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PubMed1997

Isolation and characterization of soluble boron complexes in higher plants. The mechanism of phloem mobility of boron.

Boron (B) polyol complexes have been isolated and characterized from the phloem sap of celery (Apium graveolens L.) and the extrafloral nectar of peach (Prunus persica L.). In celery the direct analysis of untreated phloem sap by matrix-assisted laser desorption-Fourier transform mass spectrometry, with verification by high-performance liquid chromagraphy and gas chromatography-mass spectrometry, revealed that B is present in the phloem as the mannitol-B-mannitol complex. Molecular modeling further predicted that this complex is present in the 3,4 3',4' bis-mannitol configuration. In the extrafloral nectar of peach, B was present as a mixture of sorbitol-B-sorbitol, fructose-B-fructose, or sorbitol-B-fructose. To our knowledge, these findings represent the first successful isolation and characterization of soluble B complexes from higher plants and provide a mechanistic explanation for the observed phloem B mobility in these species.

Biological Transport↗

PubMed1997

Molecular cloning of mannose-6-phosphate reductase and its developmental expression in celery.

Compared with other primary photosynthetic products (e.g. sucrose and starch), little is known about sugar alcohol metabolism, its regulation, and the manner in which it is integrated with other pathways. Mannose-6-phosphate reductase (M6PR) is a key enzyme that is involved in mannitol biosynthesis in celery (Apium graveolens L.). The M6PR gene was cloned from a leaf cDNA library, and clonal authenticity was established by assays of M6PR activity, western blots, and comparisons of the deduced amino acid sequence with a celery M6PR tryptic digestion product. Recombinant M6PR, purified from Escherichia coli, had specific activity, molecular mass, and kinetic characteristics indistinguishable from those of authentic celery M6PR. Sequence analyses showed M6PR to be a member of the aldo-keto reductase superfamily, which includes both animal and plant enzymes. The greatest sequence similarity was with aldose-6-phosphate reductase (EC 1.1.1.200), a key enzyme in sorbitol synthesis in Rosaceae. Developmental studies showed M6PR to be limited to green tissues and to be under tight transcriptional regulation during leaf initiation, expansion, and maturation. These data confirmed a close relationship between the development of photosynthetic capacity, mannitol synthesis, and M6PR activity.

Amino Acid Sequence↗

PubMed1997

Sugar Repression of Mannitol Dehydrogenase Activity in Celery Cells.

We present evidence that the activity of the mannitol-catabolizing enzyme mannitol dehydrogenase (MTD) is repressed by sugars in cultured celery (Apium graveolens L.) cells. Furthermore, this sugar repression appears to be mediated by hexokinases (HKs) in a manner comparable to the reported sugar repression of photosynthetic genes. Glucose (Glc)-grown cell cultures expressed little MTD activity during active growth, but underwent a marked increase in MTD activity, protein, and RNA upon Glc starvation. Replenishment of Glc in the medium resulted in decreased MTD activity, protein, and RNA within 12 h. Addition of mannoheptulose, a competitive inhibitor of HK, derepressed MTD activity in Glc-grown cultures. In contrast, the addition of the sugar analog 2-deoxyglucose, which is phosphorylated by HK but not further metabolized, repressed MTD activity in mannitol-grown cultures. Collectively, these data suggest that HK and sugar phosphorylation are involved in signaling MTD repression. In vivo repression of MTD activity by galactose (Gal), which is not a substrate of HK, appeared to be an exception to this hypothesis. Further analyses, however, showed that the products of Gal catabolism, Glc and fructose, rather than Gal itself, were correlated with MTD repression.

Journal Article↗

PubMed1997

Estimation of Polymer Rigidity in Cell Walls of Growing and Nongrowing Celery Collenchyma by Solid-State Nuclear Magnetic Resonance in Vivo.

