Phylogenomics of the benzoxazinoid biosynthetic pathway of Poaceae: Gene duplications and origin of the B× cluster

Abstract

Background: The benzoxazinoids 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) and 2,4-dihydroxy-7- methoxy-1, 4-benzoxazin-3-one (DIMBOA), are key defense compounds present in major agricultural crops such as maize and wheat. Their biosynthesis involves nine enzymes thought to form a linear pathway leading to the storage of DI(M) BOA as glucoside conjugates. Seven of the genes (B×1-B×6 and B×8) form a cluster at the tip of the short arm of maize chromosome 4 that includes four P450 genes (B×2-5) belonging to the same CYP71C subfamily. The origin of this cluster is unknown. Results: We show that the pathway appeared following several duplications of the TSA gene (α-subunit of tryptophan synthase) and of a B×2-like ancestral CYP71C gene and the recruitment of B×8 before the radiation of Poaceae. The origins of B×6 and B×7 remain unclear. We demonstrate that the B×2-like CYP71C ancestor was not committed to the benzoxazinoid pathway and that after duplications the B×2-B×5 genes were under positive selection on a few sites and underwent functional divergence, leading to the current specific biochemical properties of the enzymes. The absence of synteny between available Poaceae genomes involving the B× gene regions is in contrast with the conserved synteny in the TSA gene region. Conclusions: These results demonstrate that rearrangements following duplications of an IGL/TSA gene and of a CYP71C gene probably resulted in the clustering of the new copies (B×1 and B×2) at the tip of a chromosome in an ancestor of grasses. Clustering favored cosegregation and tip chromosomal location favored gene rearrangements that allowed the further recruitment of genes to the pathway. These events, a founding event and elongation events, may have been the key to the subsequent evolution of the benzoxazinoid biosynthetic cluster. © 2012 Dutartre et al.; licensee BioMed Central Ltd.