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Published on May 8, 2008; 10.1104/pp.108.115824

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Received January 3, 2008
Accepted April 22, 2008

Biochemical and Genomic Characterization of Terpene Synthases in Magnolia grandiflora

Sungbeom Lee and Joseph Chappell *

Plant Physiology, Biochemistry and Molecular Biology Program, Department of Plant and Soil Sciences, University of Kentucky, Lexington KY 40546-0312

* Corresponding author; email: chappell{at}uky.edu.

Southern Magnolia (Magnolia grandiflora) is a primitive evergreen tree that has attracted attention because of its horticultural distinctiveness, the wealth of natural products associated with it, and its evolutionary position as a basal angiosperm. Three cDNAs corresponding to terpene synthase (TPS) genes expressed in young leaves were isolated and the corresponding enzymes were functionally characterized in vitro. Recombinant Mg25 converted FPP (C15) predominately to {beta}-cubebene, while Mg17 converted GPP (C5) to {alpha}-terpineol. Efforts to functionally characterize Mg11 were unsuccessful. Transcript levels for all 3 genes were prominent in young leaf tissue and significantly elevated for Mg25 and Mg11 mRNAs in stamens. A putative N-terminal signal peptide of Mg17 targeted the reporter GFP protein to both chloroplasts and mitochondria when transiently expressed in epidermal cells of Nicotiana tabacum leaves. Phylogenetic analyses indicated that Mg25 and Mg11 belonged to the angiosperm sesquiterpene synthase subclass TPS-a, while Mg17 aligned more closely to the angiosperm monoterpene synthase subclass TPS-b. Unexpectedly, intron-exon organizations for the 3 Magnolia TPS genes were different from one another and from other well characterized terpene synthase gene sets. The Mg17 gene consists of 6 introns arranged in a manner similar to many other angiosperm sesquiterpene synthase, but Mg11 contains only 4 introns, and Mg25 has only a single intron located near the 5' terminus of the gene. Our results suggest that the structural diversity observed in the Magnolia TPS genes could have occurred either by a rapid loss of introns from an common ancestor TPS gene, or by a gain of introns into an intron-deficienct progenote TPS gene.




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