Characterization and Identification of PpEIN3 during the Modulation of Fruit Ripening Process by Ectopic Expressions in Tomato

Ethylene is one of the most important phytohormone in plants and plays a critical role during growth, development, maturity, and aging. The framework of the ethylene signaling pathway is well reported. Nevertheless, studies on Ethylene Insensitive 3 (EIN3), the downstream regulator of the ethylene signaling pathway, need to be investigated, especially in peach [Prunus persica (L.) Batsch]. In this study, we cloned PpEIN3 from peach and characterized it in tomato (Solanum lycopersicum L.). Our results depicted that the open-reading frame of PpEIN3 was 1875 bp, encoding a protein with 624 amino acid residues that contained a conserved EIN3 domain, a highly conserved N-terminal region, and seven DNA-binding sites. PpEIN3 showed very close association with homologous EIN genes from apple (Malus domestica Borkh.) and grapevine (Vitis vinifera L.). All investigated EIN proteins shared similar domains and structures. The PpEIN3 promoter possessed several motifs related to hormones that affect fruit development and ripening. Spatial-temporal expression analysis revealed that PpEIN3 was expressed at high levels in the late stage of fruit development vs. the early stage. In transgenic tomato, PpEIIN3 showed overexpression and the key ethylene biosynthesis genes SlACO1, SlACS1, and SlSAMS1 were upregulated and promoted early maturation in fruit. By contrast, PpEIIN3 silencing delayed ripening and reduced SlEIN3 expression in tomato. The results confirmed that PpEIN3 is a positive regulator of the ethylene signal transduction pathway, which promoted fruit ripening. Our findings provide valuable insight to the roles in ethylene signal components in the modulation of peach fruit ripening.