Abstract
Two recent hypotheses have proposed that nonβgreen plant colouration evolved as a defence against herbivores, either as protective colouration promoting handicap signals indicating plant fitness or by undermining their crypsis. The handicap hypothesis posits a coβevolutionary process between plants and herbivores, whereas the antiβcrypsis hypothesis suggests that an arms race between insects and plants is the evolutionary mechanism. Both explanations assume that insects are the evolutionary origin causing plants' colouration. Here, we propose a different hypothesis, termed the βDefence Indication hypothesisβ. This idea focuses on the multiple protective functions of anthocyanins and carotenoids as pigments, and suggests that plant colouration evolved primarily in response to various stressors. Because pigments and defensive compounds share a common biosynthesis, the production of pigments also provides elevated defensive strengths against herbivores, a process termed priming. In effect, the Defence Indication hypothesis predicts that pleiotropic effects of the pigments and, more generally, plants' shared defence responses, explain why insects might react to plant colouration. BioEssays 28:65β71, 2006. Β© 2005 Wiley Periodicals, Inc.