Toxicity of (+)- and (-)-gossypol to the plant pathogen, Rhizoctonia solani

Authors: Puckhaber, Lorraine; Dowd, Michael; Stipanovic, Robert - Bob; Howell, Charles - Charlie

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/19/2002
Publication Date: 11/29/2002
Citation: Puckhaber, L.S., Dowd, M.K., Stipanovic, R.D., Howell, C.R. 2002. Toxicity of (+)- and (-)-gossypol to the plant pathogen, Rhizoctonia solani. Journal of Agricultural and Food Chemistry. 50(24)7017-7021.

Interpretive Summary: Cottonseed contains a compound called gossypol. Gossypol can exist in two different forms. One of these forms is toxic to animals while the other is not. It might be advantageous to breed plants with very low levels of the toxic form of gossypol in the seed. However, gossypol also occurs in the root and in doing so it protects the plant from attack from germs in the soil that cause diseases in the plant. The ability of gossypol to hinder the growth of these disease-causing germs or to kill them was not known. We have now tested both forms of gossypol and an equal mixture of the different forms for their ability to hinder growth and to kill one disease causing germ that attacks cotton. We found that the different forms and the mixture are equally effective in protecting the plant. This means that breeding plants to produce low levels of the non-toxic form of gossypol will not make the plants more susceptible to these germs.

Technical Abstract: The dimeric sesquiterpene gossypol occurs naturally in cottonseed. Gossypol exists as enantiomers because of restricted rotation around the central binaphthyl bond. The (-)-enantiomer is toxic to non-ruminant animals , but the (+)-enantiomer exhibits little if any toxicity to these animals. Developing cotton plants with low levels of the (-)-enantiomer could enhance the value of cottonseed. However, gossypol may also play a role in protecting the plant from pathogens. The toxicity of gossypol to the seedling disease pathogen Rhizoctonia solani has not been investigated. We find the (+)- and (-)-enantiomers of gossypol and the racemic form are equally effective in inhibiting the growth of R. solani, and are about one-half as effective as related sesquiterpenes produced by cotton in response to attack by fungal pathogens. The enantiomers and racemate also appear to be equally toxic to R. solani, but their toxicity is significantly less than cotton¿s most potent phytoalexins.