Magical mystery tour: Salicylic acid signalling

•Salicylic acid (SA) is a major plant hormone whose level is highly controlled.•Constitutively SA-overaccumulating mutants give access to signalling upstream SA.•NPR1 is the main component controlling SA dependent gene response.•Crossing of SA overaccumulating mutants with npr1 revealed NPR1-independent pathway.•SA perception involves many targets, including NPR1 and proteins controlling NPR1.

Salicylic acid (SA) is a small phenolic compound whose therapeutic properties on human health have long been described. In plants, it is a key phytohormone that controls many physiological processes. In the last 20 years, great attention has been paid to its role in plant pathogen defence. The synthesis of SA is indeed one of the crucial ways a plant reacts to a biotic attack. SA is involved in both local and systemic resistance. SA metabolism (biosynthesis, conjugation and accumulation) and the signalling pathways that control SA levels are described here. The transcription factor NPR1 is an established cornerstone of SA signalling. Yet, both NPR1-dependent and NPR1-independent signalling pathways have been described, but very little is known about the effectors of the NPR1-independent pathway. We present work using different SA over-accumulating mutants as tools for studying the control of SA biosynthesis and the downstream effects of SA. Recently, new evidence has been provided concerning SA perception. Interestingly, NPR1 was proposed to be a SA-receptor, as were proteins involved in the control of NPR1 turnover. The different proteins involved in SA metabolism, in the regulation of SA levels and in the response to SA can define a “SA signalling module”. It is possible to use the genes encoding members of this module as indicators to identify stress situations where SA signalling is activated. This is illustrated for different biotic and abiotic stress responses.