Functional identification of MdMYB5 involved in secondary cell wall formation in apple

Being the principal elements of secondary cell wall, cellulose and lignin both play a strengthening role in plant structures and stress resistance. However, little research has been done regarding the molecular mechanisms involved in the formation of cellulose and lignin in apple. In this study, in order to better understand the regulatory network in the formation of secondary cell wall, an R2R3 MYB transcriptional factor MdMYB5 was identified and explored. The subcellular localization experiments showed that MdMYB5 could function in the nucleus. Even though lignin and cellulose content, and the expression of their biosynthesis related genes decreased in the MdMYB5-RNAi apple, the ectopic overexpression of MdMYB5 promotes lignin and cellulose content in Arabidopsis, which contributes to the dwarf phenotype. At the same time, salt and osmotic stress affect MdMYB5-RNAi apple tissue cultures. Further transcriptional activation assays carried out demonstrated that MdMYB5 could be activated by MdMYB46 and MdSND1. In conclusion, MdMYB5 was required for the normal formation of secondary cell wall in apple and could be activated by the key regulatory factors MdMYB46 and MdSND1.