Exogenous melatonin delays programmed cell death in broccoli via maintenance of ROS homeostasis and reduction of mitochondrial dysfunction

Plant senescence is the final period of the plant life cycle and represents a programmed cell death (PCD) process. Melatonin, a versatile signaling molecule, exhibits potent free radical scavenging properties and actively participates in cellular antioxidant systems, effectively retarding plant senescence throughout its development. In order to illustrate the impact of melatonin on PCD during senescence, we examined the effects of exogenous melatonin at 80 μmol L−1 on PCD and mitochondrial function in broccoli bud cells at 25 ℃. The findings revealed that melatonin treatment not only maintained the morphological integrity of chloroplasts and mitochondria but also attenuated the decline in chlorophyll content by reducing the expression of genes that metabolize chlorophyll, including BoPPH, BoPAO, and BoSGR1. In comparison to the control group, the treatment decreased the content of H2O2 and the rate of O2- production in mitochondria and simultaneously enhanced the activities of ascorbate peroxidase (APX), catalase (CAT), and superoxide dismutase (SOD), resulting in effective maintenance of reactive oxygen species (ROS) homeostasis and reduction of the level of lipid peroxidation in broccoli buds during senescence. Meanwhile, exogenous melatonin treatment up-regulated alternative oxidase (AOX) activity and its gene expression while delayed the reduction of cytochrome oxidase (COX) activity and restrained the over-opening of mitochondrial permeability transition pores (MPTP). Additionally, exogenous melatonin decelerated the increase in membrane potential and prevented mitochondria from releasing excess cytochrome c (Cyt c) into the cytoplasm. Taken together, the results indicate that melatonin treatment delayed mitochondrial dysfunction, preserved mitochondrial ROS homeostasis, and consequently postponed the initiation of PCD, ultimately inhibiting senescence in broccoli.