Transport and redistribution of selenium in the bean plant (Phaseolus vulgaris)

After incubation for 3 h with (⁷⁵Se) selenate, the selenium distribution in the bean plant (Phaseolus vulgaris L. cv. Contender) through a 29-day period showed an uneven distribution: roots and trifoliate leaves were richer in ⁷⁵Se than stem and primary leaves. The high selenium concentration of roots resulted from the retention of selenate by the root cells: at the end of the 29-day period about 60° of the radioactivity was always ethanol-soluble, and when analysed by paper chromatography, proved to be selenate. By contrast, much of the radioactivity of the leaves was ethanol-insoluble, ⁷⁵Se being quickly captured in metabolic processes which immobilize it. During plant development, a portion of the total selenium remains mobile and is continually mobilized to the younger organs which display a rapid growth rate. This delivery results from a progressive liberation of selenate retained by mature organs, especially the roots, and from turnover in older leaf tissues, especially the trifoliate leaves.     

The brain selenoproteome: priorities in the hierarchy and different levels of selenium homeostasis in the brain of selenium-deficient rats

The application of radionuclides for the localization of essential trace elements in vivo and the characterization of their binding proteins is a story of intermittently made improvements of the techniques used for their detection. In this study we present the use of neutron activation analysis and different autoradiographic imaging methods including real-time digital autoradiography to reveal new insights in the hierarchy of selenium homeostasis. Selenoproteins containing the essential trace element selenium play important roles in the CNS. Although the CNS does not show the highest selenium concentration in the case of selenium-sufficient supply in comparison with other organs, it shows a high priority for selenium uptake and retention in the case of dietary selenium deficiency. To characterize the hierarchy of selenium supply in the brain, in vivo radiotracer labeling with ⁷⁵Se in rats with different selenium status was combined with autoradiographic detection of ⁷⁵Se in brain tissue sections and ⁷⁵Se-labeled selenoproteins after protein separation by two-dimensional gel electrophoresis. This study demonstrates significant differences in the uptake of ⁷⁵Se into the brain of rats with different selenium status. A brain region-specific uptake pattern of the radiotracer ⁷⁵Se in selenium-deficient rats could be revealed and the CSF was identified as a key part of the brain selenium homeostasis.     

Selenium uptake by Butyrivibrio fibrisolvens and Bacteroides ruminicola

Abstract The uptake and incorporation of ⁷⁵[Se]selenite by Butyrivibrio fibrisolvens and Bacteroides ruminicola were by constitutive systems. Rates of uptake were higher in chemostat culture than in batch culture and there may be some inducible component. Uptake of [⁷⁵Se]selenite was distinct from sulphate or selenate transport, since sulphate and selenate did not inhibit selenite uptake, nor could sulphate or selenate uptake be demonstrated in these organisms. Selenite uptake in B. fibrisolvens had and apparent K _m of 1.74 mM and a V _max of 109 ng Se · min⁻¹· (mg protein)⁻¹. An apparent K _m of 1.76 mM and V _max of 1.5 μg Se · min⁻¹· (mg protein)⁻¹ was obtained for B. ruminicola . [⁷⁵Se]Selenite uptake by both organisms was partially sensitive to inhibition by 2,4-DNP. Uptake by B. fibrisolvens was also partially inhibited by azide and arsenate and in B. ruminicola it was partially inhibited by fluoride. CCCP, CPZ, DCCD or quinine did not inhibit uptake in either B. fibrisolvens or B. ruminicola . Selenite transport by both organisms was sensitive to IAA and NEM and was strongly inhibited by sulphite and nitrite. [⁷⁵Se]Selenite was converted to selenocystine, selenohomocystine and selenomethionine by B. fibrisolvens. B. ruminicola did not incorporate [⁷⁵Se]selenite into organic compounds, but did reduce it to red elemental selenium.     

