When some insects zero in on a flower for nectar, their ultraviolet vision is guided by a bull's-eye "painted" on the plant by chemical compounds. Now, chemical ecologists at Cornell University have discovered a second job for these compounds: warding off herbivores.
Even before a flower bud -- such as the creeping St. John's wort -- opens for business, the same chemicals, called DIPs (for dearomatized isoprenylated phloroglucinols), are both coloring the flower in patterns unrecognizable to the human eye and protecting the plant's reproductive apparatus by killing or deterring caterpillars, the scientists report in the upcoming Proceedings of the National Academy of Sciences (Vol. 98, No. 24).
"Now that we know where to look, anti-feedant chemicals like the DIPs undoubtedly will be found in other plant species, and they offer clues to more natural insect control agents," says Thomas Eisner, Cornell's Schurman Professor of Chemical Ecology and one of six authors of the report. An anti-feedant chemical discourages herbivorous insects and can harm those that don't get the message.
One place DIPs are found is in hops, the female flowers of the commercial hop, which give beer its bitter flavor and also protect against pathogenic microorganisms, Eisner says. "If your beer is safe and enjoyable to drink, you ought to thank a flower."
Also participating in the Cornell study, which was supported by grants from the National Institutes of Health, were Jerrold Meinwald, the Goldwin Smith Professor of Chemistry; Athula Attygalle, director of the Mass Spectrometry Facility in the Department of Chemistry and Chemical Biology; Mathew Gronquist, graduate student in that department; Alexander Bezzerides, graduate student in the Department of Neurobiology and Behavior; and Maria Eisner, senior research associate in that department, who is Thomas Eisner's wife and research partner. The DIP finding follows 30-year-old studies by the Eisners of floral "nectar guide" patterns that only creatures with vision in the ultraviolet part of the spectrum can see. Using combinations of special camera lenses and filters, photographic films and video imaging, the Eisners revealed a bug's-eye world where flowers display patterns that are visible only to insects. Besides making a target on the part of the flower where nectar and pollen occur, the distinctive patterns also are believed to help insects recognize a familiar flower among a field of competing images.
"But we had a nagging suspicion that the ultraviolet-absorbing pigments had other functions for the plant," recalls graduate student Bezzerides, who subsequently helped to demonstrate toxicity and a deterrent effect of the chemicals. "We wondered if the chemicals originally served the plants as a sunscreen against ultraviolet radiation."
So the Cornell biologists and chemical biologists joined forces to see what would make a caterpillar sick. Adding to their suspicion that DIPs and similar compounds might have an anti-feedant function was the finding that the compounds were particularly prevalent in plant ovary walls -- making up as much as one fifth by dry weight -- as well as in other reproductive structures such as the anthers. "Just as important as attracting pollinators to a plant is producing viable seed, so there is an evolutionary incentive to protect the reproductive apparatus from herbivores," says graduate student Gronquist, who characterized the chemicals
The flowering plant chosen for the study was Hypericum calycinum, a native of southeastern Europe that is popular with gardeners worldwide as an ornamental. When H. calycinum flowers are fully open, they appear to humans as a uniform yellow disk. But to insects with ultraviolet-sensitive eyes, the disk is highlighted by a dark, ultraviolet-absorbing center, giving the flower a bull's-eye.
While Gronquist performed analyses that led to isolation of the chemical compound, the biologists devised feeding studies. They offered to larvae of the Utheisa ornatrix moth (also called the rattlebox moth) filter-paper discs soaked with chemicals from plants the insects normally relish.
Then the caterpillars were offered paper disks also soaked with DIP chemicals. The ultraviolet-absorbing chemicals deterred most of the caterpillars. But the DIPs were lethal to those that sampled the chemical-laced paper.
The experiments showed, according to the Cornell chemical ecologists, that DIPs both contribute to the ultraviolet pattern in flowers and serve as an anti-feedant, with potentially lethal consequences. Says Eisner, "With the same chemical, the plant is saying to pollinating insects that it needs to attract, 'this bud's for you,' and to herbivores that pose a threat, 'bug off.' "
And speaking of beer, Cornell chemistry professor Meinwald notes that similar chemicals from hops, which have been used in brewing for centuries, are not in the form or quantity to harm human drinkers -- or even to deter fans of the bitter beverage. "What we use to flavor and to
preserve beer is also used by plants both to entice the pollinators and to deter the enemy," according to Meinwald. "Nature quite often has a way of using the same chemical idea to solve diverse problems."