The plastidic pentose phosphate translocator represents a link between the cytosolic and the plastidic pentose phosphate pathways in plants

Plastids are the site of the reductive and the oxidative pentose phosphate pathways, which both generate pentose phosphates as intermediates. A plastidic transporter from Arabidopsis has been identified that is able to transport, in exchange with inorganic phosphate or triose phosphates, xylulose 5-phosphate (Xul-5-P) and, to a lesser extent, also ribulose 5-phosphate, but does not accept ribose 5-phosphate or hexose phosphates as substrates. Under physiological conditions, Xul-5-P would be the preferred substrate. Therefore, the translocator was named Xul-5-P/phosphate translocator (XPT). The XPT shares only approximately 35% to 40% sequence identity with members of both the triose phosphate translocator and the phosphoenolpyruvate/phosphate translocator classes, but a higher identity of approximately 50% to glucose 6-phosphate/phosphate translocators. Therefore, it represents a fourth group of plastidic phosphate translocators. Database analysis revealed that plant cells contain, in addition to enzymes of the oxidative branch of the oxidative pentose phosphate pathway, ribose 5-phosphate isomerase and ribulose 5-phosphate epimerase in both the cytosol and the plastids, whereas the transketolase and transaldolase converting the produced pentose phosphates to triose phosphates and hexose phosphates are probably solely confined to plastids. It is assumed that the XPT function is to provide the plastidic pentose phosphate pathways with cytosolic carbon skeletons in the form of Xul-5-P, especially under conditions of a high demand for intermediates of the cycles.     

Measurement of Metabolites Associated with Nonaqueously Isolated Starch Granules from Immature Zea mays L. Endosperm

Starch granules with associated metabolites were isolated from immature Zea mays L. endosperm by a nonaqueous procedure using glycerol and 3-chloro-1,2-propanediol. The soluble extract of the granule preparation contained varying amounts of neutral sugars, inorganic phosphate, hexose and triose phosphates, organic acids, adenosine and uridine nucleotides, sugar nucleotides, and amino acids. Based on the metabolites present and on information about translocators in chloroplast membranes, which function in transferring metabolites from the chloroplast stroma into the cytoplasm, it is suggested that sucrose is degraded in the cytoplasm, via glycolysis, to triose phosphates which cross the amyloplast membrane by means of a phosphate translocator. It is further postulated that hexose phosphates and sugars are produced from the triose phosphates in the amyloplast stroma by gluconeogenesis with starch being formed from glucose 1-phosphate via pyrophosphorylase and starch synthase enzymes. The glucose 1-phosphate to inorganic phosphate ratio in the granule preparation was such that starch synthesis by phosphorylase is highly unlikely in maize endosperm.     

A comparative study of metabolite levels in plant leaf material in the dark

Metabolite levels have been compared in the dark and during photosynthesis in leaves and protoplasts from spinach, pea, wheat and barley. In protoplasts the subcellular distribution was also studied. The levels of triose phosphates and sugar bisphosphates were high in the light and low in the dark. The hexose phosphates and 3-phosphoglycerate levels in the dark were very variable depending on the plant material. In most conditions, hexose phosphates and triose phosphates were mainly in the extrachloroplast compartment, while 3-phosphoglycerate and the sugar bisphosphates were mainly in the chloroplast compartment. Leaves always had a very low triose phosphate: 3-phosphoglycerate ratio in the dark, but in protoplasts this ratio was higher. Detailed studies with spinach showed that metabolite levels were very dependent on the availability of carbohydrate in the leaf, particularly starch. Starch mobilisation is not controlled just by the availability of inorganic phosphate and accumulation of phosphorylated intermediates. Hydrolysis of starch may provide precursors for sucrose synthesis while phosphorolysis leads to provision of substrates for respiration. Starch breakdown generates high enough levels of hexose phosphate to support substantial rates of sucrose synthesis in the dark. Respiration is not greatly increased when metabolite levels are high during starch mobilisation. Higher levels of metabolites shorten the length of the induction phase of photosynthesis.Abbreviations Chl chlorophyll - DHAP dihydroxyacetone phosphate - Fru2,6bisP fructose-2,6-bisphosphate - NMR nuclear magnetic resonance - PGA 3-phosphoglyceric acid - Pi inorganic phosphate - RuBP ribulose-1,5-bisphosphate - UDPGlc uridine-5-diphosphate glucose     

