Evolutionary change from induced to constitutive expression of an indirect plant resistance.

Induced plant resistance traits are expressed in response to attack and occur throughout the plant kingdom. Despite their general occurrence, the evolution of such resistances has rarely been investigated. Here we report that extrafloral nectar, a usually inducible trait, is constitutively secreted by Central American Acacia species that are obligately inhabited by ants. Extrafloral nectar is secreted as an indirect resistance, attracting ants that defend plants against herbivores. Leaf damage induces extrafloral nectar secretion in several plant species; among these are various Acacia species and other Fabaceae investigated here. In contrast, Acacia species obligately inhabited by symbiotic ants nourish these ants by secreting extrafloral nectar constitutively at high rates that are not affected by leaf damage. The phylogeny of the genus Acacia and closely related genera indicate that the inducibility of extrafloral nectar is the plesiomorphic or 'original' state, whereas the constitutive extrafloral nectar flow is derived within Acacia. A constitutive resistance trait has evolved from an inducible one, obviously in response to particular functional demands.