Cis -regulatory variation in ISOCHORISMATE SYNTHASE 1 modulates systemic salicylic acid biosynthesis and systemic acquired resistance in plants

Systemic acquired resistance (SAR) is a long-distance immune response that protects uninfected plant tissues following a localized pathogen attack. In the model plant Arabidopsis thaliana , SAR depends on the systemic accumulation of salicylic acid (SA), mediated by the transcription factor CCA1 HIKING EXPEDITION (CHE), which activates the SA biosynthetic gene ISOCHORISMATE SYNTHASE1 ( ICS1 ). However, the conservation and functional significance of the CHE- ICS1 regulatory module across plant species remain poorly understood, particularly in the Brassicaceae, where ICS1 is the major contributor to SA biosynthesis among two known pathways: the ICS1 and PHENYLALANINE AMMONIA-LYASE (PAL) routes. Here, we identify natural variation in cis -regulatory elements within the ICS1 promoter that affects CHE binding across species. In multiple Brassicaceae species with divergent cis -element sequences, CHE fails to regulate ICS1 , leading to the absence of systemic ICS1 induction and SA accumulation following pathogen infection. Despite this deficiency, SAR still occurs in these species, albeit to a lesser extent than in species with successful systemic induction of SA mediated by an intact CHE- ICS1 regulatory module. Interestingly, introducing the CHE- ICS1 module into species lacking this interaction confers systemic SA accumulation, highlighting the potential to enhance systemic immunity through cis -element modification. Our findings demonstrate that sequence variation in a cis -regulatory element underlies the interspecies diversification of SAR regulatory mechanisms and highlight the evolutionary plasticity of plant immune signaling. This study provides a molecular framework for engineering enhanced systemic immunity in crops, particularly within the Brassicaceae, through the targeted modification of cis -regulatory elements that regulate SA biosynthesis.