Phosphate Translocator of Isolated Guard-Cell Chloroplasts from Pisum sativum L. Transports Glucose-6-Phosphate

Chloroplasts were isolated from ruptured guard-cell protoplasts of the Argenteum mutant of Pisum sativum L. and purified by centrifugation through a Percoll layer. The combined volume of the intact plastids and the uptake of phosphate were determined by silicone oil-filtering centrifugation, using tritiated water and [14C]sorbitol as membrane-permeating and nonpermeating markers and [32P]phosphate as tracer for phosphate. The affinities of the phosphate translocator for organic phosphates were assessed by competition with inorganic phosphate. The affinities for dihydroxyacetone phosphate, 3-phosphoglycerate (PGA), and phosphoenolpyruvate were in the same order as those reported for mesophyll chloroplasts of several species. However, the guard-cell phosphate translocator had an affinity for glucose-6-phosphate that was as high as that for PGA. Guard-cell chloroplasts share this property with amyloplasts from the root of pea (H.W. Heldt, U.I. Flugge, S. Borchert [1991] Plant Physiol 95: 341-343). An ability to import glucose-6-phosphate enables guard-cell chloroplasts to synthesize starch despite the reported absence of a fructose-1,6-bisphosphatase activity in the plastids, which would be required if only C3 phosphates could enter through the translocator.     

Pyridine and Adenine Nucleotide Status, and Pool Sizes of a Range of Metabolites in Chloroplasts, Mitochondria and the Cytosol/Vacuole of Avena Mesophyll Protoplasts during Dark/Light Transition: Effect of Pyridoxal Phosphate

Leaf mesophyll protoplasts of Avena sativa L. underwent dark/lighttransition in the absence or presence of pyridoxal phosphate(PLP), a specific inhibitor of the phosphate translocator ofchloroplasts. By combining rapid fractionation of protoplastswith enzymatic cycling, the contents of metabolites (adeninenucleotides, pyridine nucleotides, triose phosphates, 3-phosphoglycerate,inorganic phosphate, aspartate, malate, oxaloacetate, glutamate,2-oxoglutarate and citrate) associated with the chloroplasts,mitochondria and cytosol/vacuole were determined. Fluctuationsof metabolite pools occurred in all compartments on illumination.Mitochondria showed immediate inhibition of their tricarboxylicacid cycle activities, as indicated by a transiently increasedNADH/NAD⁺ ratio and elevated malate contents within 60 s ofillumination. During this period large transient increases intriose phosphates took place in all fractions. Incubation of intact protoplasts with PLP severely affectedthe compartmented pool sizes, producing the typical patternof inhibition of the chloroplast phosphate translocator. Themitochondrial pool sizes of the metabolites responded to light,if at all, differently than did the controls. ¹ Dedicated to Prof. Hubert Ziegler on the occasion of his 60thbirthday. (Received July 23, 1984; Accepted October 19, 1984)     

Ability of Phaeocystis sp. to grow on organic phosphates: direct measurement and prediction with the use of an inhibition constant

Batch culture experiments were performed to test the abilityof Phaeocystis sp. to grow on organic phosphates as the onlysource of phosphorus.Of nine organic phosphates tested sevensupported growth equally well as did inorganic phosphate Growthon cyclic-adenosine monophos-phate (c-AMP) was slow comparedwith inorganic phosphate, while myo-inositol hexaphosphate (phytin)was not used for growth by Phaeocystis. The relative affinityof Phaeocystis alkaline phosphatase for a number of organicphosphates, including those used in the growth tests, was determinedby measuring the ability of the organic phosphates to inhibitthe hydrolysis of a test substrate by the enzyme. The relativeaffinity was expressed by means of an inhibition constant (K₁)values for c-AMP and phytin were relatively high, indicatinglow affinity of the enzyme for these substrates K₁, values forthe other phosphates tested were much lower indicating highaffinity of the enzyme. Comparison of the results of the growthexperiments with the Kt values indicated that the latter weresuitable instruments for predicting the outcome of the growthexperiments The results imply that in natural environments Phaeocystiscan use enzyme-hydrolysable organic phosphates efficiently forgrowth.     

