What Do We Know about the Chemistry of Strawberry Aroma?

The strawberry, with its unique aroma, is one of the most popular fruits worldwide. The demand for specific knowledge of metabolism in strawberries is increasing. This knowledge is applicable for genetic studies, plant breeding, resistance research, nutritional science, and the processing industry. The molecular basis of strawberry aroma has been studied for more than 80 years. Thus far, hundreds of volatile organic compounds (VOC) have been identified. The qualitative composition of the strawberry volatilome remains controversial though considerable progress has been made during the past several decades. Between 1997 and 2016, 25 significant analytical studies were published. Qualitative VOC data were harmonized and digitized. In total, 979 VOC were identified, 590 of which were found since 1997. However, 659 VOC (67%) were only listed once (single entries). Interestingly, none of the identified compounds were consistently reported in all of the studies analyzed. The present need of data exchange between “omic” technologies requires high quality and robust metabolic data. Such data are unavailable for the strawberry volatilome thus far. This review discusses the divergence of published data regarding both the biological material and the analytical methods. The VOC extraction method is an essential step that restricts interlaboratory comparability. Finally, standardization of sample preparation and data documentation are suggested to improve consistency for VOC quantification and measurement

Functional activity, substrate specificity and influence of mutations in the N-terminus on the functional activity of DFR from <i>Angelonia</i> × <i>angustifolia</i>, varying positions between the two DFR types are marked in bold.

<p>Functional activity, substrate specificity and influence of mutations in the N-terminus on the functional activity of DFR from <i>Angelonia</i> × <i>angustifolia</i>, varying positions between the two DFR types are marked in bold.</p

Dependence of product formation through sample handling after stopping the enzyme reactions.

<p>The substrates were converted with recombinant dahlia DFR (Accession FJ216425). Substrates and products were separated on cellulose in chloroform/acetic acid/water (10∶9∶1, v:v:v). n.d: not detected.</p><p>*total amount of radioactivity was very low.</p><p>Dependence of product formation through sample handling after stopping the enzyme reactions.</p

Phylogenetic tree of amino acid sequences of DFRs from different plant species.

<p>The following sequences were used (Accession numbers in parentheses): <i>Angelonia</i> × <i>angustifolia</i> Ang.DFR2 from the present study (KF285561), <i>Antirrhinum majus</i> (X15536), <i>Perilla frutescens</i> (AB002817), <i>Forsythia</i> × <i>intermedia</i> (Y09127), <i>Solenostemon scutellarioides</i> (EF522155), <i>Sinningia cardinalis</i> (AY332536), <i>Nierembergia sp.</i> (AB078510), <i>Torenia hybrida</i> (AB012924), <i>Camellia sinensis</i> (AB018686), <i>Petunia</i> × <i>hybrida</i> (EU189078), <i>Nicotiana alata</i> (FJ969389), <i>Iochroma cyaneum</i> (GU595064), <i>Solanum pinnatisectum</i> (AY954035), <i>Lycium ruthenicum</i> (JN849097), <i>Lobelia erinus</i> (AB221076), <i>Saussurea medusa</i> (EF600682), <i>Merremia dissecta</i> (EU189077), <i>Ipomoea purpurea</i> (AF028601), <i>Rhododendron simsii</i> (AJ413278), <i>Centaurea maculosa</i> (FJ376591), <i>C. chinensis</i> (Z67981), <i>Dahlia variabilis</i> (FJ216425), <i>Chrysanthemum</i> × <i>morifolium cultivar Shenyun</i> (JF346164), <i>Helianthus annuus</i> (EU095849), <i>Cyclamen graecum</i> (AB517921), <i>Gerbera hybrid</i> cv. ‘Terra Regina’ (Z17221).</p

Characterization of recombinant DFR from <i>Angelonia</i> × <i>angustifolia</i> obtained from heterologous expression in yeast (left) and <i>E. coli</i> (right).

1<p>DHQ as a substrate,</p>2<p>DHM as a substrate.</p><p>Characterization of recombinant DFR from <i>Angelonia</i> × <i>angustifolia</i> obtained from heterologous expression in yeast (left) and <i>E. coli</i> (right).</p

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Alignment of the DFRs isolated from petals of <i>Angelonia</i> × <i>angustifolia</i>.

<p>Alignment of the DFRs isolated from petals of <i>Angelonia</i> × <i>angustifolia</i>.</p

DataSheet1.docx

<p>A recall campaign for commercial, orange flowering petunia varieties in spring 2017 caused economic losses worldwide. The orange varieties were identified as undeclared genetically engineered (GE)-plants, harboring a maize dihydroflavonol 4-reductase (DFR, A₁), which was used in former scientific transgenic breeding attempts to enable formation of orange pelargonidin derivatives from the precursor dihydrokaempferol (DHK) in petunia. How and when the A₁ cDNA entered the commercial breeding process is unclear. We provide an in-depth analysis of three orange petunia varieties, released by breeders from three countries, with respect to their transgenic construct, transcriptomes, anthocyanin composition, and flavonoid metabolism at the level of selected enzymes and genes. The two possible sources of the A₁ cDNA in the undeclared GE-petunia can be discriminated by PCR. A special version of the A₁ gene, the A₁ type 2 allele, is present, which includes, at the 3′-end, an additional 144 bp segment from the non-viral transposable Cin4-1 sequence, which does not add any functional advantage with respect to DFR activity. This unequivocally points at the first scientific GE-petunia from the 1980s as the A₁ source, which is further underpinned e.g., by the presence of specific restriction sites, parts of the untranslated sequences, and the same arrangement of the building blocks of the transformation plasmid used. Surprisingly, however, the GE-petunia cannot be distinguished from native red and blue varieties by their ability to convert DHK in common in vitro enzyme assays, as DHK is an inadequate substrate for both the petunia and maize DFR. Recombinant maize DFR underpins the low DHK acceptance, and, thus, the strikingly limited suitability of the A₁ protein for a transgenic approach for breeding pelargonidin-based flower color. The effect of single amino acid mutations on the substrate specificity of DFRs is demonstrated. Expression of the A₁ gene is generally lower than the petunia DFR expression despite being under the control of the strong, constitutive p35S promoter. We show that a rare constellation in flavonoid metabolism—absence or strongly reduced activity of both flavonol synthase and B-ring hydroxylating enzymes—allows pelargonidin formation in the presence of DFRs with poor DHK acceptance.</p