Abstract
Propamocarb (PM), a carbamate fungicide, can effectively control downy mildew on cucumber. However, due to the large-scale and high-dose use of this fungicide, PM residues have become a major problem in cucumber production. In this report, the cucumber cultivar “D0351” (with the lowest residual PM content) and the cucumber cultivar “D9320” (with the highest residual PM content) were used as experimental materials. The candidate gene CsMCF, which is related to a low residual PM content in cucumber, was screened by high-throughput tag-sequencing (Tag-Seq) and PM analysis, and its role in reducing PM residue in cucumber was explored. CsMCF was cloned and obtained. This gene contains an open reading frame of 1026 bp, encodes 341 amino acids and contains 3 Mito-carr domains. The encoded protein is a hydrophobic protein with 4 distinct transmembrane structures but no signal peptide cleavage sites. The subcellular localization of the protein is the cytoplasm. Evolutionary tree analysis showed that CsMCF had the highest homology to a gene from the melon Cucumis melo L. (XM_008464998.2). The core elements of the promoter include cis-acting elements, such as those related to salicylic acid (SA), jasmonic acid (JA), gibberellin (GA), and abscisic acid (ABA). Following PM treatment, CsMCF was significantly upregulated at most time points in different parts of the fruit, leaf, stem and root of “D0351,” while expression was downregulated at most time points in the fruit, leaf and stem of “D9320.” The order of the expression levels in different cucumber organs was as follows: fruit>leaf > stem > root. CsMCF was specifically expressed in the stems and leaves of “D0351.” The PM residues in CsMCF (+)-overexpressing T0 and T1 cucumber fruits were significantly lower than those in the wild type, while the PM residues in CsMCF (-)-overexpressing T0 and T1 cucumber fruits were significantly higher. The qRT-PCR results showed that CsMCF can respond to biotic and abiotic stresses, actively respond to PM treatment and play a role in reducing PM residues in cucumber fruits.
Highlights
Cucumber is one of the main vegetable crops grown under protected cultivation in China
Using CsMCF-F and CsMCF-R as primers and cucumber “D0351” fruit DNA as a template, we amplified a band of approximately 1000 bp by PCR (Supplementary Figure S1A)
The results showed that in CsMCF (+)-overexpression T0 plants (OE1 and OE4), the PM residues in cucumber fruits at five time points (6–72 h), but not the 1 h time point, were lower than those in the wild-type control
Summary
Cucumber is one of the main vegetable crops grown under protected cultivation in China. This vegetable is very popular because of its crispness, tenderness, fragrance and nutritional value. Addressing the issue of fungicides residues in cucumber is urgently needed. Low and high PM residues of deltamethrin, PM and myclobutanil have been identified in cucumber varieties according to this system (Liu, 2010). The researchers suggested that the PM residues in cucumber fruits were a quantitative trait controlled by multiple genes, and a PM residuerelated QTL was detected (Ma, 2010). Under PM stress, differentially expressed genes in cucumber fruits were screened by Solexa and comparative genomics methods on the basis of physiological and biochemical studies of low-PM-residue cucumbers (Wu et al, 2013). According to research on the genes responding to PM stress in cucumber varieties with different levels of PM residues [CsABC19 (Meng et al, 2016), CsWRKY30 (Li et al, 2016), CsSDH (Guo, 2013) and CsDIR16 (Liu et al, 2018)], the resistance of transgenic Arabidopsis thaliana or cucumber to PM stress can be improved significantly by the genes mentioned above
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