Cell wall composition of the yeast and mycelial forms of Yarrowia lipolytica

The cell walls of the yeast and mycelial forms of Yarrowia lipolytica were isolated and purified. Electron microscopy studies showed no differences between both types of cell walls. Chemical analysis revealed that the yeast cell wall contained 70% neutral carbohydrate, 7% amino sugars, 15% protein, 5% lipids and 0.8% phosphorus. Mycelial cell walls contained 70% carbohydrate, 14% aminosugars, 6% protein, 5% lipids and 0.6% phosphorus. Three polysaccharides: -glucan, mannan and chitin were detected. Proteins were solubilized from both cell wall fractions and separated by polyacrylamide gel electrophoresis. About 50 protein bands were detected, four of them corresponding to glycoproteins. The cell walls of the yeast and mycelial forms of Y. lipolytica were qualitatively similar and only quantitative differences were found.Abbreviations GlcNAc N-acetylglucosamine - FITC-WGA fluorescein isothiocyanate-wheat germ agglutinin - PAS periodic acid Schiff     

Ultrastructure and histochemistry of Citrus limon (L.) stigma

Citrus limon has a wet stigma which can be divided in two zones: a glandular superficial one formed by papillae, and a non-glandular one formed by parenchymatic cells. The stigmatic exudate is produced by the papillae after the latter have reached their ultimate size. The papillae of the mature pistil are of varying size and composition. Both the unicellular and multicellular ones are present. The cells at the base of the papillae are rich in cytoplasm, whereas the tip cells are vacuolated. Histochemical analysis has shown that the exudate of Citrus is composed of lipids, polysaccharides, and proteins. Our results indicate that the lipidic component is produced and secreted first, followed by production and secretion of the polysaccharidic component. The lipidic component of the exudate is produced in the basal papillae cells and accumulates as droplets in dilated parts of the smooth endoplasmic reticulum (SER). Subsequently the lipid droplets are transported to the plasma membrane, and transferred by the latter into the cell walls. Then the exudate component is accumulated in the intercellular spaces and in the middle lamellar regions of the walls. Subsequently, the polysaccharidic component of the exudate is produced and secreted by the tip cells of the papillae.Abbreviations RER rough endoplasmic reticulum - SER smooth endoplasmic reticulum     

Structure and biosynthesis of teichoic acids in the cell walls of Staphylococcus xylosus DSM 20266

The simultaneous occurrence of a N-acetylglucosaminyl poly(ribitolphosphate) (-GlcNAc) and a N-acetylglucosaminyl poly(glycerolphosphate) (-GlcNAc) in the cell walls of Staphylococcus xylosus DSM 20266 was demonstrated by different experimental lines:(1) Fractionation of extracted cell wall teichoic acid on DEAE-cellulose, (2) investigation of the composition of cell walls in the growth cycle, (3) in vitro biosynthesis using crude membranes as the source of enzyme.The polymerization of these polymers starts from CDP-ribitol and CDP-glycerol, respectively. In the presence of UDP-N-acetylglucosamine both polymers are substituted with N-acetylglucosamine at a level and with the identical anomeric configuration found in the native cell wall teichoic acids. The in vitro biosynthesis of poly(glycerolphosphate) was unique in that it was highly stimulated by UDP-N-acetylglucosamine and to a lower extent by other UDP-activated sugars. Kinetic studies have provided evidence that this stimulation is due to an increase of V _max while K _m is unchanged. Competition experiments have indicated that poly(ribitolphosphate) and poly(glycerolphosphate) were synthesized in the in vitro system in a close spatial relationship.Abbreviations ADP adenosine 5-diphospho - CDP cytidine 5-diphospho - GDP guanosine 5-diphospho - GalNAc N-acetyl-galactosamine - Glc glucose, glucosyl - GlcNAc N-acetyl-glucosamine - N acetylglucosaminyl - GlcUA glucuronic acid - Gro glycerol - Man mannose, mannosyl - Rit ribitol - SDS sodium dodecyl sulfate - UDP uridine 5-diphospho     

Cellular glycogen, β-1,2-glucan,poly-β-hydroxybutyric acid and extracellular polysaccharides in fast-growing species of Rhizobium

