Heterologous expression of a<Emphasis Type="Italic"> keto</Emphasis>hexokinase in potato plants leads to inhibited rates of photosynthesis,severe growth retardation and abnormal leaf development

In the present paper we investigated the effect of heterologous expression of a rat liver ketohexokinase in potato (Solanum tuberosum L.) plants with the aim of investigating the role of fructose 1-phosphate in plant metabolism. Plants were generated that contained appreciable activity of ketohexokinase but did not accumulate fructose 1-phosphate. They were, however, characterised by a severe growth retardation and abnormal leaf development. Studies of ¹⁴CO₂ assimilation and metabolism, and of the levels of photosynthetic pigments, revealed that these lines exhibited restricted photosynthesis. Despite this fact, the levels of starch and soluble sugars remained relatively constant. Analysis of intermediates of starch and sucrose biosynthesis revealed large increases in the triose phosphate and fructose 1,6-bisphosphate pools but relatively unaltered levels of inorganic phosphate and 3-phosphoglycerate, and these lines were also characterised by an accumulation of glyceraldehyde. The transformants neither displayed consistent changes in the activities of Calvin cycle enzymes nor in enzymes of sucrose synthesis but displayed a metabolic profile partially reminiscent of that brought about by end-product limitation, but most likely caused by an inhibition of photosynthesis brought about by the accumulation of glyceraldehyde. Analysis of the metabolite contents in lamina and vein fractions of the leaf, and of the enzymes of carbohydrate oxidation indicate that the phloem-enriched veins of ketohexokinase-expressing leaves tend toward hypoxia and indicate a problem of phloem transport.Abbreviations CaMV Cauliflower mosaic virus - DHAP Dihydroxyacetone phosphate - F1P Fructose 1-phosphate - FBP Fructose 1,6-bisphosphate - KHK Ketohexokinase - NADP-GAPdH NADP-dependent glyceraldehyde-3-phosphate dehydrogenase - PFP Pyrophosphate: fructose 6-phosphate 1-phosphotransferase - 3PGA 3-Phosphoglycerate - PEP Phosphoenolpyruvate - Rubisco Ribulose 1,5-bisphosphate carboxylase/oxygenase - SPS Sucrose phosphate synthase - SuSy Sucrose synthase     

Inorganic pyrophosphate content and metabolites in potato and tobacco plants expressing E. coli pyrophosphatase in their cytosol

Metabolite levels and carbohydrates were investigated in the leaves of tobacco (Nicotiana tabacum L.) and leaves and tubers of potato (Solanum tuberosum L.) plants which had been transformed with pyrophosphatase from Escherichia coli. In tobacco the leaves contained two- to threefold less pyrophosphate than controls and showed a large increase in UDP-glucose, relative to hexose phosphate. There was a large accumulation of sucrose, hexoses and starch, but the soluble sugars increased more than starch. Growth of the stem and roots was inhibited and starch, sucrose and hexoses accumulated. In potato, the leaves contained two- to threefold less pyrophosphate and an increased UDP-glucose/ hexose-phosphate ratio. Sucrose increased and starch decreased. The plants produced a larger number of smaller tubers which contained more sucrose and less starch. The tubers contained threefold higher UDP-glucose, threefold lower hexose-phosphates, glycerate-3-phosphate and phosphoenolpyruvate, and up to sixfold more fructose-2,6-bisphosphatase than the wild-type tubers. It is concluded that removal of pyrophosphate from the cytosol inhibits plant growth. It is discussed how these results provide evidence that sucrose mobilisation via sucrose synthase provides one key site at which pyrophosphate is needed for plant growth, but is certainly not the only site at which pyrophosphate plays a crucial role.Abbreviations Fru2,6bisP fructose-2,6-bisphosphate - Fru6P fructose 6-phosphate - FW fresh weight - Glc1P glucose-1-phosphate - Glc6P glucose-6-phosphate - PEP phosphoenolpyruvate - 3PGA glycerate-3-phosphate - PFK phosphofructokinase - PFP pyrophosphate: fructose-6-phosphate phosphotransferase - Pi inorganic phosphate - PPi inorganic pyrophosphate - UDPGlc UDP-glucose This research was supported by the Deutsche Forschungsgemein-Schaft (SFB 137) and Sandoz AG (T.J., M.H., M.S.) and by the Bundesminister für Forschung und Technologie (U.S., L.W.).     