When the growth of a plant cell ceases, its walls become more rigid and lose the capacity to extend. Nuclear magnetic resonance relaxation methods were used to determine the molecular mobility of cell wall polymers in growing and nongrowing live celery (Apium graveolens L.) collenchyma. To our knowledge, this is the first time this approach has been used in vivo. Decreased polymer mobility in nongrowing cell walls was detected through the 13C-nuclear magnetic resonance spectrum by decreases in the proton spin-spin relaxation time constant and in the intensity of a sub-spectrum corresponding to highly mobile pectins, which was obtained by a spectral editing technique based on cross-polarization rates. Flexible, highly methyl-esterified pectins decreased in relative quantity when growth ceased. A parallel increase in the net longitudinal orientation of cellulose microfibrils was detected in isolated cell walls by polarized Fourier-transformed infrared spectrometry.

Journal Article↗

PubMed1997

Subcellular localization of celery mannitol dehydrogenase. A cytosolic metabolic enzyme in nuclei.

Mannitol dehydrogenase (MTD) is the first enzyme in mannitol catabolism in celery (Apium graveolens L. var dulce [Mill] Pers. cv Florida 638). Mannitol is an important photoassimilate, as well as providing plants with resistance to salt and osmotic stress. Previous work has shown that expression of the celery Mtd gene is regulated by many factors, such as hexose sugars, salt and osmotic stress, and salicylic acid. Furthermore, MTD is present in cells of sink organs, phloem cells, and mannitol-grown suspension cultures. Immunogold localization and biochemical analyses presented here demonstrate that celery MTD is localized in the cytosol and nuclei. Although the cellular density of MTD varies among different cell types, densities of nuclear and cytosolic MTD in a given cell are approximately equal. Biochemical analyses of nuclear extracts from mannitol-grown cultured cells confirmed that the nuclear-localized MTD is enzymatically active. The function(s) of nuclear-localized MTD is unknown.

Cell Nucleus↗

PubMed2000

The sucrose transporter of celery. Identification and expression during salt stress.

In celery (Apium graveolens L.), long-distance transport of reduced carbon occurs both in the form of sucrose (Suc) and mannitol. The presence of mannitol has been related to the resistance of celery to salt stress. To investigate the transport events occurring during salt stress, we have cloned the H(+)/Suc transporter of celery AgSUT1 (A. graveolens Suc uptake transport 1) from a mature leaf cDNA library. The function of the encoded protein was confirmed by expression in yeast. AgSUT1 is a H(+)/Suc transporter with a high affinity for Suc (K(m) of 139 microM). Another closely related cDNA (AgSUT2) was also identified. AgSUT1 is mainly expressed in mature leaves and phloem of petioles, but also in sink organs such as roots. When celery plants were subjected to salt stress conditions (30 d watering with 300 mM NaCl) favoring mannitol accumulation (J.D. Everard, R. Gucci, S.C. Kann, J.A. Flore, W.H. Loescher [1994] Plant Physiol 106: 281-292), AgSUT1 expression was decreased in all organs, but markedly in roots. The results are discussed in relation to the physiology of celery.

Amino Acid Sequence↗

PubMed2000

NADPH supply and mannitol biosynthesis. Characterization, cloning, and regulation of the non-reversible glyceraldehyde-3-phosphate dehydrogenase in celery leaves.

Mannitol, a sugar alcohol, is a major primary photosynthetic product in celery (Apium graveolens L. cv Giant Pascal). We report here on purification, characterization, and cDNA cloning of cytosolic non-reversible glyceraldehyde-3-P dehydrogenase (nr-G3PDH, EC 1.2.1. 9), the apparent key contributor of the NADPH required for mannitol biosynthesis in celery leaves. As determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, purified nr-G3PDH showed a molecular mass of 53 kD. A 1,734-bp full-length cDNA clone (accession no. AF196292) encoding nr-G3PDH was identified using polymerase chain reaction and rapid amplification of cDNA ends techniques. The cDNA clone has an open reading frame of 1,491 bp encoding 496 amino acid residues with a calculated molecular weight of 53,172. K(m) values for the celery nr-G3PDH were low (6.8 microM for NADP(+) and 29 microM for D-glyceraldehyde-3-P). NADPH, 3-phosphoglycerate, and ATP were competitive inhibitors, and cytosolic levels of these three metabolites (as determined by nonaqueous fractionation) were all above the concentrations necessary to inhibit activity in vitro, suggesting that nr-G3PDH may be regulated through feedback inhibition by one or more metabolites. We also determined a tight association between activities of nr-G3PDH and mannose-6-P reductase and mRNA expression levels in response to both leaf development and salt treatment. Collectively, our data clearly show metabolic, developmental, and environmental regulation of nr-G3PDH, and also suggest that the supply of NADPH necessary for mannitol biosynthesis is under tight metabolic control.

