Cas9/sgRNA-mediated genome editing of citrus via mature tissue transformation enables both high-efficacy genome editing and early flowering

CRISPR genome editing has shown tremendous potential in genetic improvement of citrus. So far, citrus genome editing has been conducted using juvenile tissues resulting in genome-edited citrus plants that require multiple years before they can produce flowers and fruit. Here we tested whether citrus genome editing via mature tissue transformation can overcome such a hurdle. CsLOB1 is a susceptibility gene for citrus canker caused by Xanthomonas citri subsp. citri (Xcc). The transcription activator-like effector PthA4 of Xcc activates CsLOB1 by binding to the effector-binding element in its promoter (EBEpthA4-CsLOBP). In Valencia sweet orange, two CsLOB1 promoter alleles are present: TI CsLOBP, and TII CsLOBP. We specifically utilized a CRISPR/Cas9 construct (GFP-p1380N-Cas9/sgRNA:CsLOBP2) targeting EBEpthA4 in TI CsLOBP but not TII CsLOBP to test genome editing efficacy and off-target mutations. GFP-p1380N-Cas9/sgRNA:CsLOBP2 function was first validated using Xcc-facilitated agroinfiltration in Valencia leaves. The construct was subsequently introduced into Valencia mature internodal stem segments via Agrobacterium-mediated transformation, generating three independent transgenic lines (#V2, #V3 and #V5). Targeted mutations in EBEpthA4-TI CsLOBP were detected in all three lines with mutation frequencies of 100%, 21.43% and 41.94% in #V2, #V3 and #V5, respectively, while no mutations were detected in TII CsLOBP. Infection with Xcc{Delta}pthA4:dCsLOB1.3, carrying a designer TALE that specifically activates TI CsLOBP, resulted in reduced canker symptoms in #V2. Importantly, all three EBEpthA4-TI CsLOBP edited lines flowered within 15 months. In sum, these results demonstrate that CRISPR/Cas9-mediated genome modification through mature citrus transformation can achieve high genome editing efficacy and overcome the juvenility.