Leaf nitrogen dioxide uptake coupling apoplastic chemistry, carbon/sulfur assimilation, and plant nitrogen status

Emission and plant uptake of atmospheric nitrogen oxides (NO + NO₂) significantly influence regional climate change by regulating the oxidative chemistry of the lower atmosphere, species composition and the recycling of carbon and nutrients, etc. Plant uptake of nitrogen dioxide (NO₂) is concentration-dependent and species-specific, and covaries with environmental factors. An important factor determining NO₂ influx into leaves is the replenishment of the substomatal cavity. The apoplastic chemistry of the substomatal cavity plays crucial roles in NO₂ deposition rates and the tolerance to NO₂, involving the reactions between NO₂ and apoplastic antioxidants, NO₂-responsive germin-like proteins, apoplastic acidification, and nitrite-dependent NO synthesis, etc. Moreover, leaf apoplast is a favorable site for the colonization by microbes, which disturbs nitrogen metabolism of host plants. For most plant species, NO₂ assimilation in a leaf primarily depends on the nitrate (NO₃ ⁻) assimilation pathway. NO₂–N assimilation is coupled with carbon and sulfur (sulfate and SO₂) assimilation as indicated by the mutual needs for metabolic intermediates (or metabolites) and the NO₂-caused changes of key metabolic enzymes such as phosphoenolpyruvate carboxylase (PEPc) and adenosine 5′-phosphosulfate sulfotransferase, organic acids, and photorespiration. Moreover, arbuscular mycorrhizal (AM) colonization improves the tolerance of host plants to NO₂ by enhancing the efficiency of nutrient absorption and translocation and influencing foliar chemistry. Further progress is proposed to gain a better understanding of the coordination between NO₂–N, S and C assimilation, especially the investigation of metabolic checkpoints, and the effects of photorespiratory nitrogen cycle, diverse PEPc and the metabolites such as cysteine, O-acetylserine (OAS) and glutathione.     

Relationships between nitrogen uptake and carbon assimilation in whole plants of tall fescue

Abstract. The present study investigates the relationships between nitrogen uptake, transpiration, and carbon assimilation. Plants growing on nutrient solution were enclosed for 10–16 d in a growth chamber, where temperature, photon flux density, vapour saturation deficit and CO₂ concentration were controlled. One of these factors was modified every 4 to 5 d. Shoot photosynthesis and root and shoot respiration were recorded every half-hour. Nitrogen uptake from the root medium and plant transpiration were measured daily. In most cases, an increase in photon flux density led to increases in transpiration, net daily carbon assimilation, and nitrogen uptake. By modifying transpiration rate without changing photosynthesis (varying vapour saturation deficit), or by modifying transpiration and carbon assimilation in opposite ways (varying CO₂ air concentration), it was shown that nitrogen uptake does not follow transpiration, but is linked to the carbon uptake of the plant. When light was increased from low to intermediate levels, the N uptake/C assimilation ratio remained constant. At higher photon flux density, this ratio declined markedly. It is proposed that in the first case, growth is limited by carbohydrate availability, thus any increase in carbon assimilation leads to a proportional increase in nitrogen uptake, in contrast to the second situation where carbohydrates may accumulate in the plant without further nitrogen requirement.     

氮素形态对樱桃番茄果实发育中氮代谢的影响

以樱桃番茄为材料,采用基质 营养液共培养的方法,研究了全硝态氮(NO₃^-）、铵态氮和硝态氮配施（75%NO₃^-∶25%NH₄⁺）及全铵态氮（NH₄⁺）营养对樱桃番茄果实氮代谢及硝酸还原酶（NR）和谷氨酰胺合成酶（GS）基因表达的影响.结果表明： 铵态氮和硝态氮配施处理下樱桃番茄的单果质量比全硝态氮处理略有增加,且果实中NH₄⁺、总氨基酸、氮含量和氮素累积量均显著高于全硝态氮处理;全硝态氮及铵态氮和硝态氮配施处理下果实NR活性及其基因表达没有明显差异,但都显著高于全铵态氮处理;铵态氮和硝态氮配施处理下果实GS活性都高于全硝态氮处理.不同形态氮素及配施处理下,同工酶GS1（胞质型GS）和GS2（叶绿体型GS）的表达与GS的活性不一致,说明氮素对GS活性的影响主要发生在转录后水平.     

