植物铵态氮同化及其调控机制的研究进展

氮是维持植物生长发育最重要的矿质营养元素之一, 在植物整个生命进程中发挥着重要作用。在植物体内, 氮同化既是植物利用氮素的一个中心环节, 也是导致植物氮利用效率不高的因素之一。氮同化主要分为硝态氮(NO3–)和铵态氮(NH4+)同化, 其中铵态氮同化是氮同化中最为关键的一步。按照不同来源, 植物体内铵态氮同化又可分为一次同化和二次同化, 但两者都是通过谷氨酰胺/谷氨酸合成酶(GS/GOGAT)途径进行。植物铵态氮同化不仅需要大量的能量, 而且需要大量的碳源, 所以其在转录、转录后以及翻译后等各个水平上都受到严格调控。该文综述了目前关于植物铵态氮同化及其调控机制的最新研究进展。     

Relationship between growth,nitrogen fixation and assimilation in a legume (Medicago sativa L.)

Summary Symbiotic N₂ fixation, NO ₃ ⁻ assimilation and protein accumulation in the shoots were measured simultaneously in alfalfa (Medicago sativa L.) grown in the field or in pots, in order to study how the balance between the two modes of nitrogen nutrition could be influenced by agronomic factors, such as harvest, mineral nitrogen supply and drought stress. During periods of rapid growth, fixation and assimilation may function simultaneously; they are antagonistic at the beginning and at the end of the growth cycle, when the nitrogen requirement of the plant is lower. When nitrogen nutrition does not limit growth, mineral nitrogen supply favours assimilation at the expense of fixation, but does not modify the amount of nitrogen accumulated, which is adjusted to the growth capacity of the plant. After cutting, nitrate assimilation compensated for the decrease in fixation and supplied the plant with the nitrogen required by the regrowth, the proliferation of which determined the fixation recovery. Drought stress decreased N₂ fixation much more than NO ₃ ⁻ assimilation. The latter made growth recovery possible when water supply conditions became normal again. These results suggested the existence of an optimum level of nitrate assimilation, which differed depending on the age of the plants and allowed both maximum growth and fixing activity.     

The nitrogen handling characteristics of white clover (Trifolium repens L.) cultivars and a perennial ryegrass (Lolium perenne L.) cultivar

Ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) have contrasting responses to soil mineral N availability and clover has the ability to fix atmospheric N(2) symbiotically. It has been hypothesized that these differences are the key to understanding grass-clover coexistence and vegetative dynamics in pastures. However, the whole plant response of clover and ryegrass to mineral N availability has not been fully characterized and inter-cultivar variability in the N-handling dynamics of clover has not been assessed. A detailed experimental study to address these issues was undertaken. For all clover cultivars and ryegrass, mass specific mineral N uptake rates (of whole plants) were similar saturating functions of mineral N availability. For all clover cultivars total N assimilation rates, whole plant C : N ratios and root : shoot ratios were independent of mineral N availability. Clover growth rates were also independent of mineral N availability except for a slight (<10%) reduction at very low N availability levels. Specific N(2) fixation rate (whole plant) was precisely controlled to ensure fixation balanced the deficit between mineral N uptake and the total N assimilation required to maintain constant whole plant C : N ratio. There was always a deficit between N uptake and the total N assimilation required to maintain C : N ratio. Consequently, some N(2) fixation remained engaged even at high mineral N availability levels. All inter-cultivar variation in N(2) fixation dynamics could be attributed to variations in growth rate. Clover mass specific growth rate declined as plant size increased. Ryegrass specific growth rate, whole plant C : N ratio and root : shoot ratio were dependent on N availability. Increased N availability led to increased growth rate and decreased C : N and root : shoot ratios. Specific growth rate was also dependent on plant size, growth rate declining as plant size increased. It is concluded that clover inter-cultivar variation in field performance is unlikely to be a consequence of variation in N-handling characteristics. Inter-cultivar differences in growth rate are likely to be a much more important source of variation.     

Nitrate reductase activity (NRA) in Asphodelus aestivus Brot. (Liliaceae): distribution among organs,seasonal variation and differences among populations

Nitrate reductase activity (NRA) in different compartments (leaves, inflorescence stalks, flowers and tuberous roots) of Asphodelus aestivus Brot. (Liliaceae) and actual mineral nitrogen (NO₃⁻-N and NH₄⁺-N) in soil surrounding the roots were investigated over one year. Although the highest NRA was found in the leaves, the other plant compartments, such as flowers and tuberous roots, also have nitrate assimilation capacity. High nitrate assimilation capacity under suitable conditions is considered to be a good strategy for development and dominance of this species in Mediterranean environments. There was a seasonal variation in nitrate assimilation in leaves and actual NO₃⁻-N content of soils. Depending on actual nitrate content of soils, nitrate assimilation increased in winter.     

