The Effects of Cadmium-Zinc Interactions on Biochemical Responses in Tobacco Seedlings and Adult Plants

The objective of the present study was to investigate the effects of cadmium-zinc (Cd-Zn) interactions on their uptake, oxidative damage of cell macromolecules (lipids, proteins, DNA) and activities of antioxidative enzymes in tobacco seedlings as well as roots and leaves of adult plants. Seedlings and plants were exposed to Cd (10 µM and 15 µM) and Zn (25 µM and 50 µM) as well as their combinations (10 µM or 15 µM Cd with either 25 µM or 50 µM Zn). Measurement of metal accumulation exhibited that Zn had mostly positive effect on Cd uptake in roots and seedlings, while Cd had antagonistic effect on Zn uptake in leaves and roots. According to examined oxidative stress parameters, in seedlings and roots individual Cd treatments induced oxidative damage, which was less prominent in combined treatments, indicating that the presence of Zn alleviates oxidative stress. However, DNA damage found in seedlings, and lower glutathione reductase (GR) and superoxide dismutase (SOD) activity recorded in both seedlings and roots, after individual Zn treatments, indicate that Zn accumulation could impose toxic effects. In leaves, oxidative stress was found after exposure to Cd either alone or in combination with Zn, thus implying that in this tissue Zn did not have alleviating effects. In conclusion, results obtained in different tobacco tissues suggest tissue-dependent Cd-Zn interactions, which resulted in activation of different mechanisms involved in the protection against metal stress.     

Nitrite Uptake by Nitrogen-Depleted Wheat Seedlings

Intact, 14-day-old nitrogen-depleted wheat (Triticum vulgare cv. Blueboy) seedlings were exposed to solutions of 0.5 mM KNO₂, 0.05 mM CaSO₄ and 1 mM sodium 2-[N-morpholino]-ethanesulfonate, pH 6.1. Nitrite uptake was determined from depletion of the ambient solution or from incorporation of ¹⁵N in the tissue. An initial nitrite uptake shoulder was followed by a relatively slow uptake rate which subsequently increased to a substantially greater rate. This accelerated phase was maintained through 24 h. Nitrite accumulated to a slight extent in the root tissues during the first few hours but declined to low values when the accelerated rate was fully developed, indicating an increase in nitrite reductase activity paralleling the increase in nitrite uptake capacity. About 50% of the nitrogen absorbed as nitrite was translocated to the shoots by 9–12 h. Development of the accelerated nitrite uptake rate was restricted in excised roots, in intact plants kept in darkness, by 400 μg puromycin ml^?1 and by 1 mM L-ethionine. When puromycin and L-ethionine were added after the accelerated phase had been initiated, their effects were not as detrimental as when they were added at first exposure to KNO₂. The two inhibitors restricted translocation more than uptake. The data indicate an involvement of protein synthesis and a requirement for movement of a substance from shoots to roots for maximal development of the accelerated nitrite uptake phase. A requirement for protein synthesis in the transport of soluble organic nitrogen from roots to shoots is also suggested.     

Bacterial--Invertebrate Interactions in Uptake of Dissolved Organic Matter

SYNOPSIS. AS compared to integumentary uptake systems of soft-bodiedmarine invertebrates, bacterial systems, in terms of transportconstants, are much better adapted to the low concentrationsof dissolved organic nutrients encountered in coastal and offshorewaters. Bacteria respond to the presence of suitable dissolvedorganic substrates with induction, uptake and multiplication,maintaining the concentrations of dissolved organic matter (DOM)permanently low. At realistic in situ concentrations, epidermaluptake by pelagic and epibenthic animals proceeds at such lowrates that scarcely a substantial proportion of their metabolicneeds is provided by absorption. In marine sediments, wherethe life processes of bacteria and animals are closely interrelated,the macrofauna is sheltered by shells, firm tubes and burrows,which are irrigated by means of overlying water of the watercolumn. Hence, interstitial water with its sometimes higherconcentrations of DOM is scarcely available to sediment-dwellinglarger metazoans. The meiofauna mainly inhabits the few millimetersof the upper sediment layers and the thin halos surroundingirrigated macrofaunal burrows, where sufficient oxygen is available.Unless the magnitude of horizontal water movement, the amountsof diffusional nutrient supply and the percentages, by whichnutrient concentrations are reduced by meiofaunal uptake, areknown, estimations of nutritional benefits from uptake of DOMby meiofauna cannot be made. For all infaunal taxa, bacteriaappear to represent a major food supply.     

