Localisation of glutamine synthetase in chloroplasts

Physiological characterisation of Arabidopsis mutants affected in the expression of the putative regulatory protein PII

The PII signal transducing protein is involved in carbon/nitrogen (C/N) sensing in bacteria and cyanobacteria. In higher plants the function of the PII homolog GLB1 is not known. GLB1 transcripts were found in all plant organs tested, while in Arabidopsis leaves GLB1 expression and PII protein levels were not significantly affected by either the day/night cycle or N-nutrition. Its putative regulatory role in plants has been studied by analysing Arabidopsis thaliana T-DNA insertion lines in the GLB1 gene. These PII mutants showed an 80% (PIIV1 mutant) and 100% (PIIS2 mutant) reduced AtGLB1 transcript level and no detectable PII protein. They did not display an altered growth or developmental phenotype when grown under non-limiting conditions suggesting that the PII protein does not play a crucial role in plants. However, in vitro grown PII mutants did show a higher sensitivity to nitrite (NO (2) (-) ) compared to the wild-type plants. This observation is reminiscent of the role of PII in the regulation of NO (2) (-) metabolism in cyanobacteria. Furthermore, when grown hydroponically, the PII mutants displayed a slight increase in carbohydrate (starch and sugars) levels in response to N starvation and a slight decrease in the levels of ammonium (NH (4) (+) ) and amino acids (mainly Gln) in response to NH (4) (+) resupply. Although the phenotypic changes are rather small in the mutant lines, these data support the hypothesis of a subtle involvement of the PII protein in the regulation of some steps of primary C and N metabolism.

Simultaneous expression of NAD-dependent isocitrate dehydrogenase and other krebs cycle genes after nitrate resupply to short-term nitrogen-starved tobacco

Mitochondrial NAD-dependent (IDH) and cytosolic NADP-dependent isocitrate dehydrogenases have been considered as candidates for the production of 2-oxoglutarate required by the glutamine synthetase/glutamate synthase cycle. The increase in IDH transcripts in leaf and root tissues, induced by nitrate or NH4+ resupply to short-term N-starved tobacco (Nicotiana tabacum) plants, suggested that this enzyme could play such a role. The leaf and root steady-state mRNA levels of citrate synthase, acotinase, IDH, and glutamine synthetase were found to respond similarly to nitrate, whereas those for cytosolic NADP-dependent isocitrate dehydrogenase and fumarase responded differently. This apparent coordination occurred only at the mRNA level, since activity and protein levels of certain corresponding enzymes were not altered. Roots and leaves were not affected to the same extent either by N starvation or nitrate addition, the roots showing smaller changes in N metabolite levels. After nitrate resupply, these organs showed different response kinetics with respect to mRNA and N metabolite levels, suggesting that under such conditions nitrate assimilation was preferentially carried out in the roots. The differential effects appeared to reflect the C/N status after N starvation, the response kinetics being associated with the nitrate assimilatory capacity of each organ, signaled either by nitrate status or by metabolite(s) associated with its metabolism.

A rapid method for isolation of purified, physiologically active chloroplasts, used to study the intracellular distribution of amino acids in pea leaves

A procedure is described for the rapid (<5 min) isolation of purified, physiologically active chloroplasts from Pisum sativum L. Mitochondrial and microbody contamination is substantially reduced and broken chloroplasts are excluded by washing through a layer containing a treated silica sol. On average the preparations contain 93% intact chloroplasts and show high rates of (14)CO2 fixation and CO2-dependent O2 evolution (over 100 μmol/mg chlorophyll(chl)/h); they are also able to carry out light-driven incorporation of leucine into protein (4 nmol/mg chl/h). The amino-acid contents of chloroplasts prepared from leaves and from leaf protoplasts have been determined. Asparagine is the most abundant amino acid in the pea chloroplast (>240 nmol/mg chl), even thought it is proportionately lower in the chloroplast relative to the rest of the cell. The chloroplasts contain about 20% of many of the amino acids of the cell, but for individual amino acids the percentage in the chloroplast ranges from 8 to 40% of the cell total. Glutamic acid, glutamine and aspartic acid are enriched in the chloroplasts, while asparagine, homoserine and β-(isoxazolin-5-one-2-yl)-alanine are relatively lower. Leakage of amino acids from the chloroplast during preparation or repeated washing was ca. 20%. Some differences exist between the amino-acid composition of chloroplasts isolated from intact leaves and from protoplasts. In particular, γ-aminobutyric acid accumulates to high levels, while homoserine and glutamic acid decrease, during protoplast formation and breakage.

