Biochemical and cytological relationships in C4 plants

Aspartate aminotransferase from Panicum maximum Jacq. var. trichoglume Eyles, a C4 plant: purification, molecular properties, and preparation of antibody

Extracts of the leaf tissue of Panicum maximum Jacq. var. trichoglume Eyles (a phosphoenolpyruvate carboxykinase type of C4 plant) were examined and at least two isoforms of aspartate aminotransferase (EC 2.6.1.1), with different electrophoretic mobilities, were detected. The predominant isoform was purified to homogeneity from mesophyll cells. The purification procedure included fractionation with ammonium sulfate followed by chromatography on diethylaminoethyl-cellulose, Sephacryl S-300, and hydroxyapatite. The purified enzyme had specific activities of 182 and 165 mumol/min/mg protein, measured in terms of the synthesis of oxaloacetate and aspartate, respectively, at pH 8.0. The enzyme, with an apparent molecular size of 100 kDa, appears to be a dimer of a single polypeptide with a molecular size of 42 kDa. Mono specific polyclonal antibodies were raised against the 42-kDa polypeptide. Only a single stained band was detected in extracts of whole leaves by immunoblot analysis with this antibody after two-dimensional polyacrylamide electrophoresis. Furthermore, no difference in mobility was observed between the enzymes extracted from mesophyll and bundle sheath cells on native polyacrylamide gels. These findings are discussed in relation to the other isoform in the leaves of this species.

Photosynthesis in phosphoenolpyruvate carboxykinase-type C4 plants: photosynthetic activities of isolated bundle sheath cells from Urochloa panicoides

A method has been developed for rapidly preparing bundle sheath cell strands from Urochloa panicoides, a phosphoenolpyruvate (PEP) carboxykinase-type C4 plant. These cells catalyzed both HCO3(-)- and oxaloacetate-dependent oxygen evolution; oxaloacetate-dependent oxygen evolution was stimulated by ATP. For this activity oxaloacetate could be replaced by aspartate plus 2-oxoglutarate. Both oxaloacetate- and aspartate plus 2-oxoglutarate-dependent oxygen evolution were accompanied by PEP production and both were inhibited by 3-mercaptopicolinic acid, an inhibitor of PEP carboxykinase. The ATP requirement for oxaloacetate- and aspartate plus 2-oxoglutarate-dependent oxygen evolution could be replaced by ADP plus malate. The increased oxygen evolution observed when malate plus ADP was added with oxaloacetate was accompanied by pyruvate production. These results are consistent with oxaloacetate being decarboxylated via PEP carboxykinase. We suggest that the ATP required for oxaloacetate decarboxylation via PEP carboxykinase may be derived by phosphorylation coupled to malate oxidation in mitochondria. These bundle sheath cells apparently contain diffusion paths for the rapid transfer of compounds as large as adenine nucleotides.

An efficient method for the determination of K m values for HCO 3 (-) of phosphoenolpyruvate carboxylase

One of the most serious problems in obtaining estimates of the K m values for HCO 3 (-) of phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) by measurement of initial rates at varying HCO 3 (-) is the impossibility of completely excluding any contaminating HCO 3 (-) . A method is proposed which has no need for the careful control of HCO 3 (-) /CO2 contamination. The kinetic data are obtained by the evaluation of progress curves of HCO 3 (-) consumption. The method is discussed and the K m values for HCO 3 (-) of PEPCase from several C4-species are presented.

Effect of low temperature on the photosynthetic metabolism of the C4 grass Echinochloa crus-galli

CO₂ curves of photosynthesis and activities of the four C₄ enzymes and Ribulose bisphosphate carboxylase (RUBP_c) were compared in two populations of the C₄ grass Echinochloa crus-galli from contrasting thermal environments (Québec and Mississippi). Analyses were conducted both before and after 14 h of chilling at 7°C under high light conditions. This comparison provides the opportunity to assess which steps of the C₄ pathway are more susceptible to become limiting at low temperatures. Both populations maintained, after chilling, a pattern of CO₂ fixation typical of C₄ plants with photosynthesis saturating at low external CO₂ concentrations. However, the chilling treatment led to reductions in carbon uptake and in the activities of the C₄ enzymes. RUBP_c activity was not significantly affected by chilling. Reductions in photosynthesis and in C₄ enzyme activities following the chilling treatment were significantly larger for plants of the Mississippi population. The enzyme data suggest that two steps of the C₄ pathway, NADP⁺-malate dehydrogenase and pyruvate P_i dikinase, are likely to be associated with the reduction of CO₂ uptake in C₄ plants under cool conditions. When the experiment was replicated under enriched atmospheric CO₂ (675 μl l^-1 CO₂), similar differences were observed between the two populations. CO₂ enrichment resulted in an increase of activity for phospho-enol-pyruvate carboxylase and NADP⁺-malate dehydrogenase while activities of phospho-enol-pyruvate carboxylase and NADP⁺-malic enzyme were less reduced following chilling. Such an interaction was not observed for gas exchange parameters but net photosynthesis was lower when plants were grown under enriched CO₂.

