The Importance of Conserved Serine for C-Terminally Encoded Peptides Function Exertion in Apple

Background: The C-terminally encoded peptide (CEP) family has been shown to play vital roles in plant growth. Although a genome-wide analysis of this family has been performed in Arabidopsis, little is known regarding CEPs in apple (Malus domestica). Methods: Here, a comprehensive bioinformatics approach was applied to identify MdCEPs in apple, and 12 MdCEP genes were identified and distributed on 6 chromosomes. Results: MdCEP1 peptide had an inhibitory effect on root growth of apple seedlings, indicating that MdCEP1 played a negative role in root development. In addition, the serine and glycine residues remained conserved within the CEP domains, and MdCEP1 lost its function after mutation of these two key amino acids, suggesting that Ser(10) and Gly(14) residues are crucial for MdCEPs-mediated root growth of apple. Encouragingly, multiple sequence alignment of 273 CEP domains showed that Ser(10) residue was evolutionarily conserved in monocot and eudicot plants. MdCEP derivative (Ser to Cys) lost the ability to inhibit the root growth of Nicotiana benthamiana, Setaria italic, Samolous parviflorus, and Raphanus sativus L. and up-regulate the NO(3(−)) importer gene NRT2.1. Conclusion: Taken together, Ser(10) residue is crucial for CEP function exertion in higher land plants, at least in apple.