Enhanced nitrogen assimilation in transgenic mustard ... - NOPR

Indian Journal of Biotechnology Vol 13, July 2014, pp 381-387

Enhanced nitrogen assimilation in transgenic mustard (Brassica juncea L.) overexpressing high affinity sulfate transporter gene M Akmal, U Kiran, A Ali and M Z Abdin* Centre for Transgenic Plant Development, Department of Biotechnology, Faculty of Science, Jamia Hamdard New Delhi110062, India Received 16 Januaary 2014; revised 10 April 2014; accepted 15 May 2014 Nitrogen is an essential macronutrient needed by the plants to grow and survive. Due to the metabolic coupling between nitrogen and sulfur metabolism, sulfur deficiency in plants leads to nitrogen deficiency as well. In the present paper, effect of higher assimilation of sulphur, because of overexpression of LeST 1.1 gene (high affinity sulfate transporter), on nitrogen uptake and assimilation was studied in mustard [Brassica juncea (L.) Czern. & Coss.] cv. Pusa Jai Kisan. Higher nitrogen uptake under both sulfur insufficient (50 µM) and sulfur sufficient (1000 µM) conditions was observed in the transgenic plants in comparison with untransformed plants. The transgenic plants also had higher nitrate reductase, soluble protein and total nitrogen content. The results thus obtained demonstrate a positive impact of overexpression of LeST1.1 gene on the uptake and assimilation of nitrogen in B. juncea. Keywords: Nitrogen assimilation, Brassica juncea, high affinity sulfate transporter, metabolic coupling, nitrogen, sulfur metabolism

Introduction Nitrogen (N) is a major macronutrient that helps plants with rapid growth, improving quality of leaf and forage crops, and increasing seed/fruit production. It is the part of all proteins and enzymes involved in synthesis and transfer of energy. Nitrogen deficiency results in the yellowing or chlorosis of plant leaves. Nitrogen stress also reduces the amount of protein in seeds as well as in plants. It affects the standing ability of crops during the grain-fill. Thus the crop productivity relies heavily on supplementation of soil with nitrogen fertilizers. Production and application of N fertilizers consume huge amounts of energy. Excessive use of nitrogen fertilizer leads to extensive vegetative growth in the plant at the expense of flowering stage. Further, the excess nitrogen in the fertilizer is an environmental hazard and causes natural imbalance. Rainwater runoff carries the excess nitrogen from fertilizers into ground water or into surface water leading to damage to the aquatic ecosystem as well as to human societies who are depended on these areas for food1,2. Hence, the management of nitrogen utilization efficiency and _______________ *Author for correspondence: Tel: +91-11-26089688 ext.5583; Fax: +91-11-26059666 E-mail: [email protected] Conflict of interest: The authors declare that they have no conflict of interest

other essential nutrients can lead to increased growth and production efficiency of the crops and in turn be profitable for the growers. Further, it may also reduce the environmental contamination. Higher plants assimilate N and S in amounts proportional to that incorporated into amino acids and proteins3-5. S is also shown to play a significant role to the acquisition of N6,7. However, effect of S sufficiency of soil has received very less attention, as sulphate availability in soil has not been considered in the past as a limiting factor for plant growth8. Several studies have shown that S availability might influence the NUE (N use efficiency) of oilseed crops and vice versa, thus indicating that S and N availabilities may affect S and N nutrition of the plants9-12. N addition increases seed yield in S-sufficient conditions, and an optimum oil quality and maximum yield responses to both N and S applications are obtained when the amounts of available N and S are balanced13-16. Further, the decline of S supply from the soil causes substantial losses of nitrogen from agro-ecosystems to the environment13-15. These observations suggest that sufficient supply of S is required for the optimum growth and nutrient uptake ability of plants and reduction of N loads to the environment. In our previous study, it has been shown that Brassica juncea (L.) Czern. & Coss. cv. Pusa Jai Kisan had only low affinity sulfate transporter with

