Engineering Cotton to Expand Its Role as a Feed- and Food Crop.

Cotton, grown mainly for its fiber, is a major crop in the U.S. and several other, mostly poor countries. Cottonseed, a byproduct, remains an abundant but greatly underutilized source of protein because of the presence of gossypol, a toxic terpenoid that otherwise serves a beneficial, defensive function for the plant.  Annual, worldwide production of ~44 million metric tons (MMT) of cottonseed contains ~10 MMT of protein, enough to meet the basic protein requirements of 500 million people.  Therefore, elimination of gossypol from cottonseed has been a long-standing goal of geneticists.  The “glandless cotton” developed by breeders in the 1950s to meet this objective was commercially unviable because of the increased susceptibility of the plant to insect pests due to the systemic absence of glands that contain gossypol and other protective terpenoids.  Thus, the promise of cottonseed in contributing to the food requirements of the burgeoning world population remained unfulfilled.  We have employed RNA interference (RNAi) to inhibit the expression of the delta-cadinene synthase (dCS) in a seed-specific manner, thereby disrupting a key step in the biosynthesis of gossypol.  Compared to an average gossypol value of 10 μg/mg in wild-type cottonseeds, the seeds from RNAi lines showed values as low as 0.2 μg/mg.  Importantly, the levels of gossypol and related terpenoids that are derived from the same biosynthetic pathway were not diminished in the foliage and floral parts of mature plants and thus remain available for plant defense against insects and diseases. The stability of the engineered trait has been confirmed by evaluation of several lines over five generations in the greenhouse and under field conditions.  No adverse effects on fiber-quantity and -quality were observed in the engineered plants. Further, we have obtained molecular and biochemical evidence showing that the germinating, RNAi seedlings are capable of launching terpenoid-based defense response when challenged with pathogens.  Thus, the “silenced-state” of the dCS gene that existed in the seed, does not leave a lasting, residual effect that can interfere with the normal functioning of the cotton seedling during germination. Thus, it has been possible to overcome the major weakness of “glandless cotton”.  The use of reduced gossypol cottonseed either directly as food or indirectly as feed for the more efficient monogastric animals has the potential to significantly impact global food security.