A functionally conserved STORR gene fusion in Papaver species that diverged 16.8 million years ago

Abstract

The STORR gene fusion event is considered a key step in the evolution of benzylisoquinoline alkaloid (BIA) metabolism in opium poppy as the resulting bi-modular protein performs the isomerization of (S)- to (R)- reticuline which is required for morphinan biosynthesis. Our previous analysis of the opium poppy genome suggested the STORR gene fusion event occurred before a whole genome duplication event 7.2 million years ago. Here we use a combination of phylogenetic, transcriptomic, metabolomic, biochemical and genomic analysis to investigate the origin of the STORR gene fusion across the Papaveraceae family. The pro-morphinan/morphinan subclass of BIAs was present in a subset of 10 Papaver species including P. somniferum (opium poppy) and this correlated with the presence of the STORR gene fusion with one important exception. P. californicum does not produce morphinans but it does contain a STORR gene fusion that epimerizes (S)- to (R)- reticuline when heterologously expressed in yeast. The high similarity of the amino acid sequence linking the two modules of STORR along with phylogenetic gene tree analysis strongly suggests the gene fusion occurred only once and between 17-25 million years ago before the separation of P. californicum from the other Papaver species. We discovered that the most abundant BIA in P. californicum is (R)- glaucine, a member of the aporphine subclass of BIAs. Only the (S) isomer of this compound has previously been reported from nature. These results lead us to conclude that the function of the STORR gene fusion is not exclusive to morphinan production in the Papaveraceae.

Plant biosynthetic gene clusters in the context of metabolic evolution

“… 101 However, with different species selected in the analysis, it appears that the segmental duplications may instead have preceded WGDs. 96 This different interpretation highlights how evolutionary analysis is highly dependent on sample selection.…”

“…2). 71,96 Whole genome duplication (WGD) constitutes the most drastic change of genetic material where the entire genome is doubled in the progeny (Fig. 6).…”

“…47,71 Genome analysis shows that the genomic association of the separate CYP and AKR encoding genes predated the fusion event. 71,96,101 However, a fused STORR has recently been identi-ed in P. californicum, a plant that does not make (pro)morphinans, indicating it had a different role prior to the emergence of the morphinans. 96 These results suggest that STORR may have been recruited from a different BIA branch to function in (pro)morphinan formation.…”