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reduced capacity of N and S assimilation under S deficient conditions15,17. However, in other subsequent studies, it has been found that overexpression of LeST 1.1 gene (high affinity sulfate transporter from tomato) in this oil crop resulted in higher S assimilation under both S insufficient and S sufficient conditions18. Further, in the present study, we have evaluated the impact of overexpression of LeST 1.1 gene in the transgenic lines of B. juncea cv. Pusa Jai Kisan on N utilization and assimilation under both S insufficient and S sufficient conditions. Materials and Methods Plant Culture

Rooted transgenic lines (4-wk-old) of B. juncea cv. Pusa Jai Kisan overexpressing LeST 1.1 gene18 and untransformed green plants were carefully removed from the solid MS medium and cultured on Hoagland's nutrient medium19 for 10 d in an automated culture room at 25±2°C and 16/8 h photoperiod. The plants were thereafter transferred to nutrient solution containing 3 mM KNO3, 2 mM Ca (NO3)2, 1 mM NH4H2PO4, 50 µM KCl, 25 µM H3BO3, 2 µM MnCl2, 2 µM ZnCl2, 0.5 µM CuCl2, 0.5 µM (NH4)6Mo7O24, and 20 µM NaFeEDTA, and grown for 30 d. The pH of the solution was adjusted to 5.5 with KOH. MgSO4 was added as indicated in the experiments. The nutrient solution was replaced weekly20. Two experimental systems were designed. In the first one, transgenic and untransformed plants were pre-cultured on the medium without SO42- for 20 d. The plants were then transferred to nutrient solution containing 50 µM and 1000 µM SO4-2 to create sulfur insufficient and sulfur sufficient conditions, respectively17,18 and were maintained there for 7 d. Nitrate uptake was measured at the start and after every 24 h intervals during the entire period of experiment. After adjusting the nutrient solution to the original volume, an aliquot was taken from the nutrient solution and nitrate contents were analyzed by anion exchange HPLC (Waters Corporation, Massachusetts). The uptake of nitrate was calculated as the difference in ion amount (µmol) between samples taken at start and after 24 h, divided by the total plant fresh wt (fw; g) after 24 h and expressed as µmol/g fw/24 h. The nitrate contents in the nutrient solutions were determined according to the method described previously21. The anions were separated on a Spherisorb 10 µm SAX anion exchange column (4.6 mm × 250 mm; Waters Corporation, Massachusetts) and 50 mM potassium

dihydrogen phosphate (pH 3.0) was used as a mobile phase with flow rate of 1 mL/min. In the second one, after transfer to the hydroponic system, 10 plants each from transformed and untransformed groups were exposed to two different sulfate concentrations, viz., 50 µM and 1000 µM for 30 d17,18. Thereafter, these plants were assessed for soluble protein, total nitrogen and nitrate reductase activity. Total Nitrogen Content

The estimation of total nitrogen in transgenic and untransformed plants was carried out by direct determination using CHNS analyzer (Model VERIO EL-III, GMBH, Germany). The dry mater was converted into fine powder with the mortar-pestle and tightly packed in pre-weighed aluminium boats. The tungsten oxide was added to plant material for complete digestion. Sulfanilic acid was used as standard in the boat instead of biological material. Nitrate Reductase Activity (NRA)

The NR activity in the leaves was measured using method described by Hageman and Hucklesby22. The leaves were cut into 2 mm slices and placed in ice-cold incubation medium containing 3 mL of potassium phosphate buffer (pH 6.8) and 3.0 mL of 0.4 M KNO3 solution. The tubes were evacuated with a vacuum pump and then incubated in water bath at 33°C for 1 h under dark condition. The enzyme activity and the complete leaching of the nitrite in the medium were stopped by incubating the tubes in boiling water bath for 5 min. The estimation of nitrite was done as described by Evans and Nason23. An aliquot (0.2 mL) from reaction mixture was taken and 1% sulphanilamide solution in 1 N HCl (1 mL) and 0.02% N-(1-nepthyl)-ethylene diammonium dichloride (NEDD) solution in distilled water (1 mL) were added. The mixture was kept for 30 min to allow development of pink colour due to diazotization. Thereafter the volume was made up to 6 mL with double distilled water. The absorbance was measured at 540 nm. The calibration curve was prepared using sodium nitrite solution. The enzyme activity was expressed as µmol mg-1 fw min-1. Statistical Analysis

