S25 - Session P3 - Citronellol biosynthesis in Pelargonium species: a new enzyme from the P5BR subfamily controls the biosynthetic pathway and the ratio of (S) and (R) citronellol isomers

Authors: Sylvie Baudino, Laure Martinelli *, Aurélie Rius-Bony, Bernard Blerot, Giorgiana Chiereta, Jean-Louis Magnard, Frédéric Jullien, Denis Saint-Marcoux

Pelargonium genus ( Geraniaceae ) is an example of evolutionary radiation with more than 280 species indigenous of South Africa. Species are characterized by a large range of variations in leaf and floral morphology depending on abiotic environment 1 . More than 30 botanical species exhibit a remarkable scent diversity. Rose-scented hybrids of pelargonium are cultivated around the world mainly for the production of essential oils (EOs) and absolutes 2 . EO is mainly composed of geraniol, citronellol, and several derivative esters. EO quality largely depends on the geraniol/citronellol ratio 3 . As such, a better understanding of citronellol biosynthesis 4 is essential. To fully characterize the citronellol biosynthetic pathway in pelargonium, transcriptomic (RNA-Seq) and metabolomic analyses (GC-MS) were performed on 10 accessions presenting different patterns of aromatic compounds. A previous study in a scented orchid 5 led us to precisely examined correlation of candidate genes expression with the metabolomic profiles of the 10 accessions. As such, two citral reductases (CIR) belonging to a new P5 B R subfamily were characterized together with candidate alcohol dehydrogenases (ADH). The two CIRs catalyzed the reduction of citral to citronellal, but with different ratios of (S)-(-) and (R)-(+) enantiomers. One ADH both oxidized geraniol to geranial and reduced citronellal to citronellol. Transient gene expression and enzymatic activities of the corresponding proteins demonstrated that, in pelargonium, citronellol biosynthesis is a multi-step pathway with citral and citronellal as intermediate compounds. (S)-(-) citronellol exhibits a finer rose odor compared to the (R)-(+) enantiomer. Thus, understanding factors promoting the biosynthesis of (S)-(-) citronellol is of importance for pelargonium EO quality and perfume industry. Our results pave the way to investigate enzymatic variations and structural mechanisms that drive the enantiomeric catalysis of citronellol. 1. Bakker et al. (2005) . In Plant species-level systematics: new perspectives on pattern & process (No. 143, pp. 75-100). Koeltz scientific books. 2. Blerot et al. (2016). Phytochem. Rev. , 15 (5), 935-960. 3. Juliani et al. (2006). J. Essential Oil Res. , 18 . 4. Bergman et al. (2020). J. Exp. Bot. 71, 258n271. 5. Xu, et al. (2017) . Curr. Biol. 27, 1867-1877.e5.