Synthesis of acidic exopolysaccharides, neutral cellular polysaccharides and poly--hydroxybutyric acid (PHB) by Rhizobium is strongly dependent on cultural conditions and the strains used. Exopolysaccharide production by R. leguminosarum, R. Phaseoli and R. trifolii closely parallels growth, whereas R. meliloti mainly excretes (low mol wt) polysaccharides when cell propagation is limited by lack of a necessary growth element (nitrogen) and an excess of carbon source is still present in the medium.In all strains, accumulation of cellular glycogen, -1,2-glucan and PHB is initiated only under growth-limiting conditions. When the external carbon source is exhausted, glycogen and PHB are metabolized by the cells, sustaining their longevity and thus act as true reserve materials; on the other hand, -1,2-glucan and excreted polysaccharides are not utilized on further incubation of the culture.Differences exist in the nature and relative amounts of the products synthesized by strains of different species of Rhizobium. R. leguminosarum, R. phaseoli and R. trifolii synthesize a uronic acid-containing exopolysaccharide, PHB and/or glycogen, non-metabolizable capsular polysaccharide and low amounts of -1,2-glucan. R. meliloti synthesizes a uronic acid-free exopolysaccharide, PHB and/or glycogen and high concentrations of -1,2-glucan.Exopolysaccharides, -1,2-glucan and glycogen preparations were obtained by isolation and purification from cells of fast-growing species of Rhizobium and chemically characterized.     

Genetical investigations into β-glucan content in barley

Summary Random inbred lines produced by doubled haploidy (DH) and single seed descent (SSD) have been used to investigate the genetics of -glucan (gum) content in barley (Hordeum vulgare). Genetical analyses indicated that gum content is controlled by a simple additive genetic system. Significant negative genetic correlations were observed between -glucan content, thousand grain weight and height in the DH samples. These correlations were much reduced in the SSD samples and would suggest linkage of the genes controlling these characters. The presence of repulsion linkages could be exploited in a barley breeding programme by producing F1 derived DH to generate recombinants with high thousand grain weight and low -glucan content. Genetical parameters estimated from DH and F3 samples have successfully been used to predict the number of inbred lines transgressing the parental range for -glucan content and bivariate combinations involving -glucan.     

Cellular localization and cytochemical probing of resistance reactions to arbuscular mycorrhizal fungi in a ‘locus a’ myc− mutant of Pisum sativum L.

Pisum sativum L. myc^– mutants which fail to form arbuscular mycorrhiza have recently been identified amongst nod^– mutants (Duc et al., 1989, Plant Sci. 60, 215–222). The reason for this resistance to symbiotic fungi has been investigated in the case of a locus a mutant (P2) inoculated with Glomus mosseae (Nicol. and Gerd.) Gerd, and Trappe. The fungal symbiont formed viable appressoria in contact with the root surface but its development was stopped at the root epidermis. Abundant material was deposited on the inner face of root cell walls adjacent to the appressoria in the P2 mutant, but not in the wild-genotype parent cultivar (Frisson) forming a symbiotic mycorrhizal infection. Fluorescence, histochemical, cytochemical and immunocytological approaches were used to characterize the paramural deposits in epidermal and hypodermal cells of the mutant. Strong fluorescence under blue light indicated the accumulation of phenolic compounds although polymers like lignin or suberin were not localized. Proteins and glycoproteins were homogeneously distributed within the paramural deposits. In the latter, the periodic acid-thiocarbohydrazide-silver proteinate (PATAg) reaction for 1,4-polysaccharide detection showed a heterogeneous composition with electron-dense points surrounded by non-reactive material, but cytological tests for cellulose and pectin gave weak responses as compared to epidermal and hypodermal walls of the wild genotype. -1,3-Glucans indicative of callose were detected by in-situ immunolocalization in the paramural deposits below appressoria on mutant roots, but not in walls of the wild genotype. Thus, appressorium formation by G. mosseae on roots of the locus a P. sativum mutant elicits wall modifications usually associated with activation of defence responses to pathogens. It is proposed that this locus must be involved in a key event in symbiotic infection processes in P. sativum, and the possible role of complex regulatory interactions between symbiosis and defence genes in endomycorrhiza development is discussed.Abbreviations DAPI 4,6-diamino-2-phenylindole - FDA fluo-rescein diacetate - PATAg periodic acid-thiocarbohydrazide-silver proteinate The authors are grateful to C. Arnould for technical assistance, K. Niehaus for the purified Sirofluor, K. Roberts for the AFRC JIM5 antibody and J. Lherminier (INRA, Dijon, France), for useful discussion. This collaborative research programme was financially supported by MRT, INRA, EPR-Bourgogne (grant to A.G., Contrat de Plan project 3060A), EEC COST ACTION 8.10 (Endomycorrhizas) and the National Research Council of Italy, Special Project RAISA, Sub-project N.2, Paper N. 801     

