Turgor Regulation of Sucrose Transport in Sugar Beet Taproot Tissue

Self Cite

Sections of sugar beet (Beta vulgaris L.) taproot were incubated in various concentrations of mannitol. At 0.4, 0.6, and 0.8 molar, the membrane electrical potential difference (E.) averaged about -130 millivolts; at 0.2 molar, about -90 millivolts; and at 0 molar, between -60 and -80 millivolts. Additions of 10 millivolts acetate to the incubation solutions (all at pH 5) enhanced the membrane polarity to about -200 millivolts. We conclude from these and previous findings that high turgor inhibits proton extrusion in the sugar beet, but that proton extrusion can be activated in fully turgid tissue by acidification of the cytoplasm. A possible function of this turgor effect may be the control of turgor itself.sliced and pushed into Lucite tissue holders.Tissue-containing holders were incubated in a solution (referred to as IX here and in previous publications [3,7]) composed of 1 mM NaCl, 1 mm Ca(NO3)2, 1 mm KH2PO4, and 0.25 mM MgSO4. This solution was variously supplemented with mannitol, acetic acid, and NaCl as described in the individual experiments. The pH of I X was 5.0, and solutions supplemented with acetic acid were adjusted back to pH 5.0 with NaOH. After incubation times of one to several hours the tissue-containing holders were inserted into Lucite chambers fitted with inlet and outlet ports for the perfusion solutions. These chambers were used in an assembly for implanting glass microelectrodes used for the measurement of Em (3, 7).Incubation of plant tissues in solutions of mannitol, or other osmotica, sometimes stimulates acidification of the bathing media (4,5,12,17) and enhances the rate of solute uptake (2,5,12,14,17). Because many solutes are cotransported with protons across plant cell membranes (3, 7, 1 1), the observed effects of the osmotica upon plant tissues can be explained by the hypothesis that high turgor inhibits the proton extrusion pump located in the plasmalemma. The action of this pump contributes to the membrane Em' and the A[H+] across the membrane, and thereby develops the proton motive force (= Em-59ApH, units in mV, T = 250C) that drives cotransport (11,15 Figure 1. Contrary to our experience with oat coleoptiles, stable, longlasting measurements in I X with 0 M mannitol were difficult to obtain. These measurements, whether stable, unstable, long lasting, or brief were typically of a low negative value. We incorporated into Figure 1 all measurements more negative than -60 mV of at least 15-s duration. The -60 mV cutoff was chosen because negative readings up to -40 mV could be obtained by pushing the electrode into broken cell fragments. At 0.2 and 0.4 M mannitol stable measurements were easier to obtain, but at 0.6 and 0.8 M mannitol, implantations became difficult again as the tissue became flaccid. (Osmotic concentration of the cell solution ranged from 0.6 to 0.75 M [17]). Attempts to measure Em during transitions from one mannitol concentration to another failed, presumably because of the movement of the tissue during changes in turgor.Taking the measurement...

Section: Introductionmentioning

confidence: 99%

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confidence: 99%

Electrical Evidence for Turgor Inhibition of Proton Extrusion in Sugar Beet Taproot

Kinraide

Wyse²

1986

Self Cite

“…Proton extrusion across the plasma membrane is stimulated by a reduction in the external water potential mediated by any number of osmotic solutes (17,18,26,29). A substantial component of increased H+ pumping was vanadate-sensitive and it has been inferred that the plasma membrane H+-ATPase has a central function in the response to alterations in the osmotic or ionic environments (2,17,18,26).…”

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confidence: 99%

Enhanced Net K⁺ Uptake Capacity of NaCl-Adapted Cells

Watad

Reuveni²,

Bressan³

et al. 1991

Maintenance of intracellular K+ concentrations that are not growth-limiting, in an environment of high Na+, is characteristic of NaCI-adapted cells of the glycophyte, tobacco (Nicotiana tabacum/gossii). These cells exhibited a substantially greater uptake of "Rb+ (i.e. an indicator of K+) relative to unadapted cells.Potassium uptake into NaCI-adapted cells was 1.5-fold greater than unadapted cells at 0 NaCI and 3.5-fold greater when cells were exposed to 160 millimolar NaCI. The difference in net K+ uptake between unadapted and NaCI-adapted cells was due primarily to higher rates of entry rather than to reduced K+ leakage. Presumably, enhanced K+ uptake into adapted cells is a result of electrophoretic flux, and a component of uptake may be linked to vanadate-sensitive H+ extrusion.

