Serotonin has a similar effect like auxin inhibitor
Serotonin (5-hydroxytryptamine, SER) is a well-known indoleamine neurotransmitter that regulates mood, sleep and anxiety in mammals (Murch, & Saxena, 2002). It was initially identified as a vasoconstrictor substance in blood serum, named as 5-hydroxytryptamine (5-HT). It appears to play a role to cope with neuropsychiatric disorders in neuropharmacology (Weele, Anderson, & Cook, 2000). Plants can synthesize SER themselves, which is abundant in companion and xylem cells. SER may become a central role in maintaining the cellular integrity for facilitating efficient nutrient recycling from senescing leaves to sink tissues (Ramakrishna, Giridhar, & Ravishankar, 2011).
Plants are capable of taking up exogenous serotonin, which coincided with stimulated root development as a natural auxin inhibitor (Pelagio-Flores et al., 2011). It is essential for signaling and stress mitigating molecules in plant growth (Erland, Shukla, Singh, Murch, & Saxena, 2018; Erland, Murch, Reiter, & Saxena, 2015). There has been found a crosstalk between 5-HT (0.01-0.03 mM) and phytohormones such as SA, ABA, and zeatin (Erland, Shukla, Singh, Murch, & Saxena, 2018). Moreover, previous research showed that serotonin could regulate growth process including root and shoot organogenesis (Erland, Shukla, Singh, Murch, & Saxena, 2018), light mediated responses (Leclercq et al., 2002; Erland, Turi, & Saxena, 2016), maintenance of plant tissues (Erland, Turi, & Saxena, 2016), and delay of senescence (Erland, Murch, Reiter, & Saxena, 2015).
In addition, SER acts as an antioxidant to protect plants (Kang, Kim, Park, & Back, 2009; Kang et al., 2007) and responses to biotic and abiotic stresses such as cold and salt stress (Mukherjee, David, Yadav, Baluška & Bhatla, 2014). Previous research demonstrated a high level of serotonin accumulated in senescing leaves and roots as the plants aged. Also, pathogenic infection can lead to accumulated serotonin against pathogens by strengthening of the cell wall (Ramakrishna, Giridhar, & Ravishankar, 2011; Kang, Kim, Park, & Back, 2009). Absence of serotonin is suggestive of a relatively lesser sensitivity of plant tissues to environmental stress.
Furthermore, there have been interactions between serotonin and other small-molecular regulators, such as melatonin. A balance of the endogenous serotonin and melatonin may modulate plant morphogenesis in vitro (Murch, & Saxena, 2001). Thus, serotonin is implicated in several physiological roles in plants such as flowering, morphogenesis, and adaptation to environmental changes. However, these physiological functions provide new avenues to consider health effects of plant SER in human.