An investigation into the roles of photosynthesis and respiration in h efflux from aerated suspensions of asparagus mesophyll cells

Subcellular NH4 + flux analysis in leaf segments of wheat (Triticum aestivum)

•  We report the first use of tracer ¹³ NH₄ ⁺ (¹³ N-ammonium) efflux and retention data to analyse subcellular fluxes and compartmentation of NH₄ ⁺ in the leaves of a higher plant (wheat, Triticum aestivum). •  Leaf segments, 1-2 mm, were obtained from 8-d-old seedlings. The viability of the segments, and stability of NH ₄ ⁺ acquisition over time, were confirmed using oxygen-exchange and NH ₄ ⁺ -depletion measurements. Fluxes of NH ₄ ⁺ and compartment sizes were estimated using tracer efflux kinetics and retention data. •  Influx and efflux across the plasma membrane, half-lives of exchange and cytosolic pool sizes were broadly similar to those in root systems. As the external concentration of NH ₄ ⁺ ([NH ₄ ⁺ ] _o ) increased from 10 µ m to 10 m m , both influx and efflux greatly increased, with a sixfold increase in the ratio of efflux to influx. Half-lives were similar among treatments, except at [NH ₄ ⁺ ] _o  = 10 m m , where they declined. Concentrations of NH ₄ ⁺ in the cytosol ([NH ₄ ⁺ ] _c ) increased from 2.6 to 400 m m . •  Although [NH ₄ ⁺ ] _c became large as [NH ₄ ⁺ ] _o increased, the ratio of [NH ₄ ⁺ ] _c to [NH ₄ ⁺ ] _o decreased more than sixfold. The apparently futile cycling of NH ₄ ⁺ at high [NH ₄ ⁺ ] _o suggested by the large fluxes of NH ₄ ⁺ in both directions across the membrane indicate that leaf cells respond to potentially toxic NH ₄ ⁺ concentrations in a manner similar to root cells.

Studies of the Physiological and Genetic Basis of Acid Tolerance in Rhizobium leguminosarum biovar trifolii

Acid-tolerant Rhizobium leguminosarum biovar trifolii ANU1173 was able to grow on laboratory media at a pH as low as 4.5. Transposon Tn 5 mutagenesis was used to isolate mutants of strain ANU1173, which were unable to grow on media at a pH of less than 4.8. The acid-tolerant strain ANU1173 maintained a near-neutral intracellular pH when the external pH was as low as 4.5. In contrast, the acid-sensitive mutants AS25 and AS28 derived from ANU1173 had an acidic intracellular pH when the external pH was less than 5.5. The acid-sensitive R. leguminosarum biovar trifolii ANU794, which was comparatively more sensitive to low pH than mutants AS25 and AS28, showed a more acidic internal pH than the two mutants when the three strains were exposed to medium buffered at a pH of less than 5.5. The two acid-sensitive mutants had an increased membrane permeability to protons but did not change their proton extrusion activities. However, the acid-sensitive strain ANU794 exhibited both a higher membrane permeability to protons and a lower proton extrusion activity compared with the acid-tolerant strain ANU1173. DNA hybridization analysis showed that mutants AS25 and AS28 carried a single copy of Tn 5 located in 13.7-kb (AS25) and 10.0-kb (AS28) Eco RI DNA fragments. The wild-type DNA sequences spanning the mutation sites of mutants AS25 and AS28 were cloned from genomic DNA of strain ANU1173. Transfer of these wild-type DNA sequences into corresponding Tn 5 -induced acid-sensitive mutants, respectively, restored the mutants to their acid tolerance phenotypes. Mapping studies showed that the AS25 locus was mapped to a 5.6-kb Eco RI- Bam HI megaplasmid DNA fragment, whilst the AS28 locus was located in an 8.7-kb Bgl II chromosomal DNA fragment.

