Airborne Induction ofResistance to Herbivores

Plant-plant communication was first reported from the Sitka willow Salix sitchensis (Rhoades 1983), poplar Populus x euroamericana, and sugar maple Acersaccharum (Baldwin and Schultz 1983). Since then, the phenomenon has been detected in the taxonomically unrelated species Arabidopsis thaliana, black alder (Alnus glutinosa), corn (Zea mays), lima bean (Phaseolus lunatus), sagebrush (Artemisia tridentata) and wild and cultivated tobacco (Nicotiana attenuata and N. tabacum) (Shulaev et al. 1997; Karban et al. 2000; Tscharntke et al. 2001; Engelberth et al. 2004; Choh et al. 2006; Karban et al. 2006; Kost and Heil 2006; Heil and Silva Bueno 2007b; Ton et al. 2007; Godard et al. 2008). Most of these cases related to signaling among plants that belong to the same species, but plant-plant communication even occurs among different species; for example, clipping sagebrush induced resistance in neighboring tobacco plants (Karban et al. 2000; Karban 2001; see Fig.1).

The first experiments on plant-plant communication used saplings that had been kept in a closed space or trees that were growing at different distances from attacked individuals, but these reports have since been criticized for their lack of ecological realism (Baldwin and Schultz 1983) or their lack of true controls (Rhoades 1983). Later on, however, observations on black alder trees confirmed that individuals growing downwind of clipped plants became more resistant to future herbivore attack (Tscharntke et al. 2001) . While manual clipping might release unrealistically high amounts of VOCs, or even compounds that are not released when herbivores feed on plants, field studies on lima bean demonstrated that plant-plant communication also works under ecologically realistic conditions: receivers that were otherwise untreated suffered less from herbivory when they were exposed to the air that came from beetle-damaged emitters (Heil and Silva Bueno 2007b).

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