All of the nitrogen in a plant, whether derived initially from nitrate, nitrogen fixation, or ammonium ions, is converted to ammonia, which is rapidly incorporated into organic compounds through a number of metabolic pathways beginning with the activity of the enzyme glutamine synthetase (GS), which catalyzes formation of the amino acid glutamine from ammonia and glutamic acid.
An individual nitrogen atom may pass many times through the GS reaction, following uptake from the soil, assimilation, remobilization, and delivery to growing roots and leaves, and ultimately, deposition in seed as storage proteins. Thus GS is likely to be a major check-point controlling plant growth and crop productivity.
In research reported in The Plant Cell, scientists Antoine Martin and Bertrand Hirel from the National Institute of Agronomic Research (INRA) in Versailles, France, together with colleagues from institutions in the U.K., Spain, and Japan, present new information on the roles of two forms (isoenzymes) of cytosolic glutamine synthetase (GS) in maize, which underscores the importance of this enzyme and nitrogen metabolism in cereal crop productivity. Improving nitrogen use efficiency of crop plants, i.e. reducing the amount of costly nitrogen fertilizer inputs that farmers need to apply to crops while at the same time maintaining and even improving yields, is an important goal in crop research. As noted by Dr. Hirel, “a more complete understanding of the roles of GS enzymes in nitrogen metabolism and grain yield in maize and other crop plants (including rice, wheat and barley) may lead to improvements in fertilizer usage and crop yield, thus mitigating the detrimental effects of the overuse of fertilizers on the environment“.
The roles of these two GS isoenzymes, products of the Gln1-3 and Gln1-4 genes, were investigated by examining the impact of knock-out mutations on kernel yield. GS gene expression was impaired in the mutants, resulting in reduced levels of GS1 protein and activity. The gln1-4 phenotype displayed reduced kernel size whereas gln1-3 had reduced kernel number, and both phenotypes were evident in the gln1-3 gln1-4 double mutant. Shoot biomass production at maturity was not affected in either the single mutants or double mutants, suggesting that both gene products play a specific role in grain production. Levels of asparagine increased in the leaves of the mutants during grain filling, most likely as a mechanism for circumventing toxic ammonium buildup resulting from abnormally low GS1 activity. Phloem sap analysis revealed that, unlike glutamine, asparagine is not efficiently transported to developing maize kernels, which could account for the reduced kernel production in the mutants. Constitutive overexpression of Gln1-3 in maize leaves resulted in a 30% increase in kernel number relative to wild type, providing further evidence that GS1 plays a major role in kernel yield.
Some of the major cereals, such as maize, sorghum, and sugar cane, exhibit C4 photosynthesis, which enhances the efficiency of photosynthesis at high temperature (most C4 plants originated in tropical climates). In standard C3 photosynthesis (present in rice, wheat, and most temperate crop plants), CO2 entering the leaf is converted to a 3-carbon compound via the C3 pathway, utilizing energy derived from the light reactions of photosynthesis. In plants that have C4 photosynthesis, the C3 pathway enzymes are localized in specialized “bundle sheath” cells which surround the vascular tissue in the interior of the leaf. CO2 entering mesophyll cells at the leaf surface initially is converted to a 4-carbon compound, which is shuttled into the bundle sheath cells and then decarboxylated to release CO¬2. CO2 released into bundle sheath cells then enters the standard C3 pathway. This CO2-concentrating mechanism allows plants in a hot and dry climate to take up CO2 at night and store it, and release it again inside bundle sheath cells during the day, thus solving the problem of how to maintain a high concentration of CO2 inside the leaf during the daylight hours, when stomata often must be kept closed to prevent water loss. Using cytoimmunochemistry and in situ hybridization, Martin et al. found that GS1-3 is present in maize mesophyll cells whereas GS1-4 is specifically localized in the bundle sheath cells. Thus the two GS1 isoenzymes play non-redundant roles with respect to their tissue-specific localization, and the activity of both is required for optimal grain yield. This work illustrates the close coordination between nitrogen and carbon metabolism in photosynthetic tissues, and reveals that nitrogen metabolism plays a critical role in optimizing grain yields.
How brains surrender to sleep
23.06.2017 | IMP - Forschungsinstitut für Molekulare Pathologie GmbH
A new technique isolates neuronal activity during memory consolidation
22.06.2017 | Spanish National Research Council (CSIC)
An international team of scientists has proposed a new multi-disciplinary approach in which an array of new technologies will allow us to map biodiversity and the risks that wildlife is facing at the scale of whole landscapes. The findings are published in Nature Ecology and Evolution. This international research is led by the Kunming Institute of Zoology from China, University of East Anglia, University of Leicester and the Leibniz Institute for Zoo and Wildlife Research.
Using a combination of satellite and ground data, the team proposes that it is now possible to map biodiversity with an accuracy that has not been previously...
Heatwaves in the Arctic, longer periods of vegetation in Europe, severe floods in West Africa – starting in 2021, scientists want to explore the emissions of the greenhouse gas methane with the German-French satellite MERLIN. This is made possible by a new robust laser system of the Fraunhofer Institute for Laser Technology ILT in Aachen, which achieves unprecedented measurement accuracy.
Methane is primarily the result of the decomposition of organic matter. The gas has a 25 times greater warming potential than carbon dioxide, but is not as...
Hydrogen is regarded as the energy source of the future: It is produced with solar power and can be used to generate heat and electricity in fuel cells. Empa researchers have now succeeded in decoding the movement of hydrogen ions in crystals – a key step towards more efficient energy conversion in the hydrogen industry of tomorrow.
As charge carriers, electrons and ions play the leading role in electrochemical energy storage devices and converters such as batteries and fuel cells. Proton...
Scientists from the Excellence Cluster Universe at the Ludwig-Maximilians-Universität Munich have establised "Cosmowebportal", a unique data centre for cosmological simulations located at the Leibniz Supercomputing Centre (LRZ) of the Bavarian Academy of Sciences. The complete results of a series of large hydrodynamical cosmological simulations are available, with data volumes typically exceeding several hundred terabytes. Scientists worldwide can interactively explore these complex simulations via a web interface and directly access the results.
With current telescopes, scientists can observe our Universe’s galaxies and galaxy clusters and their distribution along an invisible cosmic web. From the...
Temperature measurements possible even on the smallest scale / Molecular ruby for use in material sciences, biology, and medicine
Chemists at Johannes Gutenberg University Mainz (JGU) in cooperation with researchers of the German Federal Institute for Materials Research and Testing (BAM)...
19.06.2017 | Event News
13.06.2017 | Event News
13.06.2017 | Event News
23.06.2017 | Physics and Astronomy
23.06.2017 | Physics and Astronomy
23.06.2017 | Information Technology