Molecular Mechanism of ARF5-AHL15-Mediated Auxin-Induced Embryogenic Cell Formation in Apples

The somatic embryo (SE) has bipolar characteristics, which is an ideal material for large-scale microproduction of woody plants represented by apples, and the somatic embryo is also an excellent receptor for genetic transformation. The formation of embryogenic cells is a prerequisite for somatic embryogenesis to occur. The embryogenic cells of apples cannot be obtained without induction of exogenous auxin, but how the auxin pathway regulates this process remains unknown. In this study, via RNA sequencing, <i>MdARF5</i> and <i>MdAHL15</i> were identified as differentially expressed genes involved in this process. Overexpression of <i>MdARF5</i> and <i>MdAHL15</i> induced the formation and proliferation of embryogenic cells and thus substantially shortened the induction cycle and improved the somatic embryo proliferation efficiency. A yeast one-hybrid assay showed that MdARF5 can directly bind to the promoter of <i>MdAHL15</i>. β-Glucuronidase (GUS) and dual-luciferase reporter assays revealed that MdARF5 activation of <i>MdAHL15</i> transcription was substantial. In conclusion, our results suggest that <i>MdAHL15</i> is induced by auxin and promotes the formation of embryogenic cells in early somatic embryogenesis via the positive regulation of MdARF5 in apples. The results will provide a theoretical basis for somatic embryogenesis-based development, reproduction, and transgenic breeding in apples.