Insights into the Uptake, Distribution, and Metabolism of 3-Isobutyl-2-hydroxypyrazine in Grapevine Using a Stable Isotope Tracer

Methoxypyrazines (MPs) are potent aroma compounds that have been predominately studied in grape berries but can also be detected in other vine tissues. The synthesis of MPs in berries from hydroxypyrazines by VvOMT3 is well established, but the origin of MPs in vine tissues that have negligible VvOMT3 gene expression is unknown. This research gap was addressed through the application of stable isotope tracer 3-isobutyl-2-hydroxy-[²H₂]-pyrazine (d₂-IBHP) to the roots of Pinot Meunier L1 microvines and high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) quantification of HPs from grapevine tissues following a novel solid-phase extraction method. Four weeks post-application, d₂-IBHP and its O-methylated product 3-isobutyl-2-methoxy-[²H₂]-pyrazine (d₂-IBMP) were present in excised cane, berry, leaf, root, and rachis material. Translocation of d₂-IBHP and d₂-IBMP was investigated, but results were inconclusive. Nonetheless, knowledge that d₂-IBHP, and potentially d₂-IBMP, are translocated from roots to other vine organs, including the berries, could provide opportunities for controlling MP accumulation in grapevine tissues pertinent to winemaking.

l-Serine is the Direct Precursor for the Pyrazine Ring Construction in the Biosynthesis of 3-Isobutyl-2-Methoxypyrazine in Bell Pepper Fruits (Capsicum annuum L.)

3-Isobutyl-2-methoxypyrazine (IBMP) is an extremely potent odorant and responsible for the specific aroma of many fruits and vegetables. Especially bell pepper contains high levels of IBMP, which is the character impact compound of its typical aroma. However, since the discovery of methoxypyrazines in plants in the 1960s the biosynthesis of their pyrazine ring motif remained so far unknown. Therefore, the biosynthetic pathway to IBMP was investigated by feeding experiments with stable-isotope labeled precursors. For the first time it could be shown that l-serine plays a key role in the pyrazine ring construction of 3-alkyl-2-methoxypyrazines (MPs). Based on HS-SPME-GCxGC-TOF-MS analysis, it is shown that the biosynthetic pathway to IBMP is closely linked to photorespiratory derived l-serine.

Laboratory Evidence of 2-Isobutyl-3-methoxypyrazine as a Male-Released Aggregative Cue in Labidostomis lusitanica (Germar) (Coleoptera: Chrysomelidae)

In spite of its incidence on pistachio trees, the chemical ecology of Labidostomis lusitanica (Germar) (Coleoptera: Chrysomelidae) has been neglected so far. In this work, we provide the first evidence of a biologically active male-specific compound that may be promoting field aggregation. Headspace collections through solid-phase microextraction from feral males and females reported the presence of 2-isobutyl-3-methoxypyrazine exclusively in males. Electroantennographic recordings revealed that males and females responded in a dose-dependent manner to increasing stimuli of 2-isobutyl-3-methoxypyrazine, with females overall displaying a higher response than males. In dual-choice tests, both males and females showed a significant preference for the compound in comparison to a pure air stimulus. In light of these results, the possible role of 2-isobutyl-3-methoxypyrazine as an aggregation cue in L. lusitanica is discussed.

Investigation of Biosynthetic Precursors of 3-Isobutyl-2-Methoxypyrazine Using Stable Isotope Labeling Studies in Bell Pepper Fruits (Capsicum annuum L.)

The biosynthesis of 3-isobutyl-2-methoxypyrazine (IBMP) in bell pepper fruits (Capsicum annuum L.) was investigated by in vivo feeding experiments with stable isotope-labeled precursors. Volatiles were extracted using headspace solid-phase microextraction (HS-SPME) and analyzed by comprehensive two-dimensional gas chromatography (GC×GC) coupled to a time-of-flight mass spectrometer (ToF-MS). Feeding experiments revealed incorporation of l-leucine and α-ketoisocaproic acid (α-KIC) as well as glycine and glyoxylic acid into IBMP. Furthermore, it has been shown that de novo biosynthesis of IBMP occurs in pericarp tissues of unripe bell pepper fruits, whereas pericarp tissues of ripe bell pepper fruits showed no capability of IBMP biosynthesis.

Rootstock, Vine Vigor, and Light Mediate Methoxypyrazine Concentrations in the Grape Bunch Rachis of Vitis vinifera L. cv. Cabernet Sauvignon

Ramsey rootstock has previously been implicated in an approximate 8-fold increase of 3-isobutyl-2-methoxypyrazine (IBMP) levels in the rachis (grape bunch stem) of Vitis vinifera L. cv. Shiraz scions over own-rooted Shiraz vines at harvest. IBMP extracted from rachis during red wine fermentation can contribute potent "green" flavors. Methoxypyrazines (MPs) are normally present in Cabernet Sauvignon grapes, rachis, and wines, but it is unknown whether rootstocks can influence the MP concentration in the rachis. This study considered the effect of eight rootstocks including Ramsey and own roots on the concentrations of IBMP, 3-isopropyl-2-methoxypyrazine (IPMP), and 3-sec-butyl-2-methoxypyrazine (SBMP) in the rachis and grapes of Cabernet Sauvignon over two seasons. IBMP predominated, and its concentration in rachis and berries at harvest was significantly affected by rootstock and growing season. In the 2020 vintage, light exclusion, vine vigor, and spatial variation in vine vigor were shown to significantly affect MP concentrations in rachis.

