The eTM-miR3699-MAN7 mediated cell wall degradation in regulating embryogenic cell formation during the early stage of somatic embryogenesis in apple

Abstract Somatic embryogenesis in plants requires the prior formation of embryogenic cells in plants. The remodeling of the cell wall in mature somatic cells is a prerequisite for embryogenic cell formation. However, the mechanism of this process remains unelucidated. In this study, eTM3699, miR3699 and MANNAN7 (MAN7) were identified as key regulators of embryogenic cell formation through whole-transcriptome sequencing. The dual-luciferase reporter assays and GUS histochemical staining assays, were used to identified the regulatory network of eTM3699-miR3699-MdMAN7. The overexpression and CRISPR/Cas9 mediated transgenic assays were used for functional analysis of miR3699 and MdMAN7. MdMAN7 overexpression can enhance the activity of β-mannanase, induce hemicellulose degradation, and reshape the cell wall of highly differentiated somatic cells, relieve the restriction on cell differentiation and division, ultimately positively regulating the embryogenic cell formation. Specifically, the overexpression of MdMAN7 can significantly improve the efficiency and shorten the induction cycle of somatic embryogenesis. miR3699 acted by negatively regulating MdMAN7. In addition, eTM3699 were identified as endogenous target mimics of miR3699 that bind to miR3699 to prevent cleavage of MdMAN7 and thereby positively regulate embryogenic cell formation. In conclusion, our results elucidate the mechanism of eTM-miR3699-MAN7 module regulating embryogenic cell formation during the early stage of somatic embryogenesis in apple.