Horizontal Transfer and Evolution of the Biosynthetic Gene Cluster for Benzoxazinoid

Benzoxazinoids are a class of protective and allelopathic plant secondary metabolites, first identified in maize (Zea mays) and subsequently shown to be encoded by a biosynthetic gene cluster (BGC), the Bx cluster. Data mining of mining 40 high-quality grass genomes identified complete Bx clusters (containing genes Bx1 to Bx5 and Bx8) in three genera (Zea, Echinochloa and Dichanthelium) in the Panicoideae and partial clusters in the Triticeae. The Bx cluster originated from gene duplication of native analogues of Bx genes and chromosomal translocation. An ancient Bx cluster including additional Bx genes (e.g., Bx6) is found in ancestral Panicoideae. The ancient Bx cluster was gained by the Triticeae ancestor via a horizontal transfer (HT) event from the ancestral Panicoideae and later separated into three parts on different chromosomes. Bx6 appears to have been under less constrained selection during evolution of the Panicoideae as evidenced by the fact that was translocated [~]1.31-Mb away from the Bx cluster in Z. mays, moved to other chromosomes in Echinochloa, and even lost in Dichanthelium. Further investigation indicated that intense selection and polyploidization shaped the evolutionary trajectory of the Bx cluster in the grass family. This study provides the first case of HT of BGCs among plants and sheds new insights on the evolution of BGCs. SignificanceBiosynthetic gene clustering and horizontal gene transfer are two evolutionary inventions for rapid adaption by organisms. Horizontal transfer of a gene cluster has been reported in fungi and bacteria, but not in plants up to now. By mining the genomes of 40 monocot species, we deciphered the organization of Bx gene cluster, a biosynthetic gene cluster for benzoxazinoids in grasses. We found that the Bx cluster was formed by gene duplication of native analogues of individual Bx genes and directional translocation. More importantly, the Bx cluster in Triticeae was inherited from the Panicoideae via horizontal transfer. Compared with the native analogues, Bx clusters in grasses show constrained purifying selection underscoring their significance in environmental adaption.