Distinct metabolic pathways drive monoterpenoid biosynthesis in a natural population of Pelargonium graveolens

Abstract

Pelargonium graveolens is a wild predecessor to rose-scented geranium hybrids prized for their essential oils used as fragrances and flavorings. However, little is known about their biosynthesis. Here we present metabolic evidence that at least two distinct monoterpene biosynthetic pathways contribute to their volatile profiles, namely, cyclic p-menthanes such as (−)-isomenthone and acyclic monoterpene alcohols such as geraniol and (−)-citronellol and their derivatives (referred to here as citronelloid monoterpenes). We established their common origin via the 2C-methyl-d-erythritol-4-phosphate pathway but found no indication these pathways share common intermediates beyond geranyl diphosphate. Untargeted volatile profiling of 22 seed-grown P. graveolens lines demonstrated distinct chemotypes that preferentially accumulate (−)-isomenthone, geraniol, or (−)-citronellol along with approximately 85 minor volatile products. Whole plant 13CO2 isotopic labeling performed under physiological conditions permitted us to measure the in vivo rates of monoterpenoid accumulation in these lines and quantify differences in metabolic modes between chemotypes. We further determined that p-menthane monoterpenoids in Pelargonium are likely synthesized from (+)-limonene via (+)-piperitone rather than (+)-pulegone. Exploitation of this natural population enabled a detailed dissection of the relative rates of competing p-menthane and citronelloid pathways in this species, providing real time rates of monoterpene accumulation in glandular trichomes.

Cytosolic geraniol and citronellol biosynthesis require a Nudix hydrolase in rose‐scented geranium (Pelargonium graveolens)

“…The metabolic specialization of chemotypes described in Bergman et al. (2020) is consistent with the compartmental separation of p ‐menthane and acyclic monoterpene alcohol biosynthesis proposed here.…”

“…GDP is apparently restricted to the plastid in the latter where it serves as a precursor to p-menthane monoterpenes and is not subject to PgNdx1-mediated hydrolysis. The metabolic specialization of chemotypes described in Bergman et al (2020) is consistent with the compartmental separation of p-menthane and acyclic monoterpene alcohol biosynthesis proposed here. The concentration of GDP and the observed rate of geraniol biosynthesis in the 'G' chemotype also argue against a plastid-localized GES as the sole source of geraniol in this species.…”

“…This paired analysis (EO analysis by GC‐MS and prenyl phosphate analysis by UPLC‐MS/MS) was repeated for the ‘C’, ‘G’ and ‘I’ chemotypes described previously (Bergman et al. , 2020). We observed the highest concentrations of GP in the ‘G’ chemotype (Figure 5c) (0.67 ± 0.16 µg g −1 DW), present at a ratio of 2.7:1 over GDP by mass (Figure 6).…”

“…Plants were cultivated as described in Bergman et al (2020). Briefly, wild-type P. graveolens or cultivars were maintained in BX RNA sequencing, transcriptome assembly, and molecular cloning…”

Transcriptome-Based WGCNA Analysis Reveals Regulated Metabolite Fluxes between Floral Color and Scent in Narcissus tazetta Flower

“…Floral scent is a combination characteristic determined by a complex mixture of volatile molecules, which can be classified by their biosynthetic origin into terpenes, phenylpropanoids and fatty acid derivatives [ 8 , 9 ]. Numerous structural genes responsible for the formation of volatile compounds have been identified [ 10 , 11 ], including genes responsible for the formation of phenylpropanoid and benzenoid compounds [ 12 , 13 , 14 , 15 ] and for the formation of monoterpenes and carotenoids/chlorophyll compounds [ 16 , 17 , 18 , 19 , 20 ]. To date, knowledge about the regulation of floral fragrance biosynthesis is limited, and only a few transcription factors, ODORANT1, EOBII, and PhMYB4, have been shown to be involved in the regulation of volatile biosynthesis in flowers [ 10 , 11 , 21 , 22 , 23 ].…”