Response of spinach and kidney bean plants to nitrogen dioxide

Nitrate transporters in leaves and their potential roles in foliar uptake of nitrogen dioxide

While plant roots are specialized organs for the uptake and transport of water and nutrients, the absorption of gaseous or liquid mineral elements by aerial plant parts has been recognized since more than one century. Nitrogen (N) is an essential macronutrient which generally absorbed either as nitrate (NO(-) 3) or ammonium (NH(+) 4) by plant roots. Gaseous nitrogen pollutants like N dioxide (NO2) can also be absorbed by plant surfaces and assimilated via the NO(-) 3 assimilation pathway. The subsequent NO(-) 3 flux may induce or repress the expression of various NO(-) 3-responsive genes encoding for instance, the transmembrane transporters, NO(-) 3/NO(-) 2 (nitrite) reductase, or assimilatory enzymes involved in N metabolism. Based on the existing information, the aim of this review was to theoretically analyze the potential link between foliar NO2 absorption and N transport and metabolism. For such purpose, an overview of the state of knowledge on the NO(-) 3 transporter genes identified in leaves or shoots of various species and their roles for NO(-) 3 transport across the tonoplast and plasma membrane, in addition to the process of phloem loading is briefly provided. It is assumed that a NO2-induced accumulation of NO(-) 3/NO(-) 2 may alter the expression of such genes, hence linking transmembrane NO(-) 3 transporters and foliar uptake of NO2. It is likely that NRT1/NRT2 gene expression and species-dependent apoplastic buffer capacity may be also related to the species-specific foliar NO2 uptake process. It is concluded that further work focusing on the expression of NRT1 (NRT1.1, NRT1.7, NRT1.11, and NRT1.12), NRT2 (NRT2.1, NRT2.4, and NRT2.5) and chloride channel family genes (CLCa and CLCd) may help us elucidate the physiological and metabolic response of plants fumigated with NO2.

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

Emission and plant uptake of atmospheric nitrogen oxides (NO + NO(2)) 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 (NO(2)) is concentration-dependent and species-specific, and covaries with environmental factors. An important factor determining NO(2) influx into leaves is the replenishment of the substomatal cavity. The apoplastic chemistry of the substomatal cavity plays crucial roles in NO(2) deposition rates and the tolerance to NO(2), involving the reactions between NO(2) and apoplastic antioxidants, NO(2)-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, NO(2) assimilation in a leaf primarily depends on the nitrate (NO(3) (-)) assimilation pathway. NO(2)-N assimilation is coupled with carbon and sulfur (sulfate and SO(2)) assimilation as indicated by the mutual needs for metabolic intermediates (or metabolites) and the NO(2)-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 NO(2) 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 NO(2)-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.

Effects of 60-day NO2 fumigation on growth, oxidative stress and antioxidative response in Cinnamomum camphora seedlings

OBJECTIVE

To study the oxidative stress and antioxidative response of Cinnamomum camphora seedlings exposed to nitrogen dioxide (NO(2)) fumigation.

METHODS

Measurements were made up of the growth, chlorophyll content, chlorophyll fluorescence, antioxidant system and lipid peroxidation of one-year-old C. camphora seedlings exposed to NO(2) (0.1, 0.5, and 4 microl/L) fumigation in open top chambers over a period of 60 d.

RESULTS

After the first 30 d, 0.5 and 4.0 microl/L NO(2) showed insignificant effects on the growth of C. camphora seedlings. However, exposure to 0.5 and 4.0 microl/L NO(2) for 15 d significantly reduced their chlorophyll content (P<0.05), enhanced their malondialdehyde (MDA) content and superoxide dismutase (SOD) activity (P<0.05), and also significantly reduced the maximal quantum yield of PSII in the dark [the ratio of variable fluorescence to maximal fluorescence (F(v)/F(m))] (P<0.05). In the latter 30 d, 0.5 microl/L NO(2) showed a positive effect on the vitality of the seedlings, which was reflected by a recovery in the ratio of F(v)/F(m) and chlorophyll content, and obviously enhanced growth, SOD activity, ascorbate (AsA) content and glutathione reductase (GR) activity (P<0.05); 4.0 microl/L NO(2) then showed a negative effect, indicated by significant reductions in chlorophyll content and the ratio of F(v)/F(m), and inhibited growth (P<0.05).

CONCLUSION

The results suggest adaptation of C. camphora seedlings to 60-d exposure to 0.1 and 0.5 microl/L NO(2), but not to 60-d exposure to 4.0 microl/L NO(2). C. camphora seedlings may protect themselves from injury by strengthening their antioxidant system in response to NO(2)-induced oxidative stress.