Solute distribution between vacuole and cytosol of sugarcane suspension cells: Sucrose is not accumulated in the vacuole

The compartmentation of solutes in suspension cells of Saccharum sp. during different growth phases in batch culture was determined using CuCl₂ to permeabilize the plasma membrane of the cells. The efflux of cytosolic and vacuolar pools of sugars, cations and phosphate was monitored, and the efflux data for phosphate were compared and corrected using data from compartmentation analysis of phosphate as determined by ³¹P-nuclear magnetic resonance spectroscopy. The results show that sucrose is not accumulated in the vacuoles at any phase of the growth cycle. On the other hand, glucose and fructose are usually accumulated in the vacuole, except at the end of the cell-culture cycle when equal distribution of glucose and fructose between the cytosol and the vacuole is found. Both Na⁺ and Mg²⁺ are preferentially located in the vacuoles, but follow the same tendency as glucose and fructose with almost complete location in the vacuole in the early culture phases and increasing cytosolic concentration with increasing age of the cell culture. Potassium ions are always clearly accumulated in the cytosol at a concentration of about 80 mM; only about 20% of the cellular K⁺ is located inside the vacuole. Cytosolic phosphate is little changed during the cell cycle, whereas the vacuolar phosphate pool changes according to total cellular phosphate. In general there are two different modes of solute compartmentation in sugarcane cells. Some solutes, fructose, glucose, Mg²⁺ and Na⁺, show high vacuolar compartmentation when the total cellular content of the respective solute is low, whereas in the case of ample supply the cytosolic pools increase. For other solutes, phosphate and K⁺, the cytosolic concentration tends to be kept constant, and only excess solute is stored in the vacuole and remobilized under starvation conditions. The behaviour of sucrose is somewhat intermediate and it appears to equilibrate easily between cytosol and vacuole.Abbreviation NMR nuclear magnetic resonance The very cooperative help by Dr. J. Reiner with the ³¹P-NMR measurements and the technical assistance by D. Keis are gratefully acknowledged. This research was supported by the Deutsche Forschungsgemeinschaft and by Fonds der Chemischen Industrie.     

Intercellular compartmentation of sucrose synthesis in leaves of Zea mays L.

The incorporation of ¹⁴C into sucrose and hexose phosphates during steady-state photosynthesis was examined in intact leaves of Zea mays L. plants. The compartmentation of sucrose synthesis between the bundle sheath and mesophyll cells was determined by the rapid fractionation of the mesophyll and comparison of the labelled sucrose in this compartment with that in a complete leaf after homogenisation. From these experiments it was concluded that the majority of sucrose synthesis occurred in the mesophyll cell type (almost 100% when the time-course of sucrose synthesis was extrapolated to the time of ¹⁴C-pulsing). The distribution of enzymes involved in sucrose synthesis between the two cell types indicated that sucrose-phosphate synthetase was predominantly located in the mesophyll, as was cytosolic (neutral) fructose-1,6-bisphosphatase activity. Stromal (alkaline) fructose-1,6-bisphosphatase activity was found almost exclusively in the bundle-sheath cells. No starch was found in the mesophyll tissue. These data indicate that in Zea mays starch and sucrose synthesis are spatially, separated with sucrose synthesis occurring in the mesophyll compartment and starch synthesis in the bundle sheath.     

