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

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 non-pathogenic root-colonising bacteria can induce systemic pathogen 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 been 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.