Induced resistance - orchestrating defence mechanisms through cross-talk and priming.

In nature, plants interact with a wide range of microbial pathogens and herbivorous insects. During the evolutionary arms race between plants and their attackers, primary and secondary immune responses evolved to recognise common or highly specialised features of the attacker encountered, resulting in sophisticated mechanisms of induced defence. Induced resistance mechanisms are characterised by a broad‐spectrum effectiveness and often act systemically in plant parts distant from the site of primary attack, thereby protecting the plant against subsequent invaders. Plant hormones are key players in the regulation of the defence signalling pathways involved. Because induced defence responses entail ecological fitness costs, plants must possess elaborate regulatory mechanisms that efficiently coordinate the activation of attacker‐specific defences so that fitness costs are minimised while optimal resistance is attained. A major focus in plant defence signalling research is to uncover key mechanisms by which plants tailor their responses to different attackers, and to investigate how plants cope with simultaneous interactions with multiple aggressors. Pathway crosstalk and priming for enhanced defence emerged as important regulatory mechanisms that enhance the efficiency of the plant's inducible defence arsenal. Here, we review the current knowledge on the signalling cascades involved in different types of induced pathogen and insect resistance, and the regulatory mechanisms by which plants are able to orchestrate their inducible defences in a cost‐effective manner.