In nature, plants are exposed to a multitude of enemies that feed on their leaves, stems and roots, or feast on their sap. In response to these threats, plants have evolved the capacity to produce secondary metabolites whose functions include preventing herbivores from feeding. Plants can use such defensive substances in a multifunctional manner. A team of researchers led by Tobias Köllner from the Max Planck Institute for Chemical Ecology and Matthias Erb from the University of Bern has now characterized the function of benzoxazinoids in wheat. The researchers used previously obtained, detailed knowledge about the defensive functions of benzoxazinoids in maize. In maize plants, the enzyme methyltransferase acts as a functional switch: it decides whether benzoxazinoids act as efficient toxins to protect the plant from caterpillar herbivory, or whether benzoxazinoids are less toxic, but induce callose production. Callose is used as a cell sealant that blocks sieve elements and makes it difficult for the aphids to suck phloem sap. "Our approach was to introduce the maize switch into wheat and to permanently activate it. Together with our colleagues from the Leibniz Institute of Plant Genetics and Crop Plant Research, we made transgenic wheat plants which were no longer able to choose between toxin production and defense regulation, but constantly produced the toxic form of the benzoxazinoids. This enabled us to elucidate the functions of benzoxazinoids in wheat in detail," explains Tobias Köllner.
The approach allowed for a thorough analysis of how switching between toxin production and defense regulation affects wheat resistance to lepidopteran larvae and aphids. Moreover, the scientists were able to identify the corresponding switch in wheat and to analyze it from a biochemical and phylogenetic perspective. Although maize and wheat both produce benzoxazinoids -- their most important defense, via the same, conserved core biosynthetic pathway -- in both species, the genes responsible for switching between their toxic and regulative forms are only distantly related. Thus, the two cereal species likely evolved this switch independently during the course of evolution. Scientists call this phenomenon "convergent evolution."
Read more about this study at Science Daily.