Abstract
Walnuts are among the most important nut crops grown in temperate regions of the world. Commercial production in California, and increasingly worldwide, relies on only few clonally grafted scion genotypes, particularly ‘Chandler’, and more recently clonally propagated disease-resistant rootstocks. Diseases, nematodes, insect pests, abiotic stresses, and other nutritional and environmental factors, can reduce walnut productivity and quality, affecting grower profitability. The California Walnut Breeding Program at UC Davis has developed and released scion cultivars and rootstocks to help address some of these problems. Recent sequencing of the walnut genome is expected to speed walnut breeding by facilitating development of molecular markers that can be linked to phenotypic traits, and by reducing the time needed to screen new genotypes, but conventional breeding of tree crops is still a long-term proposition. Here we describe a toolkit that uses a CRISPR-Cas9 system to enable rapid and precise editing of the genomes of currently grown commercial clonal walnut genotypes. A computational tool was developed to aid selection of guide RNAs targeting specific genomic sites and to identify potential off-target sites. As a proof-of-concept, the gene encoding phytoene desaturase (PDS) of walnut was targeted, disrupting its expression. This resulted in albino leaves; a dominant visual phenotype associated with the successful editing of this gene. This toolkit can now be adapted to other important tree crop members of the order Fagales, which includes other valuable nuts and trees species such as pecans, butternuts, chestnuts, hickory nuts, hazelnut, wingnut and oaks.
Highlights
Persian (English) walnut (Juglans regia) is a widely grown temperate-region nut crop that provides an excellent dietary source of plant-based protein and fiber, contains a rich supply of healthy oils including in particular omega-3 fatty acids, and is an important component of a heart-healthy Mediterranean diet (Vinson and Cai, 2012)
To generate a search database, we extracted the gene, mRNA, CDS, exon, and intron information from the gene model file and fetched the gene sequences from the Fasta file using in-house Perl script (Figure 2). We developed another Perl script, which extracts 23 length nucleotide sequences from the given gene sequence
Based on the availability of the complete genome sequence and JrPDS being single copy, a strategy was devised to disrupt its functionality by introducing frame shift mutations using Cas9
Summary
Persian (English) walnut (Juglans regia) is a widely grown temperate-region nut crop that provides an excellent dietary source of plant-based protein and fiber, contains a rich supply of healthy oils including in particular omega-3 fatty acids, and is an important component of a heart-healthy Mediterranean diet (Vinson and Cai, 2012). It ranks 7th in production and 5th in export among California’s top agricultural commodities, grown on 335,000 acres with a production of around 630,000 tons valued at U$1.6 billion, of which 411,000 tons are exported (CA Stat Review, 2018). Developing mutants/knockouts in specific genes can improve the interpretation of relationships between genes and phenotypic traits
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