Amino Acid Sequence↗

PubMed1983

A pathway for photosynthetic carbon flow to mannitol in celery leaves : activity and localization of key enzymes.

In the polyol producing plant, celery (Apium graveolens L.), mannitol is a major photosynthetic product and a form in which carbohydrate is translocated. Measurements of whole leaf extracts of celery indicated substantial activity of the following enzymes: mannose-6-P reductase, mannose-6-P isomerase, mannitol-1-P phosphatase, and nonreversible glyceraldehyde-3-P dehydrogenase. The activities of these enzymes were either undetectable or very low in the nonpolyol producing plants, Secale cereale L. (rye) and Vigna mungo (L.) Hepper (black gram).Mesophyll protoplasts were enzymically isolated from celery leaves, broken with a Yeda press and the intracellular localization of the above enzymes for mannitol synthesis studied following differential and/or sucrose density gradient centrifugation of the protoplast extract. These data suggested the enzymes involved in mannitol synthesis are exclusively localized in the cytoplasm. Ninety-five to 100% of the activity of these enzymes, along with the cytoplasmic marker enzyme phosphoenolpyruvate carboxylase, was found in the cytosolic fraction.We propose the pathway of photosynthetic carbon flow from triose-P to mannitol in celery occurs via fructose-6-P, mannose-6-P, and mannitol-1-P; these final reactions being catalyzed by the cytoplasmic enzymes, mannose-6-P isomerase, NADPH-dependent mannose-6-P reductase, and mannitol-1-P phosphatase, respectively. The requirement for NADPH may be met via the cytoplasmically located NADP-linked nonreversible glyceraldehyde-3-P dehydrogenase.

Journal Article↗

PubMed1984

Plant triose phosphate isomerase isozymes : purification, immunological and structural characterization, and partial amino Acid sequences.

We report the first complete purifications of the cytosolic and plastid isozymes of triose phosphate isomerase (TPI; EC 5.3.1.1) from higher plants including spinach (Spinacia oleracea), lettuce (Lactuca sativa), and celery (Apium graveolens). Both isozymes are composed of two isosubunits with approximate molecular weight of 27,000; in spinach and lettuce the plastid isozyme is 200 to 400 larger than the cytosolic isozyme. The two isozymes, purified from lettuce, had closely similar amino acid compositions with the exception of methionine which was four times more prevalent in the cytosolic isozyme. Partial amino acid sequences from the N-terminus were also obtained for both lettuce TPIs. Nine of the 13 positions sequenced in the two proteins had identical amino acid residues. The partial sequences of the plant proteins showed high similarity to previously sequenced animal TPIs. Immunological studies, using antisera prepared independently against the purified plastid and cytosolic isozymes from spinach, revealed that the cytosolic isozymes from a variety of species formed an immunologically distinct group as did the plastid isozymes. However, both plastid and cytosolic TPIs shared some antigenic determinants. The overall similarity of the two isozymes and the high similarity of their partial amino acid sequences to those of several animals indicate that TPI is a very highly conserved protein.

Journal Article↗

PubMed1987

Interaction of cell turgor and hormones on sucrose uptake in isolated Phloem of celery.