Auxin-Cytokinin Interactions in Wild-Type and Transgenic Tobacco

Cytokinins and auxins are important regulators of plant growthand development, but there is incomplete and conflicting evidencethat auxins affect cytokinin metabolism and vice versa. We haveinvestigated these interactions in Nicotiana tabacum L. by separatein planta manipulation of levels of the hormones followed byanalysis of the induced changes in the metabolism of the otherhormone. Cytokinin-overproducing plants (expressing the Agrobacte-riumtumefaciens ipt gene) had lower than wild-type levels of freeIAA, and reduced rates of IAA synthesis and turnover, but therewere no differences in the profiles of metabolites they producedfrom fed IAA. Similarly, auxin-overproducing plants (expressingthe A.tumefaciens iaaM and iaaH genes), had lower levels ofthe major cytokinins than wild-type plants and lower cytokininoxidase activity, but there were no differences in the profilesof metabolites they produced from fed cytokinins. The data demonstratethat cytokinin or auxin overproduction decreases the contentof the other hormone, apparently by decreasing its rate of synthesisand/or transport, rather than by increasing rates of turnoveror conjugation. Implications for the importance of cytokinin: auxin ratios in plant development are considered. (Received September 24, 1996; Accepted December 4, 1996)     

Kinetics of Manganese Uptake by Intact Citrus Seedlings

Uptake of manganese by intact citrus seedlings can be represented by three phases of a single, multiphasic isotherm in the range 10^?8M–2× 10^?4M. The phases are separated by marked jumps and the kinetic constants increase upon transition to higher phases.     

Uptake of Sugars by Tobacco Callus Tissue

D-Glucose and several other sugars are taken up by slices of callus tissue ofNicotiana tabacum and are both oxidized to CO₂ and incorporated into tissue components. The uptake as well as the subsequent metabolism are slow, the transport process being characterized by lack of specificity, no hyperbolic saturation, no influence of metabolic or transport inhibitors and hardly any dependence on pH. The apparent activation energy of the process was about 42 kJ mol^-1. It appears that the transport proceeds by simple diffusion through narrow intercellular spaces and possibly hydrophilic pores and that only about 10% of the tissue volume is accessible to the sugars in question.     

The Relationship Between Growth and Oxygen Uptake in Hypoxic Rice Seedlings

Atwell, B. J. and Green way, H. 1987. The relationship betweengrowth and oxygen uptake in hypoxic rice seedlings.—J.exp. Bot. 38: 454–465. Rice seedlings (Oryza saliva L.) were grown in the dark forup to 4 d in solutions containing various concentrations ofO₂. Compared with seedlings grown at 0·250 mol O₂ m^–3,the dry weight of the growing seedling was 14% lower at 0·110mol O₂ m^–3 and 60% lower at 0 mol O₂ m^–3. Decreasesin fresh weight were similar but not identical to decreasesin dry weight, possibly because leaf growth was suppressed evenabove 0·110 mol O₂ m^–3. Oxygen deficiency inhibitedroot growth more severely than coleoptile growth. Coleoptiles from seedlings grown in aerated solution were exposedto an atmosphere of pure N₂ for 30 min. Anoxia caused a declinein ATP content and energy charge, suggestive of decreased oxidativephosphorylation. It is not clear whether the decline in oxidativephosphorylation was solely responsible for impaired growth inhypoxia. In seedlings growing at O₂ concentrations less than 0·110mol O₂ m^–3, significant amounts of ethanol were synthesized.The rate of O₂ uptake decreased markedly below 0·06 molO₂ m^–3; this was presumably near the external O₂ concentrationat which oxidative phosphorylation became limited by the supplyof O₂. The stage of development of the seedlings appeared toinfluence O₂ uptake, possibly through changes in conductanceof the tissue to O₂. Uncouplers were used to confirm that thecritical O₂ concentration was dependent on O₂ diffusion ratherthan enzyme kinetics. Impaired growth above 0·110 molO₂ m^–3 may have been due to a decreased activity of oxygenasesof relatively low affinity for O₂, which in turn altered cellmetabolism. Key words: Growth, oxygen uptake, rice seedlings, hypoxia     

