To defend their vulnerable roots against the ravages of cockchafer larvae, weedy dandelions release a milky sap with a bitter taste. Researchers analyzing this dandelion latex have discovered that one single compound does most of the protecting, and it cuts down on dandelion damage remarkably well. The findings are published in PLOS Biology this week.
Plants produce over 200,000 different bioactive compounds called secondary metabolites that aren’t directly involved in development. Many of them help protect the leaves and roots from insect herbivores, though how they work below ground isn’t as well studied. Roots require a lot of protection (and are especially appetizing for grubs) because that’s where nutrients are stored; plants rely on them to fuel their springtime growth.
The common cockchafer beetle (Melolontha melolontha, also called May bug) spends the first three years of its life cycle living underground and feasting on the roots of different plants, especially that of the dandelion (Taraxacum officinale). Dandelions produce latex in all their major organs, and the highest amounts exude from wounded taproots.
To explore the defensive functions of dandelion latex against voracious larvae, Matthias Erb of the Max-Planck Institute for Chemical Ecology and colleagues separated and analyzed the various components of latex from over a dozen different types of dandelions using chromatography techniques. They found that a single substance negatively impacted cockchafer larvae growth: the sesquiterpene lactone taraxinic acid β-D-glucopyranosyl ester (or TA-G).
Then, to test the effect of TA-G deficiency on plant and insect interactions, the team identified the gene that codes for the first step of its production and silenced it. This 90 percent reduction of TA-G levels resulted in a major increase in larvae feeding. The genetically engineered TA-G-deficient lineages of dandelions suffered three times more root biomass reduction than normal dandelions. Meanwhile, the larvae didn’t feed nearly as much when the team added purified TA-G to an artificial diet in the lab.
And finally, when the team performed a garden experiment with over 2,000 individual dandelions belonging to 17 different types, they found that TA-G reduces the negative impact of cockchafer larvae in the field. Taken together, the findings demonstrate that TA-G benefits plant fitness in the presence of root eaters.