Photosynthetic characteristics and chloroplast ultrastructure of C3 and C 4 tree species grown in high- and low-light environments

Plants of the C4 tree species, Euphorbia forbesii, Sherff and the C3 tree species, Claoxylon sandwicense Muell-Arg., were grown in a full sun and a shade environment designed to simulate the understory of their native Hawiian forest habitat. When grown under shade conditions, both species exhibited a photosynthetic light response typical of shade plants with low light compensation points and low dark respiration rates. E. forbesii, however, exhibited greater acclimation of light saturated photosynthetic rates and no evidence of photoinhibition in high light. In contrast, quantum yields for CO2 uptake and chlorophyll contents were reduced in the high-light as compared to the low-light grown C. sandwicense plants. Both species exhibited similar changes in the intercellular CO2 response curves and chloroplast whole-chain electron transport capacities, suggesting that the underlying mechanisms of light acclimation are similar. Chloroplasts of E. forbesii exhibited large changes in ultrastructure, with much greater thylakoid membrane development in low than high light. In contrast, C. sandwicense exhibited different starch contents, but otherwise similar membrane development in high and low light. The results show that E. forbesii possesses a very flexible photosynthetic apparatus which may account for its ability to survive in the understory of shaded forests.

Microautoradiographic localization of imported (14)C-photosynthate in induced sink leaves of two dicotyledonous C4 plants in relation to phloem unloading

Pruned source-sink transport systems from predarkened plants of Amaranthus caudatus L. and Gomphrena globosa L. were used to study the localization of (14)C-labeled photosynthate imported into experimentally induced sink leaves by microautoradiography. During a 6-h (Amaranthus) or a 4-h (Gomphrena) transport period, (14)C-assimilates were translocated acropetally from a mature source leaf provided with (14)CO2, into a younger induced sink leaf (dark/-CO2). In addition, a young still-expanding source leaf exposed to (14)CO2 exported (14)C-assimilates basipetally into a mature induced sink leaf (dark/-CO2). Microautoradiographs showed that imported (14)C-photosynthate was strongly accumulated in the sieve element/companion cell complexes of midveins, secondary veins, and minor veins of both the mature and the expanding sink leaf. Some label was also present in the vascular parenchyma and bundlesheath cells. In petioles, (14)C-label was concentrated in the sieve element/companion cell complexes of all bundles indicating that assimilates were imported and distributed via the phloem. Moreover, a considerable amount of radioactivity unloaded from the sieve element/companion cell complexes of petiolar bundles, was densely located at sites of secondary wall thickenings of differen-tiating metaxylem vessels, and at sites of chloroplasts of the vascular parenchyma and bundle-sheath cells. These observations were more striking in petioles of Gomphrena than Amaranthus.

Aspartate aminotransferases ofPanicum miliaceum L. andPanicum antidotale retz. : Inactivation and reconstitution

L-Aspartate: 2-oxoglutarate transaminase was isolated and partially purified from leaves ofPanicum miliaceum (C4, NAD-malic enzyme type) and ofPanicum antidotale (C4, NADP-malic enzyme type). In each preparation two isoenzymes with different kinetic properties could be characterized. The enzyme activity was irreversibly inhibited by 2-aminooxyacetic acid and by 2-amino-4-methoxy-3-butenoic acid. The first inhibitor reacted with pyridoxal 5-phosphate, and its inhibition could be reversed by the exchange of the modified coenzyme. The second inhibitor binds not only to the coenzyme pyridoxal 5-phosphate, but also to the apoprotein. The results of the dissociation and reconstitution experiments were in agreement with the kinetic data, showing that the mode of inactivation was different for 2-aminooxyacetic acid and 2-amino-4-methoxy-3-butenoic acid.

The occurrence of both C3 and C 4 photosynthetic characteristics in a single Zea mays plant

The activities of the carboxylating enzymes ribulose-1,5-biphosphate (RuBP) carboxylase and phosphoenolpyruvate (PEP) carboxylase in leaves of three-week old Zea mays plants grown under phytotron conditions were found to vary according to leaf position. In the lower leaves the activity of PEP carboxylase was lower than that of RuBP carboxylase, while the upper leaves exhibited high levels of PEP carboxylase. Carbon dioxide compensation points and net photosynthetic rates also differed in the lower and upper leaves. Differences in the fine structure of the lowermost and uppermost leaves are shown. The existence of both the C3 and C4 photosynthetic pathways in the same plant, in this and other species, is discussed.