干旱胁迫下紫花苜蓿幼苗非结构性碳水化合物代谢对NO的响应

以紫花苜蓿（Medicago sativa）为材料,采用盆栽试验方法,用聚乙二醇（PEG-6000）作为渗透介质模拟干旱胁迫,外源喷施NO供体硝普钠,NO清除剂（carboxy-PTIO,cPTIO）,对紫花苜蓿幼苗叶片、根系中非结构性碳水化合物含量及相关酶活性的变化进行研究,探讨NO对紫花苜蓿耐旱机制的作用。结果表明：外源NO促进了紫花苜蓿叶片中淀粉的分解、根系中淀粉的积累,提高叶片及根系中可溶性糖（蔗糖、果糖和葡萄糖）含量,降低了渗透势,促进细胞吸水,缓解干旱造成的损伤。此外,外源NO能提高干旱胁迫下紫花苜蓿叶片中蔗糖合成酶（SS）、酸性转化酶（AI）和中性转化酶（NI）活性,降低了蔗糖磷酸合成酶（SPS）的活性,提高根系中SS、SPS和转化酶活性,使蔗糖的合成与分解处于高水平的动态平衡,增强了紫花苜蓿的抗旱性。而NO清除剂cPTIO则会不同程度的抑制紫花苜蓿幼苗中非结构性碳水化合物（NSC）及其相关酶活性。因此,NO可以通过调控NSC的代谢响应干旱胁迫,缓解干旱胁迫造成的不利影响,在紫花苜蓿的抗旱中扮演着重要的角色。     

Osmotic regulation of starch synthesis in potato tubers?

Using potato (Solanum tuberosum L.) tuber discs incubated in a range of mannitol concentrations it has been demonstrated that both sucrose uptake and the conversion of sucrose to starch are sensitive to the osmotic environment of the storage cells. Starch synthesis was optimised at 300 mM but declined sharply at both lower and higher osmotic concentrations. The decline in starch synthesis on either side of optimum was not proportional to the change in mannitol concentration, indicating different inhibitory mechanisms under low and high osmotica. The fraction of the total sucrose converted to starch i.e. the partitioning between sucrose and starch, was also influenced by osmotic environment. The amount of soluble material taken up by the storage cells, but not converted to starch, was maintained under mannitol concentrations (300–400 mM) which inhibited starch synthesis, indicating that sucrose uptake continued during declining starch synthesis. At mannitol concentrations above 400 mM, sucrose uptake was greatly enhanced but no significant change in starch synthesis occurred.     

Metabolic discrimination of sucrose starvation from<Emphasis Type="Italic">Arabidopsis</Emphasis> cell suspension by<Superscript>1</Superscript>H NMR based metabolomics

Whole cell extracts ofArabidopsis cell cultures maintained on various sucrose concentrations (0,3, and 6%) were analyzed by¹H NMR spectroscopy to determine the comprehensive metabolic change in these cultures during sucrose starvation. The amount of sucrose, glucose, and fructose in the cells decreased to almost nothing after 12 h of culture in medium without sucrose. In contrast, the total free amino acid content of the cells increased as the culture proceeded. Among the free amino acids, phenylalanine and malic acid increased the most, followed by asparagine and alanine, whereas glutamic acid did not change significantly. These results are in agreement with previous studies using HPLC.¹H NMR spectroscopy enabled measurement of changes in the sugar and free amino acid content of whole cell extracts without fractionation and complicated sample preparation. These results indicate that comprehensive metabolic changes in the cells can be determined by a simple, rapid method using whole cell extracts and¹H NMR spectroscopy.     

