1988. A reassessment of glycolysis and gluconeogenesis in higher plants. -Physiol. Plant. 72: 650-654.Sucrose is the starting point of glycolysis and end point of gluconeogenesis in higher plants. During both glycolysis and gluconeogenesis alternative enzymes are present at various steps to carry out parallel pathways; alternatives are available for utilizing nucleotide triphosphates and pyrophosphate; fructose 2.6-bisphosphate serves as a strong internal regulator; and plants use these cytoplasmic alternatives as they develop and as their environments change.
The hexose bisphosphate activation of phosphoglucomutase was investipted with both plant (pea and mung bean) and animal (rabbit muscle) sources of the enzyme. Plant phosphoglucomutase was purified about 50-fold from seeds, and to a lesser extent, from seedlings of Pisum sativum L. cv Grenadier and seedlings of Phaseolus aureus. It was found that the plant enzyme was isolated in a mostly dephosphorylated form while commercial rabbit muscle phosphoglucomutase was predominantly in the phosphorylated form. Activation studies were done using the dephosphorylated enzymes. The range of activation constant (K.) values were obtained for each bisphosphate were: for glucose 1-6-P2, 0.5 to 1.8; fructose 2,6-P2, 6 to 11.7; and fructose 1,6-P2, 7 micromolar, respectively.Fructose 2,6-P2 is known to occur in both plant and animal tissues at changing levels encompassing the K. values found in this studr, hence, these results implicate fructose 2,6-P2 as a natural activator of phosphoglucomutase, particularly in plants. Also, glucose 1,6-P2 has not been found in plants, and the method for measuring glucose 1,6-P2 by monitoring the activation of phosphoglucomutase is not specific.
Sucrose was the predominant sugar in peel samples and present in lesser amounts in pulp and juice samples of prickly pear fruit. The relative total content of glucose and fructose differed among species and within fruit tissues. Acid invertase activity was found in all fruits examined while neutral invertase activity was found in two of the three species. Red-skinned fruits had no neutral invertase activity and yellow-skinned fruits had relatively high neutral invertase activity. When both invertase activities were added, a pattern of distribution of invertase activity that corresponded with sucrose levels within fruits was observed. The overall total invertase activity levels in red-skinned fruits were about twofold that of the yellow-skinned fruits and fourfold that of purple-skinned fruits.
ABSTRACI Segments (2.5 cm) cut from 3day-old seedling roots of radish (Raphanus sativus L. 'Scarlet Globe') were cultured in medium with or without indoleacetic acid (IAA). Lateral root primordia frequency, determined for the central centimeter of segments, was dependent on IAA concentration and on conditions affecting IAA uptake. Dimethyl sulfoxide treatment, or a relatively low medium pH, greatly enhanced the response to exogenous IAA. It was concluded that a permeation barrier exists between the external medium and the hormone responsive sites within the radish seedling root.A characteristic action of IAA is to stimulate the formation of LR.3 This IAA effect has been studied using simple experimental systems employing excised or intact seedling roots (1, 10-1 3, and references cited therein). Using our system (1) based on the radish (Raphanus sativus L. 'Scarlet Globe') seedling root, we find that the response to exogenous auxin can be substantially modulated by chemical or physical factors known or presumed to influence diffusional IAA uptake by plant cells. The observations reported here indicate that there is an IAA permeation barrier between the external medium and the auxin-responsive sites involved with LR formation within the radish seedling root. MATERIALS AND METHODSThese studies were performed with roots of seedlings germinated and grown in the dark at 24C for 3 d, during which time the roots grew to an average length of 6 cm. Excised 2.5 cm segments, cut starting 0.5 to 1.5 cm behind the seedling root tip, were cultured 4 d in medium containing Murashige-Skoog salts (7), 88 mol m-3 sucrose, and 10 mol m-3 each of the buffers Mes and succinic acid. IAA RESULTS AND DISCUSSION Exogenous IAA greatly increased the frequency (i.e. the number/cm) of LRP formation, which varied from 5 cm-' in the absence of IAA to about 60 cm-' under optimal conditions. Excess IAA inhibited LRP formation. Figure 1 illustrates the effect of 30 mmol m-3 IAA, and the modulation ofthe response by pH and DMSO. Typical treatment mean frequencies of LRP formation in medium containing IAA were: 49.7 ± 3.6 SE LRP cmn' at pH 4.5 and 19.0 ± 1.9 SE LRP cm ' at pH 6. Treatment with DMSO increased the response to IAA at pH 6; a typical treatment mean frequency was 48.1 ± 3.8 SE LRP cm-'.DMSO is known to increase plasmalemma permeability to small molecules (3). In our experience, it was found most effective when given as a pulse treatment at a medium concentration of 5% (v/v) during the first 1 to 4 h of culture. Longer exposure to DMSO, or higher concentrations of DMSO, were inhibitory. DMSO by itself (i.e. with no exogenous auxin) had no promotive effects on the frequency of LRP formation. Administration of a 5% DMSO 4 h pulse treatment when medium containing 30 mmol m-3 IAA was buffered at pH 4.5, rather than 6, inhibited LRP formation.The effect of pH on IAA uptake by plant cells has been well characterized (4, 9). Since the membrane permeability coefficient to the lipophilic IAAH is on the order of 103 times that of IAA-(9), lowe...
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