SCIENCE@home

New aspect of plant–rhizobia interaction: Alkaloid biosynthesis in Crotalaria depends on nodulation

S. Irmer,Nora Podzun,D. Langel,F. Heidemann,E. Kaltenegger,Brigitte Schemmerling,C. Geilfus,C. Zörb,D. Ober

Published 2015 in Proceedings of the National Academy of Sciences of the United States of America

ABSTRACT

Significance Nodules are typical organs at the roots of legumes (Fabaceae) in which symbiotic rhizobia fix atmospheric nitrogen. The fabaceous genus Crotalaria is known to produce toxic alkaloids as part of its chemical defense. We show that, in Crotalaria, nodulation triggers the biosynthesis of these alkaloids. The first specific enzyme of their biosynthesis is of plant origin and is expressed exclusively in the nodules. 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.

PUBLICATION RECORD

CITATION MAP

EXTRACTION MAP

CLAIMS

  • No claims are published for this paper.

CONCEPTS

  • No concepts are published for this paper.

REFERENCES

Showing 1-61 of 61 references · Page 1 of 1

CITED BY

Showing 1-63 of 63 citing papers · Page 1 of 1