Goldenrod plant ‘smells’ insect foe and initiates defense

Scientists have determined how a tall goldenrod (Solidago altissima) detects its insect foe and then initiates defense to protect itself from being damage.

According to researchers at Penn State and their colleagues, tall goldenrod can protect itself by first “smelling” its attacker and then initiating its defenses. Researchers say that the gall-inducing flies (Eurosta solidaginis) are specialists that, in Pennsylvania, feed only on tall goldenrod (Solidago altissima).

The male flies emit a blend of chemicals that is attractive to females. Once the females arrive and the eggs are fertilized, the females deposit their eggs within the stem of a goldenrod plant. After the eggs hatch, the larvae begin feeding on the tissue inside the stem. Chemicals in the saliva of the larvae are thought to cause the plant to grow abnormally and form a gall, or protective casing of plant tissue, around the larvae.

Previous studies by the same team led to finding that goldenrod plants exposed to chemicals from the male flies produced greater amounts of a defense chemical known as jasmonic acid when they were damaged by herbivores.

In their current study, the scientists aimed to identify the specific chemical compounds goldenrod plants are detecting and to determine how sensitive the plants are to the compounds.

Researchers first identified the chemical compounds that make up the male fly’s chemical emission. After identifying and quantifying the compounds in the male fly emission, the researchers exposed goldenrod plants to the individual compounds and examined their defense responses. They found that the plants responded most strongly to a compound in the blend called E,S-conophthorin, which is the most abundant compound emitted by the flies.

Next, the team examined goldenrod’s sensitivity to E,S-conophthorin by exposing plants to different concentrations of the compound and measuring their defense responses. Scientists found that the plants are sensitive to even small concentrations of this compound and this indicates that the plants have developed a dedicated mechanism to perceive this compound. The results provide evidence that goldenrod can detect a single compound from the fly, supporting the idea that there is a tight co-evolutionary relationship between these two species. In other words, over time, as the fly has adapted to take advantage of the plant, the plant has adapted to protect itself from the fly.

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