Involvement of Genes on a Megaplasmid in the Acid-Tolerant Phenotype of Rhizobium leguminosarum Biovar Trifolii

The acid-tolerant Rhizobium leguminosarum biovar trifolii strain ANU1173 exhibited several new phenotypes when cured of its symbiotic (Sym) plasmid and the second largest megaplasmid. Strain P22, which has lost these two plasmids, had reduced exopolysaccharide production and cell mobility on TY medium. The parent strain ANU1173 was able to grow easily in laboratory media at pH 4.5, whereas the derivative strain P22 was unable to grow in media at a pH of <4.7. The intracellular pH of strain ANU1173 was 6.8 when the external pH was 4.5. In contrast, strain P22 had an acidic intracellular pH of <6.4 when the external pH was <5.5. Strain P22 had a dramatically increased membrane permeability to protons and decreased proton extrusion activity. Analysis with sodium dodecyl sulfate-polyacrylamide gels showed that strain P22 lacked a slow-migrating lipopolysaccharide (LPS) banding group which was present in the parent strain. Mobilization of the second largest megaplasmid of strain ANU1173 back into strain P22 restored the altered LPS structure and physiological characteristics of strain P22. Mobilization of the Sym plasmid of strain ANU1173 into strain P22 showed that the second largest megaplasmid of strain ANU1173 was required for the establishment of nitrogen-fixing nodules on Trifolium repens and Trifolium subterraneum . Furthermore, an examination of a large number of specific exopolysaccharide- or LPS-deficient Rhizobium mutants did not show a positive correlation between exopolysaccharide or LPS synthesis and acid tolerance.

Relationship between Respiration and Photosynthesis in Guard Cell and Mesophyll Cell Protoplasts of Commelina communis L

A mass spectrometric method combining (16)O/(18)O and (12)C/(13)C isotopes was used to quantify the unidirectional fluxes of O(2) and CO(2) during a dark to light transition for guard cell protoplasts and mesophyll cell protoplasts of Commelina communis L. In darkness, O(2) uptake and CO(2) evolution were similar on a protein basis. Under light, guard cell protoplasts evolved O(2) (61 micromoles of O(2) per milligram of chlorophyll per hour) almost at the same rate as mesophyll cell protoplasts (73 micromoles of O(2) per milligram of chlorophyll per hour). However, carbon assimilation was totally different. In contrast with mesophyll cell protoplasts, guard cell protoplasts were able to fix CO(2) in darkness at a rate of 27 micromoles of CO(2) per milligram of chlorophyll per hour, which was increased by 50% in light. At the onset of light, a delay observed for guard cell protoplasts between O(2) evolution and CO(2) fixation and a time lag before the rate of saturation suggested a carbon metabolism based on phosphoenolpyruvate carboxylase activity. Under light, CO(2) evolution by guard cell protoplasts was sharply decreased (37%), while O(2) uptake was slowly inhibited (14%). A control of mitochondrial activity by guard cell chloroplasts under light via redox equivalents and ATP transfer in the cytosol is discussed. From this study on protoplasts, we conclude that the energy produced at the chloroplast level under light is not totally used for CO(2) assimilation and may be dissipated for other purposes such as ion uptake.

Effects of white, blue, red light and darkness on pH of the apoplast in the Samanea pulvinus

Leaflet movements in Samanea saman (Jacq.) Merrill are driven by fluxes of K(+), anions, and water through membranes of motor cells in the pulvinus (R.L. Satter et al., 1974, J. Gen. Physiol. 64, 413-430). Extensor cells take up K(+) and swell in white light (WL) while flexor cells take up K(+) and swell in darkness (D). Excised strips of extensor and flexor motor tissue acidify their bathing medium under conditions that normally promote increase in K(+) in the intact tissue, and alkalize the medium under conditions that normally induce decrease in K(+) (A. Iglesias and R.L. Satter, 1983, Plant Physiol. 72, 564). To obtain information on pH changes in the whole pulvinus, we measured effects of light on pH of the apoplast, using liquid membrane microelectrodes sensitive to H(+). We report the following: (1) The pH of the extensor apoplast was higher than that of the flexor apoplast in WL and in D (pH gradient of 1.0 units in WL and 2.0 units in D). Apoplastic pH might affect K(+) transport through the plasma membranes of Samanea motor cells, since the conductance, gating, and selectivity of ionic channels in other systems depend upon external pH. (2) Extensor cells acidified and flexor cells alkalized their environment in response to irradiation with WL, while the reverse changes occurred in response to D. These results are consistent with the results of Iglesias and Satter (1983), and support the physiological relevance of data obtained with excised tissue. (3) The pH changes in response to irradiation with red light were similar to those obtained with D; also, the pH changes in response to blue light were similar to those obtained with WL. The pulvinus closed in red light as in darkness and opened in WL, but failed to open in blue light. The advantages and limitations of apoplastic pH measurements for assaying H(+) transport are discussed.

