Why are momilactones always associated with biosynthetic gene clusters in plants?

Abstract

There is an emerging realization that plant genomes contain biosynthetic gene clusters (BGCs) for more specialized metabolism in certain cases (1). However, while horizontal gene transfer seems to drive the assembly of self-sufficient BGCs in microbes, the limitations of strict vertical gene transmission necessitate a distinct driving force for the assembly of BGCs in plants. The evidence for horizontal gene transfer of BGCs in microbes is derived from the appearance of homologous such loci, leading to biosynthesis of the same (or very closely related) natural products, in phylogenetically distinct species. By contrast, there has been much less investigation of such occurrences in plants. This is a result of many fewer examples of BGCs, as well as instances of natural products that skip across phylogenetic distances in this kingdom. Fortunately, the momilactones provide a strikingly informative exception. These diterpenoids were originally found in rice ( Oryza sativa ), where the associated BGC was among the first few identified in plants (2), and where they serve as allelochemicals and/or phytoalexins (3, 4). Notably, momilactones are produced by not only other wild rice species within the Oryza genus but also barnyard grass ( Echinochloa crus-galli ), which falls within a separate clade of the Poaceae family. Intriguingly, beyond the expected orthologous momilactone BGCs in Oryza (5), a homologous BGC also was found in the phylogenetically more distant E. crus-galli (6). Even more interesting is the production of momilactones by the bryophyte Calohypnum plumiforme (formerly Hypnum plumaeforme ), representing a very early diverging lineage of land plants (7). In PNAS, Mao et al. (8) report that the C. plumiforme genome contains a BGC for momilactone biosynthesis. Strikingly, this not only represents identification of a BGC in a bryophyte but, further, is clearly phylogenetically distinct from the BGCs associated with momilactone biosynthesis in the Poaceae. Nevertheless, despite their clearly … [↵][1]1To whom correspondence should be addressed. Email: rjpeters{at}iastate.edu. [1]: #xref-corresp-1-1