Interactions between arthropod-induced aboveground and belowground defenses in plants.

Plants can be attacked by arthropods both above and below ground. The ensuing systemic defense response of the plant can affect even the most distant tissues. Both primary and secondary metabolic profiles of shoots can be altered upon root herbivory and vice versa (Gange and Brown, 1989; Bezemer et al., 2003; Hol et al., 2004; Schwachtje et al., 2006), making plants powerful mediators of interactions between otherwise loosely connected food webs (van der Putten et al., 2001; Bardgett and Wardle, 2003). Whereas the ecological relevance of such processes has been recognized and the role of primary and secondary metabolites acknowledged (for review, see Blossey and Hunt-Joshi, 2003; van Dam et al., 2003; Bezemer and van Dam, 2005), it remains to be explored exactly how plants coordinate their root and shoot responses against herbivores. We propose that results from current research into the mechanisms governing plant stress responses might provide several starting points to explore the physiological basis of plant-mediated aboveground and belowground interactions. Priming (Ryals et al., 1996; van Wees et al., 1999; Ton et al., 2005; Conrath et al., 2006; Frost et al., 2008) and plant volatile signaling (Engelberth et al., 2004; Heil and Kost, 2006; Ton et al., 2007) may be particularly relevant, and we attempt to place these novel insights in the context of interactions between aboveground and belowground plant defense responses. Because of the scope of this Focus Issue, we limit our review to arthropod-induced plant defense responses. We do not discuss induced changes in primary metabolites, which can be of substantial importance (Mattson, 1980; Gange and Brown, 1989; Babst et al., 2005; Schwachtje et al., 2006; Schwachtje and Baldwin, 2008). We also acknowledge the importance of putting the current findings in an appropriate ecological context (Rasmann and Agrawal, 2008) and the necessity of including microorganisms as important players in both rhizosphere and phyllosphere interactions. Several excellent reviews cover these and other intricacies of aboveground and belowground interactions (van der Putten et al., 2001; Blossey and Hunt-Joshi, 2003; van Dam et al., 2003; Bonkowski, 2004; Wardle et al., 2004).