The effect of red and far-red light on proton secretion from mesophyll-cell protoplasts of Vicia faba L

The influence of red and far-red irradiation on the transport of H(+) and (86)Rb(+) through the plasmalemma was studied using parenchymal protoplasts isolated from Vicia faba leaves. The results indicate that red light stimulates H(+) secretion and the uptake of (86)Rb(+). Moreover, it has been demonstrated that far-red irradiation acts antagonistically with respect to red light in both these processes.

A plasmamembrane redox system and proton transport in isolated mesophyll cells

Potassium ferricyanide (K(3)Fe[CN](6)) was added to aerated and stirred nonbuffered suspensions of mechanically isolated photosynthetically competent Asparagus sprengeri Regel mesophyll cells. Rates of Fe(CN)(6) (3-) reduction and H(+) efflux were measured with or without illumination. On the addition of 1 millimolar Fe(CN)(6) (3-) to nonilluminated cell suspensions acidification of the medium indicated an H(+) efflux of 1.54 nanomoles H(+)/10(6) cells per minute. Simultaneous Fe(CN)(6) (3-) reduction occurred at a rate of 1.55 nanomoles Fe(CN)(6) (3-)/10(6) cells per minute. Illumination stimulated these rates 14 to 17 times and corresponding values were 26.1 nanomoles H(+)/10(6) cells per minute and 22.9 nanomoles Fe(CN)(6) (3-)/10(6) cells per minute. These two processes appeared to be tightly coupled and were rapidly inhibited when illuminated suspensions were transferred to darkness or treated with 1 micromolar 3-(3,4-dichlorophenyl)-1,1 dimethylurea. Addition of 0.1 millimolar diethylstilbestrol eliminated ATP dependent H(+) efflux in illuminated or nonilluminated cells but had no influence on Fe(CN)(6) (3-) dependent H(+) efflux. Recent reports indicate that a transmembrane redox system spans the plasma membrane of root cells and is coupled to the efflux of H(+). The present report extends these observations to photosynthetically competent mesophyll cells. The results indicate a transport process independent of ATP driven H(+) efflux which operates with a H(+)/e(-) stoichiometry of one.

Evidence for a specific glutamate/h cotransport in isolated mesophyll cells

Mechanically isolated Asparagus sprengeri Regel mesophyll cells were suspended in 1 millimolar CaSO(4). Immediate alkalinization of the medium occured on the addition of 1 millimolar concentrations of l-glutamate (Glu) and its analog l-methionine-d,l-sulfoximine (l-MSO). d-Glu and the l isomers of the protein amino acids did not elicit alkalinization. l-Glu dependent alkalinization was transient and acidification resumed after approximately 30 to 45 minutes. At pH 6.0, 5 millimolar l-Glu stimulated initial rates of alkalinization that varied between 1.3 to 4.1 nmol H(+)/10(6) cells.minute. l-Glu dependent alkalinization was saturable, increased with decreasing pH, was inhibited by carbonyl cyanide-p-trichloromethoxyphenyl hydrazone (CCCP), and was not stimulated by light. Uptake of l-[U-(14)C]glutamate increased as the pH decreased from 6.5 to 5.5, and was inhibited by l-MSO. l-Glu had no influence on K(+) efflux. Although evidence for multiple amino acid/proton cotransport systems has been found in other tissues, the present report indicates that a highly specific l-Glu/proton uptake process is present in Asparagus mesophyll cells.

