Histamine in plants: from known roles to unknown frontiers

Histamine, a biogenic amine extensively characterized in animals for its roles in immunity, neurotransmission, and physiological regulation, was first reported in plants in 1948. Despite decades of study, its functions in the plant kingdom remain poorly understood, representing an emerging research frontier. Its presence has now been confirmed across diverse species, tissues, and developmental stages. Fluorescence-based histochemical analyses indicate the possible presence of histamine in nuclei, vacuoles, and chloroplasts, suggesting potential roles in intracellular signaling. In plants, histamine is synthesized via L-histidine decarboxylase and catabolized predominantly by diamine oxidase, with alternative pathways such as methylation and acetylation contributing to derivative formation. Exogenous histamine studies and tissue-specific quantifications indicate its involvement in seed germination, root and shoot elongation, pollen tube growth, and biomass accumulation. Levels fluctuate under abiotic stresses, including drought, salinity, and ozone exposure, highlighting a possible role in stress perception and tolerance. While its antioxidant effects remain inconclusive, limited inhibition of lipid peroxidation has been reported. Methodological considerations—including fluorescence cross-reactivity, tissue handling, derivatization, and HPLC variability—necessitate careful interpretation. This review consolidates current knowledge on histamine occurrence, metabolism, and physiological roles, while identifying major knowledge gaps: the localization of biosynthesis, the identity of putative receptors, mechanistic pathways, and the functional significance of histamine derivatives. By integrating histochemical, biochemical, and physiological evidence, this work provides a conceptual foundation for future investigations aimed at elucidating the ecological and regulatory relevance of histamine in plants.