The regulatory roles of flavonoids, melatonin, and glycine betaine in plant salt stress response.

Salt stress, one of the most detrimental environmental constraints, severely impairs plant growth and productivity by inducing ionic imbalance, osmotic stress, and oxidative damage. In response, plants synthesize a diverse array of secondary metabolites that help maintain cellular turgor, improve water uptake, restore osmotic homeostasis, scavenge reactive oxygen species, and fine-tune growth and development-collectively enhancing salt tolerance. Although considerable progress has been made in characterizing individual metabolites, the intricate interplay between the regulation of secondary metabolism and plant salt stress adaptation remains poorly understood. This review summarizes recent advances in understanding how secondary metabolites respond to salt stress, with a particular focus on flavonoids, melatonin, and glycine betaine. We highlight emerging mechanisms governing their biosynthesis and accumulation under saline conditions and discuss the regulatory networks that coordinate their functions. Finally, we identify critical knowledge gaps and propose future research directions aimed at elucidating the interaction networks among these key secondary metabolites and their upstream regulatory hubs, with the goal of breeding crops with broad-spectrum stress resistance.