Statistical analysis was carried out by two-way classification of ANOVA. The presented mean values were separated using Duncan’s Multiple Range Test (DMRT) at p ≤ 0.05.

AKMAL et al: ENHANCED NITROGEN ASSIMILATION IN TRANSGENIC MUSTARD

Results Three transgenic lines (t2, t13 & t14) of B. juncea cv. Pusa Jai Kisan overexpressing HAST gene (LeST 1.1)24 were used in the study. Sulfate (S) uptake rates, ATP-sulfurylase activity and total S content were measured in the transgenic lines and the untransformed plants. The overexpression of LeST 1.1 gene in transgenic lines had resulted in higher S assimilation under both S insufficient and S sufficient conditions as compared to the untransformed plants. The increase in sulfate uptake was observed in both the transgenic and nontransgenic plants, when they were transferred to S insufficient (50 µM S) medium, reaching the peak at 2nd d (Figs 1A & B). Thereafter, the sulfate uptake declined continuously in rate upto 6th d and finally reached at a constant level in the following days. However, under S insufficient conditions, the magnitude of increase in S uptake was higher in transgenic lines as compared with nontransgenic plants. Under S sufficient (1000 µM S) conditions, both transgenic and nontransgenic plants showed increase in S uptake, reaching to the maximum after the 2nd d. In nontransgenic plants, the S uptake declined after the 2nd d and became constant after 5th d. On the other hand, in transgenic lines, the decline in uptake rate continued till 7th d, except in t2 where the pattern was similar to nontransgenic plants. The magnitude of increase in S uptake was again higher for transgenic lines when compared with nontransgenic plants; the trend was similar as observed under S insufficient (50 µM S) conditions.

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The transgenic lines also showed higher nitrate uptake under both sulfur sufficient and insufficient conditions as compared to nontransformed plants. In sulfur insufficient conditions the N uptake in transgenic lines increased till the 2nd d of transfer and thereafter it declined. The trend observed was similar to sulfur uptake. However, the N uptake in both the transgenic and untransformed plants became constant after 4th d of transfer under both S sufficient and insufficient conditions (Figs 2A & B). Consequent upon higher N uptake in transgenic lines, nitrate reductase activity and total N content were also increased under both S insufficient and S sufficient conditions, reflecting their higher N assimilation potential (Figs 3A & B). Discussion As the nitrogen and sulfur assimilations are coupled at metabolic level, both the nutrients have synergistic relationship and of vital importance for the growth and development of plants12,24-28. The sufficient supply of S is essential to maintain the optimum growth and nutrient uptake ability of the plants, especially for the acquisition of N6. The use of sulfur fertilizer could suffice for this in general; however, the presence of low-affinity sulfate transporters in some plant genotypes may lead to poor S utilization efficiency29,30. Thus the over-expression of high affinity sulfur transporters would greatly enhance the nitrogen uptake as well as assimilation in these genotypes. Keeping the above fact in view, we have evaluated the effect of overexpression of high

Fig. 1 (A & B)―Sulfate uptake rate (µmol g-1 fw 24h-1) of transgenic lines after they were transferred into: (A) S insufficient (50 µM sulfur) condition; & (B) S sufficient (1000 µM sulfur) condition. [Transgenic lines (t2, t13 & t14) and untransformed (w) plants of B. juncea cv. Pusa Jai Kisan]

