| Ancient pagoda trees are not only the valued natural resources and historical relics, but also the real evidence of plant evolution and the genebank of precious species. In recent years, growth decline and death rise of ancient pagoda trees in many places attract great attentions in our society and management. It is of great significance to protect ancient pagoda trees resource by the means of studying the mechanisms of growth and recession and formulating effective measures to relieve senescence. Drought is a major abiotic stress limiting plant growth and productivity; most of the plants are sensitive to drought, even if it is mild drought. However, forest trees often suffer from drought duo to continuous global warming and substantial shifts in precipitation patterns, particularly in warm, arid and semi-arid regions worldwide. Drought may cause insufficient water content in soil, which will lead to hydropenia in root and death of leaves and branches. Long drought may lead to water loss of trees so drought is one of the natural factors leading to senescence. In the growth and development process of plants, nitrogen has a role to delay plant senescence. Appropriate fertilization can improve the physiological metabolism and delay senescence of leaves even the whole plant. Nitrogen needed in the growth and development process of plants comes from the uptake from soils and the transport between source tissues. The uptake and metabolism of nitogen may be negatively affected by limited water availability because ammonium or nitrate are initially dissolved in water and subsequently taken up by roots and translocated to the aboveground parts of plants. Drought and infertility are the major factors limiting plants growth in arid and semi-arid region of northwest China. So it has important and practical guiding significance for protecting ancient pagoda trees to understand the molecular mechanism of nitrogen uptake and transport responses to drought, and the relationship between it and the delay of trees senescence. Plants absorb NH4+ and NO3- by AMT and NRT,but drought can influence the expression levels of nitrogen transporters. In order to research the response mechanisms of nitrogen uptake, transport and assimilation to drought stress, Sophora japonica were used as materials to clone nitrogen transporters according to homologous species and the seedlings cultivated in greenhouse were used to determine the relative expression of nitrogen transporters, inorganic nitrogen content and activities of nitrogen metabolism enzymes in roots and leaves after mild,moderate and severe drought treatment simulated by PEG6000 for12 h, 24 h, 36 h, 48 h. The main results of this study are as follows:1. Clone of nitrogen transporters in Sophora japonica.Five nitrogen transporter genes were acquired from Sophora japonica, three of them were nitrate transprters named Sj NPF2.11, Sj NPF2.13 and Sj NPF7.2 respectively,the other two were ammonium transporters named Sj AMT1.3 and Sj AMT2.1,Sequence length of this genes were 424 bp, 490 bp, 294 bp, 407 bp and 577 bp, the homology were 83%, 88%, 91%, 87% and 87% blasted with Glycine max and cicer arietimum, respectively. The results indicated that the gene fragments cloned from Sophora japonica had high homology blasted with nitrogen transporters in Glycine max and cicer arietimum.2. Relative expression of nitrogen transporters of Sophora japonica under drought stress.In Sophora japonica, the expression of Sj NPF2.13 began to rise significantly when moderate and severe drought treatment for 36 h in roots and 24 h in leaves, and in leaves it also began to rise significantly when mild drought treatment for 48h; The expression of Sj NPF7.2 in roots began to rise significantly when moderate and severe drought treatment for 24 h, but in leaves it significantly decreased when severe drought treatment for 24 h and 48h; The expression of Sj AMT1.3 in roots rose significantly when drought treatment for 24 h and 36 h, in leaves it began to decrease significantly when severe drought treatment for 12h; The expression of Sj NPF2.11 and Sj AMT2.1 under drought stress remained unchanged. The results indicated that Sj NPF2.13 and Sj NPF7.2 participated in the NO3- transport and assimilation in vivo of Sophora japonica under drought stress, while Sj AMT1.3 participated in the NH4+ uptake of Sophora japonica root under drought stress.3. The contents of NH4+ and NO3- and the activities of NR and GS of Sophora japonica seedlings under drought stress.The NH4+ content in roots and in leaves(12h and 24h) rose significantly after mild, moderate and severe drought treatment, but in leaves it decreased significantly when moderate and severe drought treatment for 24 h and 48h; The NO3- content in roots decreased significantly when moderate and severe drought treatment for 12 h, but it rose significantly when treatment 48 h in root and 36 h in leaves by severe drought; The NR activity in roots decreased significantly when mild, moderate and severe drought treatment for 24 h and 48 h, in leaves it decreased significantly after moderate and severe drought treatment for 48h; The GS activity in roots were higher under severe and moderate than under mild treatmeat increased with drought degree when drought treatment for 12 h, it higher under moderate drought than under mild drought when treatment for 24 h, and it higher under severe drought than under control when treatment for 48 h, in leaves it rose significantly after mild, moderate and severe drought treatment for 24 h, and it rose significantly when severe drought treatment for 36 h and then became decrease when treatment for 48 h. The results indicated that Sophora japonica reduce wounding by means of reducing NO3- assimilation but increasing NH4+ uptake and assimilation under drought stress. |
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