Alyson K. Tobin

Identification and Expression Analyses of Cytosolic Glutamine Synthetase Genes in Barley (Hordeum vulgare L.)

Glutamine synthetase (GS) is a key enzyme in nitrogen (N) assimilation, particularly during seed development. Three cytosolic GS isoforms (HvGS1) were identified in barley (Hordeum vulgare L. cv Golden Promise). Quantitation of gene expression, localization and response to N supply revealed that each gene plays a non-redundant role in different tissues and during development. Localization of HvGS1_1 in vascular cells of different tissues, combined with its abundance in the stem and its response to changes in N supply, indicate that it is important in N transport and remobilization. HvGS1_1 is located on chromosome 6H at 72.54 cM, close to the marker HVM074 which is associated with a major quantitative trait locus (QTL) for grain protein content (GPC). HvGS1_1 may be a potential candidate gene to manipulate barley GPC. HvGS1_2 mRNA was localized to the leaf mesophyll cells, in the cortex and pericycle of roots, and was the dominant HvGS1 isoform in these tissues. HvGS1_2 expression increased in leaves with an increasing supply of N, suggesting its role in the primary assimilation of N. HvGS1_3 was specifically and predominantly localized in the grain, being highly expressed throughout grain development. HvGS1_3 expression increased specifically in the roots of plants grown on high NH(+)4, suggesting that it has a primary role in grain N assimilation and also in the protection against ammonium toxicity in roots. The expression of HvGS1 genes is directly correlated with protein and enzymatic activity, indicating that transcriptional regulation is of prime importance in the control of GS activity in barley.

Nitrogen and Carbon Metabolism in Plastids: Evolution, Integration, and Coordination with Reactions in the Cytosol

ABSTRACT Plastids are diverse organelles that differ in form and function depending on their location within a plant. Their evolutionary origin, as free-living cyanobacteria, has left remnants of autonomy, and whereas the majority of the genetic control now lies within the nucleus, in terms of metabolism the plastid is fundamental to the life of the cell. This chapter describes the involvement of the plastid in carbon and nitrogen metabolism, in particular nitrate and ammonium assimilation, the Calvin cycle, oxidative pentose-phosphate pathway, glycolysis, and terpenoid biosynthesis. We have selected these pathways because they provide an opportunity to describe the metabolic interchange between plastids and cytosol and show duplication of some or all of the reactions in these two subcellular compartments. We discuss current knowledge of the likely ancestry of the genes encoding these pathways and consider how this has contributed to the compartmentation of nitrogen and carbon metabolism within the cell.

Profiling of spatial metabolite distributions in wheat leaves under normal and nitrate limiting conditions

Highlights • Nitrogen and carbon assimilation interaction is essential in leaf metabolism.• Along the developing wheat leaf there is a switch from hetero- to auto-trophy.• GC–MS metabolite profiling was combined with Bayesian network (BN) correlation analysis.• Amino acid, organic acid and carbohydrate distribution changed with nitrate levels.

The Effects of Ultraviolet B Radiation on Crop Plants

Plants require light as both a source of energy for photosynthesis and as a signal to regulate their development. The quality (wavelength) of light is an important influence on whether the plant responds photosynthetically or photomorphogenically. Higher plants use light within the wavelength range of 400–700 nm for photosynthesis (defined as photosynthetically active radiation; PAR). Photomorphogenic responses, such as altered growth and/or development, tend to result from absorption of light within a much more specific range, usually as a result of absorption by photoreceptors, including phytochrome, which absorbs red (660 nm) and far-red (730 nm) light; a blue-light photoreceptor (crypto-chrome); and an as yet unidentified ultraviolet B (UV-B) photoreceptor. In this chapter, the specific responses of crop plants toward UV-B (28–320 nm) are described. In addition to photomorphogenic responses, plants may be affected as a result of the direct absorption of UV-B by a range of important molecules, including proteins, nucleic acids, and auxins. Consequently, UV-B has the potential to cause significant damage to a plant cell. Given the absolute requirement for sunlight for photosynthesis, plants have developed a range of protective and repair processes to minimize the damaging effects of UV-B. These adaptive and acclimative processes are described, along with evidence of how crop plants are likely to be affected if UV-B increases.