Direct Development of the Visual System of the Coral Reef Teleost, The Spiny Damsel, Acanthochromis polyacanthus

Unlike most marine teleosts, the coral reef-dwelling spiny damsel, Acanthochromis polyacanthus, lacks a pelagic larva dispersal phase and represents one of few examples of self recruitment onto a natal reef by a marine teleost immediately after hatching. Benthic eggs are protected by the parents, and upon hatching the young remain under parental care for several months. Visual morphogenesis of spiny damsel embryos and juveniles was examined to evaluate the potential visual capabilities of the young after emergence onto the reef. The optic primordia were visible in the embryo as hollow spheres of undifferentiated neuroblasts 2 days after fertilization (daf). Visual morphogenesis proceeded rapidly thereafter in the embryo such that at hatching (between 10 and 12daf) gross visual morphology was consistent with that reported in the majority of juvenile marine teleosts, reflecting direct development of the retina of the spiny damsel within the egg. At hatching, the outer nuclear layer comprised 2 classes of photoreceptors; cones and rods. Tangential sections of the retina revealed a square cone mosaic in which 4 double cones surrounded a single cone. This arrangement remained unchanged in all later life history intervals examined. Absolute eye size was large compared to larvae of marine pelagic spawners. Eye and lens diameters increased from 0.69 and 0.23mm, respectively, on the day of hatching (12daf), to 3.77 and 1.52mm, respectively, in a fish 131daf. Angular density of cones increased from 0.25 cones 10 visual arc^–1 in an embryo 8daf, to 1.14 cones 10 visual arc^–1 in a fish 131daf, demonstrating the potential for significant increase in spatial resolution with increasing eye size. Convergence ratios of cones to ganglion cells remained relatively constant from the time of hatching, suggesting that the determinate ganglion cell photopic receptive field was established early in development. The increase in the convergence ratios of rods: ganglion cells from 1.4 in the late stages of embryogenesis (10daf; 2 days prior to hatching) to 4.9 in a fish 103daf, demonstrated increasing scotopic ganglion cell receptive field size, with increasing age. This was a result of rod cell addition with growth. An increase in the angular density of rods from 0.18 rods 10 visual arc^–1 in an embryo 8daf, to 4.07 rods 10 visual arc^–1 in a fish 131daf, and the increase in mean scotopic light path-length from 13.3±1.1m in an embryo 8dpf, to 55±5.2m a fish 22dpf, collectively indicate the potential for increasing scotopic sensitivity during growth. On the basis of visual morphology it is predicted that newly hatched spiny damsel juveniles have substantially greater visual capabilities than first feeding larvae with a pelagic dispersal phase. In addition, we propose that the developmental trajectory of the spiny damsel is different from that of pelagic dispersing larvae and does not simply reflect displacement along a common developmental continuum by an extended embryonic duration.     

β-Glucan synthetase activity in Golgi vesicles ofPetunia hybrida

-Glucan synthetase activity has been demonstrated in a Golgi vesicle fraction isolated from pollen tubes ofPetunia hybrida. This-glucan synthetase activity differs from that of most other higher plants in its inability to incorporate [¹⁴C]glucose from GDP-[¹⁴C]glucose. UDP-[¹⁴C]glucose, however, is an appropriate glucose donor for this enzyme. The optimum conditions for this-glucan synthetase activity are: 1 mg Golgi vesicle protein/ml reaction mixture; pH=±8 and a temperature of 25°C. The newly synthesized alkali-insoluble glucan contains-1,3- as well as -1,4-glucosidic linkages.     

An alkali-soluble polysaccharide from the cell walls of Coprinus lagopus

An alkali-soluble polysaccharide was isolated from the purified mycelial walls of Coprinus lagopus. The hydrolysis products, optical rotation, and infrared spectrum indicate a -glucan. Hydrolysis of the glucan after permethylation gave only 2,3,4,6-tetra-, 2,4,6-tri-, and 2,4-di-O-methyl-d-glucose. These methylated sugars and their relative quantities reveal that the glucan is a polysaccharide containing -1,3-linked glucose units with about 14% of the sugars having 1,6-linked branch points. Partial hydrolysis of the product derived from Smith degradation of the glucan released laminaribiose and gentiobiose suggesting that the branches are generally longer than a single glucose unit.Adapted from a portion of a dissertation submitted in partial fullfillment of the requirements for the Ph.D. degree in botany at the University of Connecticut under the direction of Dr. Ralph P. Collins     

