Genome-Wide Identification and Characterization of Melon bHLH Transcription Factors in Regulation of Fruit Development

The basic helix-loop-helix (bHLH) transcription factor family is one of the largest transcription factor families in plants and plays crucial roles in plant development. Melon is an important horticultural plant as well as an attractive model plant for studying fruit ripening. However, the bHLH gene family of melon has not yet been identified, and its functions in fruit growth and ripening are seldom researched. In this study, 118 bHLH genes were identified in the melon genome. These CmbHLH genes were unevenly distributed on chromosomes 1 to 12, and five CmbHLHs were tandem repeat on chromosomes 4 and 8. There were 13 intron distribution patterns among the CmbHLH genes. Phylogenetic analysis illustrated that these CmbHLHs could be classified into 16 subfamilies. Expression patterns of the CmbHLH genes were studied using transcriptome data. Tissue specific expression of the CmbHLH32 gene was analysed by quantitative RT-PCR. The results showed that the CmbHLH32 gene was highly expressed in female flower and early developmental stage fruit. Transgenic melon lines overexpressing CmbHLH32 were generated, and overexpression of CmbHLH32 resulted in early fruit ripening compared to wild type. The CmbHLH transcription factor family was identified and analysed for the first time in melon, and overexpression of CmbHLH32 affected the ripening time of melon fruit. These findings laid a foundation for further study on the role of bHLH family members in the growth and development of melon.

Genome-wide identification of microRNAs involved in the regulation of fruit ripening and climacteric stages in melon (Cucumis melo)

Fruit ripening is influenced by multiple plant hormones and the regulation of genes. However, studies on posttranscriptional regulators (e.g., miRNAs) of fruit growth and ripening are limited. We used miRNA sequencing and degradome methods to identify miRNAs and their target genes in melon (Cucumis melo cv. Hetao melon). A total of 61 conserved miRNAs and 36 novel miRNAs were identified from fruit growth, ripening, climacteric, and postclimacteric developmental stage samples, of which 32 conserved miRNAs were differentially expressed between developmental stage samples. Sixty-two target genes of 43 conserved miRNAs and 1 novel miRNA were identified from degradome sequencing. To further investigate miRNA influencing fruit ripening, transgenic melon plants overexpressing pre-cme-miR393 (cme-miR393-OE) were generated and characterized. The results showed that fruit ripening was delayed in cme-miR393-OE transgenic lines compared to nontransgenic fruits. The target of cme-miR393 was also identified, and the expression of CmAFB2 was repressed in transgenic plants. These results provide evidence that miRNA regulates melon fruit ripening and provide potential targets to improve the horticultural traits of melon fruit.