Some factors regulating auxin translocation in intact bean seedlings

Several factors which influence the translocation patterns of stem-injected indoleacetic acid, 2,4-dichlorophenoxyacetic acid, and 2,4,5-trichlorophenoxyacetic acid in bean seedlings (Phaseolus vulgaris L. cv. Stringless Greenpod) were characterized. The acropetal translocation of auxin from the site of injection is markedly sensitive to concentration in the range of 1.0 to 5.0 micrograms per plant. The antiauxin p-chlorophenoxyisobutyric acid enhanced translocation of 2,4,5-trichlorophenoxyacetic acid to the growing shoots and primary leaves. Translocation to the roots was unaffected by p-chlorophenoxyisobutyric acid while leaching of 2,4,5-trichlorophenoxyacetic acid into the nutrient solution was enhanced slightly. Steam girdling experiments revealed that translocation to the primary leaves was in the xylem. The protein synthesis inhibitor, cycloheximide, inhibited accumulation of 2,4,5-trichlorophenoxyacetic acid in young shoots, epicotyls, and roots and enhanced accumulation in the primary leaves. The relative exchangeability of auxin between xylem and phloem is discussed in terms of regulation of auxin movements in intact bean seedlings.     

Pathway and regulation of phosphate translocation to the pods of soybean explants

We investigated the degree to which developing fruit compete directly with leaves for mineral nutrients, e.g. phosphate coming up from the roots. When soybean ( Glycine max (L.) Merrill cv. Anoka) explants cut at mid-late podfill were given a 15-min pulse of ³²Pi via the cut stem and then transferred to distilled water, 75% of the ³²P accumulated in the leaves and 21% in stem and petiole during the first hour. The amount of ³²P entering the seeds was low (1%) initially, but thereafter increased to 30% in 48 h. An accumulation of ³²P in the seed coats preceded its entry into the embryos. Disruption (with hot steam) of the phloem between the leaf and the pods after pulse labelling indicated that more than 80% of the ³²Pi pulse moved to the leaf before redistribution to the pods. Increasing "sink" size by adjusting the pod load from 1 to 2–3 did not increase the ³²P accumulated by the pods proportionally. Conversely, excision of the seeds after pulse labelling did not prevent translocation of ³²P out of the leaves. These results suggest that the rate of transport of phosphate to the pods at mid-late podfill is controlled primarily by factors in the leaves. The results are consistent with the observation that the relative size of the sink (pod load) does not regulate leaf senescence.     

Long-term effects of low levels of SO2 on bean plants (Phaseolus vulgaris). II. Immission-response effects on biomass production: quantity and quality

Bean plants ( Phaseolus vulgaris L. cv. Processer) were grown in water culture with separate air supply to roots for four to five weeks at five levels of SO₂ ranging from 10 μg m⁻³ to 950 μg m⁻³. At harvest the plant material was divided into six fractions: root, stem, fruit and leaves of three age groups.
Plants were mainly affected at and above approx. 250 μg m⁻³ SO₂. Fresh weight was reduced in mature and old leaves, and roots and fruit. Dry weight was also reduced in mature and old leaves, and roots and stem. A reduction was found in chlorophyll a and chlorophyll b in mature and old leaves, and also starch was reduced in the leaves. Sulfur content of leaves and fruit increased with exposure time and concentration, while Br, Ca, Cl, K, Mn, P and Zn increased at the highest SO₂ level only. Total (but not specific) peroxidase activity increased in all aerial fractions, i.e. soluble protein increased just like peroxidase activity. Seventeen studied amino acids all increased on the average by 38% in mature bean pods.
The observed effects may be parts of a reaction for survival and propagation of the plant, as fruit quality was not affected, indeed, it sometimes improved slightly. The latter observation is of commercial interest.     

Alteration of N nutrition in Myrica gale induces changes in nodule growth, nodule activity and amino acid composition

