Molecular characterization of benzoxazinone-deficient mutation in diploid wheat

Benzoxazinones (Bxs) are representative defensive compounds in gramineous plants, including wheat (genus Triticum) and its wild relative species (genus Aegilops). Bx production was found to be variable among three diploid wheat species with the same A genome as hexaploid wheat (2n=6x=42, genomes AABBDD). All accessions of Triticum monococcum (2n=2x=14, AA) and Triticum urartu (2n=2x=14, AA) accumulated Bxs, but 18 out of 28 accessions of Triticum boeoticum (2n=2x=14, AA) were Bx-deficient. Bx-deficient accessions were grouped into two types by genomic PCR analysis of the five Bx biosynthetic loci (TbBx1-TbBx5): those retaining all five loci (type I) and those lacking TbBx3 and TbBx4 loci (type II). Despite the Bx-deficient phenotype, all five TbBx genes were transcribed in the type-I accessions. The Bx deficiency in one accession of type I was due to the disintegration of the TbBx1, TbBx4 and TbBx5 genes due to insertions or deletions in their coding sequences. The TbBx2 and TbBx3 genes of those accessions had the complete sequences of the functional enzymes. In the type-II accessions, the remaining three genes, TbBx1, TbBx2 and TbBx5, were all transcribed, with the exception of two accessions in which either TbBx1 or TbBx5 was not transcribed. The TbBx1 coding sequence of the type-II accessions was also disintegrated, like that of the type-I accessions. These findings suggest that the Bx deficiency in T. boeoticum first resulted from disintegration of the TbBx1 coding sequence, followed by transcription failure, disintegration of the coding sequences and elimination of the TbBx1-TbBx5 genes.