Nitrite Uptake Patterns in Wheat Seedlings as Influenced by Nitrate and Ammonium

Nitrite and nitrate uptake by wheat (Triticum vulgare) from 0.5 mM potassium solutions both showed an apparent induction pattern characterized by a slow initial rate followed by an accelerated rate. The accelerated phase was more rapid for nitrate uptake, was initiated earlier, and was seriously restricted by the presence of equimolar nitrite. The accelerated phase of nitrite uptake was restricted by nitrate to a lesser extent. The two anions seem not to be absorbed by identical mechanisms. Ammonium pretreatments or prior growth with ammonium had relatively little influence on the pattern of nitrite uptake. However, prior growth with nitrate eliminated the slow initial phase and induced development of the accelerated phase of nitrite uptake. A beneficial effect was noted after 3 h nitrate pretreatment and full development had occurred by 12 h nitrate pretreatment. The evidence suggests that a small amount of tissue nitrite, which could be supplied either by absorption or by nitrate reduction, was specifically required for induction of the accelerated phase of nitrite uptake. Cycloheximide (2 μg ml^?1) seriously restricted development of the accelerated phase of nitrite uptake, but its effect was not as severe when it was added after the accelerated phase had been induced by prior exposure to nitrite or nitrate. However, translocation of ¹⁵N from the absorbed nitrite was sharply decreased under the latter conditions, indicating a difference in sensitivity of the uptake and translocation processes to cycloheximide. Potassium uptake was greater from KNO₃ than from KNO₂ and in both instances it was enhanced during the early stages of the accelerated phase of anion uptake. Moreover, addition of NaNO₃ to KNO₂ substantially increased potassium uptake. A coupling between anion and potassium uptake was therefore evident, but the coupling was not obligatory because the accelerated phase of nitrite uptake could occur in absence of rapid potassium uptake.     

The Kinetics of Chlorate Uptake by XD Tobacco Cells

The uptake of [³⁶Cl]chlorate by the 14U variant of the XD cell line of Nicotiana tobaccum L. cv Xanthi was investigated to examine the use of chlorate as a nitrate analog in transport studies. The kinetics of chlorate uptake against concentration was complex. Evidence was obtained, e.g., by means of nitrate competition, that these kinetics could be resolved into two components indicating the existence of two influx mechanisms: a saturable high affinity transport system (HATS) and a low affinity transport system (LATS) that showed first order kinetics. HATS has an apparent K_m for chlorate of 0.3 millimolar, and a marked pH dependence. The V_max dropped about fivefold as the pH was changed from the optimum pH (5.5-6.5), while the K_m remained virtually unchanged. The activity of HATS was completely inhibited by 15 millimolar nitrate and was less sensitive to chloride. LATS was inhibited by chloride and showed some inhibition by nitrate. It was concluded that [³⁶Cl]chlorate can be used as an analog for nitrate uptake studies only in a limited low concentration range where HATS is the main route for chlorate influx.     

过量表达番茄类囊体膜抗坏血酸过氧化物酶基因（StAPX）提高了烟草种苗的抗氧化能力

为了探讨叶绿体类囊体膜抗坏血酸过氧化物酶（tAPX）与其抗氧化性的关系,从番茄叶片中分离了叶绿体类囊体膜抗坏血酸过氧化物酶基因（StA跚并转入到烟草中。以野生型（WT）、转正义StAPX烟草株系T3-3和T3-6为试材,测定了外源过氧化氢诱导的氧化胁迫条件下APX酶活性、过氧化氢酶（CAT）活性、过氧化氢（H2O2）含量、叶绿素荧光参数及叶绿素含量等。Northern杂交显示StAPX因的表达受外源H2O2氧化胁迫的诱导。氧化胁迫下转基因烟草的APX酶活性和清除H2O2的能力都显著高于野生型,并且转基因烟草比野生型具有更高的PSII最大光化学效率及叶绿素含量。结果表明,．刚尸舶勺过量表达有助于提高外源H2O2诱导的转基因烟草的抗氧化能力。     

Control of Nitrate Uptake by Phloem-Translocated Glutamine in Zea mays L. Seedlings

Abstract: The putative role of glutamine, exported from leaves to roots, as a negative feedback signal for nitrate uptake was investigated in Zea mays L. seedlings. Glutamine (Gln) was supplied by immersion of the tip-cut leaves in a concentrated solution. Nitrate (NO₃⁻) uptake was measured by its depletion in amino acid-free medium. The treatment with Gln resulted in a strong inhibition of nitrate uptake rate, accompanied by a significant enrichment of amino compounds in root tissue. The effect of N-availability on NO₃⁻ uptake was determined in split-root cultures. The plants were subjected to complete or localized N supply. Inducible NO₃⁻ uptake systems were also induced in N-deprived roots when the opposite side of the root system was supplied with KNO₃. The inhibitory effect of Gln was unaffected by localized N supply on one side of the split-root. The potential role of Gln in the shoot-to-root control of NO₃⁻ uptake is discussed.     

