Targeting the “bitterness gene” by genome editing abolishes synthesis of bitter flavanones in citrus; prospects for new varieties and extended climates for cultivation

Bitterness in citrus fruit is conferred by flavanone-neohesperidosides, whose accumulation is catalyzed by a single enzyme flavanone-7-O-glucosides-1,2-rhamnosyltransferase (1,2RhaT), expressed in both leaves and fruit. To eliminate citrus bitterness, we used CRISPR/Cas9 genome editing to inactivate the 1,2RhaT gene in grapefruit (Citrus paradisi) and "Carrizo" citrange (Citrus sinensis × Citrus trifoliata). Edited lines displayed frameshift mutations that introduced premature stop codons, effectively abolishing the synthesis of the bitter neohesperidosides naringin, neohesperidin, and poncirin. Metabolomic analyses in leaves from 1,2RhaT-mutant lines confirmed the absence of bitter flavanone-neohesperidosides and a compensatory increase in the tasteless flavanone-rutinosides hesperidin, didymin, and narirutin. Since 1,2RhaT is encoded by a single gene, our findings in leaves are expected to be identical for fruit and thus demonstrate a strategy for developing non-bitter citrus cultivars while retaining health-benefitting flavonoid levels. Furthermore, cold-hardy citrus species that are currently unacceptably bitter due to high flavanone-neohesperidoside levels may become useful sources for introduction of cold-hardiness following inactivation of the 1,2RhaT gene. This approach thus paves the way for expanding grapefruit markets and breeding cold-hardy, palatable citrus varieties that are better suited to a wider range of climates.