Valentine’s Volatiles
Plenty of flowers are beautiful and expensive, but the lovely rose scent makes roses the perfect traditional gift to your Valentine. And since it is Valentine’s Day today, and this is a plant science blog, here’s a brief review of the science of floral scents and a recently published paper on the topic (roses not included).
Despite rose breeders managing to come up with flowers with stronger, subtle, or new scents from new rose varieties, the science of floral scents is not well understood. Floral scent can be under stronger natural phenotypic selection than flowers (Parachnowitsch et al. 2012; New Phyt. 195:667), but the agents of selection may be any number of organisms including pollinators and herbivores and the main influencing factor on scent evolution is not known (Theis and Adler 2012; Ecology 93:430).
The molecular mechanism and regulation of biosynthesis of the volatile, low-molecular weight compounds that cause floral scent is also fairly uncharacterised. They are mainly products of the terpenoid, fatty acid, and phenylpropanoid pathways. Recently a group from the Hebrew University of Jerusalem characterised a regulation mechanism of the phenylpropanoid volatile biosynthesis pathway (Spitzer-Rimon et al. 2012; Plant Cell 24:5089).
All phenylpropanoids share the same precursor, Phe, which is biosynthesized via the shikimate pathway. A transcriptional regulator ODORANT1 (ODO1) regulates shikimate pathway enzymes and affects metabolic flow toward phenylpropanoid production. Another transcriptional regulator, EMISSION OF BENZENOIDS II (EOBII) directly regulates ODO1’s expression, indirectly affecting the shikimate pathway and the biosynthesis of phenylpropanoid volatiles. (Van Moerkercke et al. 2001, Plant J 67:917; Verdonk et al. 2005, Plant Cell 17:1612).
Working on petunia, Spitzer-Rimon et al. identified EOBI, another regulator of floral scent. It is an R2R3-MYB-like regulatory factor (for information on R2R3-MYB factors, see Stracke et al. 2001; Curr Op Plant Biol 4:447), which acts downstream of ODO1 and is activated by EOBII. EOBI was localised exclusively to floral tissues. Silencing EOB1 down-regulates both ODO1 and genes from the shikimate and phenylpropanoid volatile biosynthesis pathways. Conversely, when ODO1 is silenced, EOBI transcripts accumulate.
Using a yeast-one-hybrid system, the authors showed that EOBI directly interacts with the promotors of volatile phenylpropanoid synthesis genes. The authors found no evidence of direct interaction between EOBI and ODO1, despite the effect EOBI has on ODO1 expression. Changing EOBI levels had no effect on EOBII.
Volatiles are only a subset of phenylpropanoid products. The phenylpropanoid pathway is physically fairly well characterised, but the regulation of carbon flux into the biosynthetsis of phenylpropanoid-based pigments, structural molecules, defence compounds, and volatiles is likely to be researched for some time. The identification of EOBI as a floral scent regulator is important, but as the authors make clear in the simple, incomplete model for regulation of volatile phenylpropanoid biosynthesis they present, it is just the beginning of understanding floral scent production.
Image credit: Bev Loyd-Roberts, via stock.xchng
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