Systemic acquired resistance: available information and open ecological questions

Systemic acquired resistance (SAR) in plants is a defence mechanism that can be induced by any one of a broad spectrum of pathogens against all of which it is then effective. The biochemical and molecular aspects of this response are rapidly becoming understood, at least for a few model organisms. The aim of this article is to draw ecologists' attention to this topic by outlining some of the ecological questions resulting from the physiological findings: 1 How widespread is SAR within the plant kingdom? 2 Does SAR fit into the framework provided by established theories on plant (anti-herbivore) defence? 3 What are the allocation costs and ultimately the fitness costs of SAR? 4 Why is SAR an inducible and not a constitutive defence? 5 What is the advantage of a defence induced systemically by a single pathogen, which then gives resistance against many others that are not yet causing any problems and which probably may never challenge the regarded plant? 6 Are there any functional or physiological relations between SAR and other biological interactions, for example mutualisms between plants and microorganisms? 7 Do plants exist in nature that are older than seedlings that have not already been induced? The many excellent reviews on the molecular aspects of SAR include those by Ryals et al. (1994), Hunt & Ryals (1996), Hunt etal. (1996), Ryals etal. (1996) and Schneider etal. (1996), and only a short overview is given here, before considering the ecological possibilities. Indeed, simply applying established ecological theories and models to the phenomenon (for example, examining SAR within the concept of fitness costs) may shed new light on this important type of plant defence.