title: Identification of a Second Site of Pyrrolizidine Alkaloid Biosynthesis in Comfrey to Boost Plant Defense in Floral Stage.
abstract: Pyrrolizidine alkaloids (PAs) are toxic secondary metabolites that are found in several distantly related families of the angiosperms. The first specific step in PA biosynthesis is catalyzed by homospermidine synthase (HSS), which has been recruited several times independently by duplication of the gene encoding deoxyhypusine synthase, an enzyme involved in the posttranslational activation of the eukaryotic initiation factor 5A. HSS shows highly diverse spatiotemporal gene expression in various PA-producing species. In comfrey (Symphytum officinale; Boraginaceae), PAs are reported to be synthesized in the roots, with HSS being localized in cells of the root endodermis. Here, we show that comfrey plants activate a second site of HSS expression when inflorescences start to develop. HSS has been localized in the bundle sheath cells of specific leaves. Tracer feeding experiments have confirmed that these young leaves express not only HSS but the whole PA biosynthetic route. This second site of PA biosynthesis results in drastically increased PA levels within the inflorescences. The boost of PA biosynthesis is proposed to guarantee optimal protection especially of the reproductive structures.
authors.affiliation: Botanisches Institut und Botanischer Garten, Universität Kiel, D-24098 Kiel, Germany.
authors.forename: Lars H
authors.identifier: http://orcid.org/0000-0002-0449-092X
authors.initials: LH
authors.lastname: Kruse
authors.affiliation: Botanisches Institut und Botanischer Garten, Universität Kiel, D-24098 Kiel, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Stegemann
authors.affiliation: Botanisches Institut und Botanischer Garten, Universität Kiel, D-24098 Kiel, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Sievert
authors.affiliation: Botanisches Institut und Botanischer Garten, Universität Kiel, D-24098 Kiel, Germany dober@bot.uni-kiel.de.
authors.forename: Dietrich
authors.identifier: http://orcid.org/0000-0002-7372-4531
authors.initials: D
authors.lastname: Ober
mesh_terms: D019883:Alkyl and Aryl Transferases / Q000235:genetics / Q000378:metabolism; D020844:Comfrey / Q000235:genetics* / Q000378:metabolism; D035264:Flowers / Q000235:genetics* / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D018515:Plant Leaves / Q000235:genetics* / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D018517:Plant Roots / Q000235:genetics / Q000378:metabolism; D011763:Pyrrolizidine Alkaloids / Q000378:metabolism*
chemical_list: D010940:Plant Proteins; D011763:Pyrrolizidine Alkaloids; D019883:Alkyl and Aryl Transferases; C028376:homospermidine synthetase