Effects Of Different Nitrogen Application On Sugar Contents And Metabolism In Apple Seedlings

Nitrogen exerts a great influence on the life process,yield formation and quality of plants.As one of the main metabolites of plants,sugar is the source of energy and basic material for the growth and development.The sorbitol-based carbohydrate metabolism in apple impacts the growth and fruit quality of plants,moreover,the ability of sorbitol and fructose metabolism is of special importance in the regulation of apple sink strength.In the mutual regulation of carbon and nitrogen metabolism,low nitrogen-promoted root growth and high nitrogen-promoted shoot growth should have a strong relationship with the sugar metabolism.It is of great significance for the efficient use of nitrogen and regulation of plant growth to explore the effects of nitrogen supply levels on carbon metabolism in apple.In this paper,Malus hupehensis(pamp)Rehd rootstock seedlings and MdFRK2 transgenic apples were chosen as study materials.The effects of sugar contents,sugar metabolism-related enzyme activities and gene expression in roots,leaves and stem tips of Malus hupehensis(pamp)Rehd seedlings were studied under different nitrogen levels incubating under hydroponics and sandy conditions respectively.Furthermore,the MdFRK2 transgenic apples were used to study how apples with altered fructose metabolism will respond to the low nitrogen level.The main results are:1.In the hydroponic seedlings of Malus hupehensis(pamp)Rehd,the decrease of nitrogen supplement can stimulate the rapid growth of apple roots.Specifically,the root total length,surface area,volume and root numbers were all increased comparing with a relative higher nitrogen level.At low nitrogen level,the genes expression of MdSOT1 and MdSUT3 which involved in assimilate unloading in roots were up-regulated.The accumulation of sorbitol and sucrose in the new fine roots was higher,and the activity and gene expression of sucrose synthase,invertase and sorbitol dehydrogenase,which are involved in the degradation of sucrose and sorbitol,were significantly increased in low nitrogen level.Additionally,the activity of fructokinase and hexokinase involved in root hexose phosphorylation and genes expression like MdFRK2 were significantly up-regulated in the low nitrogen level,and the hexose phosphate products F6 P and G6 P in the roots accumulated greatly.This increased the sugar metabolism rate of the roots and the sink strength,indicating that the utilization of sugar in the roots was accelerated by the low nitrogen in order to meet the demand for sugar under the rapid growth of roots.Moreover,the expression of MdA6 PR in leaves was significantly up-regulated at low nitrogen level,while there was no obvious change in MdSPS,suggestingthat under low nitrogen conditions,it can promote the relative synthesis of sorbitol in leaves.2.In the sand culture of two-year Malus hupehensis(pamp)Rehd under different nitrogen conditions,the study on the sugar metabolism of above-ground sink tissue stem tips and source leaf showed that the increased nitrogen level application rate promoted the growth of stem tips,while the synthesis of sorbitol and sucrose in the leaves was hampered.Under higher nitrogen levels,the expression of MdSOT and MdSUT in stem tips was up-regulated,and the activity of sucrose phosphate synthase and aldose-6-phosphate reductase increased with the increasing nitrogen level in stem tips.The main enzyme involved in the sucrose degradation of stem under high nitrogen conditions is sucrose synthase.Furthermore,the activity of fructokinase and hexokinase were also enhanced under high nitrogen level,the gene expression of MdSuSy3 and MdFRK2 were up-regulated to promote the efficient use of stem tip sugar under high nitrogen conditions and its rapid growth.3.To investigate the role of MdFRK2 in the efficient use of nitrogen and sugar under low nitrogen conditions,we investigated the response of MdFRK2 transgenic apple seedlings to low nitrogen.In the tobacco system,it was found that the nitrogen level can affect the activity of the promoter of the MdFRK2,and the activity of the promoter is enhanced with an increasing nitrogen level increased.Under low nitrogen conditions,the changes of fructokinase activity and sugar content in wild-type GL3 roots were consistent with those in only the Malus hupehensis(pamp)Rehd other than the leaves,which can be explained by the different genotype.MdFRK2 transgenic apple seedlings inhibited the increase of fructokinase activity and the reduction of fructose and glucose content in roots and leaves under low nitrogen conditions.Compared with normal nitrogen conditions,the nitrogen and chlorophyll content of transgenic apple seedlings decreased with the same decline in wild type under low nitrogen conditions,while the net photosynthetic rate of transgenic lines did not decrease significantly,although wild type reduced obviously.At the same time,the growth of roots of MdFRK2 overexpressed transgenic lines under low nitrogen conditions was significantly faster than that of wild type.These results indicate that MdFRK2 overexpressed transgenic apples can increase resistance to nitrogen stress by promoting root growth under low nitrogen conditions.