Exogenous Melatonin Regulates Superoxide Dismutase Gene Expression and Enzymatic Activity in Response to Pathogenic Fungi in Soybean Glycine max (L.) Merrill

Melatonin is a pleiotropic molecule known to play a key role in plant growth and development, as well as biotic and/or abiotic stress responses. However, melatonin specific effects on the plant defense system, particularly SOD activity are still an area of ongoing research. The results indicated a clear effect of the applied melatonin concentration in shaping the entire response to the biotic oxidative stress resulted from the two pathogenic fungi Rhizoctonia solani and Macrophomina phaseolina. The higher concentration of melatonin had a greater effect on both gene expression and SOD enzymatic activity, which indicates the melatonin importance in stimulating the defense system of soybean plants through which soybean plants try to manage the excessive ROS accumulation in response to pathogenic fungi. The different varieties showed different ability to cope with biotic stress of pathogenic fungi, especially the antioxidants enzymatic activity. Al-Shaimaa variety proved to have the most effective SOD gene expression and enzymatic activity compared to the other two Senaya22 and Lee74 soybean varieties. Melatonin concentrations in its maximum level (300 μM), was more effective in promoting the antioxidant activity in term of gene expression and enzymatic activity of SOD. Interestingly, R. solani had a greater effect on the antioxidant defense system, as it resulted in higher levels of SOD gene expression and enzymatic activity in the three soybean varieties. Further investigation maybe required to better understand the melatonin role in inducing plant antioxidant system, thus the possible protective mechanisms against plant pathogens.