Metabolomic analyses reveal potential mechanisms induced by melatonin application for tolerance of water deficit in loquat (Eriobotrya japonica Lindl.)

Melatonin participates in the response to abiotic stressors in plants and enhances tolerance to water stress tolerance in loquat seedlings; however, the underlying mechanism remains unknown. In this study, we conducted physiological and metabolomic analyses to identify the metabolic processes associated with enhanced tolerance of melatonin-treated loquat seedlings under conditions of water deficit. Loquat seedlings were pre-irrigated with either 150 μM of melatonin or water and dehydrated for the next 13 days. Our results showed that water deficit reduced the relative water content and increased the relative electrolyte leakage in the leaves. Compared with untreated plants, melatonin-treated loquat leaves exhibited lower relative conductance and higher relative water content and antioxidant enzyme activity under water deficit conditions. Additionally, after melatonin application, the total phenolic and flavonoid content in the leaves had increased on day 7 but had decreased on day 13. Metabolomic analysis on days 1 and 13 revealed 108 and 196 metabolites, respectively, that were regulated by melatonin in loquat leaves. Importantly, melatonin application enhanced phenylpropanoid biosynthesis and the tricarboxylic acid (TCA) cycle and altered flavone and flavonol biosynthesis and galactose metabolism. Melatonin-treated plants exhibited higher levels of amino acids (threonine, asparagine, glutamate, and valine) and hydroxycinnamic acid derivatives (m-coumaric acid, shikimic acid, ferulic acid, coumaroylquinic acid, and 2-hydroxycinnamic acid). Thus, this study deepens our understanding of melatonin-mediated reprograming of metabolites in loquat to cope with water deficits.