Science 339, 1202–1204 (2013)

Credit: GERALDINE WRIGHT

Most flowering plants require pollinators, which enhance a plants fecundity by mediating dispersal of pollen. Though plants produce flowers, fragrances and nectars to attract pollinators, it remains unclear whether plants use other molecular mechanisms to reinforce pollinator behavior. Wright et al. now show that certain plants produce caffeine, which may induce pollination loyalty in honeybees. LC/MS analysis of the floral nectar for four Citrus and three Coffea species revealed that these plants all produce caffeine independent of the diversity or concentration of sugars in their nectars. To examine whether caffeine could influence learning and memory in pollinators, the authors trained European honeybees (Apis mellifera) to associate floral scent with caffeine-containing sucrose solutions. High caffeine concentrations were repellent, but honeybees exposed to mock nectar dosed with submillimolar caffeine concentrations showed a conditioned response to the floral scent that persisted over days after initial exposure. Patch-clamp recordings revealed that caffeine activates adenosine receptors to influence the kinetic properties of Kenyon cells in honeybee brains, an effect similar to its mode of action in mammals. On the basis of these results, the authors propose that caffeine reinforces pollination loyalty by strengthening honeybee neuronal connections between a genera-specific floral scent and reward pathways associated with nectar sweetness.