Exogenous Dopamine and MdTyDC Overexpression Enhance Apple Resistance to Fusarium solani.

Fusarium solani, one of the main pathogenic fungi involved in apple replant disease (ARD), is a serious threat to apple growth and development. Dopamine and tyrosine decarboxylase (TyDC), a key enzyme in the dopamine synthesis pathway, have been reported to play an active role in plant responses to biotic and abiotic stresses, but little is known about the functions of dopamine and Malus domestica TyDC (MdTyDC) in the interaction between F. solani and apple roots. In this study, seedlings treated with exogenous dopamine and apple plants overexpressing MdTyDC were inoculated with F. solani; both treatments reduced the root system damage caused by F. solani. After inoculation with F. solani, exogenous dopamine increased dopamine content in the seedlings; alleviated the inhibition of biomass accumulation; increased root metabolic activity, photosynthetic efficiency, and antioxidant enzyme activities; reduced reactive oxygen species accumulation; and upregulated the expression of genes encoding chitinase, ß-1,3-glucanase, and pathogenesis-related proteins. Similar results were observed in MdTyDC-overexpressing apple plants. In addition, the overexpression of MdTyDC increased tyramine content and the deposition of cell wall-bound amines in roots. Overall, our results reveal that exogenous dopamine and overexpression of MdTyDC enhance apple resistance to F. solani, which is an important application for the prevention of ARD.

Dopamine Enhances the Resistance of Apple to Valsa mali Infection.

Apple Valsa canker is considered one of the most serious apple diseases. Dopamine is a catecholamine with key physiological functions in plants. Tyrosine decarboxylase (TYDC) is not only involved in the synthesis of dopamine in plants but may also play an important role in the resistance of plants to pathogen infection. In this study, we show that 100-µM exogenous dopamine application and Malus domestica TYDC (MdTYDC) overexpression (or overexpressing, OE) enhances the resistance of apple to Valsa mali infection, likely because the increased dopamine content reduces the accumulation of H2O2 and increases the accumulation of phenolic compounds and salicylic acid (SA) in dopamine-treated and OE apple plants. The activity of chitinase and ß-1, 3-glucanase and the expression of SA-related genes were induced more strongly by V. mali in dopamine-treated and OE apples. The dopamine content was significantly higher in dopamine-treated and OE apples than in their respective controls under both normal and inoculated conditions (P < 0.05). Overall, these findings indicate that the application of exogenous dopamine and the OE of MdTYDC may enhance the resistance of apples to V. mali infection by altering the dopamine content, which improves antioxidant capacity, promotes the accumulation of phenolic compounds and SA, and enhances the activity of disease resistance-related proteins.

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).

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.