Exogenous application of melatonin attenuates waterlogging stress through adopting quiescence adaptation technique in tomato seedlings

Waterlogging (WL) is a major limiting factor in global crop production and seriously limits growth and yield improvement in low-lying rainfed regions. Melatonin (MT) is a vital phytohormone that functions as a "master regulator" in multiple facets related to plant growth and development, in addition to maintaining a potential role in response to stresses. However, the pharmacological role of MT in attenuating waterlogging stress in tomato largely remains elucidated. The objective of the current investigation is to justify the physiological regulatory mechanism of tomato seedlings exposed to WL and the putative functions of MT to mitigate the adverse effects of WL. Tomato seedlings were grown on substrate (peat: vermiculite, 2:1, v/v) and at the 4th leaf stage subjected to waterlogging stress for 10 days and seedlings were foliar sprayed with melatonin during waterlogging stress. The results revealed that WL significantly arrested tomato seedlings growth, and reduced pigment content coincided with enhanced leaf senescence. MT supplementation attenuated WL-induced oxidative damage through increasing osmoprotectants activity, elevating antioxidant enzyme functioning synchronized with inhibiting excess reactive oxygen species (ROS) production. The concentrations of MT (28 %), abscisic acid (ABA, 170 %), and 1-aminocyclopropane-1-carboxylic acid (ACC, 129 %) were increased, while indole acetic acid (IAA, 15 %), jasmonic acid (JA, 55 %) and gibberellic acid (GA3, 26 %) content were decreased in only WL seedlings roots relative to control seedlings. The core anaerobic respiration enzyme alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC) activity were elevated by 127 % and 163 %, respectively at day 10 in WL+MT received seedlings than control. WL treatment varyingly contributed on nutrient content, as evidenced that N+, K+, and Ca2+ content decreased, whereas Mn2+, Fe2+, and Mg2+ content increased and MT addition reversed their concentrations under similar stress conditions. Exogenous MT promoted WL-tolerance of tomato by positively suppressing respiratory burst oxidase homologs (RBOH)-regulating gene expression while up-regulating ethylene biosynthesis gene transcription. Most importantly, programmed cell death (PCD) regulated enzyme caspase-3 activity concurred with PCD-induced gene (caspase-3, pirin, TBN1) expression significantly inhibited by MT application. In general, these findings reveal that external supplementation with MT can improve plant tolerance to WL through complex processes and multifaceted mechanisms.