Exogenous serotonin improves drought and salt tolerance in tomato seedlings

Serotonin and melatonin, which are indolamines present in every biological kingdom, show strong bioregulator activity in plant tissues under various stress conditions. Although numerous studies have reported the stress mitigating effects of melatonin, the effect of serotonin on plant stress physiology has rarely been studied in the current literature. In this study, the effects of exogenous serotonin application on tomato seedlings under severe salt and drought stresses were investigated by examining various morphological and physiological plant stress indicators, including tissue length and mass, relative water content (RWC), ion leakage, malonedialdehyde (MDA) and proline content, as well as some oxidative stress related gene activities, ascorbate peroxidase (APX1), superoxide dismutase (FeSOD), catalase (CAT2), glutathione reductase (GR1), delta 1-pyrroline-5-carboxylate synthase (P5CS) and 1-aminocyclopropane-1-carboxylic acid synthase 2 (ACS2). Our results showed that the tissue MDA contents decreased by 29% and 18%, the ion leakage levels decreased by 39% and 34%, whereas the relative water contents increased by 22% and 18%, respectively, under salt and drought stresses upon serotonin application. Serotonin also decreased ACS2 gene expression by 28% and 70%, while increasing FeSOD by 69 and 17%, CAT2 by 145 and 110% and GR1 gene expression by 67 and 22%, respectively, under salt and drought stresses. Also, the transcript level of P5CS increased 48% under serotonin salt coapplication. Our findings showed that similar to its close chemical relative melatonin, serotonin also exhibits stress mitigation effects and boosts the antioxidant capacity of tomato plants under most common abiotic stress conditions.