1.D.8 The Saponin (Saponin) Family
Saponins are antifungal agents found in high levels in healthy plants. They may also have insecticidal, molluscicidal and piscicidal activities. They are glycosylated compounds, widely distributed in the plant kingdom, which can be divided into three major groups, depending on the structure of the aglycone: triterpenoids, steroids, and steroidal glycoalkaloids. Digitonin is a steroid saponin from foxglove. The best studied saponins are from oats and tomatoes. Oats have both triterpenoid saponins (ie avenacins (A-1, B-1, A2 and B2) and steroidal avenacosides (avenacosides A and B). Tomatoes have the steroidal glycoalkaloid,α-tomatine. The toxic action of saponins to fungi is associated with their ability to complex with membrane steroids and cause pore-formation. Plants may protect themselves from their own saponins by sequestering them in vacuoles, the membrane of which have altered sterol compositions.