Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Abstract Only
Publication Acceptance Date: January 7, 2005
Publication Date: May 25, 2005
Citation: Liu, J., Stipanovic, R.D., Bell, A.A., Puckhaber, L.S. 2005. Gossypol pathway in cotton: Stereospecific biosynthesis of (+)-gossypol in Moco cotton [abstract]. In: Proceedings of the Beltwide Cotton Conferences, January 4-7, 2005, New Orleans, Louisiana. 2005 CDROM. Technical Abstract: Gossypol is biosynthesized by the free radical coupling of two molecules of hemigossypol to yield two optically active enantiomers, (+)-gossypol and (-)-gossypol. Most commercial cottonseeds have a (+)- to (-)-gossypol ratio of ~60:40. However, this ratio can vary from 98:2 in some moco accession to 37:63 in cultivars of G. barbadense. The biological activities of (+)- and (-)-gossypol are very distinct. Animal toxicity is mainly due to (-)-gossypol. However, the toxicity toward insects and pathogens is not affected by the (+)- to (-)-gossypol ratio. Our goal is to clone the genes controlling the (+)- to (-)-gossypol ratio in cotton, thus enabling us to alter this ratio through genetic engineering. Cotton plants making only (+)-gossypol would retain their defensive capabilities, while their seeds would have a higher nutritional value. On the other hand, cotton plants making only (-)-gossypol are of interest for pharmaceutical applications due to its anti-cancer, anti-HIV, and other properties. Our present objective is to elucidate the biochemical process governing the stereospecific coupling of hemigossypol for the formation (+)-gossypol in moco cotton, which involves currently poorly understood stereospecific coupling of high energy free radicals. For racemic gossypol formation in most commercial cultivars, the oxidative coupling of hemigossypol for the formation of gossypol involves free radical generation probably by peroxidase or less likely by Cu-containing oxidase and the random coupling of the generated high energy hemigossypol free radicals in solution. For preferential (+)-gossypol formation in moco cotton, two different mechanisms can be invoked to explain stereospecific coupling of hemigossypol. One involves a stereospecific oxidase, which generates hemigossypol free radical as well as steers the stereospecific coupling of the free radicals. This kind of mechanism has been show in operation in the stereospecific coupling 1,2,3,4,6-penta-O-galloyl-'-glucopyronose to form (S)-tellimagrandin II (Niemetz, R. et. al. 2003, Phytochemistry 62: 301-306). The other involves a non-stereospecific peroxidase or less likely oxidase for the generation of hemigossypol free radicals, and a second protein called dirigent protein to guide the stereospecific coupling of the free radicals. The dirigent protein itself does not have the ability to generate free radicals. This kind of mechanism has been show in operation in the stereospecific coupling of E-coniferyl alcohol to form (+)-pinoresinol (Davin, L.B. and Lewis, N.G. et. al. 1997, Science 275: 362-366). We have demonstrated that this second mechanism is responsible for the (+)-gossypol formation in moco cotton.