Induced systemic resistance (ISR) against pathogens - a promising field for ecological research

Local infection by single strains of pathogenic bacteria, fungi or viruses can induce plants to develop a plant-wide resistance against future attack by a variety of microbial pathogens. This phenomenon, called induced systemic resistance (ISR), has been intensively studied with respect to the underlying signalling pathways and to its potential use in crop protection, while ecological or evolutionary studies are rare. However, understanding the selective advantages of an induced as compared to constitutive resistance requires ecological and evolutionary research. The same holds true for the questions of how important this form of resistance is in natural plant populations, and how it affects other plant-insect and plant-microbe interactions. Putative fitness costs provide an explanation for why ISR is induced instead of constitutive, and they might constrain the use of ISR as preventative protection of cultivated plants. Though ISR is mainly elicited by and effective against pathogens, further biotic agents such as leaf-chewing herbivores, leaf miners, aphids and even nonpathogenic root-colonising bacteria can induce systemic pathoger resistance, while some ISR traits can have a defensive effect against herbivores. 'Cross-resistance' elicited by and effective against non-microbial plant enemies thus might add significantly to the function of ISR. On the other hand, 'trade-offs' have keen reported, i.e. increased susceptibility to herbivores in ISR-expressing plants. Finally, ISR is a rather unspecific response, being active against different microbes. It thus might have effects on mutualistic bacteria and fungi, too. The question of how expression of ISR affects the large variety of mutualistic and antagonistic plant-microbe and plant-insect interactions cannot yet be answered. This knowledge is, however, needed to obtain a risk assessment for the use of chemically induced or genetically engineered ISR in crop protection. This review aims to provide an overview and to highlight some of the many open questions which require intensive ecological research.