Factors influencing the assimilation of arsenic in a deposit-feeding polychaete

We investigated mechanisms leading to the assimilation of particle-bound arsenic (As) ingested by the deposit-feeding polychaete Alitta succinea using a radiotracer approach. The release of As from different particle types into extracted gut fluid or bovine serum albumin (BSA), a gut fluid mimic, was measured. In addition, gut fluid proteins were analyzed by separating proteins via 2D gel electrophoresis, and protein peptide sequences were determined by mass spectrometry. Major ions in the gut fluid were measured by ion chromatography and metals by mass spectrometry. Percentages of particulate As release were related to As assimilation efficiencies (AEs) in polychaetes feeding on different particle types. AEs of As were highest from radiolabeled pure diatoms (72%) and radiolabeled diatoms added to sediment (51%), lower from radiolabeled sediment (10%), and lowest from a radiolabeled iron oxide mineral, goethite (2%). It appears that As release from particles is a necessary but not sufficient requirement of As assimilation. For example, 15% of As was released from goethite into the gut fluid but only 2% was assimilated by A. succinea. Our results suggest that the likelihood of As assimilation is higher when it is bound to an organic compound of nutritional value in the ingested particles.     

Influence of vesicular arbuscular mycorrhizae and leaf age on net gas exchange of citrus leaves

The purpose of this study was to test the hypothesis that vesicular arbuscular mycorrhizal (VAM) fungi affect net assimilation of CO₂ (A) of different-aged citrus leaves independent of mineral nutrition effects of mycorrhizae. Citrus aurantium L., sour orange plants were grown for 6 months in a sandy soil low in phosphorus that was either infested with the VAM fungus, Glomus intraradices Schenck & Smith, or fertilized with additional phosphorus and left nonmycorrhizal (NM). Net CO₂ assimilation, stomatal conductance, water use efficiency, and mineral nutrient status for expanding, recently expanded, and mature leaves were evaluated as well as plant size and relative growth rate of leaves. Nutrient status and net gas exchange varied with leaf age. G. intraradices-inoculated plants had well-established colonization (79% of root length) and were comparable in relative growth rate and size at final harvest with NM plants. Leaf mineral concentrations were generally the same for VAM and NM plants except for nitrogen. Although leaf nitrogen was apparently sufficient for high rates of A, VAM plants did have higher nitrogen concentrations than NM at the time of gas exchange measurements. G. intraradices had no effect on A, stomatal conductance, or water use efficiency, irrespective of leaf age. These results show that well-established VAM colonization does not affect net gas exchange of citrus plants that are comparable in size, growth rate, and nutritional status with NM plants.     

Effect of nitrogen nutrition on amino acid composition of xylem sap and stem wood in Alnus glutinosa

Citrulline was the major amino acid in root pressure sap, stem sap and stem wood from Alnus glutinosa L. Gaertn. plants relying on fixed nitrogen or, partly or wholly, on mineral nitrogen for growth. Glutamine increased in prominence in plants assimilating mineral nitrogen but asparagine remained a relatively insignificant component. Differences in the relative amounts of the free amino compounds of stem sap from nitrogen-fixing and mineral nitrogen-fed plants were usually small compared to differences between plants fed different sources of mineral nitrogen. In contrast, relatively high values for the ratios of citrulline/total free amino nitrogen compounds and particularly of citrulline/amides in root pressure sap distinguished nitrogen-fixing plants from those receiving mineral nitrogen. Although the amino acid ratios of stem wood extracts showed closer similarity to those for root pressure sap than stem sap, the seasonal accumulation of citrulline, possibly as a storage amino acid, in stem wood from field-grown plants negated the possibility of utilising stem wood analyses as an indicator of the form of nitrogen assimilation. Comparative data on the levels of citrulline or other free amino acids in Alnus glutinosa are unlikely to be useful as an index of nitrogen fixation, under most experimental conditions.     

Application of 13C mineral carbon for assessment of the primary production of organic matter in aquatic environments

The possibility of measuring the rates of light and dark CO2 assimilation using 13C carbonate was demonstrated on Lake Kichier (Marii El). The application of methods utilizing the stable 13C and the radioactive 14C isotopes resulted in comparable values of the rates of light and dark CO2 fixation. Due to its absolute environmental safety, the method with 13C mineral carbon can be recommended as an alternative to radioisotope methods for qualitative measurements of CO2 fixation rates in aquatic ecosystems.     

