Genome-Wide Identification of the bHLH Gene Family in Kiwifruit (Actinidia chinensis) and the Responses of AcbHLH84 and AcbHLH97 Under Drought Stress

Drought stress is one of the primary environmental factors affecting plant survival rates and productivity, and it is a key bottleneck restricting the development of the world kiwifruit industry. Therefore, studying the drought resistance-related genes and drought resistance mechanisms of kiwifruit is essential. The bHLH (basic helix-loop-helix) TF family plays a crucial role in the resistance of kiwifruit to abiotic stresses such as drought stress. In this study, we analyzed the response of the AcbHLH gene in kiwifruit under drought stress based on the kiwifruit genome database, transcriptome data, and metabolome data. One hundred eighty-seven AcbHLH genes were identified via bioinformatics and divided into eighteen subfamilies via phylogenetic analysis. The cis-acting elements of the AcbHLH gene are mainly hormone-related cis-acting elements. Under drought stress, 64 AcbHLH genes were significantly different, 5 AcbHLH genes whose expression significantly differed were randomly selected for qRT-PCR verification, and the correlation between the qRT-PCR results and the transcriptome data was high. The determination of plant hormone contents revealed that the contents of plant hormones, such as JA, changed markedly before and after drought stress. Through the combined analysis of transcriptome and metabolome data, it was speculated that AcbHLH84 and AcbHLH97 have functions similar to those of the MYC2 transcription factor and are the main downstream effectors in the JA signaling pathway; these functions could be activated and participate in the JA signaling pathway and that the activation of the JA signaling pathway would inhibit the production of reactive oxygen species. In turn, the drought resistance of kiwifruit is improved. The AcbHLH84 and AcbHLH97 genes could be candidate genes for breeding new transgenic drought-resistant kiwifruit varieties.