July 8, 2015

Magnard et al. "Biosynthesis of monoterpene scent compounds in roses"

Jean-Louis Magnard1, Aymeric Roccia1,2, Jean-Claude Caissard1, Philippe Vergne2, Pulu Sun1, Romain Hecquet1, Annick Dubois2, Laurence Hibrand-Saint Oyant3, Frédéric Jullien1, Florence Nicolè1, Olivier Raymond2, Stéphanie Huguet4, Raymonde Baltenweck5, Sophie Meyer5, Patricia Claudel5, Julien Jeauffre3, Michel Rohmer6, Fabrice Foucher3, Philippe Hugueney5,*, Mohammed Bendahmane2,*, Sylvie Baudino1,*

1Laboratoire BVpam, EA3061, Université de Lyon/Saint-Etienne, 23 Rue du Dr Michelon, F-42000, Saint-Etienne, France 
2Laboratoire Reproduction et Développement des Plantes UMR Institut National de la Recherche Agronomique (INRA)–CNRS, Université Lyon 1-ENSL, Ecole Normale Supérieure de Lyon, 46 Allée d’Italie, 69364 Lyon Cedex 07, France. 
3INRA, Institut de Recherche en Horticulture et Semences (INRA, AGROCAMPUS-OUEST, Université d’Angers), SFR 4207 QUASAV, BP 60057, 49071 Beaucouzé Cedex, France
4Génomiques Fonctionnelles d’Arabidopsis, Unité de Recherche en Génomique Végétale, UMR INRA 1165–Université d’Evry Val d’Essonne–ERL CNRS 8196, Evry, France
5INRA, Université de Strasbourg, UMR 1131 Santé de la Vigne et Qualité du Vin, 28 Rue de Herrlisheim, F-68000 Colmar, France
 6Université de Strasbourg–CNRS, UMR 7177, Institut Le Bel, 4 Rue Blaise Pascal, 67070 Strasbourg Cedex, France

This outstanding report by Magnard, et al addresses the origin of monoterpenoid scent compounds in rose petals. Roses breed for sale as cut flowers are arguably the world's most important ornamental plants, and descriptions of their scents has inspired poets and artists for centuries or longer. However, the aggressive breeding which has improved the color and longevity of cut flowers has also occasionally resulted in hybrids which lack the same aromatic bouquets of more classic cultivars such as Papa Mailland. Magnard and co-workers exploited these induced genetic differences by comparing high and low scent varieties to identify differentially expressed genes that correlated with the accumulation of monoterpene alcohols typical of high quality rose scents. Geraniol, for instance, is an important monoterpene alcohol in rose oil, and geraniol synthase has been identified in basil, cinnamon, and many other plant species.  Ordinarily, we would just look at rose EST databases and look for terpene synthases, which are now some of the best studied catalysts in  the field of natural products with well known signature motifs at the amino acid level.That should lead us to a likely geraniol synthase candidate in rose.

Here is the reaction geraniol synthase catalyzes:


 The thing is, there is apparently no geraniol synthase in rose. Instead, Magnard and co-workers identified a gene from the Nudix hydrolase family that correlated with geraniol production in petals by comparing many different rose varieties, some with and some without geraniol and other aroma compounds. They used a differential transcript analysis method called differential display, which is rare to see nowadays when  people generally use NGS to sequence everything into submission and let the bioinformatics core do the statistics.

Using differential display, the identified a protein likely involved in monoterpene biosynthesis in petals. The purified protein did not make geraniol though. Instead, it made geranyl monophosphate. Incubations with whole protein extracts showed that non-specific phosphatases (or some other unidentified protein) can finish the job. This means geraniol in rose is made a completely different way than monoterpene biosynthesis in other plants species. Their proposed pathway goes like this:

The authors rationalize this seemingly more complex way of making geraniol with the observation that the Km of RhNUDX1 (the innate affinity of an enzyme for its subtrate, geranyl diphosphate in this case) is orders of magnitude lower than the geraniol synthase from basil, meaning it has a far higher affinity for its substrate than any geraniol synthase ever studied. It could make good metabolic sense to break up a traditionally slow step catalyzed by classical terpene synthases (which are pretty slow enzymes) into a two fast steps.

What else is unusual about the biosynthesis of geraniol in rose petals is that it appears to take place in the cytosol, whereas most monoterpene synthase in plants occurs in the plastid using precursors from the 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway. RhNUDX1 is a cytosolic protein and does not have a transit peptide. This leaves open the question as to whether geraniol is made with isoprenoid equivalents exported from the plastid or from the cytosolic mevalonate pathway. 

This is a pretty big paper for the field of volatile terpenoid biochemistry as it opens up a new way for monoterpenoid biosynthesis to proceed without the participation of terpene synthases, classically the commited and ever present step in any terpenoid biosynthetic pathway. Making terpenes without terpene synthases opens up all sorts of possibilities for metabolic engineering of terpenoids. I am very curious to see how many other exceptions to well established rules of terpenoid biosynthesis will be discovered in the coming years, and I congratulate the corresponding authors Hugueney, Bendahmane, and Baudino for directing this outstanding contribution to terpene chemistry and of course congratulations to first author Jean-Louis Magnard.

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