Complex nested promoters control tissue-specific expression of acetyl-CoA carboxylase genes in wheat.

Cis-acting regulatory elements of the wheat acetyl-CoA carboxylase (ACC) gene family were identified by comparing the promoter activity of 5' end gene fragments fused to a reporter gene in two transient expression systems: wheat protoplasts and epidermal cells of mature embryos. Expression of the plastid and the cytosolic ACC genes is each driven by two nested promoters responsible for the synthesis of two transcript types. The internal promoter is located in an intron removed from transcripts originating at the first promoter. These complex promoters, which are different for the cytosolic and plastid ACC genes, control tissue-specific expression of the enzymatic activity supplying cytosolic, plastid, and mitochondrial pools of malonyl-CoA. The activity of one such complex promoter, driving expression of one of the cytosolic ACC genes, was studied throughout development of transgenic wheat plants carrying a full-length promoter-reporter gene fusion. High activity of the promoter was detected in the coleoptile, in the upper sheath section of the leaf, on the top surface of the ovary, in some sections of the main veins in the lemma and glume, and in abaxial epidermis hair cells of the lemma, glume, and rachis. The findings are consistent with the developmental and environmental requirements for very-long-chain fatty acids and flavonoids, whose synthesis begins with the ACC reaction in the cytosol of these specific cell types.