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Efficient targeted mutagenesis in allotetraploid sweet basil by CRISPR/Cas9.

ABSTRACT: Sweet basil (Ocimum basilicum) is an economically important herb and its global production is threatened by basil downy mildew caused by the obligate biotrophic oomycete Peronospora belbahrii. Effective tools are required for functional understanding of its genes involved in synthesis of valuable secondary metabolites in essential oil and disease resistance, and breeding for varieties with improved traits. Clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 gene editing technology has revolutionized crop breeding and functional genomics. The applicability and efficacy of this genomic tool in the allotetraploid sweet basil were tested by editing a potential susceptibility (S) gene ObDMR1, the basil homolog of Arabidopsis DMR1 (Downy Mildew Resistant 1) whose mutations conferred nearly complete resistance against Arabidopsis downy mildew pathogen, Hyaloperonospora arabidopsidis. Two single guide RNAs targeting two different sites of the ObDMR1 coding sequence were designed. A total of 56 transgenic lines were obtained via Agrobacterium-mediated stable transformation. Mutational analysis of 54 T0 transgenic lines identified 92.6% lines carrying mutations at target 1 site, while a very low mutation frequency was detected at target 2 site. Deep sequencing of six T0 lines revealed various mutations at target 1 site, with a complete knockout of all alleles in one line. ObDMR1 homozygous mutant plants with some being transgene free were identified from T1 segregating populations. T2 homozygous mutant plants with 1-bp frameshift mutations exhibited a dwarf phenotype at young seedling stage. In summary, this study established a highly efficient CRISPR/Cas9-mediated gene editing system for targeted mutagenesis in sweet basil. This system has the capacity to generate complete knockout mutants in this allotetraploid species at the first generation of transgenic plants and transgene-free homozygous mutants in the second generation. The establishment of this system is expected to accelerate basil functional genomics and breeding.

SUBMITTER: Navet N

PROVIDER: S-EPMC7287412 | biostudies-literature | 2020 Jun

REPOSITORIES: biostudies-literature

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Efficient targeted mutagenesis in allotetraploid sweet basil by CRISPR/Cas9.

Navet Natasha N Tian Miaoying M

Plant direct 20200611 6

Sweet basil (<i>Ocimum basilicum</i>) is an economically important herb and its global production is threatened by basil downy mildew caused by the obligate biotrophic oomycete <i>Peronospora belbahrii</i>. Effective tools are required for functional understanding of its genes involved in synthesis of valuable secondary metabolites in essential oil and disease resistance, and breeding for varieties with improved traits. Clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 gene ...[more]

PMID: 32537560

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Project description:Sweet basil (Ocimum basilicum) is an economically important allotetraploid (2n = 4x = 48) herb whose global production is threatened by downy mildew disease caused by the obligate biotrophic oomycete, Peronospora belbahrii. Generation of disease resistant cultivars by mutagenesis of susceptibility (S) genes via CRISPR/Cas9 is currently one of the most promising strategies to maintain favored traits while improving disease resistance. Previous studies have identified Arabidopsis DMR6 (Downy Mildew Resistance 6) as an S gene required for pathogenesis of the downy mildew-causing oomycete pathogen Hyaloperonospora arabidopsidis. In this study, a sweet basil homolog of DMR6, designated ObDMR6, was identified in the popular sweet basil cultivar Genoveser and found to exist with a high copy number in the genome with polymorphisms among the variants. Two CRISPR/Cas9 constructs expressing one or two single guide RNAs (sgRNAs) targeting the conserved regions of ObDMR6 variants were generated and used to transform Genoveser via Agrobacterium-mediated transformation. 56 T0 lines were generated, and mutations of ObDMR6 were detected by analyzing the Sanger sequencing chromatograms of an ObDMR6 fragment using the Interference of CRISPR Edits (ICE) software. Among 54 lines containing mutations in the targeted sites, 13 had an indel percentage greater than 96% suggesting a near-complete knockout (KO) of ObDMR6. Three representative transgene-free lines with near-complete KO of ObDMR6 determined by ICE were identified in the T1 segregating populations derived from three independent T0 lines. The mutations were further confirmed using amplicon deep sequencing. Disease assays conducted on T2 seedlings of the above T1 lines showed a reduction in production of sporangia by 61-68% compared to the wild-type plants and 69-93% reduction in relative pathogen biomass determined by quantitative PCR (qPCR). This study not only has generated transgene-free sweet basil varieties with improved downy mildew resistance, but also contributed to our understanding of the molecular interactions of sweet basil-P. belbahrii.

Dataset Information

Efficient targeted mutagenesis in allotetraploid sweet basil by CRISPR/Cas9.

Publications

Efficient targeted mutagenesis in allotetraploid sweet basil by CRISPR/Cas9.

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