Evidence for a glucose 1-phosphate translocator in storage tissue amyloplasts of potato (Solanum tuberosum) suspension-cultured cells

Transport of glucose 1-phosphate (G1P) and highly purified triose phosphate into storage tissue amyloplasts was studied. Isolated amyloplasts from potato ( Solanum tuberosum L., dihaploid stock, HH 258) were transport-functional and metabolically active in starch synthesis. Fourty percent of the amyloplasts were intact and there was only a small degree (0–1.6%) of contamination by other cellular compartments. G1P showed a clear uptake pattern paralleled by starch synthesis. Uptake of triose phosphates was virtually nil. Uptake of GIP was pH dependent with a sharp maximum at pH 5.7 and showed Michaelis-Menten kinetics with an apparent K_m of 0.5 m M . Temperature influenced the rate of uptake, the highest rate being at 25°C. Fructose l-phosphate, ADP-glucose, glucose, and inorganic phosphate inhibited the uptake of G1P. Uptake was also inhibited by DIDS (1–25 μ M ) and by Phloretin (45–750 μW). It is therefore concluded that the transport of GIP across the inner amyloplast membrane is mediated by a hexose phosphate translocator selective for phosphate and glucose moieties of the molecule. Considering the low pH maximum for G1P uptake it is possible that the uptake of G1P, and eventually starch synthesis, is regulated by an acidification of the intermembrane space by proton pumps of the inner amyloplast membrane.     

Evidence for extracellular enzymic activity of the isolated perfused rat heart

1. The dissimilation of a number of externally added hexose phosphates and 5′-nucleotides by the perfused rat heart is described, and non-specific esterase and 5′-nucleotidase activity associated with the superficial cell membrane or vascular system has been demonstrated. 2. The rate of production of ¹⁴CO₂ from [U-¹⁴C]glucose 6-phosphate suggests that oxidation occurred after hydrolysis to glucose. The incorporation of isotope from [U-¹⁴C]glucose 6-phosphate into glycogen was small, and similar to that obtained with [U-¹⁴C]glucose as substrate. 3. Glucose 6-phosphate was also partially isomerized to fructose 6-phosphate. Similarly, fructose 6-phosphate was converted mainly into glucose 6-phosphate, but also into glucose and inorganic phosphate. When fructose 1,6-diphosphate was added to the perfusate, a mixture of glucose 6-phosphate, fructose 6-phosphate and triose phosphates accumulated in the medium approximately in the equilibrium proportions of the phosphohexose-isomerase and triose phosphate-isomerase reactions, together with inorganic phosphate and some glucose. Glucose 1-phosphate was hydrolysed to glucose, but was not converted into glucose 6-phosphate. Leakage of enzymes out into the perfusion fluid did not occur. 4. This demonstration that phosphohexose isomerase, triose phosphate isomerase and aldolase may react with extracellular substrates at an appreciable rate suggests that these enzymes are attached to the cell membrane.     