Phytotoxic effects of Sicyos deppei (Cucurbitaceae) in germinating tomato seeds

The phytotoxic effect of allelochemicals is referred to as allelochemical stress and it is considered a biotic stress. Sicyos deppei G. Don (Cucurbitaceae) is an allelopathic weed that causes phytotoxicity in Lycopersicon esculentum , delaying seed germination and severely inhibiting radicle growth. This paper reports in in vitro conditions, the effects of the aqueous leachate of S. deppei —throughout tomato germination times—on (1) the dynamics of starch and sugars metabolism, (2) activity and expression of the cell wall enzymes involved in endosperm weakening that allows the protrusion of the radicle, and (3) whether abscisic acid (ABA) is involved in this altered metabolic processes. Results showed that S. deppei leachate on tomato seed germination mainly caused: (1) delay in starch degradation as well as in sucrose hydrolysis; (2) lower activity of sucrose phosphate synthase, cell wall invertase, and α-amylase; being sucrose phosphate synthase (SPS) gene expression down-regulated, and the last two up regulated; (3) also, lower activity of endo β-mannanase, β-1,3 glucanase, α-galactosidase, and exo-polygalacturonase with altered gene expression; and (4) higher content of ABA during all times of germination. The phytotoxic effect of S. deppei aqueous leachate is because of the sum of many metabolic processes affected during tomato seed germination that finally is evidenced by a strong inhibition of radicle growth.     

Carbohydrate metabolism in growing rice seedlings under arsenic toxicity

We studied in the seedlings of two rice cultivars (Malviya-36 and Pant-12) the effect of increasing levels of arsenic in situ on the content of sugars and the activity of several enzymes of starch and sucrose metabolism: alpha-amylase (EC 3.2.1.1), beta-amylase (EC 3.2.1.2), starch phosphorylase (EC 2.4.1.1), acid invertase (EC 3.2.1.26), sucrose synthase (EC 2.4.1.13) and sucrose phosphate synthase (EC 2.4.1.14). During a growth period of 10-20 d As2O3 at 25 and 50 microM in the growth medium caused an increase in reducing, non-reducing and total soluble sugars. An increased conversion of non-reducing to reducing sugars was observed concomitant with As toxicity. The activities of alpha-amylase, beta-amylase and sucrose phosphate synthase declined, whereas starch phosphorylase, acid invertase and sucrose synthase were found to be elevated. Results indicate that in rice seedlings arsenic toxicity causes perturbations in carbohydrate metabolism leading to the accumulation of soluble sugars by altering enzyme activity. Sucrose synthase possibly plays a positive role in synthesis of sucrose under As-toxicity.     

Effect of sink isolation on sugar uptake and starch synthesis by potato-tuber storage parenchyma

Import into potato (Solarium tuberosum L. cv. Record) tubers was terminated by removing the sink at its connection with the stolon. The ability of discs of storage tissue from the excised tubers to take up exogenous sugars and convert them to starch was compared with that of discs from untreated tubers from the same plant population. In rapidly-growing control tubers, glucose and fructose were taken up to a greater extent than sucrose, 77% of the glucose being converted to starch within 3 h (compared with 64% and 27% for fructose and sucrose, respectively). These values fell as the tubers aged but the ranking (glucose > fructose > sucrose) was maintained, emphasising a severe rate-limiting step following the import of sucrose into the growing tuber. Sink isolation had little effect on the ability of the storage cells to take up exogenous sucrose across the plasmalemma for up to 7 d after sink isolation. However, the ability of the same cells to convert the sucrose to starch was severely inhibited within 24 h, as was the sensitivity of starch synthesis to turgor. In the case of glucose, sink isolation inhibited both the uptake and the conversion to starch, the latter being inhibited to a greater degree. A detailed metabolic study of tubers 7 d after excision showed that, with sucrose as substrate, 94% of the radioactivity in the soluble sugar pool was recovered in sucrose following sink isolation (92% in control tubers). However, with glucose as substrate, 80% of the radioactivity was recovered as sucrose following tuber excision (28% in control tubers), providing evidence that sucrose synthesis acts as a major alternative carbon sink when starch synthesis is inhibited. In the same tubers, sucrose-synthase activity decreased by 70% following sink isolation, compared with a 45% reduction in ADP-glucose pyrophosphorylase. Activities of UDP-glucose pyrophosphorylase, starch phosphorylase, starch synthase nd both PPi- and ATP-dependent phosphofructokinases remained unchanged. Acid-invertase activity increased fivefold.     