An Arabidopsis callose synthase

-1,3-glucan polymers are major structural components of fungal cell walls, while cellulosic -1,4-glucan is the predominant polysaccharide in plant cell walls. Plant -1,3-glucan, called callose, is produced in pollen and in response to pathogen attack and wounding, but it has been unclear whether callose synthases can also produce cellulose and whether plant cellulose synthases may also produce -1,3-glucans. We describe here an Arabidopsis gene, AtGsl5, encoding a plasma membrane-localized protein homologous to yeast -1,3-glucan synthase whose expression partially complements a yeast -1,3-glucan synthase mutant. AtGsl5 is developmentally expressed at highest levels in flowers, consistent with flowers having high -1,3-glucan synthase activities for deposition of callose in pollen. A role for AtGsl5 in callose synthesis is also indicated by AtGsl5expression in the Arabidopsis mpk4 mutant which exhibits systemic acquired resistance (SAR), elevated -1,3-glucan synthase activity, and increased callose levels. In addition, AtGsl5 is a likely target of salicylic acid (SA)-dependent SAR, since AtGsl5mRNA accumulation is induced by SA in wild-type plants, while expression of the nahG salicylate hydroxylase reduces AtGsl5 mRNA levels in the mpk4 mutant. These results indicate that AtGsl5is likely involved in callose synthesis in flowering tissues and in the mpk4 mutant.     

Influence of manganese on morphology and cell wall composition of Aspergillus niger during citric acid fermentation

Morphology and cell wall composition of Aspergillus niger were studied under conditions of manganese sufficient or deficient cultivation in an otherwise citric acid producing medium. Omission of Mn²⁺ (less than 10^-7 M) from the nutrient medium of Aspergillus niger results in abnormal morphological development which is characterized by increased spore swelling, and squat, bulbeous hyphae. Fractionation and analysis of manganese deficient cell walls revealed increased chitin and reduced -glucan contents as well as reduction of galactose containing polymers, as compared to cell walls from manganese sufficient grown hyphae. Addition of copper induced the same effect as manganese deficiency, both on morphology and cell wall composition. Addition of cycloheximide also produced a very similar type of morphology with increased chitin and reduced -glucan contents of the cell wall but its effect on galactose was less pronounced.Dedicated to emer. Prof. Dr. J. Kisser on the occasion of his 80th birthday     

Qualitative and quantitative composition of pigments in Phaeodactylum tricornutum (Bacillariophyceae) stressed by iron

The effect of Fe(III) deficiency on qualitative and quantitative changes in pigment composition in Phaeodactylum tricornutum Bohlin was demonstrated by HPLC and AAS. Maximum content of pigments showed the diatom cells incubated at the optimum iron concentration, i.e., 10 M. The contents of chlorophyll a, chlorophyll c ₁+c ₂, fucoxanthin, diadinoxanthin and ,-carotene were 109.99, 20.16, 40.39, 1.29 and 1.48 fg per cell, respectively. The results obtained showed that Fe(III) affected qualitative and quantitative pigment composition in P. tricornutum. The content of individual pigments, proportions between accompanying pigments and their ratios to chlorophyll a were important indicators of phytoplankton response to iron stress. The strong reduction in ,-carotene content, several times (2–5) increase in diadinoxanthin level as compared to ,-carotene, and high amount of diadinoxanthin in relation to chlorophyll a were observed in algae growing at very low Fe(III) concentrations, 0.001 and 0.01 M. The data suggested that phytoplankton pigments could be a potential physiological marker.     

Regulation of biosynthesis of aspartate family amino acids in the photosynthetic bacterium Rhodopseudomonas palustris