Phloem tissue isolated from celery (Apium graveolens L.) was used to investigate the regulation of sucrose uptake by turgor (manipulated by 50-400 milliosomolal solutions of polyethylene glycol) and hormones indoleacetic acid (IAA) and gibberillic acid (GA(3)). Sucrose uptake was enhanced under low cellular turgor (increase in the V(max)). Furthermore, enhancement of sucrose uptake was due to a net increase in influx rates since sucrose efflux was not affected by cell turgor. Manipulations of cell turgor had no effect on 3-O-methyl glucose uptake. When 20 millimolar buffer was present in uptake solutions, low turgor-induced effects were observed only at low pH range (4.5-5.5). However, the effect was extended to higher external pH (up to 7.5) when buffer was omitted from uptake solutions. A novel interaction between cellular turgor and hormone treatments was observed, in that GA(3) (10 micromolar) and IAA (0.1-100 micromolar) enhanced sucrose uptake only at moderate turgor levels. The hormones elicited little or no response on sucrose uptake under conditions of low or high cell turgor. Low cell turgor, IAA, GA(3), and fusicoccin caused acidification by isolated phloem segments in a buffer-free solution. It is suggested that enhanced sucrose uptake in response to low turgor and/or hormones was mediated through the plasmalemma H(+)-ATPase and most likely occurred at the site of loading.

Journal Article↗

PubMed1987

Facilitated transport of glucose in isolated Phloem segments of celery.

In isolated phloem segments of celery (Apium graveolens L.), a tissue highly specific for sucrose and mannitol uptake, glucose uptake occurs at very low rates and exhibits biphasic kinetics. Nonpenetrating inhibitors such as parachloromercuribenzene sulfonic acid did not inhibit glucose uptake. However, uptake was greatly inhibited by penetrating inhibitors such as N-ethylmaleimide and carbonylcyanide-m-chlorophenyl hydrazone. Carbonylcyanide-m-chlorophenyl hydrazone inhibition of uptake was reversed by washing and addition of thiol reagents to uptake solutions. Phlorizin, a competitive inhibitor of glucose caused moderate inhibition of uptake only after 3 hours of tissue exposure. Low pH, fusicoccin, and low turgor which enhance H(+)-sugar cotransport did not alter uptake rates. Furthermore, glucose did not induce alkalinization of the uptake media. Efflux analysis indicated that the presence of 50 millimolar unlabeled glucose in the wash media enhanced exchange of the labeled glucose across the tonoplast. Results indicate that the glucose carrier is not located at the plasmalemma but appears to be present at the membrane of an intracellular compartment, most likely the tonoplast. Carrier-mediated glucose transport in this tissue is proposed to be a facilitated diffusion.

Journal Article↗

PubMed1990

Direct observation of cell wall structure in living plant tissues by solid-state C NMR spectroscopy.

Solid-state (13)C nuclear magnetic resonance (NMR) spectra of the following intact plant tissues were recorded by the crosspolarization magic-angle spinning technique: celery (Apium graveolens L.) collenchyma; carob bean (Ceratonia siliqua L.), fenugreek (Trigonella foenum-graecum L.), and nasturtium (Tropaeolum majus L.) endosperm; and lupin (Lupinus polyphyllus Lindl.) seed cotyledons. All these tissues had thickened cell walls which allowed them to withstand the centrifugal forces of magic angle spinning and which, except in the case of lupin seeds, dominated the NMR spectra. The celery collenchyma cell walls gave spectra typical of dicot primary cell walls. The carob bean and fenugreek seed spectra were dominated by resonances from galactomannans, which showed little sign of crystalline order. Resonances from beta(1,4')-d galactan were visible in the lupin seed spectrum, but there was much interference from protein. The nasturtium seed spectrum was largely derived from a xyloglucan, in which the conformation of the glucan core chain appeared to be intermediate between the solution form and solid forms of cellulose.

Journal Article↗

PubMed1991

Xylulose 1,5-Bisphosphate Synthesized by Ribulose 1,5-Bisphosphate Carboxylase/Oxygenase during Catalysis Binds to Decarbamylated Enzyme.

