The genetic basis of cold tolerance in cucumber (Cucumis sativus L.)-the latest developments and perspectives.

Cold stress is one of the main causes of yield losses in plant production in temperate climate areas. Cold stress slows down and even stops plant growth and development and causes injuries that may result in the plant's death. Cucumber (Cucumis sativus L.), an economically important vegetable, is sensitive to low temperatures, thus improving cold tolerance in cucumber would benefit cucumber producers, particularly those farming in temperate climates and higher altitude areas. So far, single cucumber accessions showing different degrees of cold tolerance have been identified, and genetic studies have revealed biparentally and maternally inherited genetic factors responsible for chilling tolerance. Paternally transmitted chilling tolerance has also been suggested. Quantitative trait loci (QTL) associated with seed germination ability at low temperature and seedling recovery from chilling have been described. Several transgenic attempts have been made to improve cold tolerance in cucumber. Despite numerous studies, the molecular mechanisms of cold tolerance in cucumber have still not been sufficiently elucidated. In this review, we summarise the results of research focused on understanding the genetic basis of cold tolerance in cucumber and their implications for cucumber breeding.

Transcriptome Profiling of Cucumber (Cucumis sativus L.) Early Response to Pseudomonas syringae pv. lachrymans.

Bacterial angular leaf spot disease (ALS) caused by Pseudomonas syringae pv. lachrymans (Psl) is one of the biological factors limiting cucumber open-field production. The goal of this study was to characterize cytological and transcriptomic response of cucumber to this pathogen. Plants of two inbred lines, B10 (susceptible) and Gy14 (resistant), were grown, and leaves were inoculated with highly virulent Psl strain 814/98 under growth chamber conditions. Microscopic and transcriptional evaluations were performed at three time points: before, 1 and 3 days post inoculation (dpi). Investigated lines showed distinct response to Psl. At 1 dpi bacterial colonies were surrounded by necrotized mesophyll cells. At 3 dpi, in the susceptible B10 line bacteria were in contact with degraded cells, whereas cells next to bacteria in the resistant Gy14 line were plasmolyzed, but apparently still alive and functional. Additionally, the level of H2O2 production was higher in resistant Gy14 plants than in B10 at both examined time points. In RNA sequencing more than 18,800 transcripts were detected in each sample. As many as 1648 and 2755 differentially expressed genes (DEGs) at 1 dpi as well as 2992 and 3141 DEGs at 3 dpi were identified in B10 and Gy14, respectively. DEGs were characterized in terms of functional categories. Resistant line Gy14 showed massive transcriptomic response to Psl at 1 dpi compared to susceptible line B10, while a similar number of DEGs was detected for both lines at 3 dpi. This suggests that dynamic transcriptomic response to the invading pathogen may be related with host resistance. This manuscript provides the first transcriptomic data on cucumber infected with the pathovar lachrymans and helps to elucidate resistance mechanism against ALS disease.

Genetic mapping of psl locus and quantitative trait loci for angular leaf spot resistance in cucumber (Cucumis sativus L.).

One of the most important cucumber diseases is bacterial angular leaf spot (ALS), whose increased occurrence in open-field production has been observed over the last years. To map ALS resistance genes, a recombinant inbred line (RIL) mapping population was developed from a narrow cross of cucumber line Gy14 carrying psl resistance gene and susceptible B10 line. Parental lines and RILs were tested under growth chamber conditions as well as in the field for angular leaf spot symptoms. Based on simple sequence repeat and DArTseq, genotyping a genetic map was constructed, which contained 717 loci in seven linkage groups, spanning 599.7 cM with 0.84 cM on average between markers. Monogenic inheritance of the lack of chlorotic halo around the lesions, which is typical for ALS resistance and related with the presence of recessive psl resistance gene, was confirmed. The psl locus was mapped on cucumber chromosome 5. Two major quantitative trait loci (QTL) psl5.1 and psl5.2 related to disease severity were found and located next to each other on chromosome 5; moreover, psl5.1 was co-located with psl locus. Identified QTL were validated in the field experiment. Constructed genetic map and markers linked to ALS resistance loci are novel resources that can contribute to cucumber breeding programs.