The value of a leaf

Summary The value of a leaf to a plant depends on the fate of its exported assimilates. When these are translocated and used in the growth of new leaves they contribute to further carbon assimilation. The result is that their value to the plant is greatest while they are young. In contrast, when assimilates are translocated to storage, assimilates produced early and late in the life of a leaf are of equal value. This arguments is developed in relation to the optimal distribution of mineral resources and defenses during the life of leaves.     

Control of seed coat thickness and permeability in soybean : a possible adaptation to stress

Although the seed coat, through its thickness and permeability, often regulates seed germination, very little is known about the control of its development. Using soybean (Glycine max [L.] Merrill) explants, podbearing cuttings in which defined solutions can be substituted for the roots, we have demonstrated that cytokinin and mineral nutrients moving through the xylem can control soybean seed coat development. Lack of cytokinin and minerals in the culture solution, causes a thicker, less permeable seed coat to develop. The seeds with thickened coats will imbibe water rapidly if scarified; furthermore, these scratched seeds also germinate and produce normal plants. Inasmuch as stress (e.g. drought) decreases mineral assimilation and cytokinin production by the roots, the resulting delay in germination could be an adaptive response to stress.     

Laccaria bicolor S238N improves Scots pine mineral nutrition by increasing root nutrient uptake from soil minerals but does not increase mineral weathering

The role of ectomycorrhizal fungi on mineral nutrient mobilization and uptake is crucial for tree nutrition and growth in temperate forest ecosystems. By using a “mineral weathering budget” approach, this study aims to quantify the effect of the symbiosis with the ectomycorrhizal model strain Laccaria bicolor S238N on mineral weathering and tree nutrition, carrying out a column experiment with a quartz/biotite substrate. Each column was planted with one Scots pine (Pinus sylvestris L.) non-mycorrhizal or mycorrhizal with L. bicolor, with exception of the abiotic control treatment. The columns were continuously supplied with a nutrient-poor solution. A mineral weathering budget was calculated for K and Mg. The pine shoot growth was significantly increased (73%) when plants were mycorrhizal with L. bicolor. Whatever their mycorrhizal status, pines increased mineral weathering by factors 1.5 to 2.1. No difference between non-mycorrhizal and mycorrhizal pine treatments was revealed, however, mycorrhizal pines assimilated significantly more K and Mg. This suggests that in our experimental conditions, L. bicolor S238N improved shoot growth and K and Mg assimilation in Scots pine mainly by increasing the uptake of dissolved nutrients, linked to a better exploration and exploitation of the soil by the mycorrhizal roots.     

The decline of plant mineral nutrition under rising CO2: physiological and molecular aspects of a bad deal

The elevation of atmospheric CO₂ concentration has a strong impact on the physiology of C3 plants, far beyond photosynthesis and C metabolism. In particular, it reduces the concentrations of most mineral nutrients in plant tissues, posing major threats on crop quality, nutrient cycles, and carbon sinks in terrestrial agro-ecosystems. The causes of the detrimental effect of high CO₂ levels on plant mineral status are not understood. We provide an update on the main hypotheses and review the increasing evidence that, for nitrogen, this detrimental effect is associated with direct inhibition of key mechanisms of nitrogen uptake and assimilation. We also mention promising strategies for identifying genotypes that will maintain robust nutrient status in a future high-CO₂ world.     

Hyphomicrobium as a component of the aquatic microflora--morphological and physiological studies on two strains

Aspects of the morphology, metabolism and physiology of two oligocarbophilic strains of Hyphomicrobium (H4K and S5K), isolated from the Plusssee, were studied. Both strains are able to grow on mineral salts media without added organic carbon sources. Strain H4K grows well even in double distilled water. The two strains cannot grow on mineral media in the absence of atmospheric CO2. No growth occurred also in air purified of organic carbon, in spite of the presence of CO2. On the contrary, there is good growth in the presence of some organic compounds and without atmospheric CO2, i.e., heterotrophic metabolism without CO2 assimilation is possible. Growth was enhanced in a methanol atmosphere, and by the addition of yeast extract, methylamine, peptone and glucose. In nutrient solutions containing acetate or formate as carbon source, growth of H4K begins only after an adaptation period of ca. 4 weeks.     

Plant symbiotic mutants as a tool to analyse nitrogen nutrition and yield relationship in field-growth peas (Pisum sativum L.)

