Signaling Pathway of Serotonin in Plants: Monitoring With an Emergent Nanosensor

Serotonin, widely recognized as a mammalian pineal hormone, is also present in plants, yet its in vivo dynamics and physiological roles remain poorly understood due to the absence of real-time sensing tools. Herein, we report nitrogen-doped carbon quantum dot (N-CQD) nanosensors (∼5 nm; quantum yield 36%) for the selective detection and visualization of serotonin in plant systems. The sensing mechanism involves static-dominated mixed fluorescence quenching accompanied by a blue shift, corroborated by UV-Vis spectral changes, Stern-Volmer analysis, and fluorescence lifetime decay. The nanosensor exhibits a low detection limit of 0.391 µM and a linear response range of 4.74-75 µM. Using Arachis hypogaea seedlings as a model, stronger and more consistent serotonin-dependent fluorescence responses were observed compared with those in other plant species, enabling reliable in vivo monitoring. Real-time sensing revealed a condition-dependent regulatory role for serotonin, including growth inhibition under non-stress conditions and growth enhancement under stress, indicating a dual function in stress adaptation. Fluorescence microscopy further confirmed the intracellular association of serotonin with N-CQDs, providing direct visual evidence of its localization. This work establishes a nanosensor-based platform for real-time detection of serotonin in plants and advances understanding of serotonin-mediated signalling in plant growth and stress responses.