The four amino acids of the aspartate family (l-lysine, l-methionine, l-threonine, and l-isoleucine) are produced in bacteria by a branched biosynthetic pathway. Regulation of synthesis of early common intermediates and of carbon flow through distal branches of the pathway requires operation of a number of subtle feedback controls, which are integrated so as to ensure balanced synthesis of the several end products. Earlier studies with nonsulfur purple photosynthetic bacteria were instrumental in revealing the existence of alternative regulatory schemes, and in this communication we report on the control pattern of a representative of this physiological group not previously investigated, Rhodopseudomonas palustris. The results obtained from study of the properties of four key regulatory enzymes of the aspartate family pathway (-aspartokinase, homoserine dehydrogenase, homoserine kinase, and threonine deaminase) and of the effects of exogenous amino acids (i. e., the end products) on growth of the bacterium indicate that the control schema in Rps. palustris differs substantially from the schemes described for other Rhodopseudomonas species, but resembles the regulatory pattern observed in Rhodospirillum rubrum.Abbreviations A absorbancy - AK -aspartokinase - ASA aspartate -semialdehyde - DTT dithiothreitol - HS l-homoserine - HSDH homoserine dehydrogenase - HSK homoserine kinase - I l-isoleucine - KU Klett-Summerson photometer units - L l-lysine - M l-isoleucine - KU Klett-Summerson photometer units - L l-lysine - M l-methionine - ME -mercaptoethanol - PABA p-aminobenzoic acid - T l-threonine - TD threonine deaminase - RCV synthetic growth medium (see text) - YP agar medium containing 0.3% yeast extract, 0.3% peptone, and 1.5% agar - Y2T synthetic growth medium (see text)     

Brevibacterium divaricatum: Influence of nutritional conditions on the wall composition and release of uncross-linked peptidoglycan chains into medium

Purified cell walls, originating from penicillin-treated (3 g/ml, 1 h) and-untreated Brevibacterium divaricatum cells grown on complex (CM) and glucose minimal medium with (MM) or without (Ca-free MM) calcium carbonate, were isolated by two procedures. Electron micrographs and chemical analysis revealed no differences between identically isolated walls with respect to the presence or absence of either penicillin or calcium carbonate in the glucose growth medium. On the contrary, the appearance and peptidoglycan content of the walls was greatly dependent on the procedure used for their isolation and the walls isolated from the cells grown on complex medium contained more materials other than peptidoglycan. It was shown that the presence of calcium carbonate in the glucose minimal medium was essential for accumulation of large amounts of peptidoglycan chains into the medium. Penicillin-induced interruption of cell wall synthesis was prerequisite for manifestation of the calcium carbonate stimulating effect.Abbreviations CM complex medium - MM chemically defined minimal medium based on glucose and containing calcium carbonate - Ca-free MM MM modified only by the omission of calcium carbonate - ET-walls Enzyme treated walls - FPR-walls French press-ruptured walls     

Selective inhibition of transition from sexual agglutination to zygote formation by ethyl N-phenylcarbamate in Saccharomyces cerevisiae

Effects of ethyl N-phenylcarbamate (EPC) on the mating reaction of Saccharomyces cerevisiae were studied, with special attention on the effect on the pheromone action. EPC inhibited zygote formation at a concentration which promoted induction of sexual agglutinability. EPC enhanced agglutinability induction by pheromone, but inhibited -pheromone-induced formation of large pearshaped cells in a mating type. The enhancement of agglutinability induction was accompanied with increased production of a agglutination substance and inhibition of pheromone inactivation. EPC arrested the cell cycle of a cells probably in the step controlled by CDC19, CDC35, cAMP etc., just before the step controlled by CDC28, pheromone etc.Abbreviations EPC Ethyl N-phenylcarbamate - PBS 0.01 M phosphate buffer solution, pH 5.5 - SPB spindle pole body     

Several phenotypic changes in the cell envelope of Agrobacterium tumefaciens chvB mutants are prevented by calcium limitation

The chvB gene of Agrobacterium tumefaciens encodes a 235 kDa proteinaceous intermediate involved in the synthesis of -1,2-glucan. chvB mutants show a pleiotropic phenotype. Besides not to produce cyclic -1,2-glucan, chvB mutants have been reported to be avirulent, attachment-deficient, and nonmotile. In this study we report additional differences from the parent strain, probably all linked to changes in the cell envelope. This pleiotropic phenotype — except for attachment and virulence — could largely be prevented by growing chvB cells with low levels of calcium. Although a role for -1,2-glucan in osmoadaptation has been proposed, the mode of action of -1,2-glucan is not known. We speculate that in A. tumefaciens -1,2-glucan stabilizes membranes, which would be important especially in hypotonic media containing calcium.Abbreviations Cb carbenicillin - Km kanamycin - TCA trichloroacetic acid - K_av fraction of the stationary gel volume available for diffusion - LPS lipopolysaccharide - SDS-PAGE Sodium dodecyl sulphate polyacrylamide gel electrophoresis     

Species association among predaceous and phytophagous apple mites (Acari: Eriophyidae, Phytoseiidae, Stigmaeidae, Tetranychidae)

