OAXACA — Researchers led by Adam Steinbrenner identified and validated a cell-surface receptor in common bean plants that detects caterpillar feeding and initiates anti-herbivore defenses. Field experiments in Oaxaca, Mexico, confirmed that the receptor enables plants to attract predatory wasps when under attack.

Steinbrenner’s team discovered that common bean plants possess an inceptin receptor that specifically recognizes In11, an 11-amino acid fragment derived from chloroplast ATP synthase. Caterpillars regurgitate In11 onto leaf surfaces as they feed, and the receptor’s activation triggers a signaling cascade that up-regulates 527 genes linked to defense responses.

Because common bean plants are difficult to genetically modify, researchers used selective breeding to develop nearly identical sibling lines—with and without a functional inceptin receptor. They identified two naturally insensitive bean varieties by screening 89 Mesoamerican strains for failure to produce ethylene in response to In11, ultimately selecting a Honduran strain, W6 13807, which carries a 103-base-pair deletion in the receptor gene. This deletion produces a truncated, non-functional protein.

In controlled tests, caterpillars feeding on mutant plants lacking the functional receptor grew over 70% more in five days than those on receptor-equipped plants. Mutant plants responded to feeding as if experiencing mechanical damage rather than herbivory and failed to emit the specific blend of volatile organic compounds that signal predatory wasps.

Field experiments in Oaxaca paired sibling plants—one with and one without the receptor—and treated them with water, caterpillar oral secretions, or synthetic In11. Sentinel caterpillars were attached to leaves, and researchers observed that predatory wasps disproportionately targeted plants with functional receptors that had been exposed to In11 or oral secretions. Plants unable to detect In11 were largely ignored by wasps.

"[One] thing we didn’t know is how the plant detects the caterpillar in the first place," says Adam Steinbrenner, a biologist at the University of Washington. "We were excited to do that, but we needed the perfect comparison plants—plants lacking the receptor versus ones that have the intact receptor."