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Homospermidine in transgenic tobacco results in considerably reduced spermidine levels but is not converted to pyrrolizidine alkaloid precursors

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Abstract

Homospermidine synthase is the first specific enzyme in the biosynthesis of pyrrolizidine alkaloids. Whereas the substrates putrescine and spermidine are part of the highly dynamic polyamine pool of plants, the product homospermidine is incorporated exclusively into the necine base moiety of pyrrolizidine alkaloids. Recently, the gene encoding homospermidine synthase has been shown to have been recruited several times independently during angiosperm evolution by the duplication of the gene encoding deoxyhypusine synthase. To test whether high levels of homospermidine suffice for conversion, at least in traces, to precursors of pyrrolizidine alkaloids, transgenic tobacco plants were generated expressing homospermidine synthase. Analyses of the polyamine content revealed that, in the transgenic plants, about 80% of spermidine was replaced by homospermidine without any conspicuous modifications of the phenotype. Tracer-feeding experiments and gas chromatographic analyses suggested that these high levels of homospermidine were not sufficient to explain the formation of alkaloid precursors. These results are discussed with respect to current models of pathway evolution.

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Acknowledgments

The authors thank Monika Frey for assitence in plant transformation and Claudine Theuring and Anita Backenköhler for excellent technical assistence. This work was supported by grants from the Deutsche Forschungsgemeinschaft Priority Program 1152 “Evolution of Metabolic Diversity” (grant to DO) and the Government of Egypt (scholarship to MA).

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Correspondence to Dietrich Ober.

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Abdelhady, M.I.S., Beuerle, T. & Ober, D. Homospermidine in transgenic tobacco results in considerably reduced spermidine levels but is not converted to pyrrolizidine alkaloid precursors. Plant Mol Biol 71, 145–155 (2009). https://doi.org/10.1007/s11103-009-9514-x

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  • DOI: https://doi.org/10.1007/s11103-009-9514-x

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