Dual Mechanisms of Metabolite Acquisition by the Obligate Intracytosolic Pathogen Rickettsia prowazekii Reveal Novel Aspects of Triose Phosphate Transport

Rickettsia prowazekii is an obligate intracytosolic pathogen and the causative agent of epidemic typhus fever in humans. As an evolutionary model of intracellular pathogenesis, rickettsiae are notorious for their use of transport systems that parasitize eukaryotic host cell biochemical pathways. Rickettsial transport systems for substrates found only in eukaryotic cell cytoplasm are uncommon among free-living microorganisms and often possess distinctive mechanisms. We previously reported that R. prowazekii acquires triose phosphates for phospholipid biosynthesis via the coordinated activities of a novel dihydroxyacetone phosphate transport system and an sn-glycerol-3-phosphate dehydrogenase (K. M. Frohlich et al., J. Bacteriol. 192:4281–4288, 2010). In the present study, we have determined that R. prowazekii utilizes a second, independent triose phosphate acquisition pathway whereby sn-glycerol-3-phosphate is directly transported and incorporated into phospholipids. Herein we describe the sn-glycerol-3-phosphate and dihydroxyacetone phosphate transport systems in isolated R. prowazekii with respect to kinetics, energy coupling, transport mechanisms, and substrate specificity. These data suggest the existence of multiple rickettsial triose phosphate transport systems. Furthermore, the R. prowazekii dihydroxyacetone phosphate transport systems displayed unexpected mechanistic properties compared to well-characterized triose phosphate transport systems from plant plastids. Questions regarding possible roles for dual-substrate acquisition pathways as metabolic virulence factors in the context of a pathogen undergoing reductive evolution are discussed.     

Root Gravitropism in a Cultivar of Zea mays Whose Columella Cells Contain Starch-deficient Amyloplasts

Starch occupies 4.2 per cent of the volume of plastids in calyptrogencells in primary roots of Zea mays L. cv. vp-7 wild type. Plastidsin calyptrogen cells are distributed randomly around large,centrally located nuclei. The differentiation of calyptrogencells into columella cells is characterized by cellular enlargementand the sedimentation of plastids to the bottom of the cells.Although sedimented plastids in columella cells do not containsignificantly more starch than those in calyptrogen cells, primaryroots are graviresponsive. The onset of root gravicurvatureis not associated with a significant change in the distributionof plastids in columella cells. These results indicate thatin this cultivar of Z. mays (1) the sedimentation of plastidsin columella cells is not based upon their increased densityresulting from increased starch content alone, (2) starch-ladenamyloplasts need not be present in columella cells for rootsto be graviresponsive, and (3) the onset of root gravicurvaturedoes not require a major redistribution of plastids in columellacells. Columella cell, gravitropism (root), plastids, root cap, Zea mays     

Anaerobic Phosphate Uptake by Barley Plants

Considerable uptake of phosphate by both the shoot and roothas been demonstrated for young barley plants with their rootsin anoxic culture solution at concentrations of 1 to 10 µMorthophosphate. Consideration of the free space and passivetranspirational uptake indicates an accumulatory process, andthe immediate efflux caused by respiratory inhibitors supportsthis. Shoot uptake is much less at higher external concentrationsof phosphate and at o.I mM was only 14 per cent of the control.The root accumulation process was unimpaired at an externalconcentration of 1 µM phosphate when the whole plant wassubjected to anaerobic conditions (shoot illuminated) but undersimilar conditions at a concentration of 100 µM a considerableefflux of phosphate occurred. Analysis of the fate of phosphatetaken up from anoxic solution of phosphate (10 µM) indicatedthat there was a reduction in the level of inorganic phosphateafter 4.5 h and steady rise in sugar phosphates up to 6 h witha marked increase in the levels of glucose-6-phosphate, fructose-6-phosphate,and the phosphoglycerate fraction.     

Enzymic capacities of purified cauliflower bud plastids for lipid synthesis and carbohydrate metabolism

Isolated cauliflower (Brassica oleracea) bud plastids, purified by isopycnic centrifugation in density gradients of Percoll, were found to be highly intact, to be practically devoid of extraplastidial contaminations, and to retain all the enzymes involved in fatty acid, phosphatidic acid, and monogalactosyldiacylglycerol synthesis. Purified plastids possess all the enzymes needed to convert triose phosphate to starch and vice versa, and are capable of conversion of glycerate 3-phosphate to pyruvate for fatty acid synthesis. They are also capable of oxidation of hexose phosphate and conversion to triose phosphate via the oxidative pentosephosphate pathway. Cauliflower bud plastids prove to be, therefore, biochemically very flexible organelles.     

