Vacuolar Lucifer Yellow Uptake in Plants: Endocytosis or Anion Transport; A Critical Opinion

Because of its membrane-impermeant-properties Lucifer Yellow-CH (LY) is regarded by animal cell biologists as an ideal tracer for fluid-phase endocytosis. When presented to plant cells or protoplasts this fluoroprobe accumulates in the vacuole. On the other hand there are many cases where LY does not enter the vacuole when loaded into the plant cytosol. These, superficially divergent, results have previously been explained in terms of endocytosis whereby access to the vacuole is considered to occur through vesicle transport. This interpretation has now been challenged in three recent papers where the benzoic acid derivative, probenecid, has been shown to prevent vacuolar LY accumulation in plants. Since probenecid is a well-known inhibitor of organic anion transport in animal cells it has been argued that anion carriers capable of transporting LY might also exist at the plasma membrane and tonoplast of plant cells. Unfortunately probenecid has rarely, if ever, been used in plant transport studies. The fact that it is a weak acid, whose inhibitory effects are observed at concentrations of around 1 mM suggests that caution should prevail when interpreting results obtained with probenecid. The purpose of this article is therefore to highlight the current controversy surrounding LY uptake by plants and to critically evaluate the recent probenecid data.     

Direct and indirect interactions between ants (Pheidole megacephala), scales (Coccus viridis) and plants (Pluchea indica)

Summary This study investigated direct and indirect interactions between the ant, Pheidole megacephala (Fabr.), the green scale, Coccus viridis (Green), and the scale's host plant, Pluchea indica (L.). To examine the influence of ants on scales and host plants, scale population densities, scale mortality rates, and plant performance were studied on control host plants with ants and host plants from which ants had been removed. Plants with ants present had significantly greater scale population densities and scale reproductive rates than did plants without ants. Scale mortality from both parasitism and other causes was increased on plants without ants relative to plants with ants. Predator introduction experiments showed that P. megacephala removes predatory coccinellid larvae, even when they are covered with a protective coating. Host plants from which ants had been removed had significantly higher degrees of honeydew accumulation, which resulted in greater colonization by sooty mold and greater rates of leaf death and abscission. Ants also removed herbivorous lepidopteran larvae from plants. Results are discussed in terms of the potential of P. megacephala to exert direct and indirect positive effects on scale populations and an indirect positive effect on Pluchea indica.     

Effect of wounding on ethylene biosynthesis and senescence of detached spinach leaves

Parameters of senescence and ethylene biosynthesis pathway were screened simultaneously in detached spinach leaves and leaf discs. Senescence was enhanced by application of 1-aminocyclopropane-1-carboxylic acid (ACC) and was retarded by amino-ethoxyvinylglycine (AVG). Evidence is presented showing that the bursts of both wound- and climacteric-like ethylene promoted senescence of detached leaves and leaf discs. This ethylene-enhanced leaf senescence was dependent on: (a) ethylene production rates in the tissue; (b) the degree of wounding. Wounding resulted in elevated levels of 1-(malonylamino)cyclopropane-1-carboxylic acid (MACC), which declined in advanced stages of senescence. The results suggest that wounding might be regarded as one of the primary events in the induction of the senescence syndrome in detached leaves and leaf discs, while ethylene is implicated as a regulator of the rate of the process.     

The accumulation and metabolism of plant growth regulators during organogenesis in cultures of thin cell layers of Nicotiana tabacum

The accumulation and metabolism of exogenously applied and endogenously produced auxins and cytokinins were studied in relation to organogenesis in thin cell layers of Nicotiana tabacum L. It was shown that, in order to obtain maximal flower bud formation, both exogenous auxin and cytokinin needed to be present during the first 4 days of culture (to the formation of a subepidermal meristematic zone) whereas cytokinins needed to be present for at least 4 days more (until formation of organogenic centres). Explants taken from floral branches have higher endogenous indole-3-acetic acid (IAA) levels compared with explants from the basal part of the stem which form only vegetative buds. This might be related to a different IAA metabolism in these two types of explants as was shown by the different accumulation of exogenously applied IAA. Both 'floral' and 'vegetative' cells layers contained comparable amounts of zeatin riboside (ZR) as their major cytokinin. Free bases, zeatin (Z) and dihydrozeatin [(diH)Z], given exogenously, were largely metabolised to their respective ribosides. The observation that Z was less effective than (diH)Z in the induction of flower buds could be related to (diH)ZR apparently not being a substrate for cytokinin oxidase.     