Effects of increasing inorganic carbon supply to roots on net nitrate uptake and assimilation in tomato seedlings

We investigated the influence of an increased inorganic carbon supply in the root medium on NO^?₃ uptake and assimilation in seedlings of Lycopersicon esculentum (L.) Mill. cv. F144. The seedlings were pre-grown for 4 to 7 days with 0 or 100 mM NaCl in hydroponic culture using 0.2 mM NO^?₃ (group A) or 0.2 mM NH⁺₄ (group B) as nitrogen source. The nutrient solution for group A plants was aerated with air or with air containing 4 800 μumol mol^?1 CO₂. Nitrate uptake rate and root and leaf malate contents in these plants were determined. The plants of group B were subdivided into two sets. Plants of one set were transferred either to N-free solution containing 0 or 5 mM NaHCO₃, or to a medium containing 2 mM NO^?₃ and 5 mM NaHCO₃. Both sets of group B plants were grown for 12 h in darkness prior to 2 h of illumination, and were assayed for malate content and NO^?₃ uptake rate (only for plants grown in N-free solution). The second set of group B plants was labeled with ¹⁴C by a 1-h pulse of H¹⁴CO^?₃ which was added to a 5 mM NaHCO₃ solution containing 0 or 100 mM NaCl and 0 or 2 mM NO^?₃, and ¹⁴C-assimilates were extracted and fractionated. The roots of group B plants growing in carbonated medium accumulated twice as much malate as did control plants. This malate was accumulated only when NO^?₃ was absent from the root medium. Both a high level of root malate and aeration with CO₂-enriched air stimulated NO^?₃ uptake. Analysis of ¹⁴C-assimilates indicated that with no NO^?₃ in the medium, the ¹⁴C was present mainly in organic acids, whereas with NO^?₃, a large proportion of ¹⁴C was incorporated into amino acids. Transport of root-incorporated ¹⁴C to the shoot was enhanced by NO^?₃, while the amino acid fraction was the major ¹⁴C-assimilates in the shoot. It is concluded that inorganic carbon fixed through phosphoenolpyruvate carboxylase (EC 4.1.1.31) in roots of tomato plants may have two fates: (a) as a carbon skeleton for amino acid synthesis; and (b) to accumulate, mainly as malate, in the roots, in the absence of a demand for the carbon skeleton. Inorganic carbon fixation in the root provides carbon skeletons for the assimilation of the NH⁺₄ resulting from NO₃ reduction, and the subsequent removal of amino acids through the xylem. This ‘removal’ of NO^?₃ from the cytoplasm of the root cells may in turn increase NO^?₃ uptake.     

LEFPS1, a tomato farnesyl pyrophosphate gene highly expressed during early fruit development

Farnesyl pyrophosphate synthase (FPS) catalyzes the synthesis of farnesyl pyrophosphate, a key intermediate in sterol and sesquiterpene biosynthesis. Using a polymerase chain reaction-based approach, we have characterized LeFPS1, a tomato (Lycoperscion esculentum cv Wva 106) fruit cDNA, which encodes a functional FPS. We demonstrate that tomato FPSs are encoded by a small multigenic family with genes located on chromosomes 10 and 12. Consistent with farnesyl pyrophosphate requirement in sterol biosynthesis, FPS genes are ubiquitously expressed in tomato plants. Using an LeFPS1 specific probe, we show that the corresponding gene can account for most of FPS mRNA in most plant organs, but not during young seedling development, indicating a differential regulation of FPS genes in tomato. FPS gene expression is also under strict developmental control: FPS mRNA was mainly abundant in young organs and decreased as organs matured with the exception of fruits that presented a biphasic accumulation pattern. In this latter case in situ hybridization studies have shown that FPS mRNA is similarly abundant in all tissues of young fruit. Taken together our results suggest that several FPS isoforms are involved in tomato farnesyl pyrophosphate metabolism and that FPS genes are mostly expressed in relation to cell division and enlargement.     

