Photoregulation of Phosphoenolpyruvate Carboxylase in Salsola soda L. and Other C 4 Plants

The rate of CO2 assimilation and levels of metabolites of the C4 cycle and reductive pentose phosphate pathway in attached leaves of maize (Zea mays L.) were (17,18,25). However, recent reports revealed that there were up to 30-fold differences in PEP carboxylase activity during dark to light transitions in maize when assayed in simulated conditions of dark or light or in the presence of effectors (5, 13). Studies of metabolite levels during induction in maize leaves (9, 20, 28) provided a basis for in vitro simulation ofconditions in vivo. The total level of the C4 cycle intermediates (malate, aspartate, pyruvate, PEP, and alanine) increased during induction (28) and also with increasing CO2 concentration (31,32) as the photosynthesis rate increased. These findings suggested that input of carbon into the C4 cycle and an increase in the total level of the C4 cycle intermediates is required to increase the photosynthetic rate in maize leaves (9,28,31,32).Although the influence of light intensity on metabolite levels in C4 plants has not been reported, this is an important environmental factor controlling C4 photosynthesis. Thus, the purpose of this study was to measure changes in photosynthetic metabolite levels in maize leaves under steady-state photosynthesis with varying light intensities.Recently, studies have been conducted on changes in photosynthetic metabolite levels during the induction period (9,20,28), and under steady-state photosynthesis with normal atmospheric conditions (19, 26) and with varying CO2 concentration (31) in maize leaves. These studies have increased our understanding of C4 photosynthesis and they provide a new approach to research in C4 plant. In part they have shown that there are sufficient concentration gradients between the mesophyll and bundle sheath to facilitate intercellular transport of certain C4 cycle intermediates by diffusion during C4 photosynthesis (19,

supporting

confidence: 57%

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mentioning

confidence: 98%

Changes in Levels of Intermediates of the C₄ Cycle and Reductive Pentose Phosphate Pathway under Various Light Intensities in Maize Leaves

Usuda¹

1987

“…This modification of PEPC includes changes in enzyme activity, affinity for substrate, as well as sensitivity to metabolite effectors and to small pH changes. During the light period of C4 species, PEPC enzyme activity is increased, malate inhibition reduced, and activation by G-6-P increased (4,8). Similar changes have been observed in PEPC from CAM plants, but during the dark period rather than the light (3,9,11,17,19,20).…”

supporting

confidence: 62%

Kinetic Properties of Phosphoenolpyruvate Carboxylase in Peperomia camptotricha

Sipes¹,

Ting²

1989

Kinetic characteristics of phosphoenolpyruvate carboxylase (PEPC) from the epiphytic C3 or C4: CAM intermediate plant, Peperomia camptotricha, were investigated. Few day versus night differences in V,X,K,(PEP), or malate inhibition were observed, even in extracts from water-stressed plants which characteristically perform CAM, regardless of efforts to stabilize day/night forms. The PEPC extracted from plants during the light period remained stable, without much of an increase or decrease in activity for at least 22 hours at 0 to 40C. Extracts from mature, fully developed leaves had slightly greater PEPC activity than from very young, developing leaves. Generally, however, the kinetic properties of'PEPC extracted from mature leaves of plants grown under short day (SD), long day (LD), or 1-week waterstress conditions, as well as from young, developing leaves, were similar. The PEPC inhibitor, L-malate, decreased the V.., and increased the K,,xpEp) for all treatments. Under specific conditions, malate did not inhibit PEPC rates in the dark extracts as much as the light. The PEPC activator, glucose-6-phosphate (G-6-P), lowered the K,,,p) for all treatments. At saturafing PEP concentrations, PEPC activity was independent of pH in the range of 7.5 to 9.0. At subsaturating PEP concentrations, the pH optimum was 7.8. The rates of PEPC activity were lower in the light period extracts than the dark, at pH 7.1, but day/night PEPC was equally active at pH 7.8. At pH 7.5 and a subsaturating PEP concentration, G-6-P significantly activated PEPC. At pH 8, however, only slight activation by G-6-P was observed. The lower pH of 7.5 combined with L-malate addition, greatly inhibited PEPC, particularly in extracts from young, developing leaves which were completely inhibited at an L-malate concentration of I millimolar. However, malate did not further inhibit PEPC activity in mature leaves when assayed at pH 7.1. The fairly constant day/night kinetic and regulatory properties of PEPC from P. camptotricha are unlike those of PEPC from CAM or C4 species studied, and are consistent with the photosynthetic metabolism of this plant.PEPC' is the initial C02-fixing enzyme in C4 plants (in the light) and CAM plants (in the dark). In both C4 and CAM plants, the amount of PEPC protein does not change between '