Transport of Rb and Sr to the ear in mature,excised shoots of wheat: Effects of temperature and stem length on Rb removal from the xylem

Wheat (Triticum aestivum L. cv. ‘Arina’) shoots grown in the field were excised post-anthesis and incubated in the laboratory for 72 h standing in 2 mM RbCl+2 mM SrCl₂. Strontium is a phloemimmobile, xylem-mobile element and indicates the distribution of the xylem sap in the plant. Rubidium is easily transported in the phloem and behaves similarly to the highly mobile K as far as the redistribution within the plant is concerned, although Rb cannot substitute physiologically or biochemically for K. The Sr contents in the ear were hardly affected by stem length or by steam-girdling (phloem-interruption). Rubidium on the other hand accumulated in the stem. A peduncle length of 5 cm was sufficient to decrease the Rb concentration in the xylem by more than 50% at 25°C. Only a minor quantity of Rb reached the ear after passing through 20 cm of stem without nodes and this transport was prevented by steam-girdling. A remarkable flux of Rb into the ear was observed in shoots with a vascular connection between the flag leaf lamina and the ear. Our results suggest that Sr was transported with the transpiration stream, while Rb was rapidly eliminated from the xylem and reached the ear via the phloem. The temperature optimum for the removal of Rb from the xylem was around 35°C. The nodes may further contribute, but are not prerequisites for this redistribution. The observed transfer processes could allow a solute specific transport via the xylem and phloem of maturing cereals and may be an important factor influencing the nutrient economy in the field.     

Late-Phase Accumulation of Inositol Phosphates Stimulated by Prostaglandins D2 and F2α in Neuroblastoma × Glioma Hybrid NG108-15 Cells

The accumulation of inositol phosphates (IPs) in response to prostaglandins (PGs) was studied in NG108-15 cells preincubated with myo-[3H]inositol. As a positive control, bradykinin caused accumulation of IPs transiently at an early phase (within 1 min) and continuously during a late phase (15-60 min) of incubation in the cells. PGD2 and PGF2 alpha did not significantly cause the accumulation of IPs at an early phase but significantly stimulated inositol bisphosphate (IP2) and inositol monophosphate (IP) formation at late phase of incubation. The maximum stimulation was obtained at greater than 10(-7) M concentrations of these PGs, the levels being three-and twofold for IP2 and IP1, respectively. 9 alpha, 11 beta-PGF2 has a slight effect but PGE2 and the metabolites of PGD2 and PGF2 alpha have no effect up to 10(-6)M. The effects of PGD2 and PGF2 alpha were not additive, but the effect of each PG was additive to that of bradykinin at a late phase of incubation. Inositol 1-monophosphate was mainly identified in the stimulation by 10(-5) M PGD2 and 10(-5) M PGF2 alpha, whereas both inositol 1-monophosphate and inositol 4-monophosphate were produced in the stimulation by 10(5) M bradykinin. Depletion of extracellular Ca2+ diminished the stimulatory effect of PGD2 and PGF2 alpha and late-phase effect of bradykinin, but simple Ca2+ influx into the cells by high K+, ionomycin, or A23187 failed to cause such late-phase effects. These results suggest that PGD2 and PGF2 alpha specifically stimulate hydrolysis of inositol phospholipids.     

Relative rates of shell dissolution and net sediment accumulation - a commentary: can shell beds form by the gradual accumulation of biogenic debris on the sea floor?

Rates of shell production rarely exceed 500 g CaCO₃.m^-2yr^-1 in clastic sediments. Loss of shell carbonate by dissolution greatly exceeds loss by bioerosion and abrasion in most habitats. Rates of shell dissolution in modern sediments, estimated from rates of organic carbon degradation or measured directly, usually exceed 1000 g CaCo₃.M^-2yr^-1. This taphonomic loss is concentrated at or just below the sediment-water interface in the taphonomically-active zone (TAZ). Consequently, except where rates of shell production are very high or rates of organic carbon degradation very low, shells cannot permanently accumulate on the sea floor. Preservation requires rapid burial, usually by physical 'event' processes, to slow down taphonomic loss. Only near the base of the TAZ does the long-term sedimentation rate become an effective mediator of shell preservation as sediment accumulation gradually removes buried shell material from the taphonomically-active zone.     

