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.

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.