Proc. Natl Acad. Sci. USA http://doi.org/dw86 (2020).
Gene clusters for secondary metabolism have been found in bacteria, filamentous fungi and flowering plants. A gene cluster usually contains a set of genes encoding various enzymes that are involved in the same pathway of secondary metabolic biosynthesis. Recently, a paper involving collaboration among several groups, including Kazunori Okada’s group in the University of Tokyo, Japan, and Longjiang Fan’s group in Zhejiang University, China, reported the presence and function of a gene cluster corresponding to the biosynthesis of momilactones in a basal linage of land plants.
Momilactones (A and B forms) belong to a small class of versatile diterpenoids that can induce growth inhibition of plants and plant pathogens, and therefore function as defensive chemicals. They were first identified in cultivated rice (Oryza sativa) and later found in barnyard grass (Echinochloa crus-galli); the latter is considered one of the world’s worst weeds that compete against agricultural crops, especially cultivated rice. In addition, momilactones are produced and used by a common species of moss, Calohypnum plumiforme (or Hypnum plumaeforme). To understand the biosynthesis and evolution of momilactones, Mao and Kawaide et al. sequenced the whole genome of C. plumiforme and annotated 32,195 protein-coding genes from the genome assembly. They identified a gene cluster composed of four genes that are responsible for the biosynthesis of momilactone A, which echoes the finding of momilactone gene clusters in the grasses. However, they did not find a syntenic relationship in the gene arrangements of these different gene clusters; therefore, they hypothesized that convergent evolution is likely the mechanism driving the formation of the momilactone gene clusters in moss and grasses.