant funding from the Oregon Translational Research and Development Institute (OTRADI) to study chemical compounds produced by endophytes with the aim of examining the possibility of using such compounds as agricultural insect repellants. Dr. Ballhorn considers this shift toward applied ecology an added component to the general research focus he and his students are pursuing.
Dr. Ballhorn’s research reveals a seldom considered side of plants. Evolution has provided this kingdom with a rich and complex set of tools to help them survive and pass their genetic material on to the next generation. Plants use chemical tools to ward off attackers and attract defenders and they’ve harnessed the power of symbiotic relationships both above and below ground to provide further protection from harm. While these defenses exist to help assure the continuation of the various species, they also hold secrets that could unlock valuable medicines and state of the art technologies. It is biologists like Dr. Ballhorn who are looking closely at the lives of plants to tease out those secrets.