Experimental Context Shapes PRR-Mediated Immune Output Sensitivity in Arabidopsis

Pattern recognition receptors (PRRs) mediate plant immune responses by detecting extracellular immunogenic patterns, including microbe-associated molecular patterns (MAMPs). PRR signaling is commonly assessed using assays such as reactive oxygen species (ROS) bursts, cytosolic calcium influx, mitogen-activated protein kinase (MAPK) activation, and seedling growth inhibition (SGI), which are performed in distinct experimental systems, including seedlings grown on artificial media and soil-grown rosettes. Here, we systematically compare receptor kinase immune outputs triggered by the bacterial MAMPs elf18 and flg22 in Arabidopsis thaliana seedlings and rosettes across a range of concentrations. Rosettes exhibited greater sensitivity than seedlings in ROS assays, whereas cytosolic calcium responses measured using the Aeqcyt/pMAQ2 reporter were stronger in seedlings, correlating with reduced reporter transcript accumulation in rosette tissue. MAPK activation was consistently stronger in rosettes, whereas SGI assays revealed higher sensitivity to elf18 than flg22 in seedlings despite flg22 inducing stronger early signaling outputs. Together, these results demonstrate that canonical PRR-mediated immune outputs are differentially sensitive to experimental context and should not be interpreted as interchangeable measures of immune activation. These findings highlight the importance of considering experimental conditions when comparing immune responses across assays and developmental stages.