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Fig. 2 (A & B)―Nitrate uptake rate (µmol g-1 fw 24h-1) of transgenic lines after they were transferred into: (A) S insufficient (50 µM sulfur) condition; & (B) S sufficient (1000 µM sulfur) condition. [Transgenic lines (t2, t13 and t14) and untransformed (w) plants of B. juncea cv. Pusa Jai Kisan]

Fig. 3 (A & B)―A. Nitrate reductase activity (µmol NO2- mg-1 protein min-1) of transgenic lines and untransformed plants under S insufficient (50 µM sulfur) condition and S sufficient (1000 µM sulfur) condition; & B. Total nitrogen content (mg g-1 dw) of transgenic lines and untransformed plants under S insufficient (50 µM sulfur) condition and S sufficient (1000 µM sulfur) condition. [Transgenic lines (t2, t13 and t14) and untransformed (w) plants of B. juncea cv. Pusa Jai Kisan]

affinity sulfate transporter LeST 1.1 on uptake and assimilation of N under both S insufficient and S sufficient conditions in B. Juncea cv. Pusa Jaikisan lacking this transporter. Earlier studies have shown that B. juncea cv. Pusa Jai Kisan had only low affinity sulfate transporter with reduced capacity of N and S assimilation, especially under S deficient conditions15,17. However, in the present study, higher S assimilation was seen in the genotypes overexpressing LeST 1.1 gene under both S insufficient and S sufficient conditions (Figs 1A & B). An initial increase in the rate of S uptake observed in both untransformed plants and

transgenic lines might be due to the up regulation of native S transporter genes. Since cv. Pusa Jai Kisan had only low affinity sulfur transporter17, the magnitude of S uptake in untransformed plants was lesser. However, the presence of functional copy of LeST 1.1 gene in transgenic lines under constitutive promoter had complemented the native low affinity sulfur transporter and was responsible for the enhanced sulfur uptake in these lines18. The differences in sulfur uptake as observed among the transgenic lines could be due to the differential expression of transgene LeST 1.1 because of the positional effect and/or random integration31,32. The


transformed line t14 showed 2-fold increase in sulfate uptake rate as compared to the untransformed plants (Figs 1A & B). The enhanced sulfur uptake due to the presence of functional sulfate transporter was in accordance with earlier studies, where the sulfate uptake in transgenic Arabidopsis thaliana was enhanced by the presence of functional sulfate transporters 35S:SULTR1-1myHis and 35S: SULTR1-2mycHis33,34. The gradual decrease in S uptake rate of transgenic as well as untransformed plants after 2nd d, under both the S supply conditions (50 & 1000 µM S), could be due to the feed-back inhibition caused by accumulation of end products of S assimilation like cysteine and glutathione as reported in the earlier studies35. The transgenic lines also showed higher nitrate uptake, enhanced nitrate reductase activity and increased total N content under both sulfur sufficient and insufficient conditions (Figs 2A & B, 3A & B). Nitrate uptake by plant roots is a highly regulated process and is coordinated with many other functions involved in the N utilization by the plants36. There is a significant interdependence between nitrogen and sulfur nutrition in plants and the regulation of N metabolism is very much responsive to nutritional signals. It has been reported that sulfur deficiency limits the growth and yield of crop plants in the agricultural field, mainly because of the impaired nitrogen uptake and assimilation37-38. The studies in tobacco39 and rapeseed-mustard14 have shown that sulfur availability has a role in regulating nitrate reductase activity as well as N-assimilation, in addition to its role in regulating ATP-sulfurylase activity and S assimilation. The synthesis of cysteine as a result of incorporation of sulfide moiety into O-acetylserine appears to be the meeting point between N and S metabolism. Therefore, the higher nitrate uptake and assimilation in transgenic lines may be due to the enhancement in sulfur uptake rate because of the presence of functional LeST 1.1 gene18. The lowered activity of NR and N content in untransformed plants may not necessarily be due to the repression of the NR gene or feedback inhibition of NR by N metabolites under S deficiency, but reflect a general reduction in protein synthesis caused by the shortage of sulfur amino acids40. Sulfur plays an important role in protein synthesis as being the constituent of amino acids, namely methionine, cystine and cysteine. Also N and S, being the important constituents of chlorophyll-protein