The Effect of pH, O(2), and Temperature on the CO(2) Compensation Point of Isolated Asparagus Mesophyll Cells

The effect of pH, O(2) concentration, and temperature on the CO(2) compensation point (capital GHE, Cyrillic[CO(2)]) of isolated Asparagus sprengeri Regel mesophyll cells has been determined in a closed, aqueous environment by a sensitive gas-chromatographic technique. Measured values range between 10 and 100 microliters per liter CO(2) depending upon experimental conditions. The capital GHE, Cyrillic(CO(2)) increases with increasing temperature. The rate of increase is dependent upon the O(2) concentration and is more rapid at high (250-300 micromolar), than at low (30-60 micromolar), O(2) concentrations. The differential effect of temperature on capital GHE, Cyrillic(CO(2)) is more pronounced at pH 6.2 than at pH 8.0, but this pH-dependence is not attributable to a direct, differential effect of pH on the relative rates of photosynthesis and photorespiration, as the O(2)-sensitive component of capital GHE, Cyrillic(CO(2)) remains constant over this range. The capital GHE, Cyrillic(CO(2)) of Asparagus cells at 25 degrees C decreases by 50 microliters per liter when the pH is raised from 6.2 to 8.0, regardless of the prevailing O(2) concentration. It is suggested that the pH-dependence of capital GHE, Cyrillic(CO(2)) is related to the ability of the cell to take up CO(2) from the aqueous environment. The correlation between high HCO(3) (-) concentrations and low capital GHE, Cyrillic(CO(2)) at alkaline pH indicates that extracellular HCO(3) (-) facilitates the uptake of CO(2), possibly by increasing the flux of inorganic carbon from the bulk medium to the cell surface. The strong O(2)- and temperature-dependence of capital GHE, Cyrillic(CO(2)) indicates that isolated Asparagus mesophyll cells lack an efficient means for concentrating intracellular CO(2) to a level sufficient to reduce or suppress photorespiration.

Photosynthetic characteristics of mesophyll cells isolated from cladophylls ofAsparagus officinalis L

Intact mesophyll cells can be rapidly isolated from the cladophylls ofAsparagus officinalis by gentle scraping with a plastic card, the yield being higher than 80% on a chlorophyll basis. The cells can be stored for at least 24h without loss of photosynthetic capacity and were found to be stable under a variety of conditions. In contrast to cell preparations from other plant species, photosynthetic activity was little affected by the presence of sorbitol as an osmoticum up to a concentration of 1.5 M. Similarly, the pH value of the medium influenced photosynthesis to only a small extent at a constant [CO2] of 200 μM. The response of the cells' photosynthetic capacity to light, temperature and CO2 concentration was similar to those reported for isolated cells from other plant species. Isolated cells ofA. officinalis can be used under a large range of conditions which gives them a measure of flexibility not possible with most plant cells which have sharply defined optimal conditions for photosynthesis. The isolated cells have a photosynthetic capacity of 40-60% of that of the intact cladophyll. The loss of photosynthetic activity observed upon isolation could not be accounted for by breakage of the cells. Virtually all of the cells were shown to be intact on the basis of Evans Blue exclusion and more than 80% of the cells contained intact chloroplasts and vacuoles. The entire loss of photosynthetic activity could be accounted for by a decrease in sucrose synthesis rather than by an equal decrease in the synthesis in all products. A six- to seven fold increase in the level of(14)C in hexose phosphates in the isolated cells supports the notion of inhibition of the sucrose-synthesis pathway.

An investigation into the role of photosynthesis in regulating ATP levels and rates of h efflux in isolated meosphyll cells

Aerated and stirred 10-ml suspensions of mechanically isolated Asparagus sprengeri Regel mesophyll cells were used for simultaneous measurements of net H(+) efflux and steady-state ATP levels.Initial rates of medium acidification indicated values for H(+) efflux in the light and dark of 0.66 and 0.77 nanomoles H(+)/10(6) cells per minute, respectively. When the medium pH was maintained at 6.5, with a pH-stat apparatus, rates of H(+) efflux remained constant. Darkness or DCMU, however, stimulated H(+) efflux by 100% or more. Darkness increased ATP levels by 33% and a switch from dark to light reduced ATP levels by 31%. In the absence of aeration, illumination prevented the accumulation of respiratory CO(2) and the buffering capacity of the medium was about 50% less than that found in the nonilluminated nonaerated medium. As a result, rates of pH decline were similar even though the dark rate of H(+) efflux was approximately 50% greater.Proposals that photosynthesis stimulates H(+) efflux are based on changes in the rate of pH decline. The present data indicate that photosynthesis inhibits H(+) efflux and that changes in rates of pH decline should not be equated with changes in the rate of H(+) efflux.

