My research, in collaboration with Dr. Marlee Pierce, is directed toward understanding the mechanism of the exceptionally strong and durable resistance to bacterial blight that breeders have achieved by resistance gene pyramiding in cotton. We have shown that it involves a hypersensitive response in which the mesophyll cells closest to a microcolony of the pathogen die and accumulate fluorescent, sesquiterpenoid phytoalexins. We have determined that the phytoalexins attain concentrations in those cells that are more than high enough to account for the inhibition of the pathogen's growth when it occurs.
Isotopic labelling studies have established that the phytoalexins are biosynthesized from farnesyl diphosphate. The first two intermediates after farnesyl diphosphate have been identified, and cDNA clones have been isolated for the enzymes that catalyze their formation. A useful development from this work will be cotton lines blocked in expression of these two genes in seeds. Cottonseed oil and meal from such plants would lack toxic sesquiterpenoids and would therefore be safer ingredients for food and livestock feed.
We have developed a set of near-isogenic cotton lines with individual blight resistance genes in a common genetic background. By crossing them with each other, we have pyramided these genes to obtain a set of lines with different levels of resistance. From a subtractive cDNA library, several hundred distinct, defense-related cDNAs have been identified. They will be arrayed and used to analyze transcripts present in the pyramided lines as they resist bacterial infection to identify genes whose expression correlates with level of resistance.