Some effects of nitrate abundance and starvation on metabolism and accumulation of nitrogen in barley (Hordeum vulgare L. cv Sonja)

Metabolite responses to exogenous application of nitrogen, cytokinin, and ethylene inhibitors in relation to heat-induced senescence in creeping bentgrass

The exogenous application of ethylene inhibitors, cyotkinins, or nitrogen has previously been shown to suppress heat-induced senescence and improve heat tolerance in cool -season grasses. The objectives of this study were to examine metabolic profiles altered by exogenous treatment of creeping bentgrass with an ethylene inhibitor, cytokinin or nitrogen under heat stress and to determine metabolic pathways regulated by those compounds in association with their effectiveness for improving heat tolerance. Creeping bentgrass (Agostis stolonifera) plants (cv. Penncross) were foliar sprayed with 18 mM carbonyldiamide (N source), 25μM aminoethoxyvinylglycine (AVG, ethylene inhibitor), 25μM zeatin riboside (ZR, cytokinin), or a water control, and then exposed to 20/15°C (day/night) or 35/30°C (heat stress) in growth chambers. All three exogenous treatments suppressed leaf senescence, as manifested by increased turf quality and chlorophyll content, and reduced electrolyte leakage under heat stress. Polar metabolite profiling identified increases in the content of certain organic acids (i.e. citric and malic acid), sugar alcohols, disaccharides (sucrose), and decreased accumulations of monosaccharides (i.e. glucose and fructose) with exogenous treatment of N, AVG, or ZR at the previously mentioned concentrations when compared to the untreated control under heat stress. Nitrogen stimulated amino acid accumulation whereas AVG and ZR reduced amino acid accumulation compared to the untreated control under heat stress. These results revealed that the alleviation of heat-induced leaf senescence by N, AVG, and ZR could be due to changes in the accumulation of metabolites involved in osmoregulation, antioxidant metabolism, carbon and nitrogen metabolism, as well as stress signaling molecules.

Ammonium formation and assimilation in P(SARK)∷IPT tobacco transgenic plants under low N

Wild Type (WT) and transgenic tobacco plants expressing isopentenyltransferase (IPT), a gene encoding the enzyme regulating the rate-limiting step in cytokinins (CKs) synthesis, were grown under limited nitrogen (N) conditions. We analyzed nitrogen forms, nitrogen metabolism related-enzymes, amino acids and photorespiration related-enzymes in WT and P(SARK)∷IPT tobacco plants. Our results indicate that the WT plants subjected to N deficiency displayed reduced nitrate (NO₃⁻) assimilation. However, an increase in the production of ammonium (NH₄⁺), by the degradation of proteins and photorespiration led to an increase in the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle in WT plants. In these plants, the amounts of amino acids decreased with N deficiency, although the relative amounts of glutamate and glutamine increased with N deficiency. Although the transgenic plants expressing P(SARK)∷IPT and growing under suboptimal N conditions displayed a significant decline in the N forms in the leaf, they maintained the GS/GOGAT cycle at control levels. Our results suggest that, under N deficiency, CKs prevented the generation and assimilation of NH₄⁺ by increasing such processes as photorespiration, protein degradation, the GS/GOGAT cycle, and the formation of glutamine.

Physiology and biochemistry of source-regulated protein accumulation in the wheat grain

Wheat is unique among cereals for the baking qualities of its flour, which are dependent upon the type and concentration of its proteins. As a consequence, the grain protein concentration (GPC) is one of the main determinants of wheat international market price. More than 50-70% of the final grain N is accumulated before flowering and later remobilized to the grain, N fertilization being the common practice used to produce high GPC. However, after incremental additions of N fertilizer, GPC reaches a maximum and then remains constant, without any increase in N uptake or remobilization by the crop, thus decreasing the efficiency of N fertilizer. Although, the genetic and molecular mechanisms that regulate N uptake by the roots are being clarified quickly, the regulation and physiology of N transport from the leaves to the grain remains less clear. In this review, the possible regulatory points involved in N transport to the grain and the difficulties for increasing GPC are discussed. It has been demonstrated that protein synthesis in the grain is source-limited, and that the grain can accumulate protein limited only by the amino acids provided by the phloem. It has also been shown that there is no limitation in the amino acid/sugar ratios that can be exported to the phloem. On the other hand, NO(3)(-) uptake transporters are depressed when the plant concentration of some amino acids, such as glutamine, is high. It has also been shown that a high N supply increases cytokinins concentration, preventing leaf senescence and proteolysis. Based on this information, it is postulated that there are two main regulatory points during grain filling when plant N status is ample. On the one hand, the N uptake transporters in the roots are depressed due to the high amino acids concentration in the tissues, and N uptake is low. On the other, a high amino acids concentration keeps the cytokinins level high, repressing leaf protein degradation and decreasing amino acid export to the phloem. As a consequence, GPC cannot be increased despite the ample N supply.

