Serotonin (5-hydroxytryptamine) is a well-known neurotransmitter, hormone, and mitogenic factor that mediates a series of physiological activities in humans and animals. It also may play important roles in modulating growth, development, and defense responses in plants, such as seed germination, flowering, and abiotic stress tolerance. However, how serotonin modulates the accumulation of auxin and f reactive oxygen species in root tips is still largely unclear. 

Researchers from Xishuangbanna Tropical Botanical Garden (XTBG) investigated the involvement of reactive oxygen species and auxin in serotonin-regulated root system development in Arabidopsis. 

They found that low concentrations of serotonin had no effect on the primary root (PR) growth, whereas high concentrations of serotonin result in PR growth inhibition by reducing the length of the meristem and elongation zones.

The results suggest that high concentrations of serotonin result in stress responses in plants.  

Their further study indicated that high concentrations of serotonin inhibited PR elongation by modulating the differential distribution of O_2- and H₂O₂ via the UPB1 pathway, repressing auxin biosynthesis and affecting auxin transport via modulation of AUX1 and PIN2 abundances in root tips. 

"Based on previous studies and our results, we can conclude that high concentrations of serotonin result in stress responses by inhibiting primary root growth through regulation of the accumulation and distribution of O₂- and H₂O₂ via the UPB1 pathway and auxin biosynthesis and transport, consequently reducing root stem cell niche activity and meristematic cell division potential in root tips", said WAN Jinpeng, the first author of the study. 

The study entitled "Involvement of reactive oxygen species and auxin in serotonin-induced inhibition of primary root elongation" has been published in Journal of Plant Physiology.