Brassinosteroids as Mediators of Plant Biotic Stress Responses

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.