Chemical warfare or modulators of defence responses – the function of secondary metabolites in plant immunity

In plants, a host's responses to an attempted infection include activation of various secondary metabolite pathways, some of which are specific for particular plant phylogenetic clades. Phytochemicals that represent respective end products in plant immunity have been stereotypically linked to antimicrobial properties. However, in many cases, owing to the lack of unequivocal evidence for direct antibiotic action in planta, alternative functions of secondary metabolites should be considered. Correspondingly, recent findings have identified novel, and rather unexpected, functions of phytochemicals in plant immunity that mediate regulatory pathways for conserved defence responses. It also seems likely that these conserved responses can be regulated by clade-specific phytochemicals.

► Plant secondary metabolites can control different immune responses.
► The same metabolic pathway may have multiple functions in plant immunity.
► Particular secondary metabolites may act specifically at different infection stages.
► The same set of conserved defence responses in different plant species can be mediated by different clade-specific phytochemicals.