Changes in nodule growth and activity and in the concentrations of soluble N compounds in nodules, leaves and xylem sap under conditions of altered N nutrition in the actinorhizal plant Myrica gale L. are reported. Altering the N nutrition of symbiotic plants may alter the internal regulation of combined N which in turn may regulate nodule growth and activity. Flushing nodules daily with 100% O₂ caused a decline in amide concentration and an increase in nodule growth although plants had recovered some nitrogenase activity within 4 h of exposure to O₂. Samples of nodules, leaves and xylem sap were derivatized and amino acids identified and quantified using either reverse phase high performance liquid chromatography or gas chromatography-mass spectrometry in single ion monitoring mode. The ratio of asparagine in the nodules to that in the xylem was much higher in plants fed N (6.7 for NH⁺₄-fed and 8.3 for NO⁻₃-fed plants) than for N₂-fixing plants (2.5). Significant amounts of ¹⁵N added as ¹⁵NH⁺₄ or ¹⁵NO⁻₃ accumulated in nodules following accumulation in the shoot which is consistent with the translocation of N to the nodules via the phloem. The uptake of ¹⁵NH⁺₄ led to the synthesis and subsequent translocation of glutamine in the xylem sap. These results are discussed in terms of the feedback mechanisms that may regulate nitrogen fixation in Myrica root nodules.     

Distribution and metabolism of root-applied cytokinins in Pisum sativum

When N ⁶ [8–¹⁴C] furfuryladenine was applied to the intact root system of Pisum sativum L. cv. Meteor seedlings it was almost completely metabolised to other compounds within 24 h. Of the total activity recovered from the plants 94.5% was retained in the root system itself. ¹⁴C was recovered in a number of ethanol-soluble compounds and in ribonucleic acid, deoxyribonucleic acid and protein fractions of roots, stems, leaves and axillary buds. In rapidly growing axillary buds released from apical dominance by removal of the shoot apex the combined nucleic acid fractions accounted for 63.3% of the total ¹⁴C recovered from these organs. Xylem exudate collected from decapitated plants 0 to 12 h after supplying N ⁵[8–¹⁴C]furfuryladenine to the roots consistently contained a single major ¹⁴C-labelled compound which, in three different solvent systems, had the same Rf values as a major endogenous cytokinin isolated from the xylem of unlabelled plants. The content of N ⁶ [8–¹⁴C] furfuryladenine itself in the xylem exudate was always low and in some experiments it could not be detected.
It is suggested that part of the label from N ⁶ [8- ¹⁴CJfurfuryladenine taken up by the intact root system may have become incorporated in an endogenous cylokinin before export to the shoot.     

Selenium supplementation of infant formula: uptake and retention of various forms of selenium in suckling rats

Formula-fed infants often have lower serum selenium levels than breast-fed infants. Although no deleterious effects have been correlated to this finding, supplementation of formula with selenium is considered. In this study, we investigated the uptake and retention by suckling rat pups of ⁷⁵Se from selenite, selenate, and selenomethionine added to infant formula. The molecular distribution of ⁷⁵Se in liver, kidney, intestine, and plasma was followed by gel-filtration chromatography on Superose 12. ⁷⁵Se-uptake was most rapid from selenomethionine (70% at 1 hr), followed by selenate (51%) and selenite (29%). This difference was explained by a higher retention of ⁷⁵Se in the stomach and small intestinal wall of pups given selenite supplement. Plasma distribution of ⁷⁵Se as studied by gel filtration was also different, with a higher proportion of ⁷⁵Se from selenomethionine being protein-bound than from selenite or selenate. Similarly, a larger proportion of ⁷⁵Se from selenomethionine became protein-bound in the liver than from selenite or selenate. In conclusion, although whole body retention after 24–48 hr was similar, the metabolic fate of selenium varies considerably with the form of selenium added to formula. Further studies are needed to study the long-term consequences of selenium accumulated in different body compartments.     

Distribution of [14C]-labeled photosynthates within intensively cultured Populus clones during the establishment year

Seasonal patterns of [¹⁴C]-labeled photosynthate distribution within two intensively cultured Populus clones with contrasting phenology ( P. tristis × P. balsamifera cv. 'Tristis no. 1'; P. × euramericana cv. Eugenei) were investigated during the establishment year. During active shoot elongation upper mature leaves exported ¹⁴C acropetally to the expanding leaves and elongating internodes, and basipetally to the stem. Little ¹⁴C was exported to lower mature leaves or lateral branches. At budset the ¹⁴C export pattern shifted dramatically in the basipetal direction, i.e., to the lower stem, hardwood cutting, and roots. The timing of budset was the primary factor determining the differences between the clones, except that in all cases Tristis exported more ¹⁴C to the roots than Eugenei. After budset lower mature leaves had a similar export pattern to upper leaves, but the quantity of ¹⁴C exported to the roots was slightly higher. The results confirm the importance of autumn foliage for root growth in poplar. Clonal differences in seasonal patterns of photosynthate distribution offer potential for the poplar breeder seeking to match a clone's growth pattern with the specific growing season of the site.     