Pisum sativum L. is known for high seed and protein yields but also for.yield instability. Because legumes utilize two sources of nitrogen (atmospheric N₂ fixed in nodules and assimilation of soil mineral N), studies on their nitrogen nutrition is more complex than in other plants. In this work, pea symbiotic mutants (with no nodules at all ([Nod^-]), with inefficient nodules ([Nod⁺Fix^-]) or showing an hypernodulating and a ‘nitrate-tolerant symbiosis’ character ([Nod⁺⁺Nts]), their semi-leafless isogenic homologues and the parental control line cv Frisson were fertilized with three levels of mineral nitrogen (0, 25 or 50 g N m^-2) to generate a range of mineral nitrogen regimes in the same genetic background. Impact of the source and level of nitrogen nutrition was measured on reproductive development, growth, nitrogen accumulation and seed yield. It was shown that a N deficiency induced flowering termination. It also led to a large decrease in the number of seeds produced and the amount of N accumulated in forage and in seeds, when little effect was observed on the progression rates of reproductive stages along the stem. The single seed weight and the amount of dry matter accumulated in forage neither responded strongly to N deficiency. The source of nitrogen was shown to be of little importance to yield but the application of about 50 g N m^-2 was necessary to reach the yield of the control cv Frisson when exclusive assimilation was ensuring the N requirements of the plant. Despite the fact that the nitrate-tolerant and hypernodulating mutant P64 used in this study did not yield as well as the parent cv Frisson, it is proposed that [Nod⁺⁺Nts] characters could act as a yield regulating factor.     

静电场对植物光合器官结构和功能变化的影响

静电场促进水萝卜对矿质元素的吸收,叶片光合色素的含量及光合、呼吸活性增加,使叶片光合细胞的数目、细胞中叶绿体数目、叶绿体中基粒及类囊体片尾数都趋增加。结果表明,先合结构及活性的改善与静电场促进植物的离子吸收有关,从而改善了植物的营养状况。     

N uptake and distribution in crops: an agronomical and ecophysiological perspective

The rate of N uptake of crops is highly variable during crop development and between years and sites. However, under ample soil N availability, crop N accumulation is highly related to crop growth rate and to biomass accumulation. Critical N concentration has been defined as the minimum N concentration which allows maximum growth rate. Critical N concentration declines during crop growth. The relationship between critical N concentration and biomass accumulation over the growth period of a crop is broadly similar within major C(3) and C(4) cultivated species. Therefore, the critical N concentration concept is widely used in agronomy as the basis of the diagnosis of crop N status, and allows discrimination between situations of sub-optimal and supra-optimal N supply. The relationship between N and biomass accumulation in crops, relies on the interregulation of multiple crop physiological processes. Among these processes, N uptake, crop C assimilation and thus growth rate, and C and N allocation between organs and between plants, play a particular role. Under sub-optimal N supply, N uptake of the crop depends on soil mineral N availability and distribution, and on root distribution. Under ample N supply, N uptake largely depends on growth rate via internal plant regulation. Carbon assimilation of the crop is related to crop N through the distribution of N between mature leaves with consequences for leaf and canopy photosynthesis. However, although less commonly emphasized, carbon assimilation of the crop also depends on crop N through leaf area development. Therefore, crop growth rate fundamentally relies on the balance of N allocation between growing and mature leaves. Nitrogen uptake and distribution also depends on C allocation between organs and N composition of these organs. Within shoots, allocation of C to stems generally increases in relation to C allocation to the leaves over the crop growth period. Allocation of C and N between shoots and roots also changes to a large extent in relation to soil N and/or crop N. These alterations in C and N allocation between plant organs have implications, together with soil availability and carbon assimilation, on N uptake and distribution in crops. Therefore, N uptake and distribution in plants and crops involves many aspects of growth and development. Regulation of nitrogen assimilation needs to be considered in the context of these interregulatory processes.     

How overproduction of foreign proteins affects physiology of the recombinant strains ofHansenula polymorpha

Changes in the activity of key enzymes of the methanol utilization pathway of the recombinant strains of methylotrophic yeastHansenula polymorpha R22-2B and LAC-56 were studied at different rates of chemostat growth on methanol containing mineral media. It was shown that the strain R22-2B, initially having a 10-fold increased activity of dihydroxyacetone kinase (DHAK, a key enzyme of formaldehyde assimilation) acquired increased activity of formaldehyde dehydrogenase (FADH, a key enzyme of formaldehyde dissimilation) which resulted in the enhanced oxidation of formaldehyde to CO₂. Strain LAC-56, overproducingEscherichia coli β-galactosidase, acquired the decreased intracellular concentration of ATP which resulted in the decrease of the efficiency of formaldehyde assimilation catalyzed by DHAK and resulted in accumulation of toxic formaldehyde. As a consequence some biochemical responses occurred in cells that were directed to a diminishing of the toxic effect of accumulated formaldehyde, namely, the decreasing of methanol oxidase activity (to reduce the rate of formaldehyde synthesis), and the increasing of FADH activity (to increase the rate of formaldehyde oxidation).