In vitro and in vivo activity of Peganum harmala L. alkaloids against phytopathogenic bacteria

Medicinal plants have attracted attention in the field of plant disease control. In searching for alternative bactericides from natural origin to overcome the hazards of synthetic ones, total alkaloid extract of Peganum harmala L. seeds (TAE) specified by HPLC, was tested in vitro against four phytopathogenic bacteria: Ralstonia solanacearum Phylotype II, Erwinia amylovora, Pectobacterium carotovorum subsp. carotovorum, and Burkholderia gladioli, the causal agents of potato brown rot, pear fir blight, potato soft rot, and onion slippery skin diseases, respectively. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined in vitro. The brown rot pathogen, R. solanacearum was the most sensitive to the tested extract (MBC 150 mu g/mL) followed by B. gladioli (MBC 200 mu g/mL). The extract exhibited a marked inhibitory effect in vitro on R. solanacearum pathogen at concentrations ranging from 4 to 300 mu g/mL, but this effect on the other pathogens needed higher concentrations (50 to 300 mu g/mL). R. solanacearum cells exposed to 4 mu g/mL TAE revealed severe cellular damage, clotting of the genome, as well as disorganized cytoplasm and thickened cell wall compared to control, as shown by transmission electron microscope. In vivo, the concentration 300 mu g/mL of the extract was promising in decreasing brown rot symptoms on plants, with growth promotion by all concentrations (4, 50, 150, 300 mu g/mL). The concentration 300 mu g/mL revealed the most remarkable effect without phytotoxicity. This study revealed the antibacterial efficacy of P. harmala total alkaloids extract on phytopathogenic bacteria which might be exploited as a substitute for chemical antibacterials.