Overexpression of the tyrosine decarboxylase gene MdTyDC confers salt tolerance in apple

Dopamine mediates numerous physiological processes in plants. Tyrosine decarboxylase (TyDC, EC 4.1.1.28) is a common enzyme prevalent in plants and mediates numerous secondary reactions, but little is known about the role of TyDC in the dopamine derivative biosynthesis and function in salt tolerance. Here, apple and apple calli were utilized to study the function of MdTyDC under salt conditions. Overexpression of MdTyDC (OE) significantly increased dopamine content in apple and apple calli. The OE apple lines exhibited increased growth performance and higher photosynthetic capacity and contained more photosynthetic pigments than those of the wild-type lines (WT) under salt conditions. The OE lines had lower H2O2 and O-2. according to DAB and NBTstaining compared to the WT lines. Overexpressing MdTyDC enhanced antioxidant enzyme activities and maintained ion homeostasis, as well as increased the expression levels of associated genes. Furthermore, over-expressing MdTyDC enhanced proline content, increased fresh weight, and reduced malondialdehyde (MDA) content in apple calli; however, RNA interference lines showed higher MDA content and lower fresh weight and proline content. Our findings demonstrate that overexpressing MdTyDC induced higher dopamine levels and increased scavenging of reactive oxygen species and the Na+/K+ balance; thus, enhancing salt tolerance.