In hybrid poplar, for example, it is estimated
that 11% of the transcriptome is differentially regulated by insect feeding (Ralph
et al. 2006a). However, inducibility of a certain gene or enzyme per se is not suf-
ficient evidence for a function in plant defense. Whereas the potential contribution
of a given trait to plant resistance can be readily tested in a laboratory setting by
comparing herbivore preference and/or performance on plants that differ in the
trait of interest, a role in plant defense implies that expression of the resistance
trait is associated with a gain in plant fitness; such associations must ultimately be
demonstrated in field experiments that simulate ‘real world’ conditions (Karban and
Myers 1989).