Recruitment of alkaloid-specific homospermidine synthase (HSS) from ubiquitous deoxyhypusine synthase: Does Crotalaria possess a functional HSS that still has DHS activity?

Abstract

Quinolizidine alkaloids are the most prominent group of alkaloids occurring in legumes, except for many members of the tribe Crotalarieae that accumulate pyrrolizidine alkaloids (PAs). To study the evolution of PA biosynthesis as a typical pathway of plant secondary metabolism in this tribe, we have searched for a cDNA coding for homospermidine synthase (HSS), the enzyme catalyzing the first specific step in this biosynthesis. HSS was shown to have been recruited from deoxyhypusine synthase (DHS) by independent gene duplication in several different angiosperm lineages during evolution. Except for a cDNA sequence coding for the DHS of Crotalaria retusa, no data is available concerning the origin of PA biosynthesis within this tribe of the Fabaceae. In addition to several pseudogenes, we have identified one functional DHS in C. scassellatii and two in C. juncea. Despite C. juncea plants under study being devoid of PAs, we have found that the two sequences of C. juncea are different with respect to their genomic organization, their tissue-specific expression, and their biochemical activities. Supported by the branching pattern of a maximum likelihood analysis of these sequences, they have been classified as “class 1” and “class 2” DHS. It remains open whether the duplicated DHS belonging to class 2 is involved in the biosynthesis of PAs.