Comparative transcriptome analysis of cucumber fruit tissues reveals novel regulatory genes in ascorbic acid biosynthesis

Ascorbic acid (AsA) is one of the most abundant natural antioxidants, and it is an important indicator of the nutritional value of cucumber fruit. The aim of this study was to elucidate the regulatory mechanism affecting AsA metabolism in cucumber fruit. In this study, the AsA content in the fruit of two cucumber cultivars (H28 and H105) was significantly higher in the exocarp and endocarp than in the mesocarp. To clarify the regulation of AsA in cucumber fruit, the transcriptomes of three fruit tissues (i.e., the exocarp, mesocarp, and endocarp) of two cucumber cultivars (H28 and H105) were sequenced. Transcriptomic profiling combined with transcription factors (TFs) and correlation analysis were performed to reveal that three genes, including <i>CsaV3_5G014110</i> (<i>phosphomannomutase, PMM</i>), <i>CsaV3_2G004170</i> (<i>GDP-mannose-3', 5'-epimerase, GME</i>) and <i>CsaV3_5G006680</i> (<i>dehydroascorbate reductase, DHAR</i>), were expressed at higher level in the exocarp and endocarp than in the mesocarp. In both two cultivars, <i>CsaV3_4G028360</i> (<i>ethylene-responsive transcription factor, ERF</i>) was negatively correlated with <i>PMM</i> and <i>GME</i>, and positively correlated with <i>DHAR</i>. <i>CsaV3_6G042110</i> (<i>ethylene-responsive transcription factor, ERF</i>) was positively correlated with <i>PMM</i> and <i>GME</i>, and negatively correlated with <i>DHAR</i>. CsaV3_6G032360 (<i>mitogen-activated protein kinase, MAPK</i>) as positively correlated with <i>PMM</i>, <i>GME</i> and <i>DHAR</i>. These six genes are considered the key candidate genes for further research. This study provides insight for further study on the regulation of AsA biosynthesis in cucumber fruit and provide potential candidate genes for future genetic improvement of cucumber germplasm with enhanced AsA accumulation.