Background: Due to the variation and mutation of the races of Pseudoperonospora cubensis, downy mildew has in recent years become the most devastating leaf disease of cucumber worldwide. Novel resistance to downy mildew has been identified in the wild Cucumis species, C. hystrix Chakr. After the successful hybridization between C. hystrix and cultivated cucumber (C. sativus L.), an introgression line (IL5211S) was identified as highly resistant to downy mildew. Nucleotide-binding site and leucine-rich repeat (NBS-LRR) genes are the largest class of disease resistance genes cloned from plant with highly conserved domains, which can be used to facilitate the isolation of candidate genes associated with downy mildew resistance in IL5211S. Results: Degenerate primers that were designed based on the conserved motifs in the NBS domain of resistance (R) proteins were used to isolate NBS-type sequences from IL5211S. A total of 28 sequences were identified and named as cucumber (C. sativus = CS) resistance gene analogs as CSRGAs. Polygenetic analyses separated these sequences into four different classes. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis showed that these CSRGAs expressed at different levels in leaves, roots, and stems. In addition, introgression from C. hystrix induced expression of the partial CSRGAs in cultivated cucumber, especially CSRGA23, increased four-fold when compared to the backcross parent CC3. Furthermore, the expression of CSRGA23 under P. cubensis infection and abiotic stresses was also analyzed at different time points. Results showed that the P. cubensis treatment and four tested abiotic stimuli, MeJA, SA, ABA, and H₂O_2, triggered a significant induction of CSRGA23 within 72 h of inoculation. The results indicate that CSRGA23 may play a critical role in protecting cucumber against P. cubensis through a signaling the pathway triggered by these molecules. Conclusions: Four classes of NBS-type RGAs were successfully isolated from IL5211S, and the possible involvement of CSRGA23 in the active defense response to P. cubensis was demonstrated. These results will contribute to develop analog-based markers related to downy mildew resistance gene and elucidate the molecular mechanisms causing resistance in IL5211S in the future.

Gummy stem blight (GSB), caused by Stagonosporopsis cucurbitacearum (Didymella bryoniae), poses a growing threat to greenhouse melon production. Despite this, the defense mechanism of melon against S. cucurbitacearum remains poorly understood. In this study, by employing electron microscopy and transcriptome sequencing, we investigated the cellular ultrastructure differences and gene expression dynamics of two melon accessions, PI 420145 (resistant to GSB) and ‘Baipicui’ (susceptible), pre- and post- inoculation. Our results revealed that PI 420145 exhibits a thicker waxy layer on the leaf surface and limited conidial germination without obvious signs of cell damage compared to ‘Baipicui’. The transcriptomic analysis identified a total of 23,078 differentially expressed genes (DEGs) and 974 differentially expressed long noncoding RNAs (DELs). Specifically, phenylpropanoid biosynthesis, cyanoamino acid metabolism, and MAPK signaling–plant and plant–pathogen interactions were enriched in PI 420145, while ‘Baipicui’ displayed enrichment in metabolism and autophagy. Additionally, through lncRNA–mRNA network construction, we identified a total of 38 lncRNA–mRNA targeted regulatory relationships in the four most significant KEGG pathways for disease resistance.

The complete nucleotide sequence of the wild cucumber (C. hystrix Chakr.) chloroplast genome has been determined in this study. The genome was composed of 155,031 bp containing a pair of inverted repeats (IRs) of 25,150 bp, which was separated by a large single-copy region of 86,564 bp and a small single-copy region of 18,166 bp. The chloroplast genome contained 130 known genes, including 89 protein-coding genes, 8 ribosomal RNA genes (4 rRNA species) and 37 tRNA genes (30 tRNA species), with 18 of them located in the IR region. In these genes, 16 contained 1 intron, and 2 genes and one ycf contained 2 introns.