The role of light in nitrite reduction; Studies with leaf discs

Transcript residency on ribosomes reveals a key role for the Arabidopsis thaliana bundle sheath in sulfur and glucosinolate metabolism

Leaves of angiosperms are made up of multiple distinct cell types. While the function of mesophyll cells, guard cells, phloem companion cells and sieve elements are clearly described, this is not the case for the bundle sheath (BS). To provide insight into the role of the BS in the C3 species Arabidopsis thaliana, we labelled ribosomes in this cell type with a FLAG tag. We then used immunocapture to isolate these ribosomes, followed by sequencing of resident mRNAs. This showed that 5% of genes showed specific splice forms in the BS, and that 15% of genes were preferentially expressed in these cells. The BS translatome strongly implies that the BS plays specific roles in sulfur transport and metabolism, glucosinolate biosynthesis and trehalose metabolism. Much of the C4 cycle is differentially expressed between the C3 BS and the rest of the leaf. Furthermore, the global patterns of transcript residency on BS ribosomes overlap to a greater extent with cells of the root pericycle than any other cell type. This analysis provides the first insight into the molecular function of this cell type in C3 species, and also identifies characteristics of BS cells that are probably ancestral to both C3 and C4 plants.

The effect of oxygen on nitrate and nitrite assimilation in leaves of Zea mays L. under dark conditions

The assimilation of nitrate under dark-N2 and dark-O2 conditions in Zea mays leaf tissue was investigated using colourimetric and (15)N techniques for the determination of organic and inorganic nitrogen. Studies using (15)N indicated that nitrate was assimilated under dark conditions. However, the rate of nitrate assimilation in the dark was only 28% of the rate under non-saturating light conditions. No nitrite accumulated under dark aerobiosis, even though nitrate reduction occurred under these conditions. The pattern of nitrite accumulation in leaf tissue in response to dark-N2 conditions consisted of three phases: an initial lag phase, followed by a period of rapid nitrite accumulation and finally a phase during which the rate of nitrite accumulation declined. After a 1-h period of dark-anaerobiosis, both nitrate reduction and nitrite accumulation declined considerably. However, when O2 was supplied, nitrate reduction was stimulated and the accumulated nitrite was rapidly reduced. Anaerobic conditions stimulated nitrate reduction in leaf tissue after a period of dark-aerobic pretreatment.

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.

Nitrate, nitrite and ammonia assimilation by leaves: Effect of light, carbon dioxide and oxygen

The assimilation of nitrate, nitrite and ammonia in barley, wheat, corn and bean leaves was studied using (15)N-labelled molecules and either leaf chamber experiments with the uptake of the nitrogen species in the transpiration stream, or vacuum-infiltration experiments. The assimilation of (15)NO3 (-) into amino nitrogen was strictly dependent on light and ceased abruptly when the light was extinguished. If the leaves were exposed to air, CO2-free air or N2 there was no effect on the rate of NO3 (-) assimilation over 0.5 h. After 1.25 h of CO2-free air, NO3 (-) assimilation into amino acids was sharply reduced. Resupply of air at this time stimulated NO3 (-) assimilation and restored it to the rate observed in leaves exposed to air only. There was no recovery by tissue pretreated for 1.25 h in N2 and subsequently resupplied with air. Incorporation of (15)NO2 (-) was also markedly dependent on light with little reduction occurring in the dark. Incorporation of (15)NH4 (+) into amino acids was stimulated 5 fold by light but considerable incorporation occurred in the dark. The presence of 100 mM NO3 (-) had no effect on the rate of incorporation of (15)NO2 (-) or (15)NH4 (+). Nitrite at 1 mM had no effect on (15)NO3 (-) incorporation but at 10 mM inhibited it completely after 0.5 h. Ammonia at 1 mM had no effect on (15)NO3 (-) or (15)NO2 (-) incorporation and while 10 mM inhibited incorporation for 0.5 h this inhibition did not persist.

Nitrite reduction in leaves; Studies on isolated chloroplasts

Chloroplast preparations from spinach leaves containing a high percentage of intact chloroplasts were capable of light dependent nitrite reduction at rates around 9 μmol/mg chlorophyll/h for at least 50 min. This reduction was inhibited by DCMU but unaffected by uncouplers of photosynthetic phosphorylation. Nitrite reduction was not accompanied by a stoichiometric evolution of oxygen evolution. The disappearance of nitrite was accompanied by an approximately stoichiometric formation of reduced nitrogen.