Lymantria dispar herbivory induces rapid changes in carbon transport and partitioning in Populus nigra

We tested for rapid changes in photosynthate transport and partitioning in response to Lymantria dispar (L.) (Lepidoptera: Lymantriidae) (gypsy moth) herbivory in Populus nigra L. (Salicaceae). Transport and partitioning of [¹¹C]-photosynthate from young mature leaves were measured in vivo before and 18 h after leaf chewing by gypsy moth larvae, which were caged on three older leaves. Following herbivory, there was an increase in export speed of recently fixed carbon from younger mature leaves. The increased export speed was due to a quicker transit time of ¹¹C through the leaf, rather than a change in transport speed through the phloem. Additionally, basipetal partitioning of [¹¹C]-photosynthate was increased following herbivory. Neither of these changes was observed in control plants. This enhancement of export occurs even though herbivores are well known to induce increases in carbon allocation to secondary metabolites within leaves. Our results demonstrate that the use of non-destructive imaging of ¹¹C tracer is a powerful tool for examining plant responses to herbivory. Although the mechanisms underlying the rapid increase in carbon flux to stems and roots remain to be elucidated, our results raise the possibility of a coordinated whole plant response to herbivory. Thus, even when the herbivore specializes on only one plant tissue type, a whole plant approach may be key to understanding how plants respond to herbivory.     

The effect of phytochrome and white light of high and low intensity on the uptake and distribution of 14C-labelled kinetin in radish seedlings (Raphanus sativus)

The influence of light intensity and phytochrome on the uptake of ¹⁴C-kinetin (6-furfurylamino-[8- ¹⁴C]-purine) by the plant and the translocation of the phytochrome between the roots, the hypocotyl and the cotyledons were investigated with radish seedlings ( Raphanus sativus L. cv. Saxa Treib) grown in the dark or under white light of high (20,000 lux, 90 W m⁻²) or low intensity (2,000 lux, 14 W m⁻²). The highest uptake of labelled kinetin was found in plants grown in continuous darkness. The total uptake of kinetin was decreased by strong light and to a finally higher extent by weak light. Under white light most of the kinetin accumulated in the root, whereas in the dark an enhanced translocation of the phytohormone into the cotyledons was observed. In etiolated radish seedlings, light acting on phytochrome (daily 5 min red or far red light pulses) decreased the translocation of ¹⁴C-kinetin into the cotyledons. Under far red light a pronounced uptake of the phytohormone into the roots was found. The data are discussed with regard to the interaction of light and phytohormones on plant development.     

Sodium partitioning within the shoot of soybean

Uptake and partitioning of Na⁺ and Cl⁻ in plants of soybean ( Glycine max L. Merr. cv. Hodgson) exposed to moderate NaCl concentrations were studied over an 8-day period. Plants showed marked retention of Na⁺ in the stems and low transport to laminae of young leaves. The xylem sap ascending the main axis was progressively depleted in Na⁺. The oldest leaf greatly contributed to Na⁺ depletion of the sap flowing to younger leaves. These results in combination with estimates of phloem recirculation indicated that Na⁺ accumulation in the young leaf was prevented both by depletion of Na⁺ from the xylem stream, and by a high recirculation of Na⁺ via the phloem. However, this protection of young leaves was effective only for very mild salinity treatment.     