“…There have been reports of 2-fold changes in Vm> and a reduction in K,,,, but these changes vary between species (9,10). These reports can be criticized on the grounds that the extraction procedure lasted 15 min and that any light-induced changes might by then have been reversed.…”

mentioning

confidence: 89%

Regulation of Phosphoenolpyruvate Carboxylase Activity in Maize Leaves

Doncaster¹,

Leegood²

1987

106

The aim of this work was to investigate how light regulates the activity of phosphoenolpyruvate carboxylase in vivo in C4 plants. The properties of phosphoenolpyruvate carboxylase were investigated in extracts which were rapidly prepared (in less than 30 seconds) from darkened and illuminated leaves of Zea mays. Illumination resulted in a significant decrease in the So.5(phosphoenolpyruvate) but there was no change in V,,,. The form of the enzyme from illuminated leaves was less sensitive to malate inhibition than was the form from darkened leaves. At low concentrations of phosphoenolpyruvate, the activity of the enzyme was strongly stimulated by glucose-6-phosphate, fructose-6-phosphate, triose-phosphate, alanine, serine, and glycine and was inhibited by organic acids. The enzyme was assayed in mixtures of metabolites at concentrations believed to be present in the mesophyll cytosol in the light and in the dark. It displayed low activity in a simulated 'dark' cytosol and high activity in a simulated 'light' cytosol, but activities were different for the enzyme from darkened compared to illuminated leaves. PEP2 carboxylase occurs widely, perhaps universally, in cells of higher plants (11), but in plants with C4 and CAM modes of photosynthesis, specific isoenzymes are present in the cytosol of photosynthetic tissues at very much higher activities than in C3 photosynthetic or in nonphotosynthetic tissues (11,26). In C4 plants, PEP carboxylase is the initial step of the 'CO2 pump' which transfers CO2 from mesophyll cells and concentrates it in the bundle-sheath cells. Not only must the CO2 pump be coordinated with the rate of Calvin cycle turnover, but PEP consumption is also likely to be regulated so that photosynthesis can proceed efficiently at different irradiances, temperatures, and CO2 concentrations, and particularly to ensure that glycerate-3-P is not drained into the C4 cycle during photosynthesis (6). Since PEP also lies at an important branch-point in plant metabolism its utilization by PEP carboxylase must also be curtailed in darkness.There is good evidence that light modulation of PEP carboxylase occurs in vivo. For example, PEP has been observed to increase transiently upon illumination of maize leaves (6) (19,29,30).The evidence for regulation of the enzyme directly by light in C4 plants has, until recently, been weak. There have been reports of 2-fold changes in Vm> and a reduction in K,,,, but these changes vary between species (9, 10). These reports can be criticized on the grounds that the extraction procedure lasted 15 min and that any light-induced changes might by then have been reversed. However, Huber and Sugiyama (8) have recently reported that a change in the sensitivity of PEP carboxylase to effectors occurs during light-dark transitions. The evidence for metabolite modulation of PEP carboxylase is altogether stronger. Unlike PEP carboxylases from C3 and CAM plants, PEP carboxylase from C4 plants has a relatively low affinity for PEP (25,26), but in common with them, the enzyme f...