• costs;
  • fitness;
  • hormone crosstalk;
  • jasmonic acid;
  • priming;
  • salicylic acid

Plants activate defence responses to protect themselves against microbial pathogens and herbivorous insects. However, induction of defences comes at a price, as the associated allocation costs, autotoxicity costs and ecological costs form fitness penalties. Upon pathogen or insect attack, resources are allocated to defences instead of to plant growth and reproduction, while above- and below-ground interactions with beneficial organisms may also be disturbed. The phytohormones salicylic acid and jasmonic acid are major players in the regulation of induced defences and their associated fitness costs. Hormone-controlled signalling pathways cross-communicate, providing the plant with a finely tuned defence regulatory system that can contribute to a reduction of fitness costs by repressing ineffective defences. However, this sophisticated regulatory system causes ecological costs, because activated resistance to one organism can suppress resistance to another. Moreover, the system can be hijacked by invading organisms that manipulate it for their own benefit. Priming for enhanced defence emerged as a defence mechanism with limited fitness costs. Because priming results in a faster and stronger activation of defence only after pathogen or insect attack, the limited costs of the primed state are often outweighed by the benefits in environments with pathogen or herbivore pressure. The balance between protection and fitness is crucial for a plant's success and is therefore of great interest for plant breeders and farmers. By combining molecular knowledge and ecological relevance of defence mechanisms, one can gain fundamental insight into how and why plants integrate different immune signals to cope with their natural multitrophic environment in a cost-effective manner.