Quantitative trait locus mapping of fruit aroma compounds in cucumber (Cucumber sativus L.) based on a recombinant inbred line population.

The fresh and unique flavor of cucumber fruits, mainly composed of aldehydes and alcohols, is one of its most important fruit qualities. However, little is known about the genetic basis of aroma compounds in cucumber fruit and the related quantitative trait loci (QTLs). In this study, genomic screening of QTLs underlying aroma compounds was performed based on the genetic linkage map constructed using 1301 single-nucleotide polymorphism (SNP) markers from genotyping-by-sequencing of a recombinant inbred line (RIL) population developed from Q16 × Q24. Significant genetic variations of aroma compounds in the RIL population were observed, and a total of 28 QTLs were screened. A major QTL (qol8-2.1) related to (E,Z)-2,6-nonadien-1-ol was detected with a markedly high LOD score (10.97 in 2020 and 3.56 in 2019) between mk190 and mk204 on chromosome 2. Genome scans identified a cluster of nine lipoxygenase genes in this region. A significant positive correlation was detected between CsaV3_2G005360 (CsLOX08) and (E,Z)-2,6-nonadien-1-ol, and five amino acid variations were detected between the CsLOX08 protein sequences of the two parental lines. Based on the genome variation of CsLOX08, we developed an InDel marker. Genotyping of InDel markers was consistent with the content of (E,Z)-2,6-nonadien-1-ol in RILs, which were also verified in nine cucumber inbred lines. The results will give breeders guidance for obtaining better flavor in cucumber.

Transcriptomic and Histological Analysis of the Response of Susceptible and Resistant Cucumber to Meloidogyne incognita Infection Revealing Complex Resistance via Multiple Signaling Pathways.

The root-knot nematode (RKN), Meloidogyne incognita, is a devastating pathogen for cucumber (Cucumis sativus L.) specially in production under protected environments or continuous cropping. High level RKN resistance has been identified in African horned melon Cucumis metuliferus (CM). However, the resistance mechanism remains unclear. In this study, the comparative analysis on phenotypic and transcriptomic responses in the susceptible cucumber inbred line Q24 and the resistant CM, after M. incognita infection, was performed. The results showed that, in comparison with Q24, the CM was able to significantly reduce penetration numbers of second stage juveniles (J2), slow its development in the roots resulting in fewer galls and smaller giant cells suggesting the presence of host resistance in CM. Comparative transcriptomes analysis of Q24 and CM before and after M. incognita infection was conducted and differentially expressed genes (DEGs) associated with host resistance were identified in CM. Enrichment analyses revealed most enriched DEGs in Ca2+ signaling, salicylic acid (SA)/jamonate signaling (JA), as well as auxin (IAA) signaling pathways. In particular, in CM, DEGs in the Ca2+ signaling pathway such as those for the calmodulin and calcium-binding proteins were upregulated at the early stage of M. incognita infection; genes for SA/JA synthesis/signal transduction were markedly activated, whereas the IAA signaling pathway genes were inhibited upon infection suggesting the importance of SA/JA signaling pathways in mediating M. incognita resistance in CM. A model was established to explain the different molecular mechanisms on M. incognita susceptibility in cucumber and resistance to M. incognita infection in CM.

Relationship between key environmental factors and profiling of volatile compounds during cucumber fruit development under protected cultivation.

To provide precision management of cucumber under protected cultivation, the relationships between environmental factors and daily increment of aroma in two lines were analyzed during development, using Pearson correlation and stepwise regression. Irrespective of the line tested, the daily increment of volatile compounds and C6 aldehydes were positively correlated with average daily light intensity (X1), while the daily increment of C9 aldehydes was negatively correlated with average daily relative humidity (X4) and average daytime relative humidity (X7). X1 was considered as the most significant environmental factor which affected the daily increment of volatile compounds in both lines, X7 was the environmental factor which affected the daily increment of C9 aldehydes most in No.14-1, and X1 and X4 were the factors which affected the daily increment of C6 aldehydes and C9 aldehydes most in No.26. A range of environmental factors was forecast for the maximal daily increment of volatile compounds.