Heterosis has historically been exploited in plants; however, its underlying genetic mechanisms and molecular basis remain elusive. In recent years, due to advances in molecular biotechnology at the genome, transcriptome, proteome, and epigenome levels, the study of heterosis in vegetables has made significant progress. Here, we present an extensive literature review on the genetic and epigenetic regulation of heterosis in vegetables. We summarize six hypotheses to explain the mechanism by which genes regulate heterosis, improve upon a possible model of heterosis that is triggered by epigenetics, and analyze previous studies on quantitative trait locus effects and gene actions related to heterosis based on analyses of differential gene expression in vegetables. We also discuss the contributions of yield-related traits, including flower, fruit, and plant architecture traits, during heterosis development in vegetables (e.g., cabbage, cucumber, and tomato). More importantly, we propose a comprehensive breeding strategy based on heterosis studies in vegetables and crop plants. The description of the strategy details how to obtain F1 hybrids that exhibit heterosis based on heterosis prediction, how to obtain elite lines based on molecular biotechnology, and how to maintain heterosis by diploid seed breeding and the selection of hybrid simulation lines that are suitable for heterosis research and utilization in vegetables. Finally, we briefly provide suggestions and perspectives on the role of heterosis in the future of vegetable breeding.
Background Downy mildew (DM) is one of the most serious diseases in cucumber and brings the loss of yield and profit. Multiple QTLs for DM resistance have been detected, however, no loci related to resistance was reported using genome-wide association analysis (GWAS). In this study, the core germplasm (CG) of cucumber lines that had been constructed and resequenced were used to identify DM resistance Loci using GWAS technology. Results Thirteen loci (dmG1.1, dmG1.2, dmG2.1, dmG2.2, dmG3.1, dmG4.1, dmG4.2, dmG5.1, dmG5.2, dmG6.1, dmG6.2, dmG7.1 and dmG7.2) associated with DM resistance were detected on all the seven chromosomes. Among these loci, dmG2.1 and dmG7.1 were novel loci compared with previous studies. Based on the annotation of homologous genes in Arabidopsis and pairwise LD correlations, Csa1G575030 could be the most likely candidate gene of dmG1.2; Csa2G059820 and Csa2G060360 could be the candidate gene of dmG2.1. A WRKY transcription factor Csa5G606470 could be the candidate gene of dmG5.2. Csa7G004020 could be the candidate gene of dmG7.1. Conclusions These results identify five candidate genes for four loci related to DM resistance in cucumber which provide theoretical basis for gene cloning and genetic breeding of DM resistance in cucumber.
Gummy stem blight (GSB) is a serious fungal disease caused by Didymella bryoniae, that affects cucumber yield and quality worldwide. However, no GSB resistant genes have been identified in cucumber cultivars. In this study, the wild cucumber accession ‘PI 183967’ was used as a source of resistance to GSB in adult stems. An F2 population was mapped using resistant line ‘LM116’ and susceptible line ‘LM34’ derived from a cross between ‘PI 183967’ and ‘931’. By developing InDel and SNP markers, the gsb-s6.2 QTL on Chr. 6 was fine-mapped to a 34 kb interval harboring six genes. Gene Expression analysis after inoculation showed that two candidate genes (Csa6G046210 and Csa6G046240) were induced and differentially expressed between the resistant and susceptible parents, and may be involved in disease defense. Sequence alignment showed that Csa6G046210 encodes a Multiple myeloma tumor-associated protein, and it harbored two nonsynonymous SNPs and one InDel in the third and the fourth exons, and two InDels in the TATA-box of the basal promoter region. Csa6G046240 encodes a MYB transcription factor with six variants in the AP2/ERF and MYB motifs in the promoter. These two candidate genes lay the foundation for revealing the mechanism to GSB resistance, and may be useful for marker-assisted selection in cucumber disease-resistant breeding.
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