Expression analysis of the auxin efflux carrier family in tomato fruit development

Auxin transport network, which is important in the integration of plant developmental signals, depends on differential expression of the auxin efflux carrier PIN gene family. We cloned three tomato PIN (referred as SlPIN) cDNAs and examined their expression patterns in fruit and other organs. The expression of SlPIN1 and SlPIN2 was highest in very young fruit immediately after anthesis, whereas the expression of SlPIN3 was low at this same stage of fruit development. SlPIN2::GUS was expressed in ovules at anthesis and in young developing seeds at 4 days after anthesis, while SlPIN1::GUS was expressed in whole fruit. The DR5::GUS auxin-responsive reporter gene was expressed in the fruit and peduncle at anthesis and was higher in the peduncle 4 days after anthesis. These studies suggest that auxin is likely transported from young seeds by SlPIN1 and SlPIN2 and accumulated in peduncles where SlPIN gene expression is low in tomato. The possible role of SlPINs in fruit set was discussed.     

Seasonal nitrogen assimilation and carbon fixation in a fjordic sea loch

Carbon (C) fixation and nitrogen (N) assimilation rates havebeen estimated from ¹⁴C and ¹⁵N techniques for a 12 month periodin a Scottish sea loch. The maximum rate of nitrogen assimilated(29.92 mmol N m^–2 day^–1) was in April at the mostseaward station; similar high rates were experienced duringMay at the other stations. Carbon fixation rates were maximal(488–4047 mg C m^–2day^–1) at the time of highphytoplankton biomass (maximum 8.3 mg m^–3 chlorophylla) during May, whilst nitrate concentrations remained >0.7µ.mol l^–1. C:N assimilation ratios suggest nitrogenlimitation only during the peak of the spring bloom, althoughat times nitrogen (nitrate and ammonium) concentration fellto 0.2 µmol l^–1 in the following months. The verticalstability of the water column, influenced by tidal and riverineflushing, varied along the axis of the loch, resulting in markeddifferences between sampling stations. Although ammonium waspreferentially assimilated by phytoplankton, >50% of productionwas supported by nitrate uptake and only during the summer monthswas the assimilation of ammonium quantitatively important.     

Implication of glutamate, isocitrate and malate deshydrogenases in nitrogen assimilation in the cadmium-stressed tomato

Tomato seedlings grown on nitric medium and treated with various cadmium concentrations (0 to 50 microM) were used. Results obtained show that cadmium remains predominantly located in the roots, which then seem to play the role of trap-organs. Increasing cadmium concentration in the medium leads particularly to a decrease in NO3- accumulation, together with a decrease in the activity of glutamine synthetase and in the quantity of plastidic isoform ARNm (GS2), and, on the contrary, to an increase of the cytosolic isoform ARNm (GS1). On the other hand, stimulations were observed for NADH-dependent glutamate synthase, NADH-dependent glutamate dehydrogenase, ARNm quantity of this enzyme, ammonium accumulation, and protease activity. In parallel, stimulations were observed for NAD+ and NADP+-dependent malate dehydrogenase and NADP+-dependent isocitrate dehydrogenase. These results were discussed in relation to the hypothesis attributing to the dehydrogenase enzymes (GDH, MDH, ICDH) an important role in the plant defence processes against cadmium-induced stresses.     

Plant respiration in relation to growth, maintenance, ion uptake and nitrogen assimilation

Abstract Respiration in plants is generally observed to comprise two components: one proportional to the growth rate and the other to the plant dry mass. These components are usually interpreted as being related to the growth of new plant material and maintenance of existing plant material, respectively. By analysing data in this way, the respiratory costs of both structural synthesis and maintenance are observed to be greater in the root than the shoot. This contradicts current understanding of the biochemistry of the processes involved. The basic model is developed to incorporate three additional processes. The first is the cost of ion uptake for plant growth. The second allows for the fact that the site of nitrogen assimilation into amino acids may differ from the site of utilization for protein synthesis: when ammonium is supplied, this is incorporated immediately into amino acids owing to its toxicity to the plants; when nitrate is supplied it may be reduced either in the shoot or root, or both, and subsequently transported around the plant for utilization. The third process to be included is an energy cost for the uptake of ions to balance efflux from the root system. The theory is consistent with experimental observation and provides a means of understanding and interpreting respiration and nitrogen metabolism in plants.     

