Cross-talk between plant defence signalling pathways: boost or burden?

Plants are exposed to very different attackers, including microbial pathogens and herbivorous \ninsects. To protect themselves, plants have evolved defensive strategies to counteract potential \ninvaders. Recent advances in plant defence signalling research have revealed that plants are \ncapable of differentially activating inducible, broad-spectrum defence mechanisms, \ndepending on the type of invader encountered. The plant hormones salicylic acid (SA), \njasmonic acid (JA) and ethylene (ET) are major players in the network of defence signalling \npathways. Cross-talk between SA-, JA- and ET-dependent signalling pathways is thought to \nbe involved in fine-tuning the defence reaction, eventually leading to the activation of an \noptimal mix of defence responses to resist the intruder. Genetic engineering of the \nbiosynthetic pathways of these signalling compounds and the development of protective \nchemicals mimicking their mode of action provide useful tools for the development of new \nstrategies for crop protection. However, there is evidence for antagonism between SAdependent \nresistance to microbial pathogens and JA-dependent resistance to herbivorous \ninsects: once a plant is conditioned to express resistance against microbial pathogens it may \nbecome more susceptible to attack by herbivores, and vice versa. Yet, the evidence for tradeoffs \nbetween pathogen and insect resistance is contradictory. This review is focused on recent \nexperimental evidence on the relationship between SA-, JA- and ET-dependent induced \nresistance to microbial pathogens and herbivorous insects. In addition, we will address the \nquestion whether manipulation of defence signalling pathways, either through genetic \nengineering or through application of defence signal-mimicking plant protectants, will boost \nthe plant’s immunity to potential invaders or will be a burden in crop protection strategies.