Antibacterial activity of the flavonoid extracts from Eleutherococcus brachypus leaves and the allelopathic effects on Allium cepa

Abstract

[Objective] The study aims to explore the antibacterial activity and allelopathy of flavonoids from leaves of Eleutherococcus brachypus, and to provide the basis for the development of plant-derived antimicrobial agents and the application of their allelopathy in plant protection and agricultural production. [Methods] Leaf flavonoids was extracted by an ultrasonic-assisted method from E. brachypus leaves, then the antibacterial sphere, the minimum inhibitory concentration (MIC), the apoptosis rate, MDA content, ROS content, and the activity of antioxidant enzymes of epidermal cells from onion scaly leaves, as well as the inhibitory rate of the onion root and shoot growth were determined with different concentrations of leaf flavonoid extracts (100%, 50%, and 25%). [Results] (1) The yield of leaf flavonoids from E. brachypus was 42.3 mg/g. (2) At same concentration, the diameter of the inhibition zone was S. aureus > B. subtilis > Salmonella, respectively, and expanded with the increase in the extract concentrations with the same strain. The minimum inhibitory concentration of the three strains was also S. aureus < B. subtilis < Salmonella, respectively. (3) Each concentration of leaf flavonoids from E. brachypus induced apoptosis in the upper epidermal cells of Allium cepa. Meanwhile, as the concentration of flavonoid extracts increased, the apoptosis rate, MDA content, and H2O2 content were increased. POD activity and CAT activity were increased first and then decreased during the same time of treatment, which were higher in treatment groups than the control. (4) The growth of root and shoot of the onion scale leaves were inhibited under the treatment of flavonoids extracts, which was negatively correlated with the treatment dose. [Conclusion] The antibacterial effects of the leaf flavonoid extracts of E. brachypus on Gram-positive bacteria are better than that of Gram-negative bacteria, and the antibacterial effects are positively correlated with the concentration of flavonoid extracts. Moreover, all these changes suggest that the allelopathic mechanism of flavonoid extracts from E. brachypus may be due to the overproduction of ROS and the inhibition of antioxidant enzyme activity, which cause oxidative damage to membrane lipids and cell death.

Keywords

• eleutherococcus brachypus
• leaf flavonoid extracts
• antibacterial activity
• allelopathy
• allium cepa