BEL1-like homeodomain transcription factor SAWTOOTH1 (MdSAW1) in Malus domestica enhances the tolerance of transgenic apple and Arabidopsis to zinc excess stress.

In recent years, the phenomenon of zinc pollution in orchards has become increasingly serious, and the safety of apple production is facing a major risk. Therefore, exploring excellent genes for zinc tolerance has a positive effect on apples. Up to now, there is still a lack of attention on genes related to zinc stress tolerance in apples. In this study, the apple transcriptome map under zinc stress (1000 μM ZnSO) was generated based on high-throughput sequencing. Through transcription factor analysis and association network prediction, TALE superfamily SAWTOOTH 1 was found to have an important role in 32 up-regulated core transcription factors. Further, BEL1-like homeodomain MdSAW1 gene from Malus domestica was overexpressed in Arabidopsis seedlings ('Col-0'), apple callus tissues ('Orin'), and apple plants ('GL-3'), and the results showed that the transformed lines carried obvious tolerance to zinc stress, which was reflected in the significant reduction of relative dielectric leakage, malondialdehyde, O and HO content. The interaction between protein and DNA confirmed that MdSAW1 binds to natural resistance-associated macrophage protein NRAMP2 promoter to inhibit its transcription and thus regulate zinc ion homeostasis. In addition, overexpression of MdSAW1 increased the activity of antioxidant enzymes (superoxide dismutase, catalase, and glutathione peroxidase) and caused differences in metabolites in plants. MdSAW1 endows plants with strong tolerance to Zn stress, therefore, this study provides valuable reference for genetic improvement and environmental adaptation of fruit trees.