Nutrient-limited growth rates: quantitative benefits of stress responses and some aspects of regulation

Young sunflower plants (Helianthus annuus L.) under stress oflow nitrate or phosphate availability exhibited increases inroot: shoot ratio and in kinetic parameters for uptake. Theyshowed no significant changes in photosynthetic utilizationof either nutrient. Increases in root: shoot ratio were achievedby early and persistent suppression of shoot growth, but notroot growth. Affinity for phosphate uptake, 1/K_m(P), increasedwith phosphate stress, as did affinity for nitrate uptake, 1/K_m(N),with nitrate stress. Maximal uptake rate, V_max, for phosphateuptake increased with phosphorus stress; V_max for nitrate didnot increase with nitrogen stress. Phosphate V_max was relatedstrongly to root nutrient status. Decreases in V_max with plantage were not well explained by changes in age structure of roots.Estimated benefits of acclimatory changes in root: shoot ratioand uptake kinetics ranged up to 2-fold increases in relativegrowth rate, RGR. The relation of RGR to uptake physiology followedpredictions of functional balance moderately well, with somesystematic deviations. Analyses of RGR using growth models implyno significant growth benefit from regulating Vmax, specifically,not from down-regulating it at high nutrient availability. Quantitativebenefits of increases in root: shoot ratio and uptake parametersare predicted to be quite small under common conditions whereinnutrient concentrations are significantly depleted by uptake.The root: shoot response is estimated to confer the smallestbenefit under non-depleting conditions and the largest benefitunder depleting conditions. Even then, the absolute benefitis predicted to be small, possibly excepting the case of heterogeneoussoils. Depleting and non-depleting conditions are addressedwith very different experimental techniques. We note that atheoretical framework is lacking that spans both these cases,other than purely numerical formulations that are not readilyinterpreted. Key words: Nutrient stress, nutrient uptake, nutrient use efficiency, relative growth rate, Helianthus annuus     

Sulfate and sulfite translocation via the phosphate translocator of the inner envelope membrane of chloroplasts

The permeability of the inner envelope membranes of spinach (Spinacia oleracea) chloroplasts to sulfite and sulfate was investigated in vitro, using the technique of silicone oil centrifugal filtration. The results show that there is a permeability towards both ions, resulting in rates of uptake of about 1.0 (SO ₃ ^2- ) and 0.7 (SO ₄ ^2- ) mol mg chlorophyll^-1 h^-1 respectively (external concentration 2 mmol l^-1). The rates depend on the external concentration of the anions. Anion exchange experiments with ³⁵S-preloaded chloroplasts indicate that sulfite and sulfate are exchanged for inorganic phosphate, phosphoglyceric acid, and dihydroxyacetone phosphate with rates up to 14 nmol mg chlorophyll^-1 min^-1. There is no exchange for glucose-6-phosphate and malate. Because of the similarities to the transport of inorganic phosphate and triose phosphates the results give evidence that the phosphate translocator of the inner envelope membrane of chloroplasts is also involved in sulfite and sulfate transport — at least in part.Abbreviations DHAP dihydroxyacetone phosphate - PGA 3-phosphoglycerate - P_i inorganic phosphate - S_i sultite, sulfate     

The role of the triose-phosphate shuttle and glycolytic intermediates in fatty-acid and glycerolipid biosynthesis in pea root plastids

