Evolution of pyrrolizidine alkaloids in Phalaenopsis orchids and other monocotyledons: identification of deoxyhypusine synthase, homospermidine synthase and related pseudogenes.

In order to study the evolution of pathways of plant secondary metabolism, we use the biosynthesis of pyrrolizidine alkaloids (PAs) as a model system. PAs are regarded as part of the plant's constitutive defense against herbivores. Homospermidine synthase (HSS) is the first specific enzyme of PA biosynthesis.

Pyrrolizidine alkaloid biosynthesis in Phalaenopsis orchids: developmental expression of alkaloid-specific homospermidine synthase in root tips and young flower buds.

Pyrrolizidine alkaloids (PAs) are typical compounds of plant secondary metabolism and are believed to be part of the plant's chemical defense. Within the monocotyledonous plants, PAs have been described in only a few genera, mainly orchids, including Phalaenopsis. Because phylogenetic analyses suggest an independent origin of PA biosynthesis within the monocot lineage, we have analyzed the developmentally regulated expression of homospermidine synthase (HSS), the first pathway-specific enzyme of PA biosynthesis, at the cell level.