Nitrogen Dynamic Uptake And Genetic Expression Of Translocator Of Tree Species In Fine Roots

Nitrogen is one of the three basic elements in ecological syste andthe main limiting factor of mineral nutrition in the terrestrial ecosystem. In most plants, including woody plants, nitrogen is often in a deficit state. Plants have evolved different strategies to regulate the root system to improve the ability to absorb nitrogen and respond to nitrogen space-time distribution changes: according to the abundance,the nitrogen in soil include nitrate, ammonium, and amino acid. High and low affinity transporters belong to different family. The expression of most transporters is determined by the substrate and plant nutritional status, which constitute a complex and systematic regulation network. In addition, the plant can maintain nitrogen balance by adjusting their growth and development. Plant roots, in particular, have strong plasticity to affect nitrogen uptake from the soil. Therefore, the absorption of nitrogen in roots significantly impact plant growth and development.With the increase of the artificial forest in China, nitrogen uptake mechanism research of woody plants is international frontier science problem, also is a practical problem which needs to solve urgently in environmental resources field. Thus, three main afforestation tree species(Larix principis-rupprechtii Mayr, Cunninghamia lanceolata, and Populus simonii)were selected as research objects in this study. With the non-invasive micro-test technique(NMT), the nitrate(NO3-) and ammonium(NH4+) uptake with the change of time and space(different position from the apex) were investigated to reveal the dynamic regularity of fine root N uptake; According to genome sequence database of populus trichocarpa, the transporters in the genome(ammonium transporters and nitrate transport) and their sequences were studied; The representative genes encoding proteins associated with N uptake in poplar fine roots were selected as the research objects.The expression patterns of key N uptake genes and their expression levels in different stress were investigated with the real-time fluorescence quantitative RT-PCR technology to understand N uptake physiological ecology and molecular regulation mechanism. The main results are summarized below:1. NH4+ uptake of Prince Rupprecht’s Larch( Larix principis-rupprechtii Mayr), Chinese fir( Cunninghamia lanceolata), and Populus simonii was significantly larger than the NO3- uptake under acid condition(p H = 5.5). Significant difference was found among positions from the apex of fine roots in nitrogen uptake, and the maximum influx of NH4+ occurred near the root tip while that of NO3- occurred in elongation or mature areas. At these positions, the maximum influxes of NH4+ and NO3- maintaining steadily over time.2. Prince Rupprecht’s Larch and Chinese Fir showed different preferences for the uptake of NH4+ or NO3-.For Prince Rupprecht’s Larch, low p H(p H 4) resulted in a decrease in net ammonium uptake, which remained unchanged in Chinese Fir. Net nitrate uptake in Prince Rupprecht’s Larch and Chinese Fir was much lower in soils with p H 4 relative to those with p H 7.Low p H significantly decreased the H+-ATPase activity and the expression level of NRTs in roots of Prince Rupprecht’s Larch. Low p H decreased N assimilation in bothconifer species with the exception of NH4+ assimilation in Chinese Fir, which displayed higher glutamine synthetase(GS activities) and glutamate synthetase(GOGAT activities) at low p H. Prince Rupprecht’s Larch from the Loess Plateau takes up and assimilates a greater proportion of N as NO3-, particularly at neutral p H, whereas Chinese Fir assimilates a greater proportion of N as NH4+, particularly at low p H levels3. twenty-one ammonium transporters of P.trichocarpa can be divided into Pt AMT1 subfamily and Pt AMT2 subfamily: eight of them belong to Pt AMT1 and rest of the 13 belongs to Pt AMT2. Pt AMT1 and Pt AMT2 family may separate in an early stage and contain few common motifs. Ammonium transporters have 10-11 transmembrane domains and the three-dimensional structures of Pt AMT in same subfamily are similar. The genome of P.trichocarpa contains 68 Pt NRT1 / PTR, 6 Pt NRT2, 5 Pt NRT3 members. During the evolution process, NRT1/PTR and NRT3 but not NRT2 are expanded. Generally, soil is often under nitrogen deficiency state. Pt NRT2 family belonging to the high affinity system and members of Pt NRT2 family are key transporters to maintain plant growth and development. Therefore, the evolution Pt NRT2 family is relatively conservative.4. After salt treatment, the net NH4+ influx was markedly higher in fine roots in new roots while the NO3- uptake was reduced. The average diameter of root was significantly increased in plants with salt treatment. This may be helpful to NH4+ uptake. Salinity has detrimental effects on plant growth and development and the effects were dependent on the N source The reductions of chlorophyll content and photosynthesis were much more affected by salt treatment in the NH4+-fed plants compared to the NO3--fed plants. Salt stress significantly reduced 15 N uptake and soluble protein under NH4+ treatment, but had no obvious effect on those under NO3- treatment.Na Cl treatment increased NH4+ influx and NH4+ transporters in the roots of P. simonii. The NRT1.1 m RNA levels in both the NH4+- and NO3--fed plants were not affected under salt stress. However, the NRT2.4a transcripts were lower under salt stress. The high levels of NRT1.1 and NRT2.4a transporters in the NH4+-fed plants suggest that P. simonii might be sensitive to NO3-- deficiency. The m RNA level of the NR and Ni R transcripts was not affected under salt stress and was primarily regulated by the substrate concentration. The down-regulation of the genes involved in NH4+ assimilation(GS1.3, GS2, NADH-GOGAT and Fd-GOGAT) might be a harmful response to Na Cl toxicity. The accumulation of amino acid compounds playing an important role in osmoregulation in response to salt stress. The amino compounds were more reactive to salt stress in the NH4+-fed plants compared to the NO3--fed plants. Significant increases in response to salt stress were observed in the serine and glutamate biosynthesis groups.Salt stress decreased the root development and photosynthesis of P. simonii, depending on the N source. NH4+ uptake was enhanced, whereas NH4+ assimilation was decreased in response to salt stress. NO3-metabolism was less affected under salt treatment. The accumulation of amino acid compounds indicates an adaption in response to salt stress. Soils on the Loess Plateau in Northwest China are alkaline, and low levels of N are available. NH4+ levels are particularly low, making NO3- the most available form of N in this region. Thus, P. simonii growing in these regions exhibits different N uptake and assimilation strategies during salt stress, depending on the N source.5. P. simonii on the loess plateau maintained high N uptake and assimilation ability under low nitrogen supply to ensure normal growth. Drought stress allocated more dry matter to the roots and limited plant growth by stunting physiological activities(e.g., photosynthesis and N assimilation) and down- regulating most of the genes involved in N metabolism. In addition, NH4+ concentrations in the roots and NR activity in N assimilation were increased under drought stress when the nitrogen supply was increased. These results indicate that the negative effects of drought stress on P. simonii growth could be eased to some extent by nitrogen supply.