Inhibitory effect and possible mechanism of oregano and clove essential oils against Pectobacterium carotovorum subsp. carotovorum as onion soft rot in storage

Abstract

Soft rot caused by Pectobacterium carotovorum subsp. carotovorum (Pcc) is the most common bacterial disease of postharvest onion. This study aimed to determine the antibacterial activity of oregano and clove oils against Pcc and the control of stored soft rot decay in onion. Oregano oil had a minimum inhibitory concentration (MIC) and minimum bactericidal concentrations (MBC) of 200 μL L−1, whereas clove oil had a MIC and MBC of 400 and 500 μL L−1, respectively. Treatment with oregano and clove oil could significantly reduce Pcc cell viability and coincided with a gradient change with time. The inhibitory degree on biofilm synthesis was correlated with oregano and clove oil concentrations. Consequently, 1 × and 2 ×MIC treatments could distinctly reduce biofilm adhesion. With high concentrations of oregano and clove oil, abnormal cell morphology along with cell membrane rupture and cellular leakage were observed by scanning electron microscope (SEM), resulting in a substantial increase in extracellular conductivity. Meanwhile, secretion of extracellular cell wall degrading enzymes (CWDEs) of Pcc was declined in varying degrees, with pectinase (Pel) being significantly inhibited (29.82–100 %). In comparison to the control group, RT-qPCR data indicated that oregano and clove oil downregulated the expression of pathogenic genes, resulting in a decrease in bacterial pathogenicity. Under in vivo culturing, the soft rot decay of onion was reduced by 34.07–100 % in a concentration-dependent manner. In short, essential oils (EOs) may mitigate the severity of onion soft rot by inhibiting the Pcc virulence-associated factors such as activity, motility, toxicity, biofilm formation, and enzyme secretion to various degrees. Thus, oregano and clove oil may be used as alternative bio-fungicides to control and treat the severe disease of soft rot caused by Pcc in onions after harvest.