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complexes, electron transport as well as photophosphorylation membrane complexes and photosynthetic carbon reduction cycle complexes, play a crucial role in determining the photosynthetic capacity of the plant14,17,39,41. Consequently, with better S and N metabolism in transgenic plants due to over-expression of high affinity S transporter, the total protein content of the cells increased in the present study. Of the total protein, 50-70% of leaf soluble protein constitutes Rubisco (ribulose-1,5bisphosphate carboxilase/oxygenase) enzyme in chloroplast42. Hence, the increase in chlorophyll with soluble protein in transgenic lines of mustard has resulted in higher carbon assimilation, leading to the production of higher biomass. Conclusion In the present study, the transgenic lines of B. juncea cv. Pusa Jai Kisan showed enhanced nitrogen uptake as well as assimilation because of the presence and over expression of a functional copy of high affinity sulfur transporter (LeST 1.1) gene. Since nitrogen is an important nutrient for crop growth, development and yield, its enhanced assimilations would result in increased N incorporation in the building blocks (amino acids) of chlorophyll, enzymes and structural proteins. These transgenic plants with improved N and SUEs may profitably be cultivated in sulfur deficient areas, thus increasing the land use probability. Furthermore, the LeST 1.1 under strong root specific inducible promoter can also be transferred in other oil seed crops, vegetables and pulses, etc. that need high sulfur inputs during various phenological stages of growth and development to achieve higher S and N uptake and assimilation. Acknowledgement We thank Drs M J Hawkesford and P Buchner (Crop Genetic Improvement Centre, Rothamsted Research, West Common, Harpenden, Hertfordshire, UK for providing pBin 19-LeST 1.1cDNA construct. The first author is thankful to Council of Scientific and Industrial Research, New Delhi for the award of research fellowship for his doctoral research. References 1 Vitousek P M, Naylor R, Crews T, David M B, Drinkwater L E et al, Nutrient imbalances in agricultural development, Science, 324 (2009) 1519-1520. 2 Powlson D S, Addiscott T M, Benjamin N, Cassman K G, de Kok T M et al, When does nitrate become a risk for humans? J Environ Qual, 37 (2008) 291-295.

