The Physiological And Molecular Mechanisms Of Na~+ Involved In The Adaptation To Salt And Drought In The Succulent Xerophyte Zygophyllum Xanthoxylum

The arid and semi-arid lands account for 50%of total territory of China.The frequent drought,serious water and soil erosion and increasing desertification had seriously restricted the sustainable development for agriculture and the construction of ecological environment in North China.Given this background,seeking agriculture with high yield under drought environments has become a problem demanding prompt solution.Most of crops and forages have grown in abundant farming conditions for a long period,thus their genetic potential of stress resistance is very limited.This is also an important reason why agricultural and livestock production is restricted by drought stress.Desert xerophytes grown in extremely arid regions,however,have developed various mechanisms to adapt to harsh environments during the process of long-term evolution.Absorbing a great quantity of Na+ from low-salinity soil may be the most effective strategies for the succulent xerophytes native to the desert areas of northwest China,such as Zygophyllum xanthoxylum,to adapt arid environment;and certain concentrations of Na+ not only stimulated growth of Z.xanthoxylum,but also alleviated the deleterious impact of water deficit.However,possible physiological and molecular mechanisms involved in these observations had not been systematically explored.The aims of this Ph.D project,therefore,are to investigate the physiological and molecular mechanisms of Na+ involved in the adaptation to salt and drought environments in Z.xanthoxylum.The main findings are as follows:1.We firstly discovered the important contributions of Na+ in the osmotic adjustment in leaf of Z.xanthoxylum.We found that increasing the contribution of Na+ in the osmotic adjustment and maintaining the stable of K+ concentration in leaf may be the important strategies for Z.xanthoxylum to adapt drought environments;and certain concentration of NaCl could improve photosynthesis and water status in Z.xanthoxylum by enhancing osmotic adjustment capacity,thereby stimulating the growth and improve the drought resistance of Z.xanthoxylum.2.The transcriptome was firstly analyzed in Z.xanthoxylum under NaCl and omotic stress,and 106423 Unigenes were obtained.3.Under 50 mM NaCl and-0.5MPa osmotic stress,the expression level of many genes encoding Na+,K+ transporters and transporters for essential elements such as N,P,Ca2+,Mg2+ and microelements such as Cu2+,Zn2+ significantly increased in root and leaf,thereby enhancing the uptake and transport of nutrient elements in Z.xanthoxylwn.4.Under 50 mM NaCl and-0.5MPa osmotic stress,the expression level of a large number of genes related to ROS scavenging system significantly increased in root and leaf of Z.xathoxylum,which is beneficial to mitigate the damage of ROS to cell biomembrane system.5.50 mM NaCl significantly induced the expression of genes encoding photosynthetic electron transporters and enzymes involved in carbon assimilation,while inhibit the expression of genes related to chlorophyll catabolic,thus improving the photosynthesis of Z xanthoxylum.6.The gene encoding the plasma membrane Na+/H+ antiporter,ZxSOS1,was cloned from Z.xanthoxylun,and we found that it mainly expressed in root and was significantly induced by NaCl and osmotic stress.Using RNAi technique,we further found that under 50 mM NaCl,ZxSOS1-RNAi plants accumulated more Na+ in root but less Na+ in leaf and stem than WT,while showed a decreased spatial distribution of K+ in leaf and root but an increased distribution of K+ in stem than WT,indicating that ZxSOS1 is not only essential in long-distance transport of Na+ from root to shoot,but also vital for regulating K+ transport and distribution.7.We found that K+ channel ZxAKTl from Z.xanthoxylum mediate K+ uptake in plants:the over-expression of ZxAKTl could rescued the growth of the K+ uptake-deficient yeast strain CY162 and significantly alleviate the K+ deficiency phenotype of Arabidopsis ataktl-1 mutant under low K+ condition.8.Under 50 mM NaCl and-0.5MPa osmotic stress significantly induced the expression of genes encoding nitrate transporter involved in NO3-uptake,distribution,and vacuolar compartmentalization,thus enhancing the uptake and transport of NO3-.9.The calcium sensor CBL7 regulated high-affinity NO3-uptake by directly controlling the expression of NRT2.4 and NRT2.5.In summary,our results systematically revealed the physiological and molecular mechanisms of Na+ involved in the adaptation to salt and drought in the succulent xerophyte Z.xanthoxylum.