Inheritance and QTL mapping of cucumber mosaic virus resistance in cucumber (<i>Cucumis Sativus</i> L.)

<div><p>The commercial yield of cucurbit crops infected with Cucumber mosaic virus (CMV) severely decreases. Chemical treatments against CMV are not effective; therefore, genetic resistance is considered the primary line of defense. Here, we studied resistance to CMV in cucumber inbred line ‘02245’ using a recombinant inbred line (RIL) population generated from a cross between ‘65G’ and ‘02245’ as susceptible and resistant parents, respectively. Genetic analysis revealed that CMV resistance in cucumber is quantitatively inherited. Analysis of the RIL population revealed that a quantitative trait locus (QTL) was found on chromosome 6; named <i>cmv6</i>.<i>1</i>, this QTL was delimited by SSR9-56 and SSR11-177 and explained 31.7% of the phenotypic variation in 2016 and 28.2% in 2017. The marker SSR11-1, which is close to the locus, was tested on 78 different cucumber accessions and found to have an accuracy of 94% in resistant and moderately resistant lines but only 67% in susceptible lines. The mapped QTL was delimited within a region of 1,624.0 kb, and nine genes related to disease resistance were identified. Cloning and alignment of the genomic sequences of these nine genes between ‘65G’ and ‘02245’ revealed that Csa6M133680 had four single-base substitutions within the coding sequences (CDSs) and two single-base substitutions in its 3’-untranslated region, and the other eight genes showed 100% nucleotide sequence identity in their exons. Expression pattern analyses of Csa6M133680 in ‘65G’ and ‘02245’ revealed that the expression levels of Csa6M133680 significantly differed between ‘65G’ and ‘02245’ at 80 h after inoculation with CMV and that the expression in ‘02245’ was 4.4 times greater than that in ‘65G’. The above results provide insights into the fine mapping and marker-assisted selection in cucumber breeding for CMV resistance.</p></div

Mutation of the Csa6M133680 gene.

<p><b>a Distribution of 4 exons and 3 introns within the coding region of Csa6M133680.</b> b A single-nucleotide substitution (A / C) in exon 1, three single-nucleotide substitutions (C / T, C / T, T / A) in exon 4 and two single-nucleotide substitutions (T / A, C / G) in the 3’-untranslated region. c The four single-nucleotide substitution caused four amino acid substitutions (Lys / Thr, Ser / Leu, Pro / Leu, Asp / Glu). Note: The orange box indicates the 5’-untranslated region, the black box indicates coding regions, the black line indicates an intron, and the blue box indicates the 5’-untranslated region.</p

QTL controlling CMV resistance and its effect on cucumber seedlings.

<p>QTL controlling CMV resistance and its effect on cucumber seedlings.</p

The 0–9 infection ranking system was based on the severity of mosaic patterning and leaf distortion.

<p>0 = no symptoms; 1 = apical leaves with dispersed vein clearing or slight mottling; 3 = mosaic patches and/or necrotic spots on leaves; 5 = moderate mosaic patches and moderate distortion of the three youngest leaves; 7 = apical meristem exhibiting mosaic patches and deformation; 9 = extensive leaf mosaic patches and severe distortion of all leaves—even plant death.</p

Relative quantitative expression analysis of Csa6M133680 in ‘65G’ and ‘02245’.

<p>The data represent the expression of Csa6M133680 relative to that of actin, as measured by qRT-PCR at different times after inoculation of both ‘65G’ and ‘02245’. Note: The error bars represent the standard error of three biological replicates. **Significant difference (P<0.01). The six treatments (0 h,16 h, 40 h, 80 h, 132 h and 192 h after inoculation with CMV) of ‘65G’ and ‘02245’ corresponds to the six columns in the histogram.</p

The DIs of the parental lines and the F₁ and RIL populations.

<p>The DIs of the parental lines and the F₁ and RIL populations.</p

ELISA results for CMV and the distribution of CMV DIs.

<p>a Symptoms of the susceptible parental line ‘65G’, the resistant line ‘02245’ and their F₁ hybrid progeny after they were inoculated with CMV. b DAS-ELISA results for CMV in the leaves of P1 (‘65G’), P2 (‘02245’) and F1 plants as well as some plants within the RIL population; the result is consistent with the phenotypic identification. c Frequency distribution of the DI from CMV among the ‘65G’ב02245’ RIL population. The frequency distribution in June 2016 and June 2017 each presented a normal distribution ranging from resistant to susceptible phenotypes.</p

QTL controlling CMV resistance and its effect on cucumber seedlings during two years.

<p>One QTL was detected at the same location in June 2016 and June 2017; this QTL was identified on chromosome 6 (Chr.6). Note: The red box indicates June 2016, the blue box indicates June 2017, the black line indicates June 2016, and the red line indicates June 2017.</p