Ricin accumulation and degradation during castor seed development and late germination

The castor plant (Ricinus communis L.) is a euphorb, commonly grown for its high seed oil content. The seed also contains a deadly toxin known as ricin (RCA60) and the less toxic Ricinus communis agglutinin (RCA120) in addition to potentially harmful allergens. Using Western blotting, an antibody screening technique, ricin content was evaluated in various plant parts with a ricin A-chain specific antibody. Seed, cotyledon, root, hypocotyl, and leaf tissues were tested. Western blot results indicated that ricin is only present in the seed. In order to determine when ricin accumulates in the seed, reproductive tissue samples were harvested during seed development and upon germination. Developing seed were collected from inflorescences in 4-day intervals beginning with the day of pollination (day 0) until the seed had fully matured (day 44). Ricin was absent from the developing seed throughout development until the 28th day post-pollination. Ricin content quickly increased as the seed matured and the capsule dehydrated. Mature seed were also germinated and collected at 2-day intervals once radicle emergence began, until the 12th day, when seedlings were fully expanded. From the sixth day forward, germinating seedlings were dissected into root, hypocotyl, and cotyledonary tissue. Western blot analysis indicated that after the 6th day, ricin is degraded below levels of detection. Ricin is only detectable in the seed tissue from day 0 (ungerminated seed) through the sixth day after radicle emergence and is not detected in any of the root, hypocotyl, or cotyledonary tissue. These data suggest that ricin is generated and stored in the seed for a specific purpose; most likely the protection of the seed from predation in its quiescent state.