Illuminating plant immunity: A live sensor to watch salicylic acid in action

Salicylic acid (SA) is best known as a central hormone orchestrating plant immune response, including the hypersensitive reaction and systemic acquired resistance. Beyond defense, SA also influences plant growth and development, highlighting a delicate role in balancing the trade-off between immunity and productivity. To fully unravel how plants control these processes, tools are needed to track SA dynamics in real time and with cellular precision. This is a challenge that traditional methods, often destructive or indirect, have struggled to meet. To address this gap, Tang and colleagues developed a FRET-based sensor named SalicS1, designed specifically to visualize SA in living tissues. The sensor is built from a truncated Arabidopsis NPR1 protein linked to NIMIN1, which interact in vivo. SA binding to NPR1 disrupts this interaction, thereby decreasing the FRET emission ratio (negative ratio change sensor). By adding a nuclear localization signal, the improved version (nlsSalicS1) enabled stable in planta detection of SA across various organs, including roots, cotyledons, and mature leaves. Importantly, the sensor responsiveness can be validated through genetic manipulation: plants carrying NahG (which degrades SA) or mutations in EDS5 and PBS3 (which block SA synthesis) displayed notably lower FRET signals. Beyond its precision, nlsSalicS1 successfully detected SA responses triggered by diverse biotic stresses, from bacteria (Pseudomonas syringae), fungus (Blumeria graminis), and aphid (Brevicoryne brassicae). This powerful tool paves the way for live imaging of hormone dynamics and holds great promise for future applications in crop plants, bringing us closer to visualizing how plants fine-tune the balance between growth and defense in real time. (Summary by Ching Chan @ntnuchanlab) Science 10.1126/science.adw7650