The evolution of pyrrolizidine alkaloid biosynthesis and diversity in the Senecioneae

Pyrrolizidine alkaloids are characteristic secondary metabolites of the Asteraceae and some other plant families. They are especially numerous and diverse in the tribe Senecioneae and form a powerful defense mechanism against herbivores. Studies into the evolution of pyrrolizidine alkaloid biosynthesis using Senecio species have identified homospermidine synthase as the enzyme responsible for the synthesis of the first specific intermediate. These studies further indicated that the homospermidine synthase-encoding gene was recruited following gene duplication of deoxyhypusine synthase and that this occurred independently in several different angiosperm lineages. A review of published pyrrolizidine alkaloid data shows that the Senecioneae are characterized by a large qualitative and quantitative variation in pyrrolizidine alkaloid profiles and that these data demonstrate little phylogenetic signal. This suggests that although the first steps of this pathway are highly conserved, the diversification of secondarily derived pyrrolizidine alkaloids is extremely plastic.