Effect of osmotic stress on tolerance of air-drying and cryopreservation ofArabidopsis thaliana suspension cells

Summary Arabidopsis thaliana suspension cells were preserved in liquid nitrogen for over three years, using embedding of cells in calcium-alginate prior to subculture in sucrose-enriched medium, air-drying, and direct quenching in liquid nitrogen. Survival of cells reached 34%, yielding regrowth at the surface of all cryopreserved beads in less than 7 days. Following pretreatment and dehydration, the water content dropped from 2300% to 34% with respect to dry weight. Differential scanning calorimetry showed that glass transition occurred on cooling, followed by a slight crystallization event on rewarming. The survival of cells was independent of the cooling rate. The tolerance of the acute dehydration step increased progressively with sucrose pretreatment duration, indicating the requirement for adaptative cellular alterations. Ultrastructural studies revealed several changes in cells after sucrose pretreatment prolonged from 1 to 7 days: reversal of the initially plasmolyzed state, microvacuolation, numerous autophagic structures, scarcity of ribosomes, increase in number and size of starch grains. No cell division seemed to occur during this period. After air-drying and after a freeze-thaw cycle, followed by 24 h rehydration, regenerating cells had recovered a high level of ultrastructural organization and contained numerous polysomes suggesting an intense metabolic activity. Trehalose, a cryoprotective disaccharide not considered to be a metabolic substrate, yielded only 70% regrowth after freezing. Biochemical analysis showed that soluble sugars accumulated during the pretreatment, essentially sucrose or trehalose; the monosaccharide content also increased. In the light of these results, the action of sucrose in inducing freezing tolerance is discussed.Abbreviations HPLC high-performance liquid chromatography - LN liquid nitrogen - TTC 2,3,5-triphenyltetrazolium chloride     

Influence of the carbon source on growth and rosmarinic acid production in suspension cultures of Coleus blumei

Suspension cultures of Coleus blumei were characterized with respect to growth and rosmarinic acid formation in media with different sugars and various sugar concentrations. Sucrose is the sugar with the highest stimulating effect on growth and rosmarinic acid accumulation, followed by glucose and fructose. The sugar alcohol mannitol cannot be metabolized by the plant cells. Sucrose is cleaved into glucose and fructose by the Coleus cells. Sucrose concentrations from 1 to 5% have an increasing positive effect on growth and rosmarinic acid synthesis in the cell cultures with a maximum rosmarinic acid content of 12% of the dry weight in medium with 5% sucrose; in medium with 6% sucrose rosmarinic acid accumulation obviously did not reach its highest level in the culture period of 14 days. A very high yield of rosmarinic acid (2 mg ml^-1 suspension) could also be achieved by maintaining a sucrose concentration of 2% during the whole culture period. The start of rosmarinic acid synthesis by the cell cultures seems to be regulated by the growth limitation when a nutrient, e.g. phosphate is depleted from the medium. The rate of rosmarinic acid accumulation is related to the amount of carbon left in the medium when growth ceases.Abbreviations RA rosmarinic acid     

When growing potato tubers are detached from their mother plant there is a rapid inhibition of starch synthesis,involving inhibition of ADP-glucose pyrophosphorylase

Labelling experiments in which high-specific-activity [U-¹⁴C]sucrose or [U-¹⁴C]hexoses were injected into potato (Solanum tuberosum L. cv. Desiree) tubers showed that within 1 d of detaching growing tubers from their mother plant, there is an inhibition of starch synthesis, a stimulation of the synthesis of other major cell components, and rapid resynthesis of sucrose. This is accompanied by a general increase in phosphorylated intermediates, an increase in UDP-glucose, and a dramatic decrease of ADP-glucose. No significant decline in the extracted activity of enzymes for sucrose degradation or synthesis, or starch synthesis is seen within 1 d, nor is there a significant decrease in sucrose, amino acids, or fresh weight. Over the next 7 d, soluble carbohydrates decline. This is accompanied by a decline in sucrose-synthase activity, hexose-phosphate levels, and the synthesis of structural cell components. It is argued that a previously unknown mechanism acting at ADP-glucose pyrophosphorylase allows sucrose-starch interconversions to be regulated independently of the use of sucrose for cell growth.     