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3 Friedrich J W & Schrader L E, Sulfur deprivation and nitrogen metabolism in maize seedlings, Plant Physiol, 61 (1978) 900-903. 4 Rendig V V, Oputa C & McComb E A, Effects of sulfur deficiency on non-protein nitrogen, soluble sugars, and N/S ratios in young corn (Zea mays L.) plants, Plant Soil, 44 (1976) 423-437. 5 Ohkama-Ohtsu N & Wasaki J, Recent progress in plant nutrition research, cross-talk between nutrients, plant physiology and soil microorganisms, Plant Cell Physiol, 51 (2010) 1255-1264. 6 Habtegebrial K & Singh B R, Effects of timing of nitrogen and sulphur fertilizers on yield, nitrogen, and sulphur contents of Tef (Eragrostis tef (Zucc.) Trotter), Nutr Cycl Agroecosyst, 75 (2006) 213-222. 7 Abdallah M, Dubousset L, Meuriot F, Etienne P, Avice J C et al, Effect of mineral sulphur availability on nitrogen and sulphur uptake and remobilization during the vegetative growth of Brassica napus L., J Exp Bot, 61 (2010) 2635-2646. 8 Scherer H, Sulphur in crop production—Invited paper, Eur J Agron, 14 (2001) 81-111. 9 Fismes J, Vong P C, Guckert A & Frossard E, Influence of sulfur on apparent N-use efficiency, yield and quality of oilseed rape (Brassica napus L.) grown on a calcareous soil, Eur J Agron, 12 (2000) 127-141. 10 Kopriva S & Rennenberg H, Control of sulphate assimilation and glutathione synthesis: Interaction with N and C metabolism, J Exp Bot, 55 (2004) 1831-1842. 11 Varin S, Cliquet J B, Personeni E, Avice J C & LemauvielLavenant S, How does sulphur availability modify N acquisition of white clover (Trifolium repens L.)? J Exp Bot, 61 (2010) 225-234. 12 Fazili I S, Masoodi M, Ahmad S, Jamal A, Khan J S et al, Interactive effect of sulfur and nitrogen on growth and yield attributes of oilseed crops (Brassica campestris L. and Eruca sativa Mill.) differing in yield potential, J Plant Nutr, 33 (2010) 1216-1228. 13 Joshi N L, Mali P C & Saxena A, Effect of nitrogen and sulphur application on yield and fatty acid composition of mustard (Brassica juncea L.) oil, J Agron Crop Sci, 180 (1998) 59-63. 14 Ahmad A, Abraham G, Gandotra N, Abrol Y P & Abdin M Z, Interactive effect of nitrogen and sulphur on growth and yield of rape-seed-mustard (Brassica juncea L. Czern. & Coss. and Brassica campestris L.) genotypes, J Agron Crop Sci, 181 (1998) 193-199. 15 Ahmad A & Abdin M Z, Photosynthesis and its related physiological variables in the leaves of Brassica genotypes as influenced by sulphur fertilization, Physiol Plant, 110 (2000) 144-149. 16 Jan A, Ahmad G, Arif M, Jan M T & Marwat K B, Quality parameters of canola as affected by nitrogen and sulfur fertilization, J Plant Nutr, 33 (2010) 381-390. 17 Ahmad A, Khan I, Anjum N A, Abrol Y P & Iqbal M, Role of sulphate transporter systems in sulphur efficiency of mustard genotypes, Plant Sci, 169 (2005) 842-846. 18 Abdin M Z, Akmal M, Ram M, Nafis T, Alam P et al, Constitutive expression of high-affinity sulfate transporter (HAST) gene in Indian mustard showed enhanced sulfur uptake and assimilation, Protoplasma, 248 (2011) 591-600.

