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.
Secondary wall-associated NAC domain protein1(SND1)is a key regulator directly regulating the expression levels of MYB46 and MYB83 in the regulation network for secondary wall synthesis,especially in plant fibres.In this study,a SND1 gene was isolated from hawthorn(Crataegus pinnatifida)and named as CpSND1 because it has a conservative N-terminal DNA-binding domain with AtSND1.Arabidopsis plants overexpressing CpSND1 had similar phenotypes as plants overexpressing AtSND1,including inhibited growth,upward-curling leaves,sepal dysplasia and sterility.In addition,overexpressing CpSND1in Arabidopsis also induced the expression of downstream genes,including lignin,cellulose and xylan biosynthesis genes as well as MYB genes.Our results provided functional information of CpSND1 for future genetic engineering in hawthorn.
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.