Hormone-mediated enzyme activity in source leaves

The initial step in carbon allocation occurs in leaves and is the chemical partitioning of carbon between sucrose and starch. Sucorse phosphate synthase is one of the enzymes belived to regulate rate of sucrose synthesis. In this study, the effects of indoleacetic acid, gibberellic acid, and abscisic acid on the activity of this enzyme were investigated in source leaves of mature sugar beets. Preliminary evidence is presented that, concurrent with a modification of sucrose uptake rates, i.e., phloem loading, these plant growth substances modify the activity of sucrose phosphate synthase resulting in altered partitioning of carbon between sucrose and starch.This work was supported by National Science Foundation grant #PCM 82-39139. New Jersey Agricultural Experiment Station Paper No. 3-15192-1-85.     

Sucrose phosphate synthase and other sucrose metabolizing enzymes in fruits of various species

Recent reports have suggested that sucrose phosphate synthase (EC 2.4.1.14), a key enzyme in sucrose biosynthesis in photosynthetic “source” tissues, may also be important in some sucrose accumulating “sink” tissues. These experiments were conducted to determine if sucrose phosphate synthase is involved in sucrose accumulation in fruits of several species. Peach (Prunus persica NCT 516) and strawberry (Fragaria x ananassa cv. Chandler) fruits were harvested directly from the plant at various stages of fruit development. Kiwi (Actinidia chinensis), papaya (Carica papaya), pineapple (Ananas comosus) and mango (Mangifera indica) were sampled in postharvest storage over a period of several days. Carbohydrate concentrations and activities of sucrose phosphate synthase, sucrose synthase (EC 2.4.1.13), and acid and neutral invertases (EC 3.2.1.26) were measured. All fruits contained significant activities of sucrose phosphate synthase. Moreover, in fruits from all species except pineapple and papaya, there was an increase in sucrose phosphate synthase activity associated with the accumulation of sucrose in situ. The increase in sucrose concentration in peaches was also associated with an increase in sucrose synthase activity and, in strawberries, with increased activity of both sucrose synthase and neutral invertase. The hexose pools in all fruits were comprised of equimolar concentrations of fructose and glucose, except in the mango. In mango, the fructose to glucose ratio increased from 2 to 41 during ripening as sucrose concentration more than doubled. The results of this study indicate that activities of the sucrose metabolizing enzymes, including sucrose phosphate synthase, within the fruit itself, are important in determining the soluble sugar content of fruits of many species. This appears to be true for fruits which sweeten from a starch reserve and in fruits from sorbitol translocating species, raffinose saccharide translocating species, and sucrose translocating species.     

A role for 'futile cycles' involving invertase and sucrose synthase in sucrose metabolism of tomato fruit.

Current concepts of the factors determining sink strength and the subsequent regulation of carbohydrate metabolism in tomato fruit are based upon an understanding of the relative roles of sucrose synthase, sucrose phosphate synthase and invertase, derived from studies in mutants and transformed plants. These enzymes participate in at least four futile cycles that involve sugar transport between the cytosol, vacuole and apoplast. Key reactions are (1) the continuous rapid degradation of sucrose in the cytosol by sucrose synthase (SuSy), (2) sucrose re-synthesis via either SuSy or sucrose phosphate synthase (SPS), (3) sucrose hydrolysis in the vacuole or apoplast by acid invertase, (4) subsequent transport of hexoses to the cytosol where they are once more converted into sucrose, and (5) rapid synthesis and breakdown of starch in the amyloplast. In this way futile cycles of sucrose/hexose interchange govern fruit sugar content and composition. The major function of the high and constant invertase activity in red tomato fruit is, therefore, to maintain high cellular hexose concentrations, the hydrolysis of sucrose in the vacuole and in the intercellular space allowing more efficient storage of sugar in these compartments. Vacuolar sugar storage may be important in sustaining fruit cell growth at times when less sucrose is available for the sink organs because of exhaustion of the carbohydrate pools in source leaves.     

