Lysine metabolism in higher plants

Summary. The essential amino acid lysine is synthesised in higher plants via a pathway starting with aspartate, that also leads to the formation of threonine, methionine and isoleucine. Enzyme kinetic studies and the analysis of mutants and transgenic plants that overaccumulate lysine, have indicated that the major site of the regulation of lysine synthesis is at the enzyme dihydrodipicolinate synthase. Despite this tight regulation, there is strong evidence that lysine is also subject to catabolism in plants, specifically in the seed. The two enzymes involved in lysine breakdown, lysine 2-oxoglutarate reductase (also known as lysine α-ketoglutarate reductase) and saccharopine dehydrogenase exist as a single bifunctional protein, with the former activity being regulated by lysine availability, calcium and phosphorylation/dephosphorylation. Received December 21, 1999 Accepted February 7, 2000     

Ureide metabolism in higher plants

The synthesis, transport and assimilation of the ureides, allantoin and allantoic acid, in higher plants is reviewed. Evidence indicates that in nodulated legumes ureides are synthesized from products of N₂-fixation via purine synthesis and degradation. Their synthesis in other plants also appears to be via purine degradation but is dependent on the inorganic nitrogen source fed to the plant; greatest ureide production is associated with ammonium assimilation. The use of ureides rather than amides for N-transport from the root to the shoot via the xylem stream results in an improved carbon economy of the plant. Good evidence for the transport of ureides in the phloem is lacking for most species examined although it is assumed to be important, particularly in fruit and seed development. Ureides are stored and assimilated mainly in the shoot. The precise pathways, localization and regulation of ureide assimilation are poorly understood and require further investigation. Similarities exist between the properties of the enzymes involved in ureide assimilation in higher plants and in micro-organisms. However, the evidence that light appears to be involved in ureide assimilation in green tissues suggests that different regulatory mechanisms may exist in plants compared with micro-organisms. The economically important legume crops such as soybeans, cowpeas and Phaseolus sp. are all ureide producers. To aid our understanding of the productivity of these plants knowledge of how ureide-N is converted into seed protein is essential.     

Post-translational control of tetrapyrrole biosynthesis in plants, algae, and cyanobacteria

The tetrapyrrole biosynthetic pathway provides the vital cofactors and pigments for photoautotrophic growth (chlorophyll), several essential redox reactions in electron transport chains (haem), N- and S-assimilation (sirohaem), and photomorphogenic processes (phytochromobilin). While the biochemistry of the pathway is well understood and almost all genes encoding enzymes of tetrapyrrole biosynthesis have been identified in plants, the post-translational control and organization of the pathway remains to be clarified. Post-translational mechanisms controlling metabolic activities are of particular interest since tetrapyrrole biosynthesis needs adaptation to environmental challenges. This review surveys post-translational mechanisms that have been reported to modulate metabolic activities and organization of the tetrapyrrole biosynthesis pathway.     

Manganese in cell metabolism of higher plants

Manganese, a group VII element of the periodic table, plays an important role in biological systems and exists in a variety of oxidation states. The normal level of Mn in air surrounding major industrial sites is 0.03 μg/m³, in drinking water 0.05 mg/liter and in soil between 560 and 850 ppm. Manganese is an essential trace element for higher plant systems. It is absorbed mainly as divalent Mn²⁺, which competes effectively with Mg²⁺ and strongly depresses its rate of uptake. The accumulation of Mn particularly takes place in peripheral cells of the leaf petiole, petiolule and palisade and spongy parenchyma cells. Mn is involved in photosynthesis and activation of different enzyme systems. Mn deficiency may be expressed as inhibition of cell elongation and yield decrease. Mn toxicity is one of the important growth limiting factors in acid soils. Plant tops are affected to a greater extent than root systems. The toxicity symptoms are, in general, similar to the deficiency symptoms. Toxic effects of Mn on plant growth have been attributed to several physiological and biochemical pathways, although the detailed mechanism is still not very clear. Higher O₂ uptake and loss of control in Mn activated enzyme systems have been associated with Mn toxicity. Mn interferes with the uptake, transport and use of several essential elements including Ca, Fe, Cu, Al, Si, Mg, K, P and N. Excess of Mn reduces the uptake of certain elements and increases that of others. pH plays an important role in Mn uptake. Acidic pH causes a lack of substantial amount of nitrate as an alternative electron acceptor and leads to a high amount of Mn in leaves. High microbial activity, water logging and poorly structured soils cause severe Mn toxicity even in neutral soils. The molecular mechanism of Mn-tolerance is not yet clear. The level of tolerance is different in different species and seems to be controlled by more than one gene. Further information is required on the factors affecting the distribution, accumulation and membrane permeability of the metal in different plant parts and different species. Understanding of the genetic basis of Mn-tolerance is necessary to improve adaptation of crops against acid soils, water logging and other adverse soil conditions.     

