Dorsaf Ben Ahmed, Ikbal Chaieb, Karima Belhadj Salah, Habib Boukamcha, Hichem Ben Jannet, Zine Mighri , and Mejda Daami-Remadi
Cestrum parqui L´Hér. (Solanaceae) is used as ornamental plant in Tunisia. This plant is rich in saponin content which was largely described as a fungicidal compound synthesized by plants for defence purposes. The aim of the present work is to assess the activity of the crude saponic extract (CSE) on several bacterial and fungal agents and to study the interaction between saponin and membrane sterols in relation with their eventual inhibitory activities. Two Gram-positive (Pseudomonas aeruginosa and Escherichia coli) and two Gram-negative (Staphylococcus aureus and Enterococcus faecalis) bacteria were tested. No bacterial species was found to be sensitive to C. parqui saponins even with the highest CSE concentration used (100 mg/ml). The antifungal activity was confirmed against two plant pathogens (Fusarium solani and Botrytis cinerea) and one antagonistic agent (Trichoderma viride). Trichoderma viride exhibited a very high sensitivity to CSE (IC50 = 210 ppm) whereas F. solani and B. cinerea were found to be insensitive probably because of their ability to produce detoxifying enzymes. Thus, T. viride was used to elucidate the C. parqui saponin’s activity and to check relative assumption about the mechanisms of their action. The interaction between saponins and membrane sterol was elucidated by additions of cholesterol in the culture medium treated by CSE beforehand. The structural modification of saponins by hydrolysis resulted in loss of their antifungal activity suggesting the importance of the sugar chain in that biological activity.
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