Strengths and Limitations of Salicylic Acid Reporters.

Salicylic acid (SA) is a key phytohormone that coordinates plant innate immunity and systemic acquired resistance. Because SA levels and signaling are highly dynamic in space and time, a suite of SA-focused tools, including SA-specific microbial biosensors and SA-responsive transcriptional and chemical reporters, has been developed to study them. This review compares three classes of tools in terms of sensitivity, specificity, temporal resolution, invasiveness, quantifiability, and suitability across species. We describe developing genetically encoded sensors that can directly sense salicylic acid and report it, for example, via a fluorescence resonance energy transfer signal or another real-time output. We offer recommendations on method selection by research goal and plant species, as well as combined protocols (long-term autoluminescence plus local probes/biosensors) for cross-validation. Future work should prioritize substrate-free, quantitative SA reporters deployable in crops and the field; coupled with CRISPR-based editing and screening, these tools would enable reporter-guided discovery of immunity genes and rapid engineering of durable disease resistance.