Alterations in photosynthesis in <Emphasis Type="Italic">Arabidopsis</Emphasis> lacking IMMUTANS,a chloroplast terminal oxidase

Green and white variegation in the Arabidopsis immutans (im) mutant is caused by a nuclear recessive gene. The green sectors contain cells with normal-appearing chloroplasts, while cells in the white sectors have photooxidized plastids lacking organized lamellae. In the present experiments, we found that the green im sectors have enhanced rates of carbon assimilation (monitored by ¹⁴CO₂ uptake) and that there are corresponding increases in the activities of Rubisco and SPS, elevated starch and sucrose pool sizes, and an altered pattern of carbohydrate partitioning that favors sucrose over starch. We hypothesize that these increases are due, at least in part, to interactions with white sectors, perhaps to compensate for reductions in total source tissue. Consistent with this idea, the im white sectors accumulate low levels of sucrose and acid invertase activities are markedly increased in the white versus green cells. This suggests that there is a sucrose gradient between the green and white sectors, and that sucrose is transported from the green to white cells in response to sink demand. The expression of photosynthetic genes is not appreciably altered in the green im sectors versus wild type, but rather there is an up-regulation of genes involved in defense against oxidative stress and down-regulation of genes involved in cell wall biosynthesis. We postulate that changes in photosynthesis in the im green cells are driven by a need for photoprotection (especially early in chloroplast biogenesis) and due to source-sink interactions. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Changes in Mulberry Leaf Metabolism in Response to Water Stress

A series of experiments were conducted to characterize the water stress-induced changes in the activities of RuBP carboxylase (RuBPCO) and sucrose phosphate synthase (SPS), photosystem 2 activity, and contents of chlorophylls, carotenoids, starch, sucrose, amino acids, free proline, proteins and nucleic acids in mulberry (Morus alba L. cv. K-2) leaves. Water stress progressively reduced the activities of RuBPCO and SPS in the leaf extracts, the chlorophyll content, and PS2 activity in isolated chloroplasts. Plants exposed to drought showed lower content of starch and sucrose but higher total sugar content than control plants. While the soluble protein content decreased under water stress, the amino acid content increased. Proline accumulation (2.5-fold) was noticed in stressed leaves. A reduction in the contents of DNA and RNA was observed. Reduced nitrogen content was associated with the reduction in nitrate reductase activity. SDS-PAGE protein profile showed few additional proteins (78 and 92 kDa) in the water stressed plants compared to control plants.     

Metabolism of myo-Inositol and growth in various sugars of suspension-cultured tobacco cells

Tobacco (Nicotiana tabacum L.) cells were cultured in a liquid medium which contained sucrose as a source of carbon and energy. Various cell-wall constituents and wall precursors (L-arabinose, D-xylose, D-galactose, D-mannose, D-glucuronate, myo-inositol) were added to cells growing in this medium to by-pass possible rate-limiting steps in the relevant metabolic pathways. None of these compounds stimulated growth as measured by increase in fresh weight; myo-inositol did cause a slight increase and L-arabinose a decrease in dry weight accumulation compared to controls grown on sucrose only. Although myo-inositol was not needed for rapid growth, tracer level amounts of [2-³H]myo-inositol were rapidly absorbed and metabolized. Label was incorporated into the uronide and pentose residues of cell walls and exocellular polysaccharide.