Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 112, Issue 13, Pages 4164-4169Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1423457112
Keywords
pyrrolizidine alkaloids; alkaloid biosynthesis; plant secondary metabolism; chemical defense; root-nodule symbiosis
Categories
Funding
- Deutsche Forschungsgemeinschaft Cluster of Excellence Inflammation at Interfaces
- Deutsche Forschungsgemeinschaft Cluster of Excellence Future Ocean
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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.
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