Abstract
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. HSS is expressed in the tips of aerial roots exclusively in mitotically active cells. Raphide crystal idioblasts present within the root apical meristem do not show HSS expression. In addition, young flower buds, but not mature flowers, express HSS and have been shown by tracer feeding experiments to be able to catalyze PAs. This second site of PA biosynthesis ensures high concentrations of PAs in the reproductive structures of the Phalaenopsis flower, even after the flower opens. Thus, in spite of its identical function in PA biosynthesis, HSS shows in Phalaenopsis a completely different spatial and developmental expression pattern in comparison to other PA-producing species. These results show that the proverbial diversity of plant secondary metabolism is not just a matter of structural diversity, but is also multifaceted in terms of pathway regulation and expression.
Highlights
Pyrrolizidine alkaloids (PAs) are typical compounds of plant secondary metabolism and are believed to be part of the plant’s chemical defense
No signal with the homospermidine synthase (HSS)-specific antibody was seen in the basal part of the aerial roots, in roots entering into the substrate, or in open flowers, suggesting the specific expression of HSS in space and time (Fig. 1)
Phylogenetic analyses suggest that HSS, as the first specific enzyme of PA biosynthesis, was recruited early in the evolution of the monocots
Summary
Pyrrolizidine alkaloids (PAs) are typical compounds of plant secondary metabolism and are believed to be part of the plant’s chemical defense. In spite of its identical function in PA biosynthesis, HSS shows in Phalaenopsis a completely different spatial and developmental expression pattern in comparison to other PA-producing species. These results show that the proverbial diversity of plant secondary metabolism is not just a matter of structural diversity, but is multifaceted in terms of pathway regulation and expression. In the case of the pyrrolizidine alkaloids (PAs), we have found evidence that the ability to produce these compounds has been recruited several times independently during angiosperm evolution (Reimann et al, 2004). Our analyses of the evolution of PA biosynthesis within angiosperm plants have enabled us to show that the gene encoding homospermidine synthase (HSS) has been recruited at least four times independently from a gene encoding deoxyhypusine synthase
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