Prikhshayat Singh

Effect of photosynthesis on dark mitochondrial respiration in green cells

Green plant cells can generate ATP in both chloroplasts and mitochondria. Hence the effect of photosynthesis on dark mitochondrial respiration can be considered at a variety of levels. Turnover of ceitric acid cycle dehydrogenases, which is essential for supply of carbon skeletons for amino acid synthesis, seems to be largely unaffected during photosynthesis. The source of carbon for the anaplerotic function of the citric acid cycle in light is however, not known with certainty. NADH generated in these reactions is probably not oxidised via the mitochondrial electron transfer chain coupled to ATP synthesis. However, it may be oxidised by the alternative cyanide‐insensitive pathway, exported to the cytosol via the oxaloacetate‐malate dicarboxylate shuttle or directly utilised for cytosolic nitrate reduction. Oxidation of succinate via cytochrome oxidase may also be similarly inhibited in light. Whether increase in the cytosolic ATP/ADP ratio in light is responsible for the inhibition of mitochondrial electron transfer to O2 is not clearly established, because the ATP/ADP ratio is reported to be already quite high in the dark. Effective collaboration between photophosphorylation and oxidative phosphorylation in order to maintain the cytosolic energy charge at a present high level is discussed.

Reaction between succinate and glyoxylate as a possible source of CO2 during photorespiration in wheat leaves.

Illuminated leaves of Cs plants release recently fixed CO* during photorespiration; also known as glycolate oxidation pathway [ 11. The light reaction involves the formation of phosphoglycolate in the chloroplasts by the action of ribulose 1,5-biphosphate oxygenase. Glycolate is then produced by a phosphatase. Subsequent oxidative decarboxylation of glycolate is known to be the source of photorespiratory COZ. Thus it was shown that glycolate is transported to peroxisomes, where it is converted into glyoxylate and glycine by oxidation and aminotransferase reactions [2]. Glycine is then transported to the mitochondria, where 2 mol glycine give rise to 1 each of serine and CO,. This mechanism was confirmed in [3]. However, alternative mechanisms of COZ evolution from glycolate and glyoxylate have also been suggested. A direct nonenzymic decarboxylation of glyoxylate by HzOz was demonstrated in the peroxisomes [4]. We now suggest the possible formation of isocitrate by the condensation of succinate with glyoxylate via the reversal of the isocitrate lyase reaction. Further metabolism of isocitrate would generate CO*. The possibility of photorespiration as a source of carbon for citric acid cycle during active photosynthesis is discussed.

Regulation of nitrate assimilation in plants in light and dark

Abstract Assimilation of nitrate and nitrite accumulated in leaves was much more rapid in the light than in the dark. Under light aerobic and dark anaerobic conditions the extent of nitrate reduction was almost equal. Dark assimilation of nitrite was drastically inhibited by 2,4-dinitrophenol (2,4-DNP). Although the uncoupler did not show any significant effect on oxidation of NADH and succinate in isolated leaf mitochondria, it inhibited 14CO2 evolved from endogenously labelled sugars, probably by blocking the formation of glucose 6-phosphate (G 6-P). It is suggested that for dark assimilation of nitrite, NADPH generated in the oxidative pentose phosphate pathway is the source of reductant.

Reaction between glyoxylate and succinate in the mitochondria of wheat leaves

Abstract In wheat leaf mitochondria glyoxylate significantly stimulated 14 CO 2 evolution from [1,4− 14 C]succinate when the oxidation of the latter was inhibited by malonate. Conversely decarboxylation of [1− 14 C]glyoxylate was stimulated by succinate. These results indicated the reversibility of isocitrate lyase reaction in vivo. This was further confirmed by demonstrating the incorporation of both [1− 14 C]glyoxylate and [1,2− 14 C]glyoxylate into isocitrate and other citric acid cycle intermediates. The possible role of photorespiratory substrates such as glyoxylate as a source of carbon for citric acid cycle in light is discussed.

Changes in Leaf Nitrate Reductase Activity In Vivo and In Vitro During Light-Dark Transitions

Nitrate reductase activity in the leaves of a number of plants after transfer from light to dark was assayed both by in vivo and in vitro methods. The initial activity persisted during the dark phase for a considerable length of time and declined gradually. After exposure to light again, the NR activity increased rapidly. The possibility of nitrate assimilation in complete darkness is discussed.

Dissociation of cytochrome oxidase-carbon monoxide complex in complete darkness

Abstract In the leaves of a number of plant species, dissociation of CO from the active site of cytochrome a 3 -CO complex in complete darkness was observed when excess O 2 was supplied. This was detected by direct O 2 uptake and by the in vivo nitrate reductase test. Thus, in contrast to animal mitochondria, in plants the cytochrome a 3 -CO complex appears to be quite unstable, even in the dark.

Metabolism of Glycine in Roots of Wheat and Rice Seedlings

Wheat and rice roots have been shown to decarboxylate both the carbon atoms of glycine. Cold formate and serine significantly depressed (14)CO(2) evolution from (2-(14)CO) glycine. It is therefore likely that C(1)-tetrahydrofolate pool is involved in 2-C-glycine metabolism.

Differential Carbon Monoxide Sensitivity of Cytochrome-Oxidase in the Leaves of Tall and Dwarf Wheat Cultivars

Differential response in the leaves of tall and dwarf wheat to CO, an inhibitor of cytochrome oxidase and to SHAM, an inhibitor of alternative oxidase appears to be correlated with presence of Rht dwarfing genes. This was detected by in vivo nitrate reductase assay after CO treatment and direct O2 uptake in presence of SHAM. Pretreatment of the leaves with Triton X-100 at a concentration which specifically inhibits the accessibility of exogenous NAD(P)H to alternative oxidase, Significantly enhanced the CO response as assessed by in vivo NR assay. This supports the hypothesis that the competition for NADH between NR and mitochondrial respiration is regulated by NADH-dehydrogenase located on the outer surface of inner mitochondrial membrane.

Redox status of cytochrome oxidase in darkened leaf of C4 crop plants.

Light is essential for growth, development and various metabolic processes in plant. One-third of the whole intact leaf blades of pearlmillet and maize were covered (treated leaf) with a black opaque plastic sheet at the middle region for 15 days. The leaf samples were taken from three regions: basal, middle and distal; from treated and parallel untreated leaves (control). Oxygen uptake was measured from all the three regions by taking randomized leaf discs. Oxygen uptake was nearly the same in all the regions of treated and parallel untreated leaf in pearlmillet and maize. Carbon monoxide used at 0.5 mM concentration with pearlmillet inhibited oxygen uptake slightly (22%) in covered leaf blade, whereas the inhibition with maize leaf at 1.12 mM CO was significantly higher (45%). However, CO did not inhibit oxygen uptake in untreated leaf from pearlmillet and maize. In contrast. cyanide brought about 33%, inhibition in oxygen uptake at 0.25 mM with pearlmillet and 60%, with maize at 0.4 mM, irrespective of the fact whether a portion of the leaf blade was covered or not with an opaque sheet. The results indicate that removing light from a portion of the leaf blade alters the redox state of the whole leaf in terms of an increase in the level of the ferrocytochrome a3 component of cytochrome c oxidase (cytochrome aa3).