Cell wall integrity, genotoxic injury and PCD dynamics in alfalfa saponin-treated white poplar cells highlight a complex link between molecule structure and activity

In the present work, eleven saponins and three sapogenins purified from Medicago sativa were tested for their cytotoxicity against highly proliferating white poplar (Populus alba L.) cell suspension cultures. After preliminary screening, four saponins with different structural features in terms of aglycone moieties and sugar chains (saponin 3, a bidesmoside of hederagenin; saponins 4 and 5, monodesmoside and bidesmoside of medicagenic acid respectively, and saponin 10, a bidesmoside of zanhic acid) and different cytotoxicity were selected and used for further investigation on their structure-activity relationship. Transmission Electron Microscopy (TEM) analyses provided for the first time evidence of the effects exerted by saponins on plant cell wall integrity. Exposure to saponin 3 and saponin 10 resulted into disorganization of the outer wall layer and the effect was even more pronounced in white poplar cells treated with the two medicagenic acid derivatives, saponins 4 and 5. Oxidative burst and nitric oxide accumulation were common hallmarks of the response of white poplar cells to saponins. When DNA damage accumulation and DNA repair profiles were evaluated by Single Cell Gel Electrophoresis, induction of single and double strand breaks followed by effective repair was observed within 24 h. The reported data are discussed in view of the current issues dealing with saponin structure-activity relationship.