Translocation of photoassimilate from leaves of two popyploid genotypes of tall fescue differing in photosynthetic rates

The photosynthetic rate of a decaploid genotype (1-16-2) of tall fescue ( Festuca arundinacea Schreb.) is about twice that of a common hexaploid genotype (V6-802) (Plant Physiol. 72: 16–21, 1983). Translocation of photosynthate out of the leaves is a possible means of regulating carbon assimilation. To evaluate this possibility, we have examined a) translocation velocity, b) time course of translocation from leaves, c) photoassimilate partitioning pattern into whole plants in pulse and chase experiments, and d) interveinal distances between two ploidy genotypes. Most of the ¹⁴C accumulated in sucrose, and the labelled carbon moved down the leaf blades at similar velocities (6 to 10 cm h⁻¹) in both genotypes. Recent ¹⁴C assimilate was rapidly translocated from the fed area of the leaf blade. For example, the decaploid and the common hexaploid had translocated 40 and 26% of the ¹⁴C, respectively, at 6 h, and 79 and 49% of the ¹⁴C, respectively, at 24 h. Partitioning of ¹⁴C among plant organs was considerably different between the genotypes after a 24 h chase. For example, out of the total ¹⁴C recovered from the whole plant, the decaploid had retained 40% in the labelled leaf with 10, 33 and 29% in other leaves, stem bases and roots, respectively; whereas the hexaploid had retained 91% in the labelled leaf with 4, 3 and 2% in other leaves, stem bases and roots, respectively. However, the higher rate of translocation was correlated with greater interveinal distances in the decaploid genotype. These results suggested that the higher translocation percentage in the decaploid than the hexaploid genotype was due to greater sink activity.     

Control of phloem unloading by action potentials in Mimosa

In the sensitive plant, Mimosa pudica , action potentials arise when the leaves are touched and they trigger a sudden decrease in turgor of the pulvinar motor cells, which causes the leaf to close. These potentials may travel through the phloem and they appear to influence pulvinar phloem unloading after stimulation. Mature leaves were exposed to ¹⁴CO₂ and phloem translocation was observed by autoradiography. In unstimulated pulvini, labeled photoassimilates were restricted to the phloem. However, after stimulation, the ¹⁴C-label appeared to be concentrated in the extensor region of the motor cortex. Since stimulation elicits an action potential, it is suggested that it also triggers phloem unloading of sucrose in the pulvini.     

On the mechanism of rejuvenation of ageing detached bean leaves by low-concentration stressors

The effect of low concentrations of some stress-inducing compounds of different toxicity and chemical nature, such as Cd and Pb salts or DCMU, was investigated on the senescence of chloroplasts in detached primary leaves of bean ( Phaseolus vulgaris L . ). After 1 week of senescence followed by root development from the petiole, these agents stimulated chlorophyll accumulation and photosynthetic activity (¹⁴CO₂ fixation) as compared to the control, thus inducing rejuvenation. Low-concentration stressors increased the level of active cytokinins in roots and leaves during the treatment, as monitored by the Amaranthus betacyanin bioassay and high-pressure liquid chromatography. The lithium ion, an inhibitor of the PIP₂-IP₃/DAG signal transduction pathway, abolished the stimulating effect of stressors, both in roots (retarding cytokinin synthesis) and consequently also in leaves (reducing cytokinin-dependent chlorophyll accumulation). This suggests the involvement of the PIP₂-IP₃/DAG signal transduction pathway in generation of these consecutive organ-specific responses.     

The directional control of sucrose and asparagine transport in lupin by abscisic acid

Injections of exogenous abscisic acid into the primary flowerhead of Lupinus luteus cv. Weiko III reduced the movement of ¹⁴C-sucrose into the flowerhead from the uppermost leaves. Sucrose transported from below the lateral branches subtending the flowerhead, was diverted into the lateral branches by injection of the exogenous abscisic acid into the flowerhead. ¹⁴C-sucrose was also diverted from a lateral branch injected with exogenous abscisic acid to all other parts of the plant, particularly the main stem and leaves, and the roots. Transport of ¹⁴C-asparagine administered at the cotyledonary node was directed from the flowerhead into the subtending lateral branches by injection of abscisic acid into the flowerhead. Transport of both ¹⁴C-sucrose and ¹⁴C-asparagine into the flowerhead was reduced at least three fold at physiological levels of abscisic acid. No significant correlation was found between the amount of ¹⁴C-asparagine entering a sink and the dry weight of the tissues of that sink. It is concluded that distribution of ¹⁴C-sucrose and ¹⁴C-asparagine between the flowerheads and lateral branches of L. luteus is actively and dynamically controlled and that abscisic acid levels play a significant part in that control. It is suggested that the relative levels of endogenous abscisic acid in plant organs could serve as an important factor in the directional control of assimilate transport in plants.     

