MdWRKY71 as a positive regulator involved in 5-aminolevulinic acid-induced salt tolerance in apple

5-Aminolevulinic acid (ALA), is a novel plant growth regulator that can enhance plant tolerance against salt stress. However, the molecular mechanism of ALA is not well studied. In this study, ALA improved salt tolerance of apple (Malus × domestica 'Gala') when the detached leaves or cultured calli were used as the materials. The expression of MdWRKY71, a WRKY transcription factor (TF) gene was found to be responsive to NaCl as well as ALA treatment. Functional analysis showed that overexpressing (OE)-MdWRKY71 significantly improved the salt tolerance of the transgenic apple, while RNA interfering (RNAi)-MdWRKY71 reduced the salt tolerance. However, exogenous ALA alleviated the salt damage in the RNAi-MdWRKY71 apple. When MdWRKY71 was transferred into tobacco, the salt tolerance of transgenic plants was enhanced, which was further improved by exogenous ALA. Subsequently, MdWRKY71 bound to the W-box of promoters of MdSOS2, MdNHX1, MdCLC-g, MdSOD1, MdCAT1 and MdAPX1, transcriptionally activating the gene expressions. Since the genes are responsible for Na⁺ and Cl⁻ transport and antioxidant enzyme activity respectively, it can be concluded that MdWRKY71, a new TF, is involved in ALA-improved salt tolerance by regulating ion homeostasis and redox homeostasis. These results provided new insights into the transcriptional regulatory mechanism of ALA in enhancing apple salt tolerance.