Abstract
The uptake of sucrose, 3-O-methylglucose (3-O-MeG), and valine were studied in discs and in purified plasma membrane vesicles (PMV) prepared from sugar beet (Beta vulgaris L.) exporting leaves. The uptake capacities of freshly excised leaf discs were compared with the uptake in discs that had been floated for 12 h on a simple medium (aging) and with discs excised from leaves that had been cut from the plant 12 h before the experiments (cutting). After cutting, sucrose uptake amounted to twice the uptake measured in fresh discs, whereas the uptake of 3-O-MeG and valine remained unaffected. In aged leaf discs, there was a general stimulation of uptake, which represented 400, 300, and 400% of the uptake measured in fresh discs for sucrose, 3-O-MeG, and valine, respectively. Sucrose uptake in fresh discs was sensitive to N-ethylmaleimide (NEM), to p-chloromercuribenzenesulfonic acid (PCMBS), and to mersalyl acid (MA). Although cutting induced the appearance of a sucrose uptake system that is poorly sensitive to NEM but sensitive to PCMBS and MA, aging induced the development of an uptake system that is sensitive to NEM but poorly sensitive to PCMBS and MA. Autoradiographs of discs fed with [14C]sucrose show that cutting resulted in an increase of vein labeling with little effect in the mesophyll, whereas aging induced an increase of labeling located mainly in the mesophyll. The data show that cutting is sufficient to induce dramatic and selective changes in the uptake properties of leaf tissues and that the effects of cutting and aging on the uptake of organic solutes are clearly different. Parallel experiments were run with purified PMV prepared from fresh and cut leaves. The uptake of sugars and amino acids was studied after imposition of an artificial proton motive force (pmf). Comparison of the uptake properties of PMV and of leaf tissues indicate that the recovery of the sucrose uptake system in PMV is better than the recovery of the hexose and of the valine uptake systems. As observed with the leaf discs, cutting induced a 2-fold increase of the initial rate of sucrose uptake in PMV but did not affect the uptake of valine and 3-O-MeG. Cutting induced an increase of both Vmax and Km of the sucrose transport system in PMV. Measurements of the pmf imposed on the vesicles indicated that the increase of sucrose uptake induced by cutting was not due to a better integrity of the vesicles. Hexoses did not compete with sucrose for uptake in PMV from fresh and cut leaves, and maltose was a stronger inhibitor of sucrose uptake in PMV from cut leaves than in PMV from fresh leaves. The sensitivity of sucrose uptake to NEM, PCMBS, and MA in PMV from fresh and cut leaves paralleled that described above for the corresponding leaf discs. These data show that (a) the changes induced by cutting on sucrose uptake by leaf discs are due to membrane phenomena and not to the metabolism of sucrose; (b) the study of sucrose uptake with PMB gives a good account of the physiological situation; and (c) the specific effects induced by cutting on the sucrose uptake system are not lost during the preparation of the PMV.
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Selected References
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