Exogenous melatonin and serotonin improve boron toxicity tolerance in tomato seedlings

Boron toxicity is an abiotic stress restricting agricultural production in arid and semiarid parts of the world. This study aimed to investigate the effects of serotonin and melatonin on tomato seedlings under short (5 days) and long-term (8 days) boron toxicity applied as 7.5 mM boric acid in the nutrient solution. Shoot and root lengths, dry weights and water contents, ion leakage levels, malondialdehyde, proline and relative water contents were used as morphological and physiological stress indicators. Expression levels of the enzymatic antioxidative defense genes, FeSOD, CAT2, GR1, APX1, P5CS, ethylene biosynthesis gene ACS2, DEADBOX RNA helicase, and two protein kinase genes CPK2 and MPK3 were also investigated. Melatonin application (10 µM) completely reversed necrotic and chlorotic lesions on leaves, while serotonin application (5 µM) partly ameliorated the visible boron toxicity symptoms. Both indoleamines reduced membrane damage and increased osmoprotectant proline levels under long-term boron toxicity. Exogenous melatonin and serotonin applications also reduced ACS2 gene expression while increasing the transcript levels of CPK2, as well as enzymatic antioxidative defense system genes under long-term boron toxicity. The study showed that both indoleamine compounds interacted with early and late stress responses and successfully mitigated boron toxicity stress in tomato.