New study uncovers plant defense mechanisms against pests

 Since the beginning of crop domestication many centuries ago, farmers have battled a multitude of threatening pests that feed on and destroy their harvests. Throughout the 1900s, widespread manufacturing of synthetic pesticides offered powerful protection against pests, but their high costs, variable effectiveness, and negative environmental impacts have demanded the development of better approaches for crop defense. Recent advances in plant genetics make it easier to identify genes that naturally provide resistance to insect herbivory, allowing breeders to cultivate crops with better defenses.

Scientists from BTI, the Max Planck Institute for Chemical Ecology, the University of Neuchatel Institute of Biology, and the U.S. Department of Agriculture uncovered an underlying biochemical pathway for maize defense against the corn leaf aphid Rhopalosiphum maidis, a harmful sap-sucking insect. Their findings, published June 2013 in Plant Cell, offer new insights into natural variation of plant defense responses and carve the way for new opportunities to improve crop defense systems through breeding.

The various abilities of plants to resist pests can be visually observed in any garden or natural ecosystem; one plant may suffer severe pest damage while surrounding plants appear unharmed. Natural variation in resistance, even with plants of the same species, evolved over time across Earth’s vast and diverse terrain, where an array of plant-insect interactions can be linked to different geographical regions. According to BTI scientist Georg Jander, “Understanding the complex genetic and biochemical pathways behind plant resistance gives us the opportunity to breed insect-resistant plants that require less use of insecticides.”

To investigate natural variation in aphid resistance, researchers fed 25 genetically diverse lines of maize to aphids and measured aphid reproduction on each line. They also analyzed levels of specific defense chemicals produced by maize; a group of metabolites called benzoxazinoids that are activated by insect feeding. Their results linked aphid reproductive success to the relative abundance of different types of these metabolites.

Using quantitative genetic approaches, the researchers located a group of defense genes that regulate the metabolites and discovered a process that actually increases maize sensitivity to aphids. Investigating the activity of these genes, researchers found a previously unidentified (methyltransferase) enzyme that converts one benzoxazinoid into another and causes higher aphid-sensitivity. Maize varieties with a natural knockout mutation in the defense gene linked to this enzyme expressed lower amounts of the methylated benzoxazinoid and surprisingly, higher aphid resistance.

According to Jander, this finding raised the question: “Why would any plant have a gene that makes it more aphid-sensitive?” It turns out that, although the gene increases aphid sensitivity, the methylated form of the benzoxazinoid provides enhanced caterpillar resistance. In other words, with this gene turned on, plants are able to respond more rapidly to a caterpillar attack. For plants that are not exposed to a caterpillar threat, however, the natural knockout mutation comes into play. It suppresses production of the methylated benzoxazinoid, turns off caterpillar defense, and allows for stronger aphid defense. By uncovering the complexities of the benzoxazinoids methylation gene, scientists revealed an intriguing trade-off in plant defense; the maize can be either resistant to aphids or caterpillars, but not both.

This knowledge of a new mechanism of aphid resistance in maize helps to explain natural variations in plant-insect defense strategies and opens the door for further research to identify other maize benzoxazinoid genes and their functions in plant defense. By knowing which genes are required for defense against specific insect pests, it will be possible to breed maize in a more targeted manner for resistance to the local insects.

Support for this research comes from the US National Science Foundation Award, the Defense Advanced Research Projects Agency, and a Friedrich Wilhelm Bessel Research Award from the Alexander von Humboldt Foundation.