Update on Plant Defense Priming against Herbivores Plant Defense Priming against Herbivores : Getting Ready for a Different Battle 1

Plants have evolved various strategies to defend themselves against herbivores and pathogens. Although some of these strategies are constitutive, i.e. present at all times, others are induced only in response to herbivore feeding or pathogen infection. The induction of direct and indirect plant defenses in response to herbivory and other biotic stresses is well established (Karban and Baldwin, 1997), and a wide array of studies have documented such induced defenses (Schultz and Baldwin, 1982; Haruta et al., 2001; van Dam et al., 2004; Miranda et al., 2007). Induced defenses should be adaptive when: (1) defenses are costly to implement, (2) there is spatial or temporal variability in the distribution of herbivores or pathogens so that plants do not always experience attack, and (3) there are tradeoffs between defenses against different enemies (e.g. defenses against herbivores and pathogens or against different herbivore species) so that defense against one increases susceptibility to another. Although induced defenses allow plants to avoid the costs of implementing defenses in the absence of enemies, plants may suffer considerable damage during the time required to mount defenses once attack occurs. To compensate for this vulnerability, some plants appear to prime specific defenses in response to environmental cues that reliably indicate an increased probability of attack before they actually experience an herbivore or pathogen. In everyday language, to prime means to prepare or make ready. In plant defense, priming is a physiological process by which a plant prepares to more quickly or aggressively respond to future biotic or abiotic stress (Fig. 1). The condition of readiness achieved by priming has been termed the ‘‘primed state’’ (Conrath et al., 2006). Priming may be initiated in response to an environmental cue that reliably indicates an increased probability of encountering a biotic stress, but a primed state may also persist as a residual effect following an initial exposure to the stress. For example, the classic pathogen-induced hypersensitive response is often induced with greater efficiency in plants that have previously experienced pathogen attack (Kuc, 1987), and similar effects may occur in plant-herbivore interactions (Karban and Baldwin, 1997). In the context of long-lived plants such as trees, a primed state may persist across multiple growing seasons, a phenomenon commonly referred to in the ecological literature as ‘‘delayed induced resistance’’ (Haukioja et al., 1985; Zvereva et al., 1997). Because priming initiates a state of readiness that does not confer resistance per se but rather allows for accelerated induced resistance once an attack occurs, one presumed benefit of priming is that it does not impose the costs associated with full implementation of an induced defense response. To date, priming has most often been considered in the context of plant-pathogen interactions (for review, see Conrath et al., 2002, 2006), but plants can also be primed by signals associated with herbivore feeding. This review focuses specifically on priming responses to herbivores to highlight current knowledge and directions for future research on defense priming in plant-herbivore interactions.