Inhibition of NOminusb3 Uptake by Various Phloem-Translocated Amino Acids in Soybean Seedlings

We have examined the effect of different amino acids on NO     

Transport Interactions between Paraquat and Polyamines in Roots of Intact Maize Seedlings

Interactions between absorption of paraquat and the polyamines putrescine, cadaverine, and spermine in roots of intact maize (Zea mays L. cv 3377 Pioneer) seedlings were examined. Concentration-dependent kinetics for paraquat and putrescine influx were similar and both kinetic curves could be resolved into a linear and a saturable component. The linear component was previously shown to represent cell wall/membrane binding. The saturable components for paraquat and putrescine uptake, which represent influx across the plasmalemma, had K_m values of 98 and 120 micromolar, respectively, and V_max values of 445 and 456 nanomoles per gram fresh weight per hour, respectively. Lineweaver-Burk transformation of the saturable component of paraquat influx in the presence of varying concentrations of putrescine indicated that the diamine competitively inhibited the saturable component of paraquat uptake. Reciprocal experiments similarly demonstrated that paraquat competitively inhibited the saturable component of putrescine uptake. Competitive inhibition of both paraquat and putrescine influx could also be demonstrated with the diamine cadaverine, which has a charge distribution similar to that of paraquat and putrescine. In contrast, the larger, tetravalent polyamine spermine appeared to noncompetitively inhibit the influx of paraquat and putrescine. These results strongly suggest that paraquat enters maize root cells via a carrier system that normally functions in the transport of diamines with a charge distribution similar to that of paraquat.     

Lysine Uptake in Cultured Trypanosoma cruzi: Interactions of Competitive Inhibitors*

SYNOPSIS Three sites are involved in lysine transport in Trypanosoma cruzi, as inferred from interactions of inhibitory neutral amino acids. Phenylalanine and tyrosine inhibit one site; proline and the straight-chain amino acids, alanine, methionine and cysteine inhibit another; and glycine and the branched-chain valine, leucine, and isoleucine inhibit a 3rd. The curved rather than straight line obtained with a Lineweaver-Burk plot of uptake rates presumably results from the functioning of qualitatively different transport sites. Blocking every site but one results in the linear double-reciprocal plot characteristic of adsorption kinetics. The partial competitiveness of most of the inhibitions appears to denote qualitatively different sites for lysine transport and the reaction of the inhibitor with more than one site. Since most of the inhibitions were partially competitive, the specificities of the 3 lysine transport sites must overlap considerably.     

The Influence of Arginine-specific Reagents on Nitrate Uptake by Corn Seedlings

The effects of three arginine-specific reagents on uptake were studied using corn seedlings (Zea mays L., GoldenCross Bantam). In the presence of borate, 0.25 mM 2, 3-butanedione(BD) and 1.0 mM 1, 2-cyclohexanedione (CHD) inhibited uptake by 76% and 68%, respectively, compared tothe controls. However, in the absence of borate, only 18% and38% inhibition was observed for 0.25 mM BD and 1.0 mM CHD, respectively.Similarly, 0.5 mM phenylglyoxal (PGO) resulted in 75% inhibition.The degree of inhibition of nitrate uptake exhibited a concentration-dependencewith respect to the reagents. Corn seedlings are 2- or 3-foldmore sensitive to BD than to PGO and CHD, respectively, presumablydue to the unfavourable steric effects of the benzal ring. Uptakeof was partially restored after removal of BD, CHD, and PGO from the uptake medium. No significant differenceswere observed for the ATPase and plasma membrane-associatedvanadate-sensitive H⁺-ATPase or K⁺-stimulated ATPase activityin homogenates and microsomal fractions prepared from corn seedlingswhich had been incubated for 2 h in the presence or absenceof 0.5 mM BD or 1.0 mM PGO. This suggests that inhibition ofnitrate uptake by the arginine-specific reagents was not causedby the indirect effect of their binding and inhibiting H⁺-ATPase.The fact that the arginine-specific reagents strongly inhibit uptake indicates that the transport system has arginine residues at or near the activesite. Key words: Arginine-specific reagents, borate effect, nitrate uptake, reversibility     

Copper Uptake by Ryegrass Seedlings; Contribution of Cell Wall Adsorption

Net copper uptake by cellulose discs, isolated root cell walls,and by live and dead roots of whole ryegrass seedlings, werestudied using ⁶⁴Cu as a tracer. Uptake by cellulose discs stoppedafter around 10 h while uptake by isolated root cell walls continuedfor up to 50 h. An initial fast phase of uptake consisting predominantlyof cell wall adsorption was similar in live and dead tissuefor up to 19 h. A slower phase of uptake continued for up to50 h, greater in live than in dead tissue, the slower phaseof uptake in live tissue consisting of both a living and a deadcomponent. Based on these results, an alternative to the desorptionmethod for estimating the apoplastic contribution to total copperuptake is presented. Time-course studies with seedlings givena variety of growing solution/uptake solution regimes, and therelationship between copper uptake and external copper concentration,for short (4.8 h) and long (42.4 h) term uptakes, suggest thatdiffering contributions of cell wall adsorption and symplasmicabsorption may be responsible for differing effects of externalcopper concentration on uptake being expressed by the same tissue.Water flux had little effect on total uptake of copper althougha possible effect on absorption could not be ruled out. Key words: Copper uptake, cell wall adsorption, ryegrass seedlings