From passive life forms to living organisms that can sense a myriad of external signals and
alter their development, our perception of how plants interact with the environment has changed
profoundly. We now know that plants can promptly respond to biotic stressors by quickly reallocating
resources from growth to defensive traits. Which signals do plants use to finely tune their “growth
versus defense balance”? Hormones seem to fulfill this role, since they are associated with almost every
process in plant development, and also with stress responses. For this reason, plants are constantly
modulating hormonal pathways in order to better allocate internal resources. Here, we discuss how
brassinosteroids (BRs), steroidal plant hormones known to be potent growth regulators, work as strong
mediators of plant biotic stress responses. Interaction between BRs and other stress hormones, like
jasmonates and salicylic acid, important to build-up defensive barriers necessary to cope with insects
and microbes are also discussed. Finally, we present evidence that these plant steroids are not only
directly involved in defense responses against pest and pathogens, but they are also key regulators in the
resource allocation decision.
Keywords: Insect herbivory, jasmonates, resource allocation, salicylic acid, systemic acquired resistance (SAR).