Root respiratory characteristics associated with plant adaptation to high soil temperature for geothermal and turf-type Agrostis species

Respiration is a major avenue of carbohydrates loss. The objective of the present study was to examine root respiratory characteristics associated with root tolerance to high soil temperature for two Agrostis species: thermal Agrostis scabra, a species adapted to high-temperature soils in geothermal areas in Yellowstone National Park, and two cultivars ('L-93' and 'Penncross') of a cool-season turfgrass species, A. stolonifera (creeping bentgrass), that differ in their heat sensitivity. Roots of thermal A. scabra and both creeping bentgrass cultivars were exposed to high (37 degrees C) or low soil temperature (20 degrees C). Total root respiration rate and specific respiratory costs for maintenance and ion uptake increased with increasing soil temperatures in both species. The increases in root respiratory rate and costs for maintenance and ion uptake were less pronounced for A. scabra than for both creeping bentgrass cultivars (e.g. respiration rate increased by 50% for A. scabra upon exposure to high temperature for 28 d, as compared with 99% and 107% in 'L-93' and 'Penncross', respectively). Roots of A. scabra exhibited higher tolerance to high soil temperature than creeping bentgrass, as manifested by smaller decreases in relative growth rate, cell membrane stability, maximum root length, and nitrate uptake under high soil temperature. The results suggest that acclimation of respiratory carbon metabolism plays an important role in root survival of Agrostis species under high soil temperatures, particularly for the thermal grass adaptation to chronically high soil temperatures. The ability of roots to tolerate high soil temperatures could be related to the capacity to control respiratory rates and increase respiratory efficiency by lowering maintenance and ion uptake costs.

The export of amino acid in the phloem is altered in wheat plants lacking the short arm of chromosome 7B

Grain protein content is one of the major determinants of the baking and nutritional quality of wheat. It has previously been reported that the ditelosomic line of wheat (Triticum aestivum L.) CSDT7BL, where the short arm of chromosome 7B is missing, shows a lower grain protein concentration than the normal line, but a similar grain yield. In the present paper the growth and nitrogen (N) metabolism of wheat plants cv. Chinese Spring (CS) and its ditelosomic line CSDT7BL were compared. When plants were grown to maturity in pots with different N supplements, the wild-type line showed a higher grain protein concentration and a lower straw N concentration than the ditelosomic line at every N level analysed, suggesting a deficiency in the N remobilization capacity. When 15-d-old plants were grown in a growth cabinet in pots with sand, and supplied with nutrient solutions of different nitrate concentrations, the ditelosomic line showed no differences in N uptake per unit of root dry weight, nitrate reductase activity, nitrate, total N concentration or free amino acid concentration. However, the ditelosomic line showed a decreased capacity to export amino acids in the phloem under high N, independently of the N source. This deficiency was also observed under dark-induced senescence. The diminished export of amino acids to the phloem was principally caused by a decrease in the export of Glu, Asp, and Gln. It is suggested that the decrease in grain protein concentration in the ditelosomic line is a consequence of defective export in the phloem of these amino acids.

Effects of nutrient depletion on growth of Holcus lanatus L. and Festuca ovina L. and on the ability of their roots to absorb nitrogen at warm and cool temperatures

To examine the ecological relevance of slow growth for survival in unproductive environments, responses to nutrient depletion were compared between two ecologically contrasted grasses with reference to their ability to utilize a nutrient flush. Dry weight growth, ability of roots to absorb nitrogen and subsequent distribution between root and shoot were determined for Holcus lanatus L. and Festuca ovina L. before and after subjecting them to nutrient depletion at warm and cool temperatures. Initially, the relative growth rates (RGR) of H. lanatus were much higher than those of F. ovina, but after 21 days of nutrient depletion, F. ovina attained higher RGR. During nutrient depletion, F. ovina maintained higher shoot N concentrations than H. lanatus, mainly due to lower rates of dry matter accumulation in F. ovina. Maintenance of N concentrations in F. ovina may contribute to sustaining of the shoot activity under N-depleted conditions. Nutrient depletion caused an increase in the proportion of N allocated to roots relative to shoots, but F. ovina exhibited less plasticity than H. lanatus. Following nutrient depletion, the ability of roots to absorb nitrate declined markedly in H. lanatus and to a much lesser extent in F. ovina. The ability to absorb ammonium was also reduced at warm temperatures, while at cool temperatures it was unchanged in H. lanatus, but increased in F. ovina. These results suggest that F. ovina has a greater potential to exploit 'nutrient flushes' than H. lanatus under unproductive conditions, especially at cool temperatures.