**5. Conclusions**

The bHLH gene family plays an important role in improving plant response to stress and plant development. In this study, we identified 117 *PebHLHs* from the genome of the passion fruit, and the genomic information about PebHLH family members was analyzed. *PebHLHs* could respond to abiotic stresses, including drought, high salinity, cold and hightemperature stresses, and they were differentially expressed in different fruit developmental stages. We were concerned about a gene *PebHLH56* that was induced under cold stress. The pCAMBIA1304-*PebHLH56p* transgenic *Arabidopsis* had a significantly deeper GUS staining than the control under cold stress, and the GUS enzyme activity also increased. The RT-qPCR result showed that the expression of *PebHLH56* was induced by cold stress. This study provides new insights into the regulatory functions of *PebHLHs* during abiotic stress and fruit ripening and screens a stress-resistance gene as a candidate member to improve the understanding of *PebHLH* gene family characteristics and evolution.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/horticulturae9020272/s1, Table S1. Basic information of bHLH genes identified in passion fruit; Table S2. The transcriptome data of passion fruit bHLHs under the drought, salt, cold and high-temperature treatment; Table S3. qPCR data of passion fruit bHLHs under the drought, salt, cold and high-temperature treatment; Table S4. The transcriptome data of passion fruit in the three stages of fruit ripening; Table S5. qPCR data of passion fruit bHLHs in the three stages of fruit ripening; Table S6. A list of the oligo primers of PebHLHs used for qRT-PCR.

**Author Contributions:** Y.X., W.Z., F.M., B.W., D.H., B.X. and W.X. performed experiments, D.C., S.S. and P.S. analyzed the data; Y.X. and S.S. drafted the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work was sponsored by Hainan Provincial Natural Science Foundation (321RC1088), Project of Sanya Yazhou Bay Science and Technology City (SCKJ-JYRC-2022-84, SCKJ-JYRC-2022-93), and National Natural Science Foundation of China (32260737).

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
