Expression of the rice cytoplasmic cysteine synthase gene in tobacco reduces ozone-induced damage

Cysteine serves as a precursor for the synthesis of various sulfur-containing metabolites, and the cysteine synthase (CS) gene plays a central role in the sulfur cycle in nature. In the present study, rcs1, a cytosolic CS gene of rice, was introduced into the genome of tobacco (Nicotiana tabacum). The tolerance of wild-type tobacco plants as well as of the resulting transgenic tobacco plants overexpressing the rcs1 gene to toxic levels of ozone (O₃, 0.15 μ mol⁻¹) was measured after various lengths of exposure. Leaf lesions in plants exposed for 2 weeks to O₃ were more prevalent in the leaves of the wild-type plants than in those of the transgenic tobacco plants. Transgenic tobacco plants showed a higher growth rate and a higher chlorophyll content than the wild-type plants. Cysteine synthase activity and cysteine and glutathione contents were higher in transgenic plants than in wild-type plants irrespective of the length of the O₃ treatment. Our results indicate that the CS gene plays a role in the protection of the plant against toxic O₃ gas, probably through the mechanism of an over-accumulation of such sulfur-rich antioxidants as cysteine and glutathione.     

干旱胁迫与施氮对麻疯树幼苗渗透调节物质积累的影响

采用盆栽控水的方法,研究干旱胁迫(80％ FC、60％ FC、40％ FC和20％ FC)及施氮(N0 0 g·pot-1、N1 1.2 g·pot1、Nm3.6 g·pot-1和Nh6.0 g·pot-1)对麻疯树幼苗叶、茎和根部主要渗透调节物质积累的影响.结果表明:干旱胁迫条件下,麻疯树幼苗茎和根部的游离脯氨酸、可溶性蛋白和茎部可溶性糖大量积累,叶片中脯氨酸含量也随干旱胁迫程度的增加大幅度上升;Na+、Ca2+和Mg2+在麻疯树幼苗叶、茎和根中大量积累,而K+仅在茎中大量积累,叶片和根部K+含量略微上升.施氮对植株渗透调节物质的影响与干旱胁迫强度和施氮水平有关.在80％ FC和60％ FC水分条件下,增加N肥施用量能明显促进麻疯树幼苗各组分渗透调节物质的积累;在40％ FC水分条件下,Nh处理对渗透调节物质积累的促进作用减弱;而在20％ FC条件下,N1处理植株的渗透调节能力较高,Nm和Nh处理对植株渗透调节的促进作用不明显甚至转为抑制.     

Effect of ammonium salt on enzymes of ammonium assimilation in maize seedlings

Glutamate dehydrogenase (GDH E.C. 1.4.1.2.4), glutamine synthetase (GS E.C. 6.3.1.2) and glutamate synthase (glutamine oxoglutarate amino transferase, GOGAT E.C. 2.6.1.53) activities, protein and organic nitrogen contents and growth of roots and shoots of maize seedlings raised in dark at 25±2°C in half strength Hoagland’s solution containing different ammonium salts as source of nitrogen, were determined to assess the contribution of alternate pathways in ammonium assimilation. Ammonium nitrate or in some cases ammonium chloride appeared to be the best source for both root and shoot growth and for increase in protein, total nitrogen and the enzymes of ammonium assimilation. In roots, NH₄-nitrogen appeared to be assimilated by both GDH as well as GS-GOGAT pathways specially in the dark grown seedlings, while in shoots it was primarily by GS-GOGAT pathway.     

Content of adenine nucleotides and orthophosphate in exporting and importing mature maize leaves

Experiments were conducted to determine if nitrate (¹⁵N-labeled) was taken up and assimilated by intact soybean (Glycine max [L.] Merr. cv Williams) plants during extended periods of dark. Nitrate was taken up by soybean roots throughout a 12-hour dark period. The ¹⁵N-labeled nitrogen was also translocated to the plant shoots, but at a slower rate than the rate of accumulation in the roots. Much of the nitrogen (¹⁵N-labeled) was present in a nonreduced form, although considerable soluble-reduced nitrogen (¹⁵N-labeled) accumulated throughout the dark period. The ¹⁵N-labeled, soluble-reduced nitrogen fraction accounted for nearly 30% of the total ¹⁵N found in plant roots and more than 63% of the total ¹⁵N found in plant tops after 12 hours of dark. This provided evidence that intact soybean plants take up and metabolize significant quantities of nitrate to reduced N forms in the dark.

In addition to nitrate influx during the dark, it was shown that there was a concomitant loss of ¹⁵N-labeled nitrogen compounds from previously ¹⁵N-labeled plants to a natural abundance ¹⁵N nutrient solution. Thus, evidence was obtained which indicated that light was not directly essential for flux and reduction of nitrate by intact soybean plants.

