Cloning And Function Analysis Of NO₃^-Transporter Gene ZxNRT1.5 In The Desert Plant Zygophyllum Xanthoxylum

The frequent drought and irrigation-induced secondary soil salinity severely restrict the sustainable development of agriculture in northern China.The xerophytes that are widely distributed in desert regions,have evolved multiple mechanisms to resist to various environmental stresses including drought and soil salinity.The study on adaptive mechanisms of xerophytes to drought and salinity,and exploration of superior gene resources in these species,would provide important theoretical basis for genetically improvements of stress resistance in common forages and crops.Zygophyllum xanthoxylum is a dominant species distributed throughout the desert regions of northwest China,with excellent adaptability to salt and drought stresses as well as barren soils.Our previous studies have showed that Z.xanthoxylum could efficiently sequestrate large amounts of sodium?Na⁺?into vacuole and thereby,use Na⁺as a beneficial osmotium to adapt to salt and drought environments;meanwhile,Na⁺could enhance nitrogen absorption and consequently,improve the drought resistance in Z.xanthoxylum.However,the synergistic mechanism between nitrogen transport and Na⁺accumulation in Z.xanthoxylum is still unclear.By using the model plant Arabidopsis which shows decreased tissue NO₃^-concentrations under salt and drought stresses and exhibits very limited salt tolerance and drought resistance,researchers have demonstrated that the NO₃^-transporter AtNRT1.5 mediating NO₃^-load into xylem can affects Na⁺transport and distribution and therefore,is involved in salt tolerance,and meanwhile,negatively regulates drought resistance in plants.However,the possible roles of NRT1.5 homologous protein from halophytes and xerophytes that possess strong stress tolerance in NO₃^-,Na⁺transport and salt tolerance and drought resistance still remain elusive.In the present study,the homologous gene of NRT1.5 in the desert plant Z.xanthoxylum that exhibits strong stress tolerance and could absorb a great quantity of Na⁺and maintain stable tissue NO₃^-concentrations under adversity was cloned,its expression pattern and locations were systemically analyzed,and the possible roles of this gene in NO₃^-transport,Na⁺accumulation and plant stress resistance were investigated by the overexpression of this gene in Arabidopsis.The main results are as follows:1.We firstly cloned the nitrate transporter gene ZxNRT1.5 from Z.xanthoxylum.The full length of ZxNRT1.5 is 2277 bp,it encodes 597 amino acids,containing 12transmembarne domains.ZxNRT1.5 was preferentially expressed in roots,and in contrast to AtNRT1.5 in the salt-sensitive glycophyte Arabidopsis,the expression of ZxNRT1.5 in roots was significantly induced by salt treatments and osmotic stress.2.The In Situ PCR result showed that ZxNRT1.5 was mainly expressed in the root stele,and its encoding protein was located on the cell plasma membrane.3.The overexpression of ZxNRT1.5 in roots of model plant Arabidopsis showed that,ZxNRT1.5 is not only increased the NO₃^-transport from roots into shoots,but also could regulate the expression of Na⁺transport-related genes including AtHKT1;1 and affect the membrane hyperpolarization to activate Cl^-efflux channels and as a consequence,substantially increased Na⁺and Cl^-accumulations in tissues and improved plant salt tolerance.4.Under osmotic stress,the expression of ZxNRT1.5 in roots of Arabidopsis could enhance NO₃^-absorption and transport ability,and led the plants to absorb more Na⁺from medium with extremely low Na⁺concentration and accumulate them in shoots as an beneficial osmoticum,resulting in a substantial improvement overall growth of Arabidopsis.All the above results indicated that ZxNRT1.5 could synergistically regulate the transport of NO₃^-and Na⁺,and therefore,affect the salt tpolerance and drought resistance of plants.