Salicylic acid in plant defence-the players and protagonists

Salicylic acid (SA) is synthesised by plants in response to challenge by a diverse range of phytopathogens and is essential to the establishment of both local and systemic-acquired resistance (SAR). SA application induces accumulation of pathogenesis-related (PR) proteins. Mutations leading to either reduced SA production or impaired SA perception enhance susceptibility to avirulent and virulent pathogens. However, our knowledge of the primary signalling components activating SA biosynthesis and linking to PR proteins accumulation is rudimentary. We review progress towards characterising key players (NPR1, MPK4) and processes (methylation, amino acid conjugation, S-nitrosylation) contributing to SA-signalling and perception pathways. Further, we examine emerging data on how pathogens have evolved strategies (e.g. ABA modulation and coronatine production) to suppress SA-mediated plant defence.