Nitrite reductase gene enrichment improves assimilation of NO(2) in Arabidopsis

Transgenic plants of Arabidopsis bearing the spinach (Spinacia oleracea) nitrite reductase (NiR, EC 1.7.7.1) gene that catalyzes the six-electron reduction of nitrite to ammonium in the second step of the nitrate assimilation pathway were produced by use of the cauliflower mosaic virus 35S promoter and nopaline synthase terminator. Integration of the gene was confirmed by a genomic polymerase chain reaction (PCR) and Southern-blot analysis; its expression by a reverse transcriptase-PCR and two-dimensional polyacrylamide gel electrophoresis western-blot analysis; total (spinach + Arabidopsis) NiR mRNA content by a competitive reverse transcriptase-PCR; localization of NiR activity (NiRA) in the chloroplast by fractionation analysis; and NO(2) assimilation by analysis of the reduced nitrogen derived from NO(2) (NO(2)-RN). Twelve independent transgenic plant lines were characterized in depth. Three positive correlations were found for NiR gene expression; between the total NiR mRNA and total NiR protein contents (r = 0.74), between the total NiR protein and NiRA (r = 0.71), and between NiRA and NO(2)-RN (r = 0.65). Of these twelve lines, four had significantly higher NiRA than the wild-type control (P < 0.01), and three had significantly higher NO(2)-RN (P < 0.01). Each of the latter three had one to two copies of spinach NiR cDNA per haploid genome. The NiR flux control coefficient for NO(2) assimilation was estimated to be about 0.4. A similar value was obtained for an NiR antisense tobacco (Nicotiana tabacum cv Xanthi XHFD8). The flux control coefficients of nitrate reductase and glutamine synthetase were much smaller than this value. Together, these findings indicate that NiR is a controlling enzyme in NO(2) assimilation by plants.

Tansley Review No. 24 Why are atmospheric oxides of nitrogen usually phytotoxic and not alternative fertilizers?

Atmospheric pollution by the oxides of nitrogen, NO and NO₂ , can cause reductions in growth but rarely visible injury. This review considers their uptake into foliage, as well as their subsequent metabolism and physiology, and attempts to explain why these gases are often phytotoxic. The combined stresses of resisting cellular acidification, enhanced levels of nitrite (and ammonia), and the direct interference of the free radical ('N=O) with critical enzymes, reaction centres and regulatory mechanisms are thought to be the main reasons why oxides of nitrogen, especially NO, inhibit growth. If other air pollutants such as SO₂ are also present with NO or NO₂ then free radical-induced injury, similar to that caused by O₃ alone, also occurs. CONTENTS Summary 395 I. Introduction 396 II. Uptake and cycling of oxides of nitrogen 396 III. Biochemical responses to NO and NO₂ 405 IV. Physiological responses to NO and NO₂ 410 V. Combinations of NO and NO₂ with other pollutants 416 VI. Recapitulation and beyond 418 Acknowledgements 420 References 420.

Effects of nitrogen dioxide on growth and yield of black turtle bean (Phaseolus vulgaris L.) cv. 'Domino'

Twenty-six-day-old black turtle bean cv. 'Domino' plants were exposed to nitrogen dioxide (0.0, 0.025, 0.05 and 0.10 microl liter(-1)), 7 h per day for 5 days per week for 3 weeks, under controlled environment. Data were collected on net photosynthesis rate (PN), stomatal resistance (SR), and dark respiration rate (DR), immediately after exposure, 24 h after the termination of exposure and at maturity (when the leaves had just started turning yellow), using a LICOR 6000 Portable Photosynthesis System. Chlorophyll-a (Ch-a), chlorophyll-b (Ch-b), total chlorophyll (tot-Ch) and leaf nitrogen were measured immediately after exposure and at maturity. Growth characteristics-relative growth rate (RGR), net assimilation rate (NAR), leaf area ratio (LAR) and root: shoot ratio (RSR)-were computed for treated plants. Net photosynthesis rate increased by 53% in 0.10 microl liter(-1) NO2 treated plants immediately after exposure compared to control plants. Dark respiration rates were also higher in treated plants. Ch-a, Ch-b and tot-Ch showed significant increases with 0.1 microl liter(-1) NO2 treatment immediately after exposure. Foliar nitrogen content showed an increase in treated plants both immediately after exposure and at maturity. Increases were also seen in RGR and NAR. Plant yield increased by 86% (number of pods), 29% (number of seeds) and 46% (weight of seeds), respectively. Nitrogen dioxide stimulated the overall plant growth and crop yield.