Synthesis, transport and accumulation of quinolizidine alkaloids in Lupinus albus L. and L. angustifolius L

Each of the principal quinolizidine alkaloids (QA) found in both xylem and phloem exudates together with extracts from all component organs collected from bitter (cv. Lupini) and sweet (cv. Ultra) cultivars of Lupinus albus L. were quantified by gas chromatographic analyses throughout reproductive development. In addition to establishing the major translocated QA species estimates for fluxes of QA to developing fruits based on their sap composition and water economy showed that around half of the QA that accumulated in fruit tissues was due to synthesis in situ and half to translocation principally by phloem. Detailed analyses of QA in transport fluids and component organs were extended to reciprocal homo- and hetero-grafts using bitter (cv. Fest) and sweet (cv. Danja) cultivars of L. angustifolius L. These data confirmed that the majority of QA were synthesized in shoot tissues. In both lupin species feeding and analysis of deuterated QA (lupanine and 13-hydroxylupanine) were used as tracers to demonstrate direct redistribution of alkaloids by translocation from mature leaves in phloem.     

Transport of zinc and manganese to developing wheat grains

An understanding of the transport pathway used by Zn and Mn to enter developing grains may allow measures to increase the Zn and Mn content of wheat grain grown on Zn/Mn deficient soils. For this reason, transport of Zn and Mn into developing grains of wheat ( Triticum aestivum L. cv. Aroona) was investigated. Detached ears (18–22 days post-anthesis) were cultured for 48 h in a solution containing 185 kBq of ⁶⁵Zn and 185 kBq of ⁵⁴Mn. Transport of ⁶⁵Zn to the grain was unaffected by removal of glumes but was slightly reduced after the lemma was removed. Heat girdling the peduncle slightly reduced the amount of ⁶⁵Zn transported to the grain, whilst heat girdling the rachilla reduced transport of ⁶⁵Zn to the grain to a greater degree, suggesting phloem transport to the rachilla. The transport inhibitor CCCP (carbonyl cyanide m -chlorophenyl hydrazone) blocked ⁶⁵Zn transport to grain but not to lemma and glumes. Removing glumes and lemma and heat girdling the peduncle did not affect transport of ⁵⁴Mn, but transport was slightly affected by heat girdling the rachilla, indicating xylem transport. CCCP blocked transport of ⁵⁴Mn into the grain but not to lemma and glumes. It was concluded that xylem-to-phloem transfer of Zn occurs in the rachis and to a lesser extent in peduncle and lemma. The results suggest that the lemma may be an important site for phloem loading when the concentration of Zn within the xylem is high. The data also suggest that Mn was predominantly translocated to the spikelets in the xylem, but that transport to the grain was dependent upon membrane transport before entering the grain. Phloem loading of Mn into the grain vascular system may have occurred at the site of xylem discontinuity in the floral axis.     

Localisation of translocated 14C in roots and root exudates of field-grown maize

The roots of a mature, field-grown maize plant are dimorphic: the primary root and those from the oldest nodes are bare with a heavily lignified cortex arid sloughed epidermis; those from younger nodes, except for a bare elongation zone, have an intact epidermis surrounded by a persistent soil sheath. Sheathed roots consistently have more layers of cortical cells, but the ratio of volumes of cortex to stele (ca 4) and the cross-sectional area of phloem (ca³× 10⁻² mm²) are similar in each type. Assimilated carbon (from ¹⁴ C applied to a small area of one leaf) was translocated to all roots and actively metabolized in cortex and stele of both types. After 1 to 2 days the proportion of ¹⁴C exuded from a given length of mature root into its soil sheath, or into the adjacent unattached soil in the case of bare roots, was the same (5%) in both root types when compared with the ethanol-soluble ¹⁴C in the tissues of this length. Up to 75% of the ethanol-soluble label in the roots was in a cationic fraction (amino acids and unidentified compounds), ca 1% was in an anionic fraction and the remainder was in a neutral fraction (sugars). Approximately equal amounts of soluble ¹⁴C were found in the stele, cortex and laterals.