For decades, scientists have understood that plants can release volatile organic compounds to attract the natural enemies of herbivores. However, the exact mechanism behind this process remained unclear until now.
Researchers at the University of Washington have pinpointed a single immune receptor in common bean plants responsible for orchestrating their anti-caterpillar defense system.
The team discovered that when an herbivorous insect like a caterpillar feeds on a plant, it introduces its saliva into the plant’s damaged tissues. This saliva contains biological clues called HAMPs (herbivore-associated molecular patterns), which include a peptide called inceptin and an 11-amino acid fragment named In11.
These molecules are fragments of the ATP synthase found in chloroplasts – essentially pieces of one of the plant’s own proteins. As the caterpillar ingests the leaf, its gut enzymes chop up the plant’s cellular engines, releasing their pieces onto the leaf’s surface at extremely small concentrations.
Over millions of years, plants like the common bean have evolved a specialized cell-surface receptor called the inceptin receptor to detect In11. When this receptor interacts with In11, it sets off a signaling cascade in the plant’s cells, initiating immune responses.
The researchers used selective breeding to introduce modifications into the common bean plant, as they were difficult to genetically modify using modern techniques like gene silencing. They found a natural mutant that was unable to detect the caterpillar’s saliva and sequenced its genome, discovering a 103-base-pair deletion in the gene that encodes the inceptin receptor.
To test the effect of this dysfunctional receptor on the plant’s defenses, the team bred plants for their experiment. They created sibling plants that were nearly identical genetically except for the presence or absence of the functional inceptin receptor.
When these two siblings were put side by side in the lab and in the field, it turned out the consequences of having a broken inceptin alarm were rather grave for the bean plants.
The researchers found that when caterpillars fed on the mutant beans with inactive inceptin receptors, their growth rate was over 70 percent higher than on the plants with a functional receptor. This was because the plants without the receptor failed to mount targeted responses against the herbivores.
Instead, they reacted as if they were just being mechanically wounded by the wind or a passing animal.
Another consequence for In11 insensitive beans was that they were unable to summon predatory wasps. The researchers packed up their sibling bean lines and headed to an experimental agricultural field in Oaxaca, Mexico, where they placed pairs of bean plants out in the open.
They treated the plants with synthetic In11 peptide or actual caterpillar oral secretions and observed the results.
The findings have significant implications for our understanding of plant-insect interactions and may lead to new strategies for crop protection.
Source: Original article