Behavior of 3-isobutyl-2-hydroxypyrazine (IBHP), a key intermediate in 3-isobutyl-2-methoxypyrazine (IBMP) metabolism, in ripening wine grapes

3-Isobutyl-2-hydroxypyrazine (IBHP) is thought to be a key intermediate in both the biosynthesis and degradation of the herbaceous smelling 3-isobutyl-2-methoxypyrazine (IBMP), but its behavior during the growing season is not well understood. First, an improved method for IBHP quantification was developed. A deuterated IBHP standard was added to samples prior to isolation by mixed-mode cation exchange solid phase extraction. Extracts were silylated prior to quantification by GC-MS. A limit of detection of ca. 20 ng/L could be achieved for a 100 mL juice sample. This method was used to quantify IBHP during the 2010 growing season in berries of two clones of Cabernet franc in the Finger Lakes region of New York and of Merlot grown in the California Central Valley. For all three sources, IBHP was detectable at the earliest sampling point, and its concentration per berry increased to a maximum around veraison, 208-477 pg/berry. On a per berry basis, IBHP peaked and began to decline 1-2 weeks after IBMP, indicating that previous studies that sampled preveraison fruit have missed the true maximum value of IBHP. The highest per berry concentration of IBHP observed was in the California Merlot. However, after veraison, IBHP declined more rapidly in the California Merlot than in the New York Cabernet franc, such that the Merlot had the lowest IBHP concentration at harvest. Thus, IBHP at harvest cannot be used as a proxy for IBMP at veraison as was previously suggested.

Correlation of 3-isobutyl-2-methoxypyrazine to 3-isobutyl-2-hydroxypyrazine during maturation of bell pepper (Capsicum annuum) and wine grapes (Vitis vinifera)

Environmental factors affecting degradation of 3-isobutyl-2-methoxypyrazine (IBMP, "green pepper aroma") in wine grapes (V. vinifera) are widely studied, but the degradation pathway is not defined. We hypothesized that IBMP is demethylated to 3-isobutyl-2-hydroxypyrazine (IBHP) during fruit maturation effectively reversing the final putative step of IBMP biosynthesis. A quantification method for IBHP was developed using solid-phase extraction coupled to one- or two-dimensional gas chromatography-mass spectrometry with a recovery of ca. 80%. IBMP and IBHP in bell peppers (Capsicum annuum) and V. vinifera (cv. 'Cabernet Franc', 'Riesling', 'Pinot noir') were then measured at different maturities. IBMP and IBHP were inversely correlated in both bell peppers (R2=0.958) and Cabernet Franc grapes (R2=0.998) over a range of maturities. In bell peppers, we observed a significant decline in IBMP (125 to 15 ng/mL) and increase in IBHP (undetectable to 42 ng/mL) during ripening. In grapes, all cultivars had comparable IBHP concentrations preveraison (64 to 88 pg/mL) but differed in IBHP concentration by 2 orders of magnitude at the final sampling point (undetectable to 235 pg/mL). Higher preveraison IBMP was correlated with higher final IBHP across the three grape cultivars, with the order Cabernet Franc>Riesling>Pinot noir for both IBMP and IBHP. Acid hydrolysis resulted in a significant increase (33%) in IBHP in Cabernet Franc, indicating that IBHP exists partially in a bound form in grapes.

Two O-methyltransferases involved in the biosynthesis of methoxypyrazines: grape-derived aroma compounds important to wine flavour

Methoxypyrazines (MPs) are volatile, grape-derived aroma compounds that contribute to the distinct herbaceous characters of some wines. Although the full pathway leading to MP production has not been elucidated, there is strong evidence that the final step involves the methylation of non-volatile hydroxypyrazine (HP) precursors. Two cDNA encoding O-methyltransferases (OMTs) that have homology to an enzyme previously purified and shown to catalyse the methylation of HPs were isolated from Cabernet Sauvignon. Recombinant protein from the cDNAs (VvOMT1 and VvOMT2) was produced in E. coli and activity assays demonstrated that both encode OMTs able to methylate HPs to produce MPs, however both showed greatest activity against the flavonol quercetin. VvOMT1 has higher catalytic activity against isobutyl hydroxypyrazine compared to isopropyl hydroxypyrazine, whereas the converse is true for VvOMT2. The timing of the expression of VvOMT1 in the skin and the flesh of developing Cabernet Sauvignon grape berries was associated with the period of MP accumulation in these tissues, while VvOMT2 expression was greatest in roots, which were found to contain high levels of MPs. The MP composition of these tissues also reflects the relative levels of expression of these genes and their substrate preference. The identification of genes responsible for MP production in grapevine will help in understanding the effect of different viticultural and environmental factors on MP accumulation.