From signalling to resistance; delineating the multifaceted role of salicylic acid as the key modulator of biotic stress tolerance.

The sessile nature of plants and complete absence of humoral immunity equips them with complex mechanism of resilience against diverse stress factors. The innate immune system in plants mostly utilises transcriptional reprogramming during stress exposures. Biotic interactions in plants require multifaceted gene expressional plasticity due to constant co-evolutionary arms-race between the plants and pathogen. Classical 'zig-zag' model of plant immunity reports biphasic induction of immune response in plant-microbe interaction. Reactive oxygen species (ROS) and salicylic acid (SA) play central roles in pathogen associated molecular pattern triggered immunity and effector triggered immunity. Role of SA in modulating defence signalling pathway and activating systemic acquired resistance is well established in model and crop plants. However, integration of SA-mediated defence signalling pathways in developing sustainable resistance response is still scarce. SA-mediated immune signalling in plants is not mutually exclusive and relies on highly coordinated transcriptional overlaps and hormonal crosstalk. This review discusses about SA biosynthesis, its evolutionary perspective, the signalling overlaps of salicylic acid with ROS, other plant hormones and also their roles in transgenerational priming. We decipher the integral SA signalling nodes during in planta infection, which could be utilised in future crop management programmes.