Zero-Bitter Brassica: TGG1/TGG2 Myrosinase Double Knockout Eliminates Isothiocyanate-Mediated Bitterness While Preserving Glucosinolate-Derived Health Benefits

Zero-Bitter Brassica: TGG1/TGG2 Myrosinase Double Knockout Eliminates Isothiocyanate-Mediated Bitterness While Preserving Glucosinolate-Derived Health Benefits Priyanca Iyengar Ford | Scoville Splice LLC | 2026-11-01 | ORCID: 0000-0003-0395-1752 OVERVIEW Brassica vegetables—broccoli, kale, Brussels sprouts, cauliflower—are among the most nutritionally dense crops consumed globally, yet widespread consumer rejection driven by bitter and pungent off-notes remains a substantial barrier to intake. This paper presents a precision CRISPR-based strategy to eliminate that bitterness at its enzymatic source, without altering the glucosinolate content that underlies the health benefits of these vegetables. A critical mechanistic distinction guides this work: glucosinolates themselves are not bitter. Bitterness in Brassica arises from isothiocyanates (ITCs), the hydrolysis products generated when the enzyme myrosinase (THIOGLUCOSIDE GLUCOHYDROLASE 1 and 2; TGG1/TGG2) cleaves intact glucosinolates upon tissue damage. ITCs—principally sinigrin-derived allyl-ITC and glucoraphanin-derived sulforaphane-ITC—activate the TAS2R38 bitter taste receptor, producing the characteristic pungent bitterness that deters vegetable consumption, particularly in children and bitter-sensitive adults (those with PAV haplotype of TAS2R38). By knocking out TGG1 and TGG2, ITCs are never formed; glucosinolates remain intact and accumulate at WT levels. MOLECULAR STRATEGY The TGG1/TGG2 double knockout targets the catalytic nucleophile glutamate (Glu167 equivalent) shared across both myrosinase homeologs. Brassica oleracea is an allotetraploid (AABB genome, ~630 Mb, TO1000 reference assembly), requiring simultaneous editing of four homeologs: BraTGG1a and BraTGG1b (B. rapa-derived A subgenome) and BolTGG2a and BolTGG2b (B. oleracea-derived B subgenome). DNA-free ribonucleoprotein (RNP) delivery via PEG-mediated protoplast transformation is employed to avoid stable transgene integration, opening a potential non-GMO regulatory pathway under both EU and USDA APHIS frameworks. Four guide RNAs were designed targeting conserved catalytic regions across all four homeologs, with Doench 2016 efficiency scores ≥0.60 and CFD off-target scores ≥0.85. Expected edit types are frameshift-inducing small indels at the catalytic exon, confirmed by Sanger sequencing and myrosinase activity assay (4-methylumbelliferyl glucosinolate substrate, fluorometric). Multiplexed delivery via single transformation event is modeled on prior quadruple homeolog edits in allopolyploid wheat (Zhang et al. 2016, Nat Plants 2:16112). Precedent for TGG1/TGG2 double knockout phenotype was established in Arabidopsis thaliana by Barth & Jander (2006, Plant J 47:771–783), demonstrating complete elimination of myrosinase activity and ITC production with no visible growth or reproductive penalty. Translation to Brassica oleracea is supported by the high conservation of TGG1/TGG2 catalytic domains (87–93% identity) and the established role of cell-type-specific myrosin cells in ITC-mediated defense (Husebye et al. 2002, Plant Physiol 128:1180–1188). IFI-B PROJECTIONS Bitterness reduction is quantified using the Iyengar Flavor Index – Bitter subscale (IFI-B), which normalizes bitter receptor (TAS2R38) activation against the 1 µM quinine reference (IFI-B = 1,000). The reference and methodology for all IFI subscales are defined in Paper 4 (IFI Methodology; DOI: 10.5281/zenodo.PAPER04) and should be read as a prerequisite for interpreting these values. Projected IFI-B values across three major Brassica crops:- Broccoli: WT IFI-B ~720 → TGG KO IFI-B ~32 (ITC: 180 → 8 µg/g FW)- Kale: WT IFI-B ~960 → TGG KO IFI-B ~42 (ITC: 240 → 10 µg/g FW)- Brussels sprouts: WT IFI-B ~1280 → TGG KO IFI-B ~48 (ITC: 320 → 12 µg/g FW) Residual IFI-B in the KO genotypes reflects phenolic bitter compounds (flavonoids, hydroxycinnamic acids) that are not ITC-mediated and not targeted by this edit. GLUCOSINOLATE PRESERVATION AND HEALTH BENEFIT AMPLIFICATION An unexpected benefit of the TGG KO strategy is enhanced sulforaphane bioavailability. In WT plants, myrosinase rapidly converts glucoraphanin to sulforaphane immediately upon chewing, but gastric acid and rapid transit limit absorption. In TGG KO plants, intact glucoraphanin passes to the lower gut where colonic microbiota (Bacteroidetes, Firmicutes expressing bacterial myrosinase-like enzymes) convert it more slowly and completely. Published pharmacokinetic comparisons of myrosinase-intact vs. myrosinase-knockout delivery routes (Vermeulen et al. 2008, Mol Nutr Food Res 52:827–836) support a +53% increase in urinary sulforaphane metabolite recovery in the KO delivery model. Glucosinolate content is unchanged; the health benefit is preserved and the delivery route is actually improved. Note: Capsicum bitterness is entirely mechanistically distinct. Capsicum does not produce glucosinolates or myrosinase; Capsicum bitterness (where present at very high SHU) is phenolic-mediated via CaMYB31/CaF3H pathway activity. The TGG strategy does not apply to Capsicum. CAPSICUM NOTE — FULL SPECTRUM PLATFORM This paper is one component of the Capsaicin Design Platform's full-spectrum flavor engineering series, which addresses heat (Papers 1–2; DOI: 10.5281/zenodo.23743695, 10.5281/zenodo.23749878), sweetness (Paper 3), bitterness across multiple crop species (this paper, Paper 8), umami (Paper 5), kokumi (Paper 7), cooling (Paper 9), and TRPA1-mediated pungency (Paper 10). All papers are interconnected via the Iyengar Flavor Index framework (Paper 4, IFI Methodology). INTELLECTUAL PROPERTY NOTE Syngenta, Corteva, and BASF hold extensive IP covering glucosinolate pathway engineering in Brassica (glucosinolate content modification, myrosinase-associated proteins, sinigrin/glucoraphanin conversion). This paper is published as prior art to establish independent and contemporaneous development of the TGG1/TGG2 myrosinase knockout strategy for ITC elimination. Freedom-to-operate (FTO) analysis with respect to competitor IP estates is ongoing. The DNA-free RNP edit strategy may navigate certain transgenic IP claims. No provisional patent has been filed for this work at time of preprint submission. LIMITATIONS This work presents computational design and FBA-informed projections. Experimental validation in Brassica oleracea protoplasts and regenerated plants is required to confirm: (1) successful quadruple homeolog editing efficiency; (2) complete ITC elimination vs. partial myrosinase knockdown; (3) absence of growth, yield, or defense penalties; (4) microbiome-mediated sulforaphane recovery in animal feeding trials. Consumer acceptance studies (sensory panels, preference testing in PAV vs. AVI TAS2R38 haplotype subjects) are a critical next step. KEYWORDS Brassica oleracea, myrosinase, TGG1, TGG2, glucosinolate, isothiocyanate, IFI-B, TAS2R38, sulforaphane, CRISPR, DNA-free editing, RNP, allotetraploid, non-GMO, broccoli, kale, Brussels sprouts, Iyengar Flavor Index, bitter taste, flavor engineering, allyl isothiocyanate, glucoraphanin RELATED RECORDS - Paper 1 (Scoville Splice main paper): DOI 10.5281/zenodo.23743695- Paper 2 (Beyond Peppers — cross-species): DOI 10.5281/zenodo.23749878- Paper 4 (IFI Methodology — defines measurement standard): DOI 10.5281/zenodo.PAPER04- Paper 5 (Umami Engineering — same series): DOI 10.5281/zenodo.PAPER05- Paper 7 (Kokumi Engineering — CaSR synergy): DOI 10.5281/zenodo.PAPER07- Paper 8 (Astringency Reduction — complementary bitterness reduction): DOI 10.5281/zenodo.PAPER08