Removal of a 10-kb Gret1 transposon from VvMybA1 of Vitis vinifera cv. Chardonnay

A New Approach for CRISPR/Cas9 Editing and Selection of Pathogen-Resistant Plant Cells of Wine Grape cv. 'Merlot'

Grape is one of the most economically significant berry crops. Owing to the biological characteristics of grapes, such as the long juvenile period (5-8 years), high degree of genome heterozygosity, and the frequent occurrence of inbreeding depression, homozygosity during crossbreeding leads to loss of varietal characteristics and viability. CRISPR/Cas editing has become the tool of choice for improving elite technical grape varieties. This study provides the first evidence of a decrease in the total fraction of phenolic compounds and an increase in the concentration of peroxide compounds in grape callus cells upon the addition of chitosan to the culture medium. These previously unreported metabolic features of the grape response to chitosan have been described and used for the first time to increase the probability of selecting plant cells with MLO7 knockout characterised by an oxidative burst in response to the presence of a pathogen modulated by chitosan in the high-metabolite black grape variety 'Merlot'. This was achieved by using a CRISPR/Cas9 editing vector construction with the peroxide sensor HyPer as a reporter. This research represents the first CRISPR/Cas9 editing of 'Merlot', one of the most economically important elite technical grape varieties.

Editing VvDXS1 for the creation of muscat flavour in Vitis vinifera cv. Scarlet Royal

Muscat flavour represents a group of unique aromatic attributes in some grape varieties. Biochemically, grape berries with muscat flavour produce high levels of monoterpenes. Monoterpene biosynthesis is mainly through the DOXP/MEP pathway, and VvDXS1 encodes the first enzyme in this plastidial pathway of terpene biosynthesis in grapevine. A single-point mutation resulting in the substitution of a lysine with an asparagine at position 284 in the VvDXS1 protein has previously been identified as the major cause for producing muscat flavour in grapes. In this study, the same substitution in the VvDXS1 protein was successfully created through prime editing in the table grape Vitis vinifera cv. 'Scarlet Royal'. The targeted point mutation was detected in most of the transgenic vines, with varying editing efficiencies. No unintended mutations were detected in the edited alleles, either by PCR Sanger sequencing or by amplicon sequencing. More than a dozen edited vines were identified with an editing efficiency of more than 50%, indicating that these vines were likely derived from single cells in which one allele was edited. These vines had much higher levels of monoterpenes in their leaves than the control, similar to what was found in leaf samples between field-grown muscat and non-muscat grapes.

Targeted deletion of grape retrotransposon associated with fruit skin color via CRISPR/Cas9 in Vitis labrascana 'Shine Muscat'

Transposition of transposable elements affect expression levels, splicing and epigenetic status, and function of genes located in, or near, the inserted/excised locus. For example, in grape, presence of the Gret1 retrotransposon in the promoter region of the VvMYBA1a allele at the VvMYBA1 locus suppress the expression of the VvMYBA1 transcription factor gene for the anthocyanin biosynthesis and this transposon insertion is responsible for the green berry skin color of Vitis labrascana, 'Shine Muscat', a major grape cultivar in Japan. To prove that transposons in grape genome can be removed by genome editing, we focused on Gret1 in the VvMYBA1a allele as a target of CRISPR/Cas9 mediated transposon removal. PCR amplification and sequencing detected Gret1 eliminated cells in 19 of 45 transgenic plants. Although we have not yet confirmed any effects on grape berry skin color, we were successful in demonstrating that cleaving the long terminal repeat (LTR) present at both ends of Gret1 can efficiently eliminate the transposon.

Gene Editing Profiles in 94 CRISPR-Cas9 Expressing T0 Transgenic Tobacco Lines Reveal High Frequencies of Chimeric Editing of the Target Gene

Chimeric editing is often reported in gene editing. To assess how the general chimeric editing is, we created a transgenic tobacco line carrying a marker, beta-glucuronidase gene (gusA), introduced a CRISPR-Cas9 editing vector into the transgenic tobacco line for knocking out gusA, and then investigated the gusA editing efficiencies in T0 and subsequent generations. The editing vector carried a Cas9 gene, which was driven by the cauliflower mosaic virus 35S promoter, and two guide RNAs, gRNA1 and gRNA2, which were driven by Arabidopsis U6 (AtU6) and U3 (AtU3) promoter, respectively. The two gRNAs were designed to knock out a 42-nucleotide fragment of the coding region of gusA. The editing vector was transformed into gusA-containing tobacco leaves using Agrobacterium tumefaciens-mediated transformation and hygromycin selection. Hygromycin-resistant, independent T₀ transgenic lines were used to evaluate gusA-editing efficiencies through histochemical GUS assays, polymerase chain reactions (PCR), and next-generation sequencing of PCR amplicons. Profiles of targeted sequences of 94 T₀ transgenic lines revealed that these lines were regenerated from non-edited cells where subsequent editing occurred and created chimeric-edited cells in these lines during or after regeneration. Two of them had the target fragment of 42 bp pairs of nucleotides removed. Detail analysis showed that on-target mutations at the AtU6-gRNA1 site and the AtU3-gRNA2 site were found in 4.3% and 77.7% of T₀ transgenic lines, respectively. To overcome the issue of extremely low editing efficiencies in T₀ lines, we conducted a second round of shoot induction from the chimeric line(s) to enhance the success of obtaining lines with all or most cells edited. The mutation profiles in T₀ transgenic lines provide valuable information to understand gene editing in plant cells with constitutively expressed CRISPR-Cas9 and gRNAs.