title: New aspect of plant-rhizobia interaction: alkaloid biosynthesis in Crotalaria depends on nodulation.
abstract: Infection of legume hosts by rhizobial bacteria results in the formation of a specialized organ, the nodule, in which atmospheric nitrogen is reduced to ammonia. Nodulation requires the reprogramming of the plant cell, allowing the microsymbiont to enter the plant tissue in a highly controlled manner. We have found that, in Crotalaria (Fabaceae), this reprogramming is associated with the biosynthesis of pyrrolizidine alkaloids (PAs). These compounds are part of the plant's chemical defense against herbivores and cannot be regarded as being functionally involved in the symbiosis. PAs in Crotalaria are detectable only when the plants form nodules after infection with their rhizobial partner. The identification of a plant-derived sequence encoding homospermidine synthase (HSS), the first pathway-specific enzyme of PA biosynthesis, suggests that the plant and not the microbiont is the producer of PAs. Transcripts of HSS are detectable exclusively in the nodules, the tissue with the highest concentration of PAs, indicating that PA biosynthesis is restricted to the nodules and that the nodules are the source from which the alkaloids are transported to the above ground parts of the plant. The link between nodulation and the biosynthesis of nitrogen-containing alkaloids in Crotalaria highlights a further facet of the effect of symbiosis with rhizobia on the ecologically important trait of the plant's chemical defense.
authors.affiliation: Biochemical Ecology and Molecular Evolution, Botanical Institute and Botanical Gardens, Kiel University, 24118 Kiel, Germany;
authors.forename: Simon
authors.initials: S
authors.lastname: Irmer
authors.affiliation: Biochemical Ecology and Molecular Evolution, Botanical Institute and Botanical Gardens, Kiel University, 24118 Kiel, Germany;
authors.forename: Nora
authors.initials: N
authors.lastname: Podzun
authors.affiliation: Biochemical Ecology and Molecular Evolution, Botanical Institute and Botanical Gardens, Kiel University, 24118 Kiel, Germany;
authors.forename: Dorothee
authors.initials: D
authors.lastname: Langel
authors.affiliation: Biochemical Ecology and Molecular Evolution, Botanical Institute and Botanical Gardens, Kiel University, 24118 Kiel, Germany;
authors.forename: Franziska
authors.initials: F
authors.lastname: Heidemann
authors.affiliation: Biochemical Ecology and Molecular Evolution, Botanical Institute and Botanical Gardens, Kiel University, 24118 Kiel, Germany;
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Kaltenegger
authors.affiliation: Biochemical Ecology and Molecular Evolution, Botanical Institute and Botanical Gardens, Kiel University, 24118 Kiel, Germany;
authors.forename: Brigitte
authors.initials: B
authors.lastname: Schemmerling
authors.affiliation: Institute of Plant Nutrition and Soil Science, Kiel University, 24118 Kiel, Germany; and.
authors.forename: Christoph-Martin
authors.initials: CM
authors.lastname: Geilfus
authors.affiliation: Institute of Crop Science, Quality of Plant Products, University Hohenheim, 70593 Stuttgart, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Zörb
authors.affiliation: Biochemical Ecology and Molecular Evolution, Botanical Institute and Botanical Gardens, Kiel University, 24118 Kiel, Germany; dober@bot.uni-kiel.de.
authors.forename: Dietrich
authors.initials: D
authors.lastname: Ober
mesh_terms: D019883:Alkyl and Aryl Transferases / Q000378:metabolism; D029864:Crotalaria / Q000378:metabolism* / Q000382:microbiology; D018076:DNA, Complementary / Q000378:metabolism; D009584:Nitrogen / Q000737:chemistry; D055170:Plant Root Nodulation; D018517:Plant Roots / Q000378:metabolism; D016133:Polymerase Chain Reaction; D011763:Pyrrolizidine Alkaloids / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D012231:Rhizobium / Q000502:physiology*; D013559:Symbiosis
chemical_list: D018076:DNA, Complementary; D011763:Pyrrolizidine Alkaloids; D011994:Recombinant Proteins; D019883:Alkyl and Aryl Transferases; C028376:homospermidine synthetase; D009584:Nitrogen
keywords: alkaloid biosynthesis; chemical defense; plant secondary metabolism; pyrrolizidine alkaloids; root-nodule symbiosis