Winter wheat (Triticum aestivum L.) was grown for 4 years in multi-factorial field trials at Rothamsted, southern England. Thirty nine elite commercial cultivars (primarily short-straw) were grown including those released in the UK over a 25-year period, a selection of continental varieties, and three older, tall varieties. Varieties spanned the quality spectrum from 'bread' to 'feed'. The crops were given ammonium nitrate at five rates in the range 0-350 kg-N/ha as a 3-way split. The aim was to quantify the genotypic variation in total nitrogen uptake by grain and straw (total-Nup), and in nitrogen utilization efficiency for grain yield (grain yield per unit of N taken up) (grain-NutE). Depending on treatment, grain yield ranged from 2.1 to 11.8 t/ha (85% DM), grain %N from 1.1% to 2.8% (in DM), total-Nup from 31 to 264 kg-N/ha, and grain-NutE from 27 to 77 kg-DM/kg-N. There were significant varietal differences in total N-uptake and grain-NutE both between 'tall' and 'short' varieties and within 'short' varieties. The best short varieties took up 31-38 kg/ha more N than the worst, and grain-NutE was 24-42% better, depending on N-rate. Up to 77% of the variation in grain-NutE was accounted for by yield. All interactions between the factors 'Variety', 'Year', and 'N-rate' were highly significant, but only 'Year × N-rate' made an important contribution to the variation. There was a near-functional inverse relationship between grain-NutE and grain %N; high-quality wheat (high grain %N) can be expected to have a low grain-NutE. The four key variables determining N-efficiency in a wheat crop-grain yield, grain %N, total N-uptake and nitrogen harvest index (NHI)-are ultimately constrained by the law of conservation of matter. Improving grain-NutE for fixed total-Nup and NHI can only be achieved at the expense of grain %N. To improve grain-NutE and maintain grain %N requires a simultaneous increase in NHI and grain starch yield which may be difficult to achieve in practice. The law of conservation of matter ultimately sets a limit on the physiological and agronomic processes that determine crop N requirements. A high yield of high-quality grain (high grain %N) requires a high input and uptake of nitrogen.

Interactions between sulfur (S) nutritional status and sulfate transporter expression in field-grown wheat (Triticum aestivum) were investigated using Broadbalk +S and 2S treatments (S fertilizer withheld) at Rothamsted, United Kingdom. In 2008, S, sulfate, selenium (Se), and molybdenum (Mo) concentrations and sulfate transporter gene expression were analyzed throughout development. Total S concentrations were lower in all tissues of 2S plants, principally as a result of decreased sulfate pools. S, Se, and Mo concentrations increased in vegetative tissues until anthesis, and thereafter, with the exception of Mo, decreased until maturity. At maturity, most of the S and Se were localized in the grain, indicating efficient remobilization from vegetative tissues, whereas less Mo was remobilized. At maturity, Se and Mo were enhanced 7-and 3.7-fold, respectively, in 2S compared with +S grain, while grain total S was not significantly reduced. Enhanced expression of sulfate transporters, for example Sultr1;1 and Sultr4;1, in 2S plants explains the much increased accumulation of Se and Mo (7-and 3.7-fold compared with +S in grain, respectively). Sultr5;2 (mot1), thought to be involved in Mo accumulation in Arabidopsis (Arabidopsis thaliana), did not fully explain patterns of Mo distribution; it was expressed in all tissues, decreasing in leaf and increasing in roots under 2S conditions, and was expressed in florets at anthesis but not in grain at any other time. In conclusion, S fertilizer application has a marked impact on Mo and Se distribution and accumulation, which is at least partially a result of altered gene expression of the sulfate transporter family.

2-carboxy-D-arabinitoI-l-phosphate (CAIP) bound toRubisco eitber in leaf extracts or after purification can be displaced by SO4^ ions. Tbus, treatment of leaf extracts witb a buffer containing 200 mol m"^d isplaces any 4 p y bound CAIP and enables measurement of maximum carboxylation potential. In tobacco leaves, tbe activity following treatment witb SO4^ ions ('maximal activity') is greater tban tbe total Rubisco activity. Tbe ratio of tbe two activities altered in a dynamic way witb fluctuations in irradiance. Even in species wbicb do not produce significant amounts of CAIP, tbe maximal activity greatly exceeded tbe total activity. Anion excbange separation of components in acid extracts confirmed tbe absence of CAIP in tobacco leaves barvested above an irradiance of 300 //mol quanta m"^ s~*, but tbe presence of anotber inbibitor of Rubisco. Tbese results are consistent witb tbe regulation of Rubisco activity by inbibitors otber tban CAIP wbicb, like CAIP, can be displaced by SO4^~ ions.

