Endogenous serotonin induced by cold acclimation increases cold tolerance by reshaping the MEL/ROS/RNS redox network in Kandelia obovata

Cold stress severely limits the distribution of mangrove species worldwide and it remains unclear how mangroves respond and adapt to cold temperatures. In this study, we investigated the effects of cold acclimation and/or inhibition of serotonin levels on reactive oxygen species (ROS), reactive nitrogen species (RNS), melatonin (MEL) and serotonin (SER) accumulation during cold stress in Kandelia obovata. Morphologic observation and parameter analysis revealed that cold acclimation mitigated the photoinhibition of photosystem I (PSI) and photosystem II (PSII), maintained optimal ROS and RNS redox homeostasis, and increased the contents of SER and MEL in leaves. This suggests that cold acclimation reshapes the MEL/ROS/RNS redox network. In particular, the tryptophan/tryptamine/Ser/N-acetylserotonin/MER pathway was identified as a branch of the MEL synthesis pathway. Inhibition of endogenous SER exacerbated damage caused by cold stress, indicating the crosstalk of SER synthesis and cold acclimation. In this study, we report a coordinated regulation of cold stress by a complex defense network in K. obovata.