Study On The Function Of Dopamine In Response To Low Nitrogen And Alkaline Stress In Apples

Apple（Malus domestica Borkh.）is one of the four main fruits in the world.The loess Plateau in northwest China is very suitable for apple cultivation due to its abundant sunlight,deep soil layer,dry climate and large temperature difference between day and night.However,the low nitrogen（N）and organic N in the soil and the soil impoverishment caused by soil erosion restricted the sustainable development of apple in northwest China.Dopamine is a kind of catecholamine,which has strong antioxidant and reducing ability.In plants,dopamine is involved in the ROS scavenging process and can interact with hormones to affect plant growth and development.However,there have been few reports on the effect of dopamine on plant growth under low N stress.Therefore,in this study,the effect of exogenous dopamine on the growth of M.hupehensis under low N stress was firstly studied through hydroponic experiments.In addition,transgenic apple plants with key enzymes（tyrosine decarboxylase,TyDc）in the process of dopamine synthesis were used as test materials and treated with low N stress,preliminarily revealing the regulation mechanism of the overexpressed MdTyDc on N metabolism in response to low N stress and alkali stress in apples.The main results are as follows:1.The effect of exogenous dopamine on plant growth under low N stress was studied by hydroponic experiment with apple rootstock M.hupehensis.The results showed that the exogenous application of 100μmol L^-1 dopamine significantly reduced the inhibition of plant growth under low N stress.Exogenous dopamine can not only modify root system structure under low N stress,but also have positive effects on the uptake,transport,and distribution of N,P,and K.In addition,exogenous dopamine also enhanced the activity of enzymes involved in N metabolism（nitrate reductase,nitrite reductase,glutamine synthetase,glutamate synthetase）under low N stress.qRT-PCR further proved that exogenous dopamine promoted the expression of ethylene signaling genes（ERF1、ERF2、EIL1、ERS2、ETR1 and EIN4）.These results suggested that exogenous dopamine can mitigate low N stress by regulating the absorption of mineral nutrients,possibly through the regulation of the ethylene signaling pathway.2.The effect of MdTyDc overexpression（OE）in regulating apple response to low N stress was studied by hydroponic experiments and low N treatment to callus with MdTyDc overexpressing apple lines and callus.The results showed that the overexpression of MdTyDc could significantly alleviate the effect of low N stress on the growth of apple plants.The net photosynthetic rate of overexpressed lines under low N stress was higher than that of the WT plants.Compared with the WT lines,the leaves of OE lines maintained a higher Fv/Fm value,which was conducive to maintaining normal photosynthesis of the plants.N content and N metabolism related enzyme activity in the OE lines were also higher than those in the WT plants.In addition,exogenous dopamine had complementary effects on the Ri-MdTyDc callus growth.3.The effect of MdTyDc overexpression on N metabolism of apple under alkali stress was studied by hydroponics experiment with MdTyDc transgenic apple lines.The results showed that after 15 days of hydroponic alkali stress,the overexpression of MdTyDc could alleviate the effect of alkali stress on the growth of apple plants.The relative electrolyte leakage（REL）of OE lines under alkali stress was lower than that of WT plants,while the root activity was higher than that of WT lines.The net photosynthetic rate and chlorophyll content of the OE lines were also higher than those of the WT lines.Compared with the WT,the Na⁺/K⁺ratio was lower in the root,stem and leaf organs of the OE lines,while the anion content and proline content were higher,which was beneficial to maintain the normal ion homeostasis of the plants.In addition,N content and N metabolism related enzyme activity in the OE lines were higher than those in the WT lines.Further study found that the expression level of SOS signaling system genes was higher than WT plants under alkali stress.The expression patterns of other ion transport-related genes,such as MdHKT1 and MdAKT1,were similar to MdSOS3.