Dual role of MdSND1 in the biosynthesis of lignin and in signal transduction in response to salt and osmotic stress in apple

Clarifying the stress signal transduction pathway would be helpful for understanding the abiotic stress resistance mechanism in apple (Malus × domestica Borkh.) and could assist in the development of new varieties with high stress tolerance by genetic engineering. The key NAC transcription factor SND1, which is involved in the lignin biosynthesis process in apple, was functionally analyzed. The results of the stress treatments indicated that MdSND1 could be induced by salt, mannitol and ABA. Compared with wild-type GL-3 plants, MdSND1-overexpressing apple plants with greater antioxidant capacity and lignin were more resistant to salt and simulated osmotic stress, while RNAi plants were more vulnerable. Additionally, molecular experiments confirmed that MdSND1 could regulate the biosynthesis of lignin by activating the transcription of MdMYB46/83. Moreover, genes known to be involved in the stress signal transduction pathway (MdAREB1A, MdAREB1B, MdDREB2A, MdRD29A, and MdRD22) were screened for their close correlations with the expression of MdSND1 and the response to salt and osmotic stress. Multiple verification tests further demonstrated that MdSND1 could directly bind to these gene promoters and activate their transcription. The above results revealed that MdSND1 is directly involved in the regulation of lignin biosynthesis and the signal transduction pathway involved in the response to both salt and osmotic stress in apple.