Small RNAs in tomato fruit and leaf development

Tomato fruit and leaf development offers excellent systems to study the evolution of gene regulation underlying development of different organs. We have identified over 350 and 700 small RNAs from tomato fruit and leaf, respectively. Except for conserved microRNAs, more than 90% of the small RNAs are unique to tomato. We confirmed expression of some conserved as well as novel putative microRNAs by Northern hybridization. Our results help form a basis for comparative studies on how small RNA-mediated gene expression has contributed to the evolution of common and distinct developmental pathways of fruits and leaves. We have established a website (http://ted.bti.cornell.edu/digital/sRNA/) for public access to all of our small RNA sequences, their expression patterns in respective tissues, and their matching genes or predicted target genes in a searchable manner.     

Abscisic acid triggers whole-plant and fruit-specific mechanisms to increase fruit calcium uptake and prevent blossom end rot development in tomato fruit

Calcium (Ca) uptake into fruit and leaves is dependent on xylemic water movement, and hence presumably driven by transpiration and growth. High leaf transpiration is thought to restrict Ca movement to low-transpiring tomato fruit, which may increase fruit susceptibility to the Ca-deficiency disorder, blossom end rot (BER). The objective of this study was to analyse the effect of reduced leaf transpiration in abscisic acid (ABA)-treated plants on fruit and leaf Ca uptake and BER development. Tomato cultivars Ace 55 (Vf) and AB2 were grown in a greenhouse environment under Ca-deficit conditions and plants were treated weekly after pollination with water (control) or 500 mg l(-1) ABA. BER incidence was completely prevented in the ABA-treated plants and reached values of 30-45% in the water-treated controls. ABA-treated plants had higher stem water potential, lower leaf stomatal conductance, and lower whole-plant water loss than water-treated plants. ABA treatment increased total tissue and apoplastic water-soluble Ca concentrations in the fruit, and decreased Ca concentrations in leaves. In ABA-treated plants, fruit had a higher number of Safranin-O-stained xylem vessels at early stages of growth and development. ABA treatment reduced the phloem/xylem ratio of fruit sap uptake. The results indicate that ABA prevents BER development by increasing fruit Ca uptake, possibly by a combination of whole-plant and fruit-specific mechanisms.     

Nitrogen uptake,assimilation and transport in barley in the presence of atmospheric nitrogen dioxide

Summary Exposure of the leaves of young barley plants to nitrogen dioxide (NO₂) was shown to affect the rate of translocation of N, the form in which it is transported in the xylem stream and the partitioning of N between roots and shoots. Following its entry through the leaves, NO₂ is assimilated by the plant into reduced nitrogenous compounds which accounted for the major increases in plant N content and growth. The various effects of atmospheric NO₂ upon barley seedlings were strongly influenced by nitrate supply to the roots.     

Genetic engineering of parthenocarpic fruit development in tomato

Parthenocarpy was engineered in two genotypes of Lycopersicon esculentum Mill. by using the DefH9-iaaM chimeric gene. The parthenocarpic trait consists of fruit set and growth in the absence of fertilization. Seedless parthenocarpic fruits were obtained from emasculated flowers, and fruits with seeds from pollinated flowers. All parthenocarpic tomato plants analysed expressed the DefH9-iaaM gene during flower development. The fruit set percentage of emasculated transgenic flowers was similar to that of control plants. In 7 out of 8 independent transgenic plants, the fresh weight of fruits derived from pollinated or emasculated flowers did not significantly differ from that of fruits obtained by pollination of the control plants. The pH of the parthenocarpic fruit was generally unaffected and the soluble solid concentration was either unchanged or increased. Thus, the DefH9-iaaM gene is a genetic tool that might be used to improve tomato productivity.     

