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, MdARF5 and MdAHL15 were identified as differentially expressed genes involved in this
process. Overexpression of MdARF5 and MdAHL15 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 MdAHL15. β-Glucuronidase (GUS) and dual-luciferase reporter assays
revealed that MdARF5 activation of MdAHL15 transcription
was substantial. In conclusion, our results suggest that MdAHL15 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.