Overexpression of the tyrosine decarboxylase gene MdTyDC in apple enhances long-term moderate drought tolerance and WUE.

Drought stress affects the apple yield and quality. Tyrosine decarboxylase (TyDC) plays a fundamental role in many secondary metabolite reactions in higher plants (including those involving dopamine). Our aims of this study are: 1) to identify the role of TyDC in dopamine derivative biosynthesis and its function in long-term moderate drought conditions; and 2) to explore the role of MdTyDC in plant growth and development as well as the drought stress response. Wild type and three independently apple plants overexpression of MdTyDC were treated for long-term moderate drought stress. The growth and physiological parameters of apple plant, photosynthetic capacity, antioxidant enzymes activity, water use efficiency (WUE), stomatal behavior, amino acid content and dopamine content were detected under long-term moderate drought stress. Overexpression of MdTyDC (OE) in apple showed better growth performance, higher photosynthetic capacity and higher capacity for photochemical reactions than wild type lines (WT). Under long-term moderate drought stress, OE lines showed higher WUE, increased ABA content, decreased stomatal aperture, higher antioxidant activity, lower accumulation of ROS and increases in amino acids, such as proline, phenylalanine and lysine. In addition, qRT-PCR revealed higher gene expression of MdTyDC and dopamine content in OE compared with WT lines under long-term moderate drought stress. These results indicate that MdTyDC confers long-term moderate drought tolerance by improving photosynthetic capacity, WUE, antioxidant activity, dopamine content and changing the contents of amino acids (such as proline accumulation).

Heterologous Expression of the Melatonin-Related Gene HIOMT Improves Salt Tolerance in Malus domestica.

Melatonin, a widely known indoleamine molecule that mediates various animal and plant physiological processes, is formed from N-acetyl serotonin via N-acetylserotonin methyltransferase (ASMT). ASMT is an enzyme that catalyzes melatonin synthesis in plants in the rate-determining step and is homologous to hydroxyindole-O-methyltransferase (HIOMT) melatonin synthase in animals. To date, little is known about the effect of HIOMT on salinity in apple plants. Here, we explored the melatonin physiological function in the salinity condition response by heterologous expressing the homologous human HIOMT gene in apple plants. We discovered that the expression of melatonin-related gene (MdASMT) in apple plants was induced by salinity. Most notably, compared with the wild type, three transgenic lines indicated higher melatonin levels, and the heterologous expression of HIOMT enhanced the expression of melatonin synthesis genes. The transgenic lines showed reduced salt damage symptoms, lower relative electrolyte leakage, and less total chlorophyll loss from leaves under salt stress. Meanwhile, through enhanced activity of antioxidant enzymes, transgenic lines decreased the reactive oxygen species accumulation, downregulated the expression of the abscisic acid synthesis gene (MdNCED3), accordingly reducing the accumulation of abscisic acid under salt stress. Both mechanisms regulated morphological changes in the stomata synergistically, thereby mitigating damage to the plants' photosynthetic ability. In addition, transgenic plants also effectively stabilized their ion balance, raised the expression of salt stress-related genes, as well as alleviated osmotic stress through changes in amino acid metabolism. In summary, heterologous expression of HIOMT improved the adaptation of apple leaves to salt stress, primarily by increasing melatonin concentration, maintaining a high photosynthetic capacity, reducing reactive oxygen species accumulation, and maintaining normal ion homeostasis.

MdTyDc Overexpression Improves Alkalinity Tolerance in Malus domestica.

Tyrosine is decarboxylated to tyramine by TYDC (Tyrosine decarboxylase) and then hydroxylated to dopamine, which is involved in plant response to abiotic stress. However, little is known about the function of MdTyDc in response to alkaline stress in plants. In our study, it was found that the expression of MdTyDc was induced by alkaline stress. Therefore, the apple plants overexpressing MdTyDc was treated with alkali stress, and we found that MdTyDc played an important role in apple plants' resistance to alkali stress. Our results showed that the restriction on the growth, the decrease of membrane permeability and the accumulation of Na+ were alleviated to various degrees in MdTyDc transgenic plants under alkali stress. In addition, overexpression of MdTyDc enhanced the root activity and photosynthetic capacity, and improved the enzyme activity related to N metabolism, thus promoting N absorption. It is noteworthy that the dopamine content of these three transgenic lines is significantly higher than that of WT. In summary, these findings indicated that MdTyDc may enhance alkaline tolerance of apples by mediating dopamine content, mainly by maintaining high photosynthetic capacity, normal ion homeostasis and strong nitrogen absorption capacity.