Active extrusion of protons into deionized water by roots of intact maize plants

The investigations were focussed on the question as to whether roots of intact maize plants (Zea mays L. cv Blizzard) release protons into deionized H(2)O. Plants in the six to seven leaf stage depressed the pH of deionized H(2)O from 6 to about 4.8 during an experimental period of 4 hours. Only one-third of the protons released could be ascribed to the solvation of CO(2) in H(2)O. The main counter anions released were Cl(-), NO(3) (-), and SO(4) (2-). At low temperature (2 degrees C), the H(+) release was virtually blocked while a relatively high amount of K(+) was released. The presence of K(+), Na(+), Ca(2+), and Mg(2+) in the external solution increased the H(+) secretion significantly. Addition of vanadate to the outer medium inhibited the H(+) release while fusicoccin had a stimulating effect. Substituting the nutrient solution of deionized H(2)O resulted in a substantial increase of the membrane potential difference from -120 to -190 millivolts. The experimental results support the conclusion that the H(+) release by roots of intact maize plants is an active process driven by a plasmalemmalocated ATPase. Since the net H(+) release was not associated with a net uptake of K(+), it is unlikely to originate from a K(+)/H(+) antiport.

Disulfiram metabolism in isolated mesophyll cells and inhibition of photosynthesis and cyanide-resistant respiration

Tetraethylthiuram disulfide (disulfiram) stimulated medium acidification when added at a concentration of 0.4 millimolar to illuminated or nonilluminated suspensions of Asparagus sprengeri Regel mesophyll cells. Similar concentrations inhibited photosynthesis and cyanide-resistant respiration. The reduction product of disulfiram, diethyldithiocarbamic acid, accumulated in concentrations sufficient to account for the observed acidification.

Stimulation of h efflux and inhibition of photosynthesis by esters of carboxylic acids

Suspensions of mechanically isolated Asparagus sprengeri Regel mesophyll cells were used to investigate the influence of various carboxyester compounds on rates of net H(+) efflux in the dark or light and photosynthetic O(2) production. Addition of 0.15 to 1.5 millimolar malathion, alpha-naphthyl acetate, phenyl acetate, or p-nitrophenyl acetate stimulated H(+) efflux and inhibited photosynthesis within 1 minute. In contrast, the more polar esters methyl acetoacetate or ethyl p-aminobenzoate had little or no effect on either of these two processes. A 0.15 millimolar concentration of alpha-naphthylacetate stimulated the normal rate of H(+) efflux, 0.77 nanomoles H(+) per 10(6) cells per minute by 750% and inhibited photosynthesis by 100%. The four active carboxyester compounds also stimulated H(+) efflux after the normal rate of H(+) efflux was eliminated with 0.01 milligrams per milliliter oligomycin or 100% N(2). Oligomycin reduced the ATP level by 70%. Incubation of cells with malathion, alpha-naphthyl acetate, or p-nitrophenyl acetate resulted in the generation of the respective hydrolysis products ethanol, alpha-naphthol, and p-nitrophenol. It is proposed that inhibition of photosynthesis and stimulation of H(+) efflux result when nonpolar carboxyester compounds enter the cell and generate acidic carboxyl groups when hydrolyzed by esterase enzymes.

