MdCIB1, an apple bHLH transcription factor, plays a positive regulator in response to drought stress

Abstract Drought restricts the plant growth and productivity worldwide. The basic helix-loop-helix (bHLH) transcription factors play key roles in response to various abiotic stresses. In this study, we identified and characterized a bHLH transcription factor, cryptochrome-interacting bHLH 1 (MdCIB1), from apple (Malus domestica). MdCIB1 had a typical bHLH domain and was localized in the nucleus. Moreover, MdCIB1 also interacted with MdCRY2, and participated in blue light response and flowering time regulation. RT-qPCR analysis showed that PEG 6000 notably induced the transcript level of MdCIB1. MdCIB1-overexpressing apple calli displayed improved tolerance to PEG 6000 treatment. Ectopic expression of MdCIB1 in Arabidopsis improved drought stress resistance, as reflected by better physiological indexes under drought stress, such as longer roots, higher chlorophyll levels and survival rate, and lower water loss rate of leaves than Col control. Further investigation demonstrated that MdCIB1 played a positive regulator in ABA sensitivity and ABA-mediated stomatal closure. In addition, MdCIB1 transgenic Arabidopsis exhibited higher proline content, lower malondialdehyde (MDA) content, and H2O2 and O2− accumulation, as well as increased the activities of antioxidant enzymes under drought stress, which were able to protect plants from osmotic and oxidative damages. Besides, the transcripts of ROS scavenging-related genes and stress-responsive genes were also significantly upregulated in MdCIB1 transgenic lines. In summary, our study demonstrated that MdCIB1 plays a positive role in drought resistance.