“…Finally, although Komor (10) did not pretreat his Ricinus cotyledons with high sugar levels, he did not report using an osmoticum to standardize the osmolality of each treatment. It has recently been shown that the osmolality of the uptake solution can have a pronounced effect on the uptake kinetics of some tissues ( 19).If, during the photosynthetic period, sucrose is transported across the tonoplast of mesophyll cells by a permease, then the vacuolar sucrose levels should be in near chemical potential equilibrium with the cytosol. Thus, by monitoring changes in the gross sucrose levels of the leaf, one should be able to obtain an approximate measure of the cytosolic sucrose concentration.…”

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confidence: 99%

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confidence: 99%

Influence of Internal Sugar Levels on Apoplasmic Retrieval of Exogenous Sucrose in Source Leaf Tissue

Wilson

Lucas²

1987

Sugar levels in Beta vulgaris leaves were increased by heat-girdling the petiole and returning the plant to the controlled-environment chamber for 10 and 34 hours. After 10 hours, sucrose influx into the treated leaves was similar to the controls, although sucrose levels increased from 2.1 to 5.3 micromoles per milligram chlorophyll. However, after a 34-hour treatment, sucrose levels increased from 2.1 to 11.5 micromoles per milligram chlorophyll. In this instance, sucrose influx decreased relative to the untreated controls. Decreasing sugar levels by DCMU treatment resulted in a small stimulation of sucrose influx. A similar DCMU treatment applied to leaves of Allium cepa also resulted in an increase in sucrose influx. However, in A. cepa we could not attribute this increase to a lowering of sugar levels, as the kinetic profiles obtained from control leaves did not vary from each other throughout the day, despite considerable changes in sugar levels. Additionally, it appeared that sucrose uptake in onion may be set at some point and remains invariant throughout the day. Similar studies were also conducted on discs cut from mature leaves of Spinacia oleracea var America. Between I and 8 hours after the onset of the photoperiod, the sucrose content of the spinach leaves increased from 2.6 to 93 micromoles per milligram chlorophyll. A comparison of the kinetic profiles obtained from leaf discs, taken at these times, indicated that sucrose uptake was not influenced by these changes in internal sugar levels. The relationship between the above findings and 'trans' inhibition of exogenous sucrose uptake is discussed. Although intermediate changes in sugar levels in sugar beet leaves did not appear to affect sucrose influx, autoradiographic studies revealed that these changes dramatically affected the partitioning of exogenously supplied I'4Clsucrose. Our results indicate that while intermediate changes in internal sugar levels have little effect on sucrose influx across the plasmalemma, they may dramatically affect partitioning between the phloem and the mesophyll vacuole.Phloem transport is a complex and highly regulated phenomenon which involves a variety oforganelles, cell types, specialized anatomical structures, and cellular processes. An important facet of phloem transport which is in need of further study is the elucidation of the process(es) involved in the movement ('loading') of sucrose from the mesophyll into the sieve tubes. In a recent study, we proposed that this process of loading should be considered in a broader perspective than just the movement of sucrose into the sieve-element companion-cell complex (18). A working model of phloem loading must integrate not only the transport processes across the plasma membrane and tonoplast of a number of cell types, but also the biochemical and biophysical processes involved in regulating these fluxes (18).'Supported by National Science Foundation Grant PCM-83-15408.At the biochemical level, the regulatory mechanisms involved in both the partitioning of ...