Increasing demands for productivity together with environmental concerns about fertilizer use dictate that the future sustainability of agricultural systems will depend on improving fertilizer use efficiency. Characterization of the biological processes responsible for efficient fertilizer use will provide tools for crop improvement under reduced inputs. Transcriptomic and metabolomic approaches were used to study the impact of nitrogen (N) and sulphur (S) deficiency on N and S remobilization from senescing canopy tissues during grain filling in winter wheat (Triticum aestivum). Canopy tissue N was remobilized effectively to the grain after anthesis. S was less readily remobilized. Nuclear magnetic resonance (NMR) metabolite profiling revealed significant effects of suboptimal N or S supply in leaves but not in developing grain. Analysis of amino acid pools in the grain and leaves revealed a strategy whereby amino acid biosynthesis switches to the production of glutamine during grain filling. Glutamine accumulated in the first 7 d of grain development, prior to conversion to other amino acids and protein in the subsequent 21 d. Transcriptome analysis indicated that a down-regulation of the terminal steps in many amino acid biosynthetic pathways occurs to control pools of amino acids during leaf senescence. Grain N and S contents increased in parallel after anthesis and were not significantly affected by S deficiency, despite a suboptimal N:S ratio at final harvest. N deficiency resulted in much slower accumulation of grain N and S and lower final concentrations, indicating that vegetative tissue N has a greater control of the timing and extent of nutrient remobilization than S.

cDNAs encoding a high-affinity sulfate transporter and an adenosine 5#-phosphosulfate reductase from potato (Solanum tuberosum L. cv Désirée) have been cloned and used to examine the hypothesis that sulfate uptake and assimilation is transcriptionally regulated and that this is mediated via intracellular O-acetylserine (OAS) pools. Gas chromotography coupled to mass spectrometry was used to quantify OAS and its derivative, N-acetylserine. Treatment with external OAS increased sulfate transporter and adenosine 5#-phosphosulfate reductase gene expression consistent with a model of transcriptional induction by OAS. To investigate this further, the Escherichia coli gene cysE (serine acetyltransferase EC 2.3.1.30), which synthesizes OAS, has been expressed in potato to modify internal metabolite pools. Transgenic lines, with increased cysteine and glutathione pools, particularly in the leaves, had increased sulfate transporter expression in the roots. However, the small increases in the OAS pools were not supportive of the hypothesis that this molecule is the signal of sulfur (S) nutritional status. In addition, although during S starvation the content of S-containing compounds decreased (consistent with derepression as a mechanism of regulation), OAS pools increased only following extended starvation, probably as a consequence of the S starvation. Taken together, expression of these genes may be induced by a demand-driven model, via a signal from the shoots, which is not OAS. Rather, the signal may be the depletion of intermediates of the sulfate assimilation pathway, such as sulfide, in the roots. Finally, sulfate transporter activity did not increase in parallel with transcript and protein abundance, indicating additional posttranslational regulatory mechanisms.Higher plants use inorganic sulfate as their major source of sulfur (S), which is reduced and assimilated to Cys. Sulfate is actively taken up into the root by high-affinity sulfate transporters, and transport of sulfate around the plant depends upon a gene family of sulfate transporters (Hawkesford, 2003). The cloning of high-affinity Takahashi et al., 2000), low-affinity (Smith et al., 1995;Takahashi et al., 2000), and organelle-specific (Kataoka et al., 2004b) sulfate transporters has provided molecular probes with which to study the regulation of S uptake and the control of S assimilation.Reductive sulfate assimilation is a multistep pathway in which sulfate is activated, reduced to sulfide, and incorporated into Cys, which may be used for the synthesis of other S-containing compounds (Leustek and Saito, 1999;Hawkesford and Wray, 2000;Leustek et al., 2000). Reductive assimilation occurs in most tissues, but predominantly in plastids of green tissue where the supply of the required ATP and redox equivalent is plentiful. The final step of Cys biosynthesis is catalyzed by the Ser acetyltransferase (SAT; EC 2.3.1.30)/ O-acetylserine (thiol) lyase (OASTL; EC 4.2.99.8) bienzyme complex (Bogdanova and Hell, 1997). SAT acetylates L-Ser using acetyl-CoA to form ...

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The initial steps of carbon assimilation and photorespiration are catalysed by ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39). Natural variation in the kinetic properties of the enzyme suggest that it is possible to alter the enzyme to favour the carboxylation activity relative to oxygenation. Mutagenesis in vitro of the gene encoding the large subunit of the enzyme from Anacystis nidulans has been used to modify catalytic properties. Residues at the C-terminal end of loop 6 of the β/α barrel structure of the large subunit influence specificity towards the gaseous substrates, CO2 and O2. None of the residues altered by mutagenesis appear to interact directly with the transition state analogue and their effect on the reaction of the enediolate intermediate with the gaseous substrates and stabilization of the resulting transition state intermediates by lysine 334 must be indirect. Interactions with other parts of the enzyme must also be important in determining substrate specificity. Backbone carbonyl groups close to lysine 334 interact with lysine 128; mutation of lysine 128 to residues of less positive polarity reduces enzyme activity and favours oxygenation relative to carboxylation. The likely effects on assimilation rates of altering the kinetic properties of Rubisco have been modelled. A leaf with cyanobacterial Rubisco may out-perform a higher plant Rubisco at elevated CO2 and cool temperatures.

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