Genomic evidence for convergent evolution of gene clusters for momilactone biosynthesis in land plants

Abstract

Momilactones are bioactive diterpenoids that contribute to plant defense against pathogens and allelopathic interactions between plants. Both cultivated and wild grass species of <i>Oryza</i> and <i>Echinochloa crus-galli</i> (barnyard grass) produce momilactones using a biosynthetic gene cluster (BGC) in their genomes. The bryophyte <i>Calohypnum plumiforme</i> (formerly <i>Hypnum plumaeforme</i>) also produces momilactones, and the bifunctional diterpene cyclase gene <i>CpDTC1/HpDTC1,</i> which is responsible for the production of the diterpene framework, has been characterized. To understand the molecular architecture of the momilactone biosynthetic genes in the moss genome and their evolutionary relationships with other momilactone-producing plants, we sequenced and annotated the <i>C. plumiforme</i> genome. The data revealed a 150-kb genomic region that contains two cytochrome P450 genes, the <i>CpDTC1</i>/<i>HpDTC1</i> gene and the "dehydrogenase momilactone A synthase" gene tandemly arranged and inductively transcribed following stress exposure. The predicted enzymatic functions in yeast and recombinant assay and the successful pathway reconstitution in <i>Nicotiana benthamiana</i> suggest that it is a functional BGC responsible for momilactone production. Furthermore, in a survey of genomic sequences of a broad range of plant species, we found that momilactone BGC is limited to the two grasses (<i>Oryza</i> and <i>Echinochloa</i>) and <i>C. plumiforme</i>, with no synteny among these genomes. These results indicate that while the gene cluster in <i>C. plumiforme</i> is functionally similar to that in rice and barnyard grass, it is likely a product of convergent evolution. To the best of our knowledge, this report of a BGC for a specialized plant defense metabolite in bryophytes is unique.