P-238 - Ancestral sequence reconstruction reveals the evolutionary origin of phytoalexin production in Solanaceae

Solanaceae (including tomato, potato, tobacco and bell pepper) produce sesquiterpenoid phytoalexins, which are a class of defense compounds that derive from farnesyl diphosphate (FPP). These compounds can differ between species, but are each critical for immunity, as they can thwart or hamper infection by various fungal and bacterial pathogens. FPP is initially converted either to 5-epi-aristolochene by epi-aristolochene synthase (EAS), or into premnaspirodiene by vetispiradiene synthase (VS). Nicotiana and Capsicum species preferentially produce EAS pathway compounds such as capsidiol, whereas Solumn species produce premnapirodiene derivatives such as solavetivone, lubimin and rishitin. To elucidate the evolutionary origin of these two pathways, we have reconstructed an ancestral terpene synthase which predates the speciation into Nicotiana, Solanum and Capsicum. Heterologous expression of this ancient gene in yeast revealed that the original phytoalexins in ancient Solanaceae were premnaspirodiene derivatives, formed by a terpene synthase similar to modern VS, whereas the EAS pathway arose via the mutation of an ancestral VS-like gene. These findings shed light on the evolutionary origin of phytoalexins in Solanaceae and help tracing the evolutionary pressures that led to this development.