Nitrate, nitrite and ammonia assimilation by leaves: Effects of inhibitors

The assimilation of H(14)CO3 (-), (15)NO3 (-), (15)NO2 (-) and (15)NH4 (+) by barley (Hordeum vulgare L.) leaf segments in the presence of a number of metabolic inhibitors was studied in experiments where the substrates and inhibitors were vacuum infiltrated into the tissue. 3-(3',4'-Dichlorophenyl)-1,1-dimethylurea (DCMU), carbonyl-cyanide-m-chlorophenylhydrazone (CCCP), and iodoacetate (IOA) inhibited (14)CO2 fixation and (15)NO3 (-) and (15)NO2 (-) assimilation in the light. (15)NH4 (+) assimilation in the light was only inhibited 70% by 10(-4)M DCMU. (15)NH4 (+) assimilation was stimulated by 10(-5)M CCCP but was inhibited by concentrations of CCCP above 5×10(-5)M. In double-label experiments (15)NO3 (-) assimilation was less sensitive than (14)CO2 fixation to both DCMU and CCCP. CCCP but not DCMU stimulated NO2 (-) accumulation in dark and IOA was inhibitory. The rate of NO2 (-) accumulation in the light in the presence of DCMU or atrazine was similar to that in the dark and in all cases NO2 (-) accumulation was inhibited about 90% by oxygenation of the medium. The results indicate that the assimilation of all nitrogen species is closely linked to photosynthetic electron transport, that rate of assimilation of nitrogen species is independent of rate of assimilation of CO2, and that the dark in-vivo nitrate reduction is a useful analogue of the mechanism operating in the light only if electron flow to oxygen is impaired.

Light-dependent synthesis of glutamine in pea-chloroplast preparations

Isolated-chloroplast preparations obtained from Pisum sativum (L.) plants synthesized L-[(14)C]glutamine from L-[(14)C]glutamate in the light. In the dark appreciable synthesis of glutamine occurred only in the presence of added ATP. Photoconversion of glutamate to glutamine was inhibited by millimolar concentrations of ammonium or nitrite, and by DCMU. The inhibition due to DCMU, NO2 (-), or NH4 (+) was markedly relieved by ATP. ATP-dependent synthesis of glutamine in the dark was not inhibited by ammonium or nitrite ions.The data suggest that ATP necessary to support chloroplastic glutamine synthesis may be derived from noncyclic or pseudocyclic phosphorylation. Since high ammonia concentrations appear to inhibit chloroplastic glutamine synthesis by uncoupling photophosphorylation, it is possible that ATP synthesized outside the chloroplast may be required to drive the rapid synthesis of glutamine that occurs in leaves subjected to toxic levels of ammonia.

Glutamine as a major acceptor of reduced nitrogen in leaves

The feeding of K(15)NO3 via the xylem stream to photosynthesising leaves of Datura stramonium L. shows a massive routing of (15)N to glutamine, increasing with rise in the N-feeding level (between 0 and 400 μg N ml(-1)). At each feeding level the glutamine and glutamate pools have very similar (15)N enrichment despite disparity in pool size. Amino-compound pool-size and (15)N enrichment do not increase above the 300 μg ml(-1) feeding level which corresponds to the normal level of nitrate-N in the xylem stream of the experimental plants. Thus, the rôle of glutamine as an N storage compound and ammonia "detoxifier" in many plants extends to the incorporation of photosynthetically reduced N newly produced in Datura leaves; the level of N incorporation into glutamine is apparently dependent on the level of nitrate feeding and the degree of induction of nitrate-reducing enzymes.