Rearrangement of the genes for the biosynthesis of benzoxazinones in the evolution of Triticeae species

Gramineous plants, including the major agricultural crops wheat (Triticum aestivum L.), rye (Secale cereale L.) and maize (Zea mays L.), accumulate benzoxazinones (Bxs) as defensive compounds. Previously, we isolated cDNAs of the Bx biosynthetic genes in wheat, TaBx2—TaBx5, that encode the enzymes catalyzing the sequential hydroxylation of indole to Bxs. In this study we isolated a cDNA of TaBx1, which encodes the first enzyme of the Bx pathway of wheat. The level of identity (80%) in deduced amino-acid sequence between TaBx1 and the corresponding maize gene Bx1 was as high as those shown between TaBx2—TaBx5 and the corresponding maize genes Bx2—Bx5, respectively. Southern blot analysis using the TaBx1—TaBx5 cDNAs as probes was conducted with aneuploid lines of hexaploid wheat in order to determine sub-chromosomal locations of the five Bx biosynthetic genes in Triticeae species. In wheat, TaBx1 and TaBx2 co-existed in specific regions of chromosomes 4A, 4B and 4D, and TaBx3—TaBx5 were localized together in the distal regions of the short arms of chromosomes 5A, 5B and 5D. TaBx3 and TaBx5 were found to have duplicated loci in the long arm and the short arm of chromosome 5B, respectively. In rye, homoeoloci of TaBx1 and TaBx2 were located on chromosome 7R and those for TaBx3—TaBx5 were located on chromosome 5R. In barley, no Southern blot band was detected with any of the probes under the highly stringent hybridization conditions, suggesting that the non-Bx phenotype of barley is attributable to the loss of Bx biosynthetic genes.