FORUM On integrating molecular and ecological studies of plant resistance: variety of mechanisms and breadth of antagonists PAUL E. HATCHER and NIGEL D. PAUL* Department of Agricultural Botany, School of Plant Sciences, University of Reading, 2 Earley Gate, Whiteknights, Reading, RG6 6AU, UK; and *Division of Biological Sciences, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, LA1 4YQ, UK Introduction We are grateful that an attempt has been made (Heil 1999) to place the issue of systemic acquired resistance (SAR) within an ecological context. However, although this review raises some interest- ing points for further action and provides food for thought, we believe that the author may have under- estimated both the changes in conceptual frame- works that are needed in order to incorporate SAR within such a context, and the eorts already being made in this area. The investigation of induced resistance mechan- isms in plants is a fast-moving area of research and several excellent reviews on aspects considered in Heil (1999) have subsequently appeared (e.g. Agrawal et al. 1999; Bostock 1999; Maleck & Dietrich 1999; Pieterse & van Loon 1999). Indeed, a further review could usefully explore the evidence available for the points that Heil raises at the begin- ning of his article. Our article does not purport to be such a review; rather we wish to raise, in reply, two important points that have been generally over- looked in this debate. First, any understanding of SAR in its ecological context involves integrating this resistance within the framework of the other mechanisms by which the plant modi®es its interac- tions with consumers. Second, this wider context for SAR must consider the potential for very broad interphylum eects. We demonstrate these points from our studies on the interactions between the large perennial herb Rumex obtusifolius and its consumers, notably the leaf-feeding chrysomelid beetle Gastrophysa viridula and the leaf-infecting rust fungus Uromyces rumicis. This is one of the few tripartite systems that has been intensively studied and that can also provide information on multiple, interacting resistances in plants. More than one resistance Current molecular studies of SAR exist in an ecolo- gical vacuum. They often use young parts, even cotyledons, generally of fast-growing annuals (e.g. Arabidopsis) or crop plants that are grown under strictly de®ned, controlled-environment conditions that usually dier substantially from conditions in the ®eld. We understand that there are good bio- chemical reasons for concentrating on plant parts with such properties, but it will make the transition to ecological studies all the more dicult. Furthermore, most laboratory studies of SAR con- sidered this phenomenon in isolation, i.e. without reference to the broader defence mechanisms of the plant. The plant is likely to have a number of strate- gies to deal with predators, whose potential diversity is evident, for example, in Rumex where we have found three resistance mechanisms. Age-speci®c con- stitutive resistance to infection by U. rumicis is observed in undamaged and uninfected young devel- oping leaves of R. obtusifolius (Hatcher et al. 1995a); however, these leaves become susceptible to the fungus as they age. Localized induced resistance against rust has been demonstrated following arti®- cial damage, insect chewing damage and rust infec- tion (Hatcher et al. 1994b; 1995a). Resistance is induced when leaves are `challenged' in these ways but is localized to undamaged areas of the aected leaf. Systemic induced resistance against rust (i.e. resistance in unchallenged leaves on challenged plants) follows insect chewing damage and rust infection, but cannot be stimulated by arti®cial damage (Hatcher et al. 1994b; 1995a). In addition to these resistance mechanisms, a fourth, resource depletion, may lead to resistance, although it may not have been selected for as such. For example, infection by the fungus U. rumicis causes a depletion in leaf nitrogen and changes in carbohydrate composition (Hatcher et al. 1995b; Hatcher & Ayres 1998) that are correlated with the reduced growth rates and fecundity of G. viridula fed on these leaves (Hatcher et al. 1994a). Correspondence: Dr Paul Hatcher (fax 0118 935 1804; e-mail p.e.hatcher@rdg.ac.uk). Journal of Ecology 2000, 88, 702±706 # 2000 British Ecological Society