C4 photosynthesis in Spartina townsendii at low and high temperatures

The metabolism of (14)CO2 in the cool temperate saltmarsh grass Spartina townsendii was investigated in plants grown in their natural habitats at two temperatures. Both in the spring at 10°C and in the late summer at 25°C radioactivity was initially incorporated into the organic acids malate and aspartate and then transferred to 3-phosphoglycerate in the manner characteristic of the C4 pathway of photosynthesis. Metabolism was not disrupted at the lower temperature as in some C4 plants. Radioactivity was transferred more slowly from malate into alanine, glycine and serine at 10°C, but sugars were labelled equally at both temperatures.

C4-Dicarboxylic acid metabolism in bundle-sheath chloroplasts, mitochondria and strands of Eriochloa borumensis Hack., a phosphoenolpyruvate-carboxykinase type C4 species

C4-acid metabolism by isolated bundlesheath chloroplasts, mitochondria and strands of Eriochloa borumensis Hack., a phosphoennolpyruvate-carboxykinase (PEP-CK) species, was investigated. Aspartate, oxaloacetate (OAA) and malate were decarboxylated by strands with several-fold stimulation upon illumination. There was strictly light-dependent decarboxylation of OAA and malate by the chloroplasts, but the chloroplasts did not decarboxylate aspartate in light or dark. PEP was a primary product of OAA or malate decarboxylation by the chloroplasts and its formation was inhibited by 3-(3,4-dichlorophenyl)-1, 1-dimethylurea or NH4Cl. There was very little conversion of PEP to pyruvate by bundle-sheath chloroplasts, mitochondria or strands. Decarboxylation of the three C4-acids by mitochondria was light-independent. Pyruvate was the only product of mitochondrial metabolism of C4-acids, and was apparently transaminated in the cytoplasm since PEP and alanine were primarily exported out of the bundle-sheath strands. Light-dependent C4-acid decarboxylation by the chloroplasts is suggested to be through the PEP-CK, while the mitochondrial C4-acid decarboxylation may proceed through the NAD-malic enzyme (NAD-ME) system. In vivo both aspartate and malate are considered as transport metobolites from mesophyll to bundle-sheath cells in PEP-CK species. Aspartate would be metabolized by the mitochondria to OAA. Part of the OAA may be converted to malate and decarboxylated through NAD-ME, and part may be transported to the chloroplasts for decarboxylation through PEP-CK localized in the chloroplasts. Malate transported from mesophyll cells may serve as carboxyl donor to chloroplasts through the chloroplastic NAD-malate dehydrogenase and PEP-CK. Bundle-sheath strands and chloroplasts fixed (14)CO2 at high rates and exhibited C4-acid-dependent O2 evolution in the light. Studies with 3-mercaptopicolinic acid, a specific inhibitor of PEP-CK, have indicated that most (about 70%) of the OAA formed from aspartate is decarboxylated through the chloroplastic PEP-CK and the remaining (about 30%) OAA through the mitochondrial NAD-ME. Pyruvate stimulation of aspartate decarboxylation is discussed; a pyruvate-alanine shuttle and an aspartate-alanine shuttle are proposed between the mesophyll and bundle-sheath cells during aspartate decarboxylation through the PEP-CK and NAD-ME system respectively.

Distribution and structure of the plasmodesmata in mesophyll and bundle-sheath cells of Zea mays L

In leaf blades of Zea mays L. plasmodesmata between mesophyll cells are aggregated in numerous thickened portions of the walls. The plasmodesmata are unbranched and all are characterized by the presence of electron-dense structures, called sphincters by us, near both ends of the plasmodesmatal canal. The sphincters surround the desmotubule and occlude the cytoplasmic annulus where they occur. Plasmodesmata between mesophyll and bundle-sheath cells are aggregated in primary pit-fields and are constricted by a wide suberin lamella on the sheath-cell side of the wall. Each plasmodesma contains a sphincter on the mesophyll-cell side of the wall. The outer tangential and radial walls of the sheath cells exhibit a continuous suberin lamella. However, on the inner tangential wall only the sites of plasmodesmatal aggregates are consistently suberized. Apparently the movement of photosynthetic intermediates between mesophyll and sheath cells is restricted largely or entirely to the plasmodesmata (symplastic pathway) and transpirational water movement to the cell walls (apoplastic pathway).

C4-Pathway photosynthesis in Portulaca oleracea and the significance of alanine labelling

Portulaca oleracea L. has some features common to C4 species but it has been suggested from other studies that a substantial proportion of assimilated carbon dioxide is incorporated into pyruvate and alanine by routes not involving C4 acids or 3-phosphoglycerate. The present paper reports that enzyme activities in P. oleracea leaves, and the labelling patterns observed during assimilation of (14)CO2, are entirely consistent with the operation of the C4 pathway, with most of the label appearing initially in the C-4 of C4 acids, followed by the prominent labelling of 3-phosphoglycerate, hexose phosphates and then sucrose and starch after longer periods. In contrast to other recent observations, neither alanine nor pyruvate was a prominent early-labelled product, and the observed labelling of alanine was consistent with it being derived from radioactive carbon initially incorporated into C4 acids.