Light-Stimulated Changes in the Acidity of Suspensions of Oat Protoplasts: Dependence upon Photosynthesis

Light induced an alkalinization and stimulated a subsequent acidification of the medium surrounding oat (Avena sativa L. cv Garry) leaf protoplasts. Blue light was less effective than would be predicted from photosynthetic action spectra. Nonetheless, 3-(3,4-dichlorophenyl)-1,1-dimethylurea prevented alkalinization and reduced acidification to the dark rate for protoplast suspensions exposed to all light regimes tested.Alkalinization increased in parallel with initial rates of O(2) evolution as the quantum flux density of white light was raised to 75 microeinsteins per square meter per second. Alkalinization was accompanied by a decrease in the CO(2) content of the medium; therefore, it was attributed to photosynthetically induced CO(2) uptake. The effect of CO(2) depletion on the acidity of the medium appeared to be mainly restricted to the first 15 minutes of exposure to light. Consequently, subsequent pH changes primarily reflected a constant net proton efflux. Acidification occurred in the dark, but rates of acidification increased in response to increased light approximately in parallel with changes in a concomitant net O(2) efflux. The results indicated that protoplasts could acidify the medium in response to nonphotosynthetic activity, but that photosynthesis mediated light stimulation of acidification.

H fluxes in excised samanea motor tissue : I. Promotion by light

Previous investigators revealed that white light-promoted leaflet opening in Samanea saman (Jacq) Merrill depends upon K(+) uptake by extensor cells and efflux from flexor cells of the pulvinus, while dark-promoted closure depends upon K(+) fluxes in the opposite directions. We now monitored H(+) fluxes during pulvinar movement to test a model proposing coupled H(+)/K(+) fluxes. H(+) fluxes were monitored by measuring changes in the pH of a weakly buffered solution (initial pH = 5.5) bathing excised strips of extensor or flexor tissue. White light at hour 3 of the usual dark period promoted pulvinar opening, H(+) efflux from extensor cells and uptake by flexor cells, while darkness at hours 2 to 4 of the usual light period promoted pulvinar closure, H(+) uptake by extensor cells and efflux from flexor cells. The following conditions altered H(+) fluxes during dark-promoted closure. (a) Light reversed the directions of the fluxes in both extensor and flexor cells. (b) Anoxia increased the rate of H(+) uptake by extensor cells and promoted H(+) uptake (rather than efflux) by flexor cells, consistent with an outwardly directed H(+) pump. KCN showed similar effects initially, but they were transient. (c) Increase in external pH from 5.5 to 6.7 promoted H(+) efflux (rather than uptake) by extensor cells and increased the rate of H(+) efflux from flexor cells, presumably by decreasing the rate of inward diffusion. (d) Change in external K(+) did not alter H(+) fluxes by extensor cells, but removal of external K(+) decreased the rate of H(+) efflux from flexor cells by 70%. These observations support a model for coupled H(+)/K(+) fluxes in pulvinar cells during light-and dark-promoted leaflet movements.

Role of o(2) and mitochondrial respiration in a photosynthetic stimulation of oat protoplast acidification of a surrounding medium

Some photosynthetically stimulated acidification of the medium by oat (Avena sativa L. cv Garry) leaf protoplasts required respiration. The requisite respiration (a) had a low apparent affinity for O(2), (b) was blocked by cyanide plus salicylhydroxamic acid, (c) characterized protoplasts and mitochondria isolated from protoplasts, (d) could be induced in leaf segments, and (e) appeared to result from an inhibition of mitochondrial respiration that included the cytochrome pathway.Carbon monoxide and cyanide prevented acidification of weakly photosynthesizing suspensions. Salicylhydroxamic acid had no effect on acidification, indicating a specific dependence upon cyanide-sensitive respiration. Photosynthesis stimulated acidification through stable products, and exogenously supplied O(2) stimulated acidification. The acidification response to O(2) was additive to the response to photosynthesis at subsaturating levels of light, indicating a common mode of action. Oligomycin prevented stimulation of acidification by low levels of photosynthetic activity; this stimulation appeared to be due to O(2)-induced increases in mitochondrial energy production. Oligomycin only partially inhibited stimulation of acidification by higher levels of light; this stimulation appeared to be partially dependent upon photophosphorylation. Therefore, oligomycin-sensitive acidification of the medium appeared to reflect changes in mitochondrial energy production in photosynthesizing protoplasts.