Subcellular location of the tetrapyrrole synthesis enzyme porphobilinogen deaminase in higher plants: an immunological investigation

A recombinant plasmid, pArab8, harbouring the cDNA encoding the mature form of the tetrapyrrole synthesis enzyme porphobilinogen deaminase (EC 4.3.1.8; also known as hydroxymethylbilane synthase) from Arabidopsis thaliana (L.) Heynh. has been constructed, and used to transform Escherichia coli. The porphobilinogen deaminase protein from Arabidopsis was overexpressed in this strain, and purified to homogeneity (3000-fold) with a yield of 20%. Antibodies were raised against the purified plant enzyme, and used in Western blot analysis, immunoprecipitation of enzyme activity and immuno-gold electron microscopy. The results indicate that the enzyme is confined to plastids in both leaves and roots. The implications of this finding for plant tetrapyrrole synthesis are discussed.Abbreviations DEAE diethylaminoethyl - FPLC fast protein liquid chromatography - PBG porphobilinogen This work was supported by Science and Engineering Research Council (SERC) and Agricultural and Food Research Council (AFRC) grants to P.M.J. and an AFRC grant to A.G.S. The protein sequencing was carried out by Mr Lawrence Hunt of the SERC MRI Protein Sequencing Unit (Director Dr M.G. Gore) at Southampton University. We acknowledge the Wellcome Foundation for financial support of the Protein and Nucleic Acid Chemistry Facility at the University of Cambridge, where the oligonucleotide primers were synthesised.     

Novel metabolism of nitrogen in plants

Our previous study showed that approximately one-third of the nitrogen of 15N-labeled NO2 taken up into plants was converted to a previously unknown organic nitrogen (hereafter designated UN) that was not recoverable by the Kjeldahl method (Morikawa et al., 2004). In this communication, we discuss metabolic and physiological relevance of the UN based on our newest experimental results. All of the 12 plant species were found to form UN derived from NO2 (about 10-30% of the total nitrogen derived from NO2). The UN was formed also from nitrate nitrogen in various plant species. Thus, UN is a common metabolite in plants. The amount of UN derived from NO2 was greatly increased in the transgenic tobacco clone 271 (Vaucheret et al., 1992) where the activity of nitrite reductase is suppressed less than 5% of that of the wild-type plant. On the other hand, the amount of this UN was significantly decreased by the overexpression of S-nitrosoglutathione reductase (GSNOR). These findings strongly suggest that nitrite and other reactive nitrogen species are involved in the formation of the UN, and that the UN-bearing compounds are metabolizable. A metabolic scheme for the formation of UN-bearing compounds was proposed, in which nitric oxide and peroxynitrite derived from NO2 or endogenous nitrogen oxides are involved for nitrosation and/or nitration of organic compounds in the cells to form nitroso and nitro compounds, including N-nitroso and S-nitroso ones. Participation of non-symbiotic haemoglobin bearing peroxidase-like activity (Sakamoto et al., 2004) and GSNOR (Sakamoto et al., 2002) in the metabolism of the UN was discussed. The UN-bearing compounds identified to date in the extracts of the leaves of Arabidopsis thaliana fumigated with NO2 include a delta2-1,2,3-thiadiazoline derivative (Miyawaki et al., 2004) and 4-nitro-beta-carotene.     