19 Hoagland D R & Arnon D I, The water-culture method for growing plants without soil, circular 347, revised January 1950 (The College of Agriculture, University of California, Berkeley, California, USA) 1950, p 31. 20 Blake-Kalff M M, Harrison K R, Hawkesford M J, Zhao F J & McGrath S P, Distribution of sulfur within oilseed rape leaves in response to sulfur deficiency during vegetative growth, Plant Physiol, 118 (1998) 1337-1344. 21 Maas F M, Hoffmann I, Van Harmelen M J & De Kok L J, Refractometric determination of sulphate and other anions in plants separated by High-Performance Liquid Chromatography, Plant Soil, 91 (1986) 129-132. 22 Hageman R & Hucklesby D, Nitrate reductase from higher plants, Methods Enzymol, 23 (1971) 491-503. 23 Evans H J & Nason A, Pyridine nucleotide-nitrate reductase from extracts of higher plants, Plant Physiol, 28 (1953) 233-254. 24 Fazli I S, Abdin M Z, Jamal A & Ahmad S, Interactive effect of sulphur and nitrogen on lipid accumulation, acetyl-CoA concentration and acetyl-CoA carboxylase activity in developing seeds of oilseed crops (Brassica campestris L. and Eruca sativa Mill.), Plant Sci, 168 (2005) 29-36. 25 Fazili I S, Masoodi M, Ahmad S, Jamal A, Khan J S et al, Oil biosynthesis and its related variables in developing seeds of mustard (Brassica juncea L.) as influenced by sulphur fertilization, J Crop Sci Biotechnol, 13 (2010) 39-46. 26 Thomas R L, Sheard R W & Moyer J R, Comparison of conventional and automated procedures for nitrogen, phosphorus, and potassium analysis of plant material using a single digestion, Agron J, 59 (1967) 240-243. 27 Jamal A, Fazli I S, Ahmad S, Abdin M Z & Yun S J, Effect of sulphur and nitrogen application on growth characteristics, seed and oil yields of soybean cultivars, Korean J Crop Sci, 50 (2005) 340-345. 28 Jamal A, Fazli I S, Ahmad S & Abdin M Z, Interactive effect of nitrogen and sulphur on yield and quality of groundnut (Arachis hypogea L.), Korean J Crop Sci, 51 (2006) 519-522. 29 Hawkesford M J, Plant responses to sulphur deficiency and the genetic manipulation of sulphate transporters to improve S-utilization efficiency, J Exp Bot, 51 (2000) 131-138. 30 Heiss S, Schäfer H J, Haag-Kerwer A & Rausch T, Cloning sulfur assimilation genes of Brassica juncea L.: Cadmium differentially affects the expression of a putative low-affinity sulfate transporter and isoforms of ATP sulfurylase and APS reductase, Plant Mol Biol, 39 (1999) 847-857. 31 Pröls F & Meyer P, The methylation patterns of chromosomal integration regions influence gene activity of transferred DNA in Petunia hybrida, Plant J, 2 (1992) 465-475. 32 Koncz C, Martini N, Mayerhofer R, Koncz-Kalman Z, Körber H et al, High-frequency T-DNA-mediated gene tagging in plants, Proc Natl Acad Sci USA, 86 (1989) 8467-8471. 33 Maruyama-Nakashita A, Inoue E, Watanabe-Takahashi A, Yamaya T & Takahashi H, Transcriptome profiling of sulfurresponsive genes in Arabidopsis reveals global effects of sulfur nutrition on multiple metabolic pathways, Plant Physiol, 132 (2003) 597-605. 34 Yoshimoto N, Inoue E, Watanabe-Takahashi A, Saito K & Takahashi H, Posttranscriptional regulation of high-affinity sulfate transporters in Arabidopsis by sulfur nutrition, Plant Physiol, 145 (2007) 378-388. 35 Lappartient A G & Touraine B, Glutathione-mediated regulation of ATP sulfurylase activity, SO42-uptake, and


oxidative stress response in intact canola roots, Plant Physiol, 114 (1997) 177-183. 36 Lejay L, Tillard P, Lepetit M, Olive F D, Filleur S at al, Molecular and functional regulation of two NO3-uptake systems by N-and C-status of Arabidopsis plants, Plant J, 18 (1999) 509-511. 37 Zhao F, Evans E J, Bilsborrow P E & Syers J K, Influence of sulphur and nitrogen on seed yield and quality of low glucosinolate oilseed rape (Brassica napus L.), J Sci Food Agric, 63 (1993) 29-37. 38 Sexton P J, Paek N C & Shibles R M, Effects of nitrogen source and timing of sulfur deficiency on seed yield and expression of 11S and 7S seed storage proteins of soybean, Field Crops Res, 59 (1998) 1-8.

387

39 Reuveny Z, Dougall D K & Trinity P M, Regulatory coupling of nitrate and sulfate assimilation pathways in cultured tobacco cells, Proc Natl Acad Sci USA, 77 (1980) 6670-6672. 40 Prosser I M, Purves J V, Saker L R & Clarkson D T, Rapid disruption of nitrogen metabolism and nitrate transport in spinach plants deprived of sulphate, J Exp Bot, 52 (2001) 113-121. 41 Abrol Y P, Chatterjee S R, Kumar P A & Jain V, Improvement in nitrogen use efficiency: Physiological and molecular approaches, Curr Sc, 76 (1999) 1357-1364. 42 Feller U, Anders I & Mae T, Rubiscolytics: fate of Rubisco after its enzymatic function in a cell is terminated, J Exp Bot, 59 (2008) 1615-1624.