Current Status of Viral Resistance Molecular Breeding in Cucumbers (Cucumis sativus L.)

The cucumber (Cucumis sativus var. sativus) is a member of the Cucurbitaceae family. The cucumber is an important vegetable crop cultivated worldwide because it is not only rich in minerals and vitamins, but also low in calories and is linked by many consumers. While growing and developing, a cucumber is subject to various stressors, including divergent diseases caused by pathogens and environmental conditions detrimental to life. As a result, these biotic and abiotic stressors certainly affect the quality and productivity of cucumbers. It is widely believed that viral diseases are one of the most serious pathogens due to increasing numbers of virus-transmitting vectors caused by global warming. To efficiently manage viral diseases, chemical methods are not feasible, so breeding virus-resistant cultivar(s) is the only viable option. Because cucumbers have a relatively small diploid genome size, a relatively short life cycle, and a self-fertilization system, it is advantageous for genetic and genome research. The draft genome for cucumber was the first to be made public among vegetable crops. Accumulated genomic resources in the post-genomics era have been successfully applied for breeding complex traits, especially disease resistance, and complex genetic inheritance. Furthermore, it is now possible to identify the molecular markers quickly and easily or clone candidate genes based on fine genetics maps, genome sequences, high-throughput sequencing and multi-omics technologies, and bioinformatics tools. The utilization of molecular markers tightly linked to the viral disease resistance trait is necessary for the current viral disease resistance cucumber breeding program. For this purpose, highly saturated genetic and physical maps and fine mapping technologies are needed, although the marker-assisted selection is frequently available in cucumbers. Only a few genes involved in viral resistance have been identified in cucumbers, although many efforts have been devoted to quantitative trait locus mapping of virus resistance. This review focused on the latest studies related to viral disease resistance in cucumbers. This will be a good opportunity to get efficient clues for developing viral disease resistance cucumber cultivar(s).