*Article* **Characterization of the Passion Fruit (***Passiflora edulis* **Sim) bHLH Family in Fruit Development and Abiotic Stress and Functional Analysis of** *PebHLH56* **in Cold Stress**

**Yi Xu 1,2, Weidong Zhou 3, Funing Ma 1,2, Dongmei Huang 1, Wenting Xing 1, Bin Wu 1, Peiguang Sun 1, Di Chen 1, Binqiang Xu <sup>1</sup> and Shun Song 1,2,\***


**Abstract:** Abiotic stress is the focus of research on passion fruit characters because of its damage to the industry. Basic helix-loop-helix (bHLH) is one of the Transcription factors (TFs) which can act in an anti-abiotic stress role through diverse biological processes. However, no systemic analysis of the passion fruit bHLH (PebHLH) family was reported. In this study, 117 PebHLH members were identified from the genome of passion fruit, related to plant stress resistance and development by prediction of protein interaction. Furthermore, the transcriptome sequencing results showed that the *PebHLHs* responded to different abiotic stresses. At different ripening stages of passion fruit, the expression level of most *PebHLHs* in the immature stage (T1) was higher than that in the mature stage (T2 and T3). Eight *PebHLHs* with differentially expressed under different stress treatments and different ripening stages were selected and verified by qRT-PCR. In this research, the expression of one member, *PebHLH56,* was induced under cold stress. Further, the promoter of *PebHLH56* was fused to β-Galactosidase (GUS) to generate the expression vector that was transformed into *Arabidopsis*. It showed that *PebHLH56* could significantly respond to cold stress. This study provided new insights into the regulatory functions of *PebHLH* genes during fruit maturity stages and abiotic stress, thereby improving the understanding of the characteristics and evolution of the *PebHLH* gene family.

**Keywords:** bHLH; passion fruit; abiotic stress; cold; genome-wide analysis
