Combination of melatonin and plant growth promoting rhizobacteria improved the growth of Spinacia oleracea L. under the arsenic and cadmium stresses

Arsenic and cadmium are among highly toxic elements that causes detrimental effects to the plants, animals as well as human beings. Melatonin is a natural molecule that works as a strong antioxidant by forming antioxidant cascade reaction by scavenging free radicals of reactive oxygen species (ROS). In the same way, plant growth promoting rhizobacteria (PGPR) colonize plant roots and enhance plant growth by number of mechanisms. The objective of the current research was to study the impact of melatonin and Bacillus licheniformis in combination on Spinacia oleracea L. in the presence of arsenic (As) and cadmium (Cd) stress. Spinacia oleracea L. seeds were germinated in pots consisting of control and three different doses of arsenic and cadmium irrigated with combination of melatonin and Bacillus licheniformis. The results of the study showed that the combination of melatonin and Bacillus licheniformis has the potential to alleviate the toxic effects of As and Cd. Plant morphological and physiological parameters like plant height, root length, fresh weight, dry weight, chlorophyll content were enhanced because of combine application of melatonin and Bacillus licheniformis. Superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) activity increased because of combine application of melatonin and Bacillus licheniformis while malondialdehyde (MDA) content is decreased. In response to different stress conditions, expression of genes encoding the superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) is increased in different As and Cd treatments. Results show that highest translocation factor (TF) 0.44 was observed at 125 mg/kg Cd and highest bioconcentration factor (BCF) 0.46 at 75 mg/kg Cd. All three treatments have TF and BCF less than one thus making Spinacia oleracea L. useful for uptake by human beings. Additionally, it was found the melatonin content in Spinacia oleracea L. was increased under As and Cd stress and exogenous melatonin application.