The capacity of the triose-phosphate shuttle and various combinations of glycolytic intermediates to substitute for the ATP requirement for fatty-acid and glycerolipid biosynthesis in pea (Pisum sativum L.) root plastids was assessed. In all cases, ATP gave the greatest rates of fatty-acid and glycerolipid biosynthesis. Rates of up to 66 and 27 nmol·(mg protein)^–1·h^–1 were observed for the incorporation of acetate and glycerol-3-phosphate into lipids in the presence of ATP. In the absence of exogenously supplied ATP, the triose-phosphate shuttle gave up to 44 and 33% of the ATP-control activity in promoting fatty-acid and glycerolipid biosynthesis from acetate and glycerol-3-phosphate, respectively. The optimum shuttle components were 2 mM dihydroxyacetonephosphate (DHAP), 2 mM oxaloacetic acid and 4 mM inorganic phosphate (referred to as the DHAP shuttle). Glyceraldehyde-3-phosphate, as a shuttle triose, was approximately 82% as effective as DHAP in promoting fatty-acid synthesis while 2-phosphoglycerate, 3-phosphoglycerate, and phosphoenolpyruvate were only 27–37% as effective as DHAP. When glycolytic intermediates were used as energy sources for fatty-acid synthesis, in the absence of both exogenously supplied ATP and the triose-phosphate shuttle, phosphoenolpyruvate, 2-phosphoglycerate, fructose-6-phosphate and glucose-6-phosphate each gave 48%, 17%, 23% and 17%, respectively, of the ATP-control activity. Other triose phosphates tested were much less effective in promoting fatty-acid synthesis. When exogenously supplied ATP was supplemented with the DHAP shuttle or glycolytic intermediates, the complete shuttle increased fatty-acid biosynthesis by 37% while DHAP alone resulted in 24% stimulation. Glucose-6-phosphate, fructose-6-phosphate and glycerol-3-phosphate similarly all improved the rates of fatty-acid synthesis by 20–30%. In contrast, 3-phosphoglycerate, 2-phosphoglycerate and phosphoenolpyruvate all inhibited fatty-acid synthesis by approximately 10% each. The addition of the DHAP shuttle and glycolytic intermediates with or without exogenously supplied ATP caused an increase in the proportion of radioactive oleate and a decrease in the proportion of radioactive palmitate synthesized. The use of these alternative energy sources resulted in higher amounts of free fatty acids and triacylglycerol, and lower amounts of diacylglycerol and phosphatidic acid. The data presented here indicate that ATP is superior in promoting in-vitro fatty-acid biosynthesis in pea root plastids; however, both the triose-phosphate shuttle and glycolytic metabolism can produce some of the ATP required for fatty-acid biosynthesis in these plastids.Abbreviations DHAP dihydroxyacetonephosphate - Fru6P fructose-6-phosphate - G3P glycerol-3-phosphate - Glc6P glucose-6-phosphate - OAA oxaloacetate - PEP phosphoenolpyruvate - 2PGA 2-phosphoglycerate - 3PGA 3-phosphoglycerate - 3PGalde glyceraldehyde-3-phosphate This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada.     

Subcellular distribution of enzymes of the oxidative pentose phosphate pathway in root and leaf tissues

The subcellular distribution of enzymes of the oxidative pentose phosphate pathway was studied in plants. Root and leaf tissues from several species were separated by differential centrifugation into plastidic and cytosolic fractions. In all tissues studied, glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were found in both plastidic and cytosolic compartments. In maize and pea root, and spinach and pea leaf, the non-oxidative enzymes of the pentose phosphate pathway (transaldolase, transketolase, ribose 5-phosphate isomerase, ribulose 5-phosphate 3-epimerase) appear to be restricted to the plastid. In tobacco leaf and root, however, the non-oxidative enzymes were found in the cytosolic as well as the plastidic compartments. In the absence of ribose 5-phosphate isomerase and ribulose 5-phosphate 3-epimerase in the cytosol, the product of the oxidative limb of the pathway (ribulose 5-phosphate) must be transported into a compartment capable of utilizing it. Ribulose 5-phosphate was supplied to isolated intact pea root plastids and was shown to be capable of supporting nitrite reduction. The kinetics of ribulose 5-phosphate-driven nitrite reduction in isolated pea root plastids suggested that the metabolite was translocated across the plastid envelope in a carrier-mediated transport process, indicating the presence of a translocator capable of transporting pentose phosphates.Keywords: Pentose phosphate, subcellular, plastid, ribulose 5-phosphate, compartmentation      

