Fruit spine density is an important commercial trait for cucumber (Cucumis sativus L.). Most North China-type cucumbers that are grown over large areas have a dense-spine phenotype, which directly affects the appearance quality, storage, and transportation of the fruits. Here, we isolated a novel few spines mutant (fs2) from the wild-type (WT) inbred line WD1, a North China-type cucumber with high density fruit spines, by an ethyl methanesulfonate (EMS) mutagenesis treatment. Genetic analysis revealed that the phenotype of fs2 is controlled by a single recessive nuclear gene. We fine-mapped the fs2 locus using F₂ and BC₁ populations (1,802 and 420 individuals, respectively), which showed that the candidate gene of FS2 (Csa4G652850) encodes an ARID-HMG transcription factor containing an AT-rich interaction domain (ARID) and a high mobility group box domain (HMG). One SNP (C to T) and one InDel (a 40-bp deletion) in the coding region of FS2 result in amino acid variation and premature translation termination in the fs2 mutant, respectively. FS2 was found to be highly expressed in the apical buds and young ovaries. In addition, experiments suggest that FS2 participates in the regulation of fruit spine initiation by activating the expression of the Tril gene in cucumber. This work provides not only an important reference for understanding the molecular mechanisms of fruit spine development but also an important resource for fruit appearance quality breeding in cucumber.

Cucumber is an important vegetable worldwide, and powdery mildew (PM) is a common and serious disease of cucumbers. Breeding disease-resistant cucumber varieties is the most advantageous strategy to control this disease. In recent years, exploration and identification of cucumber PM resistance genes have achieved great advancement, and many genes have been cloned and verified using different methods. However, the resistance mechanism of cucumber PM is still unclear, and many ambiguities need to be elucidated urgently. In this review, we summarized the research advances in PM resistance in cucumbers, including genetic analysis, quantitative trait locus mapping, map-based cloning, transcriptomics, mlo-mediated PM resistance, and mining of noncoding RNAs involved in resistance. Finally, the research directions and the problems that need to be solved in the future were discussed.

Tnt1 is an active retrotransposon originally identified in tobacco (Nicotiana tabacum L.) (Grandbastien et al., 1989), but its transposition activity could be activated through tissue culture in other plant species. The insertions are stable and inheritable in the progeny, which has made it a valuable and versatile tool for developing insertional mutagenesis libraries in several plant species. Here, we explored its utility for mutagenesis in cucumber (Cucumis sativus L.). T₃ Tnt1 transgenic cucumber plants were subjected to tissue culture to regenerate self-pollinated progeny. With PCR and analyses and Southern hybridization, we found regenerated plants maintained the original Tnt1 insertion and created new insertions suggesting characteristic re-transposition activity of Tnt1 during this process. Using genome walking, some flanking sequences of Tnt1 insertions were recovered in regenerated plants. The results demonstrated that Tnt1 could be stably inherited and re-transposable during tissue culture in cucumber and that it is feasible to use for developing an insertional mutagenesis library for cucumber.