Exploring the role of serotonin as a plant stress modulator.

Serotonin (5-hydroxytryptamine), an indoleamine with a dual evolutionary legacy in animals and plants, has transcended its initial classification as a secondary metabolite to emerge as a central regulator of plant stress adaptation. This review moves beyond cataloging stress-associated effects to propose a unified framework for serotonin as a dynamic signaling and metabolic hub. I synthesize evidence that serotonin's role is defined not merely by its antioxidant capacity, but by its sophisticated integration into the core stress-signaling circuitry of plants. The key to this function is its inducible biosynthesis via the tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H) pathway, which is activated by diverse stressors through reactive oxygen species (ROS), phytohormone, and calcium-dependent signals. I critically analyze its multifaceted mechanisms: (1) direct and indirect ROS scavenging; (2) precise modulation of phytohormone networks (auxin, abscisic acid, jasmonic acid, salicylic acid), where it acts less as a hormone and more as a hormone signal modulator, notably fine-tuning root architecture and stomatal aperture; (3) regulation of ion transporter activity (e.g., SOS1, HMAs) for ionic homeostasis; and (4) epigenetic and transcriptional reprogramming of stress-responsive genes. A dedicated section clarifies the synergistic yet distinct partnership with melatonin, distinguishing serotonin's rapid, localized actions from melatonin's longer-term, systemic roles. I further explore serotonin's emerging functions in biotic stress as an antimicrobial compound and defense pathway potentiator. This integrative synthesis aims to reframe serotonin from a protective molecule to a master regulator at the nexus of plant stress perception and adaptive response.