Predator–predator, predator–prey, and prey–prey associations among nine species of mites were studied in a plot of 100 Red Delicious apple (Malus pumila Miller) trees from 1990 to 1997. In 1990, seven-year-old trees were inoculated with Panonychus ulmi (Koch), Tetranychus urticae Koch (Acari: Tetranychidae) or both, and sprayed with azinphosmethyl (alone or plus endosulfan), or nothing. The species Zetzellia mali (Ewing) (Acari: Stigmaeidae), Amblyseius andersoni Chant (Acari: Phytoseiidae), Eotetranychus sp., Bryobia rubrioculus (Scheuten) (Acari: Tetranychidae), and Aculus schlechtendali Nalepa (Acari: Eriophyidae) were already present or immigrated into plots, and Galendromus occidentalis (Nesbitt) and Typhlodromus pyri Scheuten (Acari: Phytoseiidae) were introduced. Yule's V association index was used to measure positive, neutral, or negative interspecific associations for each species pair, because of its robustness with spatially autocorrelated data. We found that pesticide and release treatments did not greatly affect the association results, but there were strong seasonal differences. Predator–predator associations were the strongest and most consistent, showing negative associations in the early and mid seasons, and neutral ones in late season. Negative associations of T. pyri with other predators were the strongest, which is consistent with evidence that this mite can detect other predators on a leaf. Predator–prey seasonal associations were mixed, with some positive and others negative, with most significant associations occurring in the mid season. One prey–prey interaction was positive, again in mid season, most likely because of similar habitat preferences.     

Cell wall polysaccharides of four strains ofPaecilomyces variotii

The main polysaccharidic fractions extracted with 1m NaOH from cell wall material of four different strains ofPaecilomyces variotii were characterized as an -(13)-glucan (F1I) amounting from 33.2 to 39.1% and a -glucan-chitin complex (F4) representing from 42.7 to 47.3%. The water-soluble fraction, F1S, was composed of mannose, glucose, and mainly galactose in all the strains studied. The F1S fractions of the four strains were analyzed by NMR spectroscopy showing analogous structural features in all the strains. This fraction was purified through Sepharose CL-6B and methylated. The purified material (F1S-B) mainly contained (16)- and (12,6)-linked mannopyranose, (15)- and (16)-linked galactofuranose, and terminal residues of glucopyranose and galactofuranose. The proportion, the chemical composition, and the structure of each fraction revealed a homogeneous cell wall composition in these strains.     

Observations on the cell-wall compositions of the bracket fungi Laetiporus sulphureus and Piptoporus betulinus

Homogenized tissues and their alkali-soluble and alkali-insoluble fractions of fruiting bodies of the basidiomycetes Laetiporus sulphureus and Piptoporus betulinus were investigated using X-ray diffraction, infrared spectrometry and chemical methods. The presence of (13)--d-glucan, (13)--d-glucan and chitin was established. The relative amounts of these polysaccharides were different in the two species and differences were also found between context and trama. The proportion of (13)--d-glucan was exceptionally high in the context of L. sulphureus (about 78%). In addition, the trama of both species contained a substance resembling a cyclic wax by its X-ray pattern and solubility properties. The substances identified are considered to belong to the hyphal wall     

Plasmodesmatal frequency and radial translocation rates in ray cells of poplar (Populus x canadensis Moench ‘robusta’)

The minimum radial translocation rate of sugars has been determined from the starchaccumulation rate for the wood rays of Populus x canadensis Moench robusta, and related to ultrastructural peculiarities of the cell walls to be passed. The minimum radial flux or flow of sugars through the tangential walls, the pit fields, and per plasmodesma was 80.7 pmol · cm^-2 · s^-1, 400 to 800 pmol · cm^-2 · s^-1, and 1.0 to 1.7 · 10^-7 pmol · plasmodesma^-1 · s^-1, respectively. These values exclude a transmembrane flux mechanism and indicate that the radial translocation in this tissue must proceed via plasmodesmata. In the isolation cells of the ray center we found 39 plasmodesmata per m² of pit field, 8.0 per m² of tangential wall, and 1.98% of the wall occupied by plasmodesmata. Cells of the ray margins show plasmodesmata on only 1.16% of their tangential wall area and thus appear to be slightly inferior for radial translocation. As judged from both the observed plasmodesmatal frequencies and the translocation rates, the ray parenchyma cells are comparable to cells specialized in short-distance translocation.Abbreviations CCR contact-cell row - IC isolation cell - ICR isolation-cell row