Branched-chain amino acid metabolism in higher plants

Valine, leucine and isoleucine contain short branched carbohydrate residues responsible for their classification as branched-chain amino acids (BCAA). Among the proteinogenic amino acids, BCAA show the highest hydrophobicity and are accordingly the major constituents of transmembrane regions of membrane proteins. BCAA cannot be synthesized by humans and thus belong to the essential amino acids. In contrast, plants are able to synthesize these amino acids de novo and are an important source for these compounds in the human diet. However, BCAA cannot only be synthesized in plants, leucine and probably also valine and isoleucine can also be degraded. Many enzymes operating in turnover are found in mitochondria, while some catabolizing activities are located in peroxisomes. The breakdown of BCAA is physically separated from their biosynthesis in chloroplasts. Additionally, in the order of the Capparales, enzymes of the leucine metabolism seem to be evolutionary related to or may even participate in the methionine chain elongation pathway, the early part of the biosynthesis of aliphatic glucosinolates. In summary, in higher plants a complex network of pathways interferes with the homeostasis of Val, Leu and Ile.     

Origins and metabolism of formate in higher plants

Formate, a simple one-carbon compound, is readily metabolized in plant tissues. In greening potato tubers, similar to some procaryotes, formate is directly synthesized via a ferredoxin-dependent fixation of CO₂, serving as the main precursor for carbon skeletons in biosynthetic pathways. In other plant species and tissues, formate appears as a side-product of photorespiration and of fermentation pathways, but possibly also as a product of direct CO₂ reduction in chloroplasts. Formate metabolism is closely related to serine synthesis and to all subsequent reactions originating from serine. Formate may have a role in biosynthesis of numerous compounds, in energetic metabolism and in signal transduction pathways related to stress response. This review summarizes the current state of formate research, physiological/biochemical and molecular aspects.     

Novel inhibitors of ethylene production in higher plants

Of a number of O-substituted hydroxylamine derivatives, N-benzyloxycarbonyl-L-a-aminooxy-propionicacid and -aminooxyacetic acid inhibited ethylene productionby etiolated mung bean hypocotyls by 50% at 3 and 6 µmconcentrations, respectively. Their potency is thus similarto that of aminoethoxyvinylglycine (50% inhibition at 2 µM),the most potent inhibitor of ethylene production hitherto known.Methionine partially alleviated inhibition of ethylene productionby a-aminooxy-acetic acid. The results are in agreement withthe postulated involvement of pyridoxal phosphate in ethylenebiosynthesis. (Received August 31, 1979; )     

Glycerol metabolism in higher plants: glycerol kinase

Glycerol kinase activity was identified in extracts of higher plant seeds and seedlings, and was partially purified and characterized from cucumber radicle tissue. The enzyme was localized in the post-mitochondrial supernatant of the cell, and catalyzed the formation of glycerol-3-phosphate. The pH optiumum was 9.0. ATP, CTP, GTP or UTP could be used as the phosphoryl group donor. The Km for glycerol was 55 microM and Km values for the nucleoside triphosphates were 145-620 microM. The Vmax for the reaction was 40-78 pmol product per min. Kinetic data indicate that the enzyme has a sequential mechanism.     

Sulfur metabolism in higher plants: potential for phytoremediation

Sulfur is a major nutrient for all organisms. Plant species have a high biodiversity in uptake, metabolization and accumulation of sulfur so that there are potentials to use plants for phytoremediation of sulfur-enriched sites. A survey of soils enriched with sulfur either naturally or by human activities shows that a surplus of sulfur is mostly accompanied with a surplus of other chemical elements which may limit phytoremediation because these co-occurring elements are more toxic to plants than sulfur. In addition, the accumulation of the other elements makes the plant material (phyto-extraction) less suitable for the use as fodder and for human consumption.     

Glutathione metabolism and possible biological roles in higher plants

Synthesis of glutathione in plants seems to proceed in the same series of enzyme catalysed reactions observed in animal cells; the pathway of glutathio     

Recent developments in understanding the regulation of starch metabolism in higher plants

This article reviews current knowledge of starch metabolism in higher plants, and focuses on the control and regulation of the biosynthetic and degradative pathways. The major elements comprising the synthetic and degradative pathways in plastids are discussed, and show that, despite present knowledge of the core reactions within each pathway, understanding of how these individual reactions are co-ordinated within different plastid types and under different environmental conditions, is far from complete. In particular, recently discovered aspects of the fine control of starch metabolism are discussed, which indicate that a number of key reactions are controlled by post-translational modifications of enzymes, including redox modulation and protein phosphorylation. In some cases, enzymes of the pathway may form protein complexes with specific functional significance. It is suggested that some of the newly discovered aspects of fine control of the biosynthetic pathway may well apply to many other proteins which are directly and indirectly involved in polymer synthesis and degradation.