Studies of the Enzymic Capacities and Transport Properties of Pea Root Plastids

Plastids have been isolated from pea (Pisum sativum L.) roots with a high degree of purity and intactness. In these plastids, the activity of enzymes involved in carbohydrate metabolism have been analyzed and corrected for cytosolic contamination. The results show that fructose-1,6-bisphosphatase, NAD-glyceraldehyde phosphate dehydrogenase, and phosphoglyceromutase are not present in pea root plastids. Transport measurements revealed that inorganic phosphate, dihydroxyacetone phosphate (DHAP), 3-phosphoglycerate, 2-phosphoglycerate, phosphoenolpyruvate, and glucose-6-phosphate (Glc6p) are transported across the envelope in a counterexchange mode. Transport of glucose-1-phosphate was definitely excluded. The oxidation of Glc6P by intact plastids resulted almost exclusively in the formation of DHAP. The parallel measurement of DHAP formation and NO2- consumption during Glc6P-supported nitrite reduction yielded a ratio of NO2-reduced/DHAP formed of 1.6, which is relatively close to the theoretical value of 2.0. These results show that the oxidation of Glc6P, involving the uptake of Glc6P and the release of DHAP, and the reduction of NO2- are very tightly coupled to each other.     

Energy Supply for ATP-Synthase Deficient Chloroplasts of Chlamydomonas reinhardii

The mutant F54 of the unicellular green alga Chlamydomonas reinhardiiis not able to perform photophos-phorylation. Nevertheless,it grows on acetate and the chloroplasts accomplish most oftheir energy-requiring synthetic processes. However, no light-dependentchloroplast protein synthesis could be detected in intact F54chloroplasts isolated from a cell wall-deficient double mutantF54-cw-15. Exogenous ATP was not able to induce this in organelloprotein synthesis to an appreciable degree. In contrast, thestrictly ATP-dependent protein synthesis was stimulated veryefficiently by glyceraldehyde-3-phosphate, dihydroxy-acetonephosphate and glycerol-3-phosphate, but strongly inhibited by3-phosphoglycerate. These compounds can be transported acrossthe envelope membrane by the triose phosphate translocator.Pyridoxal phosphate, a specific inhibitor of the translocator,abolished the stimulation by triose phosphates. Spermidine,which activates initiation of translation in chloroplasts, enhancedtriose phosphate-stimulated protein synthesis even further.In the dark, no stimulation was observed, indicating that alight-dependent reaction was also involved in this kind of ATPproduction in chloroplasts. The results suggest that chloroplastsdefective in photophosphorylation recruit their energy via anATP shuttle which was shown in this study to import rather thanexport ATP across the chloroplast envelope. (Received August 21, 1997; Accepted November 18, 1997)     

Root Graviresponsiveness and Cellular Differentiation in Wild-type and a Starchless Mutant of Arabidopsis thaliana

Primary roots of a starchless mutant of Arabidopsis thalianaL. are strongly graviresponsive despite lacking amyloplastsin their columella cells. The ultrastructures of calyptrogenand peripheral cells in wild-type as compared to mutant seedlingsare not significantly different. The largest difference in cellulardifferentiation in caps of mutant and wild-type roots is therelative volume of plastids in columella cells. Plastids occupy12.3% of the volume of columella cells in wild-type seedlings,but only 3.69% of columella cells in mutant seedlings. Theseresults indicate that: (1) amyloplasts and starch are not necessaryfor root graviresponsiveness; (2) the increase in relative volumeof plastids that usually accompanies differentiation of columellacells is not necessary for root graviresponsiveness; and (3)the absence of starch and amyloplasts does not affect the structureof calyptrogen (i.e. meristematic) and secretory (i.e. peripheral)cells in root caps. These results are discussed relative toproposed models for root gravitropism. Arabidopsis thaliana, gravitropism (root), plastids, root cap, stereology, ultrastructure