     

Cyanogenic glycosides and their metabolic enzymes in barley, in relation to nitrogen levels

The possible role for cyanogenic glycosides as nitrogen storage compounds was studied in barley, Hordeum vulgare (cv. Golf), cultivated under different nitrogen regimes. Cyanogenic glycosides were absent in seeds and roots but were synthesized in seedlings where they accumulated at a level of about 150 nmol shoot⁻¹ in control plants and 110 nmol shoot⁻¹ in nitrogen-starved plants. An enzyme involved in the breakdown of cyanogenic glycosides, β-glucosidase (EC 3.2.1.-) exhibited high activity in seeds and was also detected in roots and shoots. The activity of β-cyanoalanine synthase (EC 4.4.1.9), which is involved in the metabolism of HCN, was low in seeds but very high in roots and shoots. There was no correlation between the activities of the two enzymes and the content of cyanogenic glycosides or nitrogen. The relative content of nitrogen in cyanogenic glycosides never exceeded 0.3% of total nitrogen, and the amount of cyanogenic glycosides decreased at a low rate even at a stage when nitrogen limitation inhibited growth.     

Evaluation of effects of arsenic on carbon, nitrogen, and sulfur metabolism in two contrasting varieties of Brassica juncea

This study evaluated the effects of arsenic (As) exposure on carbon, nitrogen, and sulfur (CNS) metabolism in Brassica juncea. Two contrasting, tolerant (TPM-1) and sensitive (TM-4), varieties of B. Juncea were selected and grown either in control sand (150 g) or in sand containing 10 mg of arsenate. Harvesting was performed at 7 and 15 days and various metabolites and enzymes of CNS as well as γ-aminobutyric acid (GABA) metabolism were analyzed. At 7 days, TM-4 showed significantly higher As accumulation and stressed phenotype with increase in superoxide radicals, malondialdehyde, and cell death, as compared with TPM-1. However, the level of hydrogen peroxide was higher in TPM-1 than in TM-4. The level of GABA and the activity of glutamate decarboxylase increased in both roots and shoots of TPM-1, but not in TM-4. The level of nitrate and sulfate increased and decreased in shoots of TPM-1 and TM-4, respectively. The supply of fumarate and succinate was maintained in both shoots and roots of TPM-1 while it was only in shoots of TM-4. There was significant alteration in the profile of amino acids and in sulfur and nitrogen metabolism. However, at 15 days, As accumulation of both varieties was found to be similar along with an increase in GABA, nitrate, and sulfate in both shoots and roots except sulfate in TM-4. Supply of fumarate and succinate was also maintained and other responses were found to be similar in TPM-1 and TM-4. The study demonstrates that responses of CNS metabolism differ in varietal and time-dependent manner.     

Light regulation of assimilatory sulphate reduction in Arabidopsis thaliana

Adenosine 5'-phosphosulphate reductase (APR) is considered to be a key enzyme of sulphate assimilation in higher plants. We analysed the diurnal fluctuations of total APR activity and protein accumulation together with the mRNA levels of three APR isoforms of Arabidopsis thaliana. The APR activity reached maximum values 4 h after light onset in both shoots and roots; the minimum activity was detected at the beginning of the night. During prolonged light, the activity remained stable and low in shoots, but followed the normal rhythm in roots. On the other hand, the activity decreased rapidly to undetectable levels within 24 h of prolonged darkness both in shoots and roots. Subsequent re-illumination restored the activity to 50% in shoots and to 20% in roots within 8 h. The mRNA levels of all three APR isoforms showed a diurnal rhythm, with a maximum at 2 h after light onset. The variation of APR2 mRNA was more prominent compared to APR1 and APR3. 35SO42- feeding experiments showed that the incorporation of 35S into reduced sulphur compounds in vivo was significantly higher in light than in the dark. A strong increase of mRNA and protein accumulation as well as enzyme activity during the last 4 h of the dark period was observed, implying that light was not the only factor involved in APR regulation. Indeed, addition of 0.5% sucrose to the nutrient solution after 38 h of darkness led to a sevenfold increase of root APR activity over 6 h. We therefore conclude that changes in sugar concentrations are also involved in APR regulation.     

Role of evergreen foliage in the nitrogen economy during shoot growth ofTernstroemia gymnanthera,a warm-temperate broadleaf tree

Accumulation and redistribution of nitrogen were examined during the shoot growth ofTernstroemia gymnanthera, a warm-temperate evergreen broadleaf tree species. Measurements and analyses were confined to the shoot units comprising 2-year-old, 1-year-old and developing current shoots with the foliage of respective ages. Budbreak occurred in early May and nitrogen was rapidly translocated into curent shoots with the progress of their growth. In all of the old organs of the shoot unit, nitrogen concentrations decreased gradually from the time of budbreak to early July. During this period, those old organs supplied more than 60% of the amount of nitrogen needed for the developing current shoots within the same shoot unit. The rest was supplied from the basal organs outside the shoot units comprising branches older than 2 years, stem and roots, by redistribution and/or by absorption from soil. Old leaves, mainly 1-year-old ones, provided about 72% of the total nitrogen derived from the old organs in the shoot units. It was concluded that the evergreen broadleaves served as a large source of nitrogen for the early shoot growth.