Xylulose 1,5-bisphosphate (XuBP) is synthesized from ribulose 1,5-bisphosphate (RuBP) at carbamylated catalytic sites on ribulose 1,5-bisphosphate carboxylase (Rubisco) with significant amounts of XuBP being formed at pH less than 8.0. XuBP has been separated by high performance liquid chromatography and identified by pulsed amperometry from compounds bound to Rubisco during catalysis with the purified enzyme and from celery (Apium graveolens var Utah) leaf extracts. XuBP does not bind tightly to carbamylated sites, but does bind tightly to decarbamylated sites. Upon incubation of fully activated Rubisco with 5 micromolar XuBP, loss of activator CO(2) occurred before XuBP bound to the enzyme catalytic sites, even in the presence of excess CO(2) and Mg(2+). Binding of XuBP to decarbamylated Rubisco sites was highly pH dependent. At pH 7.0 and 7.5 with 10 millimolar MgCl(2) and 10 millimolar KHCO(3), the apparent dissociation constant for XuBP, K(d), was 0.03 micromolar, whereas at pH 8.0 and 8.5, the apparent K(d) was 0.35 and 2.0 micromolar, respectively. This increase in K(d) with pH was a result of a decrease in the association rate constant and an increase in the dissociation rate constant of XuBP bound to decarbamylated sites on Rubisco. The K(d) of 2-carboxyarabinitol 1-phosphate binding to carbamylated sites was only slightly pH dependent.

Journal Article↗

PubMed1992

Mannitol Synthesis in Higher Plants : Evidence for the Role and Characterization of a NADPH-Dependent Mannose 6-Phosphate Reductase.

Mannitol is a major photosynthetic product in many algae and higher plants. Photosynthetic pulse and pulse-chase (14)C-radiolabeling studies with the mannitol-synthesizing species, celery (Apium graveolens L.) and privet (Ligustrum vulgare L.), showed that mannose 6-phosphate (M6P) and mannitol 1-phosphate were among the early photosynthetic products. A NADPH-dependent M6P reductase was detected in these species (representing two different higher plant families), and the enzyme was purified to apparent homogeneity (68-fold with a 22% yield) and characterized from celery leaf extracts. The celery enzyme had a monomeric molecular mass, estimated from mobilities on sodium dodecyl sulfate-polyacrylamide gels, of 35 kilodaltons. The isoelectric point was pH 4.9; the apparent K(m) (M6P) was 15.8 millimolar, but the apparent K(m) (mannitol 1-phosphate) averaged threefold higher; pH optima were 7.5 with M6P/NADPH and 8.5 with mannitol 1-phosphate/NADP as substrates. Substrate and cofactor requirements were quite specific. NADH did not substitute for NADPH, and there was no detectable activity with fructose 6-phosphate, glucose 6-phosphate, fructose 1-phosphate, mannose 1-phosphate, mannose, or mannitol. NAD only partially substituted for NADP. Mg(2+), Ca(2+), Zn(2+), and fructose-2,6-bisphosphate had no apparent effects on the purified enzyme's activity. In vivo radiolabeling results and the enzyme's kinetics, specificity, and distribution (in two-plant families) all suggest that NADPH-dependent M6P reductase plays an important role in mannitol biosynthesis in higher plants.

Journal Article↗

PubMed1992

Induction of somatic embryogenesis using side chain and ring modified forms of phenoxy Acid growth regulators.

The induction of somatic embryo development in cell cultures of alfalfa (Medicago sativa), celery (Apium graveolens), and lettuce (Lactuca sativa) was compared for 2,4-dichlorophenoxy-acetic acid (2,4-D) and various phenoxy acid growth regulators. Tests using a series of straight chain extensions to the phenoxy acid side chain indicate that phenoxybutanoic acid is active, whereas the phenoxypropanoic and phenoxypentanoic analogs are inactive for the induction of alfalfa embryogenesis. Side branching on the carbon adjacent to the phenoxy group results in optically active compounds. Racemic mixtures and the (+) enantiomers of the compounds are active for alfalfa embryo induction, whereas the (-) enantiomers are inactive and apparently do not inhibit embryogenesis in any way. Development of alfalfa embryos, as measured by plantlet formation from individual embryos, is improved by 4-(2,4-dichlorophenoxy)butanoic acid and with side branching at the carbon adjacent to the phenoxy group compared with induction with 2,4-D. Similarly, substituted phenoxy acids also enhance somatic embryo development in celery and lettuce when compared with 2,4-D. These results are discussed with reference to earlier studies on the structure activity of various synthetic auxins during cell elongation and with reference to the possible importance of auxin metabolism on subsequent somatic embryo development.