Temperature dependence of inorganic nitrogen uptake and assimilation in Antarctic sea-ice microalgae

Summary The influence of temperature on NO ₃ ^- and NH ₄ ⁺ uptake, and the activity of the assimilatory enzyme NO ₃ ^- reductase (NR) was compared to inorganic C uptake (photosynthesis) in natural assemblages of Antarctic sea-ice microalgae. NO ₃ ^- and NH ₄ ⁺ uptake reached a maximum between 0.5°–2.0°C and 2.0°–3.0°C, respectively, which was close to that for photosynthesis (2.5°–3.0°C). NR showed a distinctly higher temperature maximum (10.0°–12.0°C) and a lower Q₁₀ value than inorganic N and C transport. Our data imply that, owing to differential temperature characteristics between N transport and N assimilation at in situ temperature (-1.9°C), the incorporation of extracellular NO ₃ ^- into cellular macromolecules, may be limited by transport of NO ₃ ^- into the cell rather than the intracellular reduction of NO ₃ ^- to NH ₄ ⁺ . Despite differences in temperature maxima between N transport and N assimilation, the overall low temperature maxima of inorganic N metabolism characterizes Antarctic sea-ice microalgae as psychrophilic. Our study is the first to examine the temperature dependence of inorganic N uptake and assimilation in sea-ice microbial communities.     

Economy of water, carbon, and nitrogen in the developing cowpea fruit

The nutritional economy of the fruit of cowpea (Vigna unguiculata (L.) Walp cv Vita 3) was assessed quantitatively from intake and utilization of carbon, nitrogen, and water. Fruits failed to make net gains of CO₂ from the atmosphere during daytime, although pod photosynthesis did play a role in the fruit's carbon economy by refixing a proportion of the fruit's respired CO₂. Of every 100 units by weight of carbon entering the fruit, 70.4 were finally incorporated into seeds, 10.3 remained as nonmobilizable material in pod walls, and the remaining 19.3 were lost in fruit respiration. Phloem supplied 97% of the fruit's carbon and 72% of its nitrogen. The xylem contribution of nitrogen occurred mainly in early growth. Ninety-six% of the fruit's nitrogen was incorporated into seeds, approximately 10% of this mobilized from the senescing pod. The mean transpiration ratio of the fruit was very low—8 milliliters water transpired per gram dry matter accumulated. Models of carbon, nitrogen, and water flow were constructed for the two consecutive 11 day periods of fruit development, and indicated a considerably greater entry of water through xylem and phloem than could be accounted for in changes in fruit tissue water and transpiration loss. This discrepancy was greater in the second half of fruit growth and was interpreted as evidence that a significant fraction of the water entering the fruit through phloem cycled back to the parent plant via the xylem.     

Diel changes of uptake of inorganic carbon and nitrogen by phytoplankton, and the relationship between inorganic carbon and nitrogen uptake in Lake Nakanuma, Japan

Diel changes of uptake of inorganic carbon and nitrogen wereexamined in a small freshwater lake, Lake Nakanuma, Japan, bythe ¹³C and ¹⁵N method. Experiments were earned out in spring,summer and autumn in 1984. Carbon and nitrogen uptake in thelight incubation showed maxima around noon at the three seasons.Carbon uptake ceased at night, but ammonium uptake was fairlylarge at night. In the dark incubation carbon uptake did notoccur. Ammonium uptake showed a maximum at dusk in the darkexperiments. Diel changes of nitrate uptake were less clearthan those of ammonium uptake. These results indicate that nitrogenuptake partly depended on the carbon uptake. Then, we triedto explain the diel changes of nitrogen uptake, assuming thatthe nitrogen uptake partly depends on stored carbohydrate. Thediel changes may be elucidated by the sum of three terms: oneis the term of decay of stored carbohydrate, the second is theterm which indicates cumulative increase of stored carbohydrateand the third is the term which directly depends on light.