METABOLISM OF THE POTATO SAPONINS ALPHA-CHACONINE AND ALPHA-SOLANINE BY GIBBERELLA-PULICARIS

Potato tubers accumulate varying amounts of several saponins preferent ially in the peel. These compounds are toxic to living cells containin g sterols in their plasma membrane and are therefore thought to be pre formed chemical defence compounds. Two strains of the potato pathogen Gibberella pulicaris (Fusarium sambucinum), R-6380 and R-7843, were an alysed for their ability to metabolize the most predominant saponins f ound in tubers, alpha-chaconine and alpha-solanine. The first compound is degraded by both strains via removal of alpha-1,2-L-rhamnose leadi ng to beta(2)-chaconine. This product is converted to the aglycone, so lanidine, which is further metabolized to unknown products. The releas e of alpha-1,2-L-rhamnose is also the first step in the break down of a-solanine by strain R-6380, followed by the removal of the beta-1,3-b ound glucose molecule leading to gamma-solanine, which is not metaboli zed any further. Strain R-7843 is not able to metabolize alpha-solanin e. Crude protein extracts of the culture quid of both strains containe d enzymes able to convert alpha-chaconine to beta(2)-chaconine, but wi th no alpha-solanine metabolic activity. This result indicates that G. pulicaris excretes enzymes specific for different saponins. (C) 1997 Elsevier Science Ltd.