Journal Article↗

PubMed2004

Arabidopsis POLYOL TRANSPORTER5, a new member of the monosaccharide transporter-like superfamily, mediates H+-Symport of numerous substrates, including myo-inositol, glycerol, and ribose.

Six genes of the Arabidopsis thaliana monosaccharide transporter-like (MST-like) superfamily share significant homology with polyol transporter genes previously identified in plants translocating polyols (mannitol or sorbitol) in their phloem (celery [Apium graveolens], common plantain [Plantago major], or sour cherry [Prunus cerasus]). The physiological role and the functional properties of this group of proteins were unclear in Arabidopsis, which translocates sucrose and small amounts of raffinose rather than polyols. Here, we describe POLYOL TRANSPORTER5 (AtPLT5), the first member of this subgroup of Arabidopsis MST-like transporters. Transient expression of an AtPLT5-green fluorescent protein fusion in plant cells and functional analyses of the AtPLT5 protein in yeast and Xenopus oocytes demonstrate that AtPLT5 is located in the plasma membrane and characterize this protein as a broad-spectrum H+-symporter for linear polyols, such as sorbitol, xylitol, erythritol, or glycerol. Unexpectedly, however, AtPLT5 catalyzes also the transport of the cyclic polyol myo-inositol and of different hexoses and pentoses, including ribose, a sugar that is not transported by any of the previously characterized plant sugar transporters. RT-PCR analyses and AtPLT5 promoter-reporter gene plants revealed that AtPLT5 is most strongly expressed in Arabidopsis roots, but also in the vascular tissue of leaves and in specific floral organs. The potential physiological role of AtPLT5 is discussed.

Animals↗

PubMed2001

Identification of a mannitol transporter, AgMaT1, in celery phloem.

A celery petiole phloem cDNA library was constructed and used to identify a cDNA that gives Saccharomyces cerevisiae cells the ability to grow on mannitol and transport radiolabeled mannitol in a manner consistent with a proton symport mechanism. This cDNA was named AgMaT1 (Apium graveolens mannitol transporter 1). The expression profile in source leaves and phloem was in agreement with a role for mannitol in phloem loading in celery. The identification in eukaryotes of a mannitol transporter is important because mannitol is not only a primary photosynthetic product in species such as celery but is also considered a compatible solute and antioxidant implicated in resistance to biotic and abiotic stress.

Amino Acid Sequence↗

PubMed1988

A study of allergens in celery with cross-sensitivity to mugwort and birch pollens.

Sixty-one sera with positive RAST to mugwort pollen (Artemisiae vulgaris) were submitted to RASTs for birch pollen (Betula verrucosa) and celery (Apium graveolens). In 36 cases RAST results were positive for celery. In addition, 23 sera presented specific IgE to birch pollen. The binding of specific IgE to individual allergens in celery, mugwort pollen and birch pollen was studied by the immunoblotting technique. This involved electrophoretic separation of allergenic extracts, electrotransfer of proteins onto nitrocellulose sheets and sensitive immunoenzymatic detection. Eighteen sera had specific IgE binding to two celery components of molecular weight around 15 kD. All these sera also detected a 15 kD allergen in mugwort and two allergens in birch of 14 kD and 16 kD molecular weight. The sera that did not detect the 15 kD bands in celery failed to react with both the 15 kD mugwort component and the 14 and 16 kD birch components. Specific cross-inhibitions of the detection of these allergens on immunoblots were obtained by pre-incubation of the sera with crude extract of the three species. These results strongly suggest that such allergens display some structural identity and that they could be at the origin of some cases of crossed hypersensitivity to celery, mugwort pollen and birch pollen.

Allergens↗