Control of photosynthesis by the carbohydrate level in leaves of the C4 plant Amaranthus edulis L.

Photosynthesis was studied in relation to the carbohydrate status in intact leaves of the C₄ plant Amaranthus edulis. The rate of leaf net CO₂ assimilation, stomatal conductance and intercellular partial pressure of CO₂ remained constant or showed little decline towards the end of an 8-h period of illumination in ambient air (340 bar CO₂, 21% O₂). When sucrose export from the leaf was inhibited by applying a 4-h cold-block treatment (1°C) to the petiole, the rate of photosynthesis rapidly decreased with time. After the removal of the cold block from the petiole, further reduction in photosynthetic rate occurred, and there was no recovery in the subsequent light period. Although stomatal conductance declined with time, intercellular CO₂ partial pressure remained relatively constant, indicating that the inhibition of photosynthesis was not primarily caused by changes in stomatal aperture. Analysis of the leaf carbohydrate status showed a five- to sixfold increase in the soluble sugar fraction (mainly sucrose) in comparison with the untreated controls, whereas the starch content was the same. Leaf osmotic potential increased significantly with the accumulation of soluble sugars upon petiole chilling, and leaf water potential became slightly more negative. After 14 h recovery in the dark, photosynthesis returned to its initial maximum value within 1 h of illumination, and this was associated with a decline in leaf carbohydrate levels overnight. These data show that, in Amaranthus edulis, depression in photosynthesis when translocation is impaired is closely related to the accumulation of soluble sugars (sucrose) in source leaves, indicating feedback control of C₄ photosynthesis. Possible mechanisms by which sucrose accumulation in the leaf may affect the rate of photosynthesis are discussed with regard to the leaf anatomy of C₄ plants.Abbreviations and symbols A net CO₂ assimilation rate - Ci intercellular CO₂ partial pressure - PEP phosphoenolpyruvate - RuBP ribulose-1,5-bisphosphate - water potential - osmotic pressure     

Metal-protein interactions in transport,accumulation, and excretion of metals

The binding of Cu(II), Zn(II), Ni(II), and Cd(II) to protein components in serum, placenta, kidney, and urine was investigated at physiological pH, using radioisotopes as tracers. All the four metals were bound to albumin and other macromolecules in serum. However, small amounts were also bound to low molecular weight components of the size 1500–10000 daltons. The nature of the Cu(II)-binding to α-fetoprotein suggests its important role as the Cu(II)-transporting protein in fetal life. Metal binding to placental components were studied using both rat placenta and isolated human trophoblast cells. Studies of metal binding targets in kidney resulted in the isolation of a 4000 daltons acidic polypeptide which binds Ni(II) and Cd(II) with K_app=1.1×10⁻⁵ and 2.3×10⁻⁵, respectively. Studies of metal binding substances in urine reveals the major amounts of these metals binding to substances of molecular weight 500–5000 daltons. Preliminary amino acid analysis suggests these components rich in acidic amino acids, similar to what has been found with kidney polypeptide. There may be a general role for such compounds in the handling of metals in the process of excretion.     

Postnatal accumulation of zinc by the rat hippocampus

Timm's staining material has been detected in the rat hippocampus as early as day 1 postnatally. However, staining was diffuse and widespread and light granulation was observed only in the mossy fiber layer. By day 6 postnatally most diffuse staining had disappeared and the characteristic pattern of granulation had intensified in the mossy fiber layer. Pronounced staining of the mossy fibers became apparent from day 6. Electron microscopic autoradiography indicated that⁶⁵Zn injected intraperitoneally into suckling pups became localized largely in the axons and axon terminals of the mossy fiber layer in the CA3 and CA4 regions of the Horn of Ammon. In vitro studies with hippocampal slices have demonstrated that zinc is accumulated by an active transport system, but the kinetic characteristics of this uptake do not appear to alter with age. Zinc located intracellularly in the hippocampus appears to be associated mainly with large molecular weight ligands, with more than 75% of newly acquired zinc being bound to substances having molecular weights greater than 70,000 Daltons.