d and ctions efenses

emerged that enables a comprehensive analysis of all the Review TRENDS in Ecology and Evolution Vol.20 No.11 November 2005restricted to the site of attack, but result instead inmicroorganisms [5], plants have evolved a variety of chemical defenses to repel, deter or kill their enemies [6]. These defense strategies can be divided into direct defenses (directly impacting the enemy; e.g. trichomes and toxins; see Glossary) and indirect defenses (which include compounds that enable plants to recruit carnivores or enhance their effectiveness; e.g. extrafloral nectar and volatiles) (Box 1). Rather than maintaining high (con-stitutive) levels of these indirect defense chemicals, many plants increase their levels of defense only when they are attacked. Such ‘induced defenses ’ (Box 1) have been studied intensively, primarily in an aboveground context [6,7]. Roots, however, also have direct and indirect induced defenses [8,9]. Because many induced defenses are no