Physical barriers to carotenoid bioaccessibility. Ultrastructure survey of chromoplast and cell wall morphology in nine carotenoid-containing fruits and vegetables

BACKGROUND

The ultrastructural characterisation of cellular components is a key element in revealing the bases for differences in nutrient bioaccessibility among fruits and vegetables and their derived products. Together, cell walls and chromoplasts constitute the two major physical barriers to carotenoid release from the food matrix (structure) during digestion. In general, larger cells with thinner cell walls are most likely to fail under mechanical pressure. In relation to chromoplasts, the substructures plastoglobuli, crystals and membranes give decreasing rates of carotenoid solubilisation when exposed to digestive forces.

RESULTS

This paper describes cell wall and chromoplast structures in nine carotenoid-storing raw fruits and vegetables. Watermelon and melon cells were shown to have the largest cells concomitant with thin, non-fibrous cell walls, while carrot, hypodermal grapefruit and sweet potato cells were smallest with fibrous or dense cell walls. Mango fruit showed the highest proportion of globules to other substructures. Carrot, papaya and tomato contained many crystalline structures. Finally, watermelon, mango and butternut squash developed a high proportion of membranous structures.

CONCLUSION

A more precise description of the physical characteristics of foods that stand as barriers to bioaccessibility can help in understanding which are more or less inhibitory for particular foods.

Physical barriers to carotenoid bioaccessibility. Ultrastructure survey of chromoplast and cell wall morphology in nine carotenoid-containing fruits and vegetables

BACKGROUND

The ultrastructural characterisation of cellular components is a key element in revealing the bases for differences in nutrient bioaccessibility among fruits and vegetables and their derived products. Together, cell walls and chromoplasts constitute the two major physical barriers to carotenoid release from the food matrix (structure) during digestion. In general, larger cells with thinner cell walls are most likely to fail under mechanical pressure. In relation to chromoplasts, the substructures plastoglobuli, crystals and membranes give decreasing rates of carotenoid solubilisation when exposed to digestive forces.

RESULTS

This paper describes cell wall and chromoplast structures in nine carotenoid-storing raw fruits and vegetables. Watermelon and melon cells were shown to have the largest cells concomitant with thin, non-fibrous cell walls, while carrot, hypodermal grapefruit and sweet potato cells were smallest with fibrous or dense cell walls. Mango fruit showed the highest proportion of globules to other substructures. Carrot, papaya and tomato contained many crystalline structures. Finally, watermelon, mango and butternut squash developed a high proportion of membranous structures.

CONCLUSION

A more precise description of the physical characteristics of foods that stand as barriers to bioaccessibility can help in understanding which are more or less inhibitory for particular foods.

In situ characterisation of a microorganism surface by Raman microspectroscopy: the shell of Ascaris eggs

Intestinal nematodes are very common human parasites and a single species, Ascaris lumbricoïdes, is estimated to infect a quarter of the world's population. A sticky external layer covers their eggs. This work shows that Raman vibrational confocal spectroscopy is able to give information on the biochemical composition of the shell of Ascaris eggs. The biochemical localised characterisation of Ascaris eggs was performed directly on the eggs in their aqueous environment. The studied parasites came from two origins: dissections of adult females and extractions from biosolid sludges. The presence of mucopolysaccharides, proteins and chitin in the shell was demonstrated. The presence of ascaroside compounds was shown particularly via the narrow and intense bands from the organised long CH2 chains. To the best of our knowledge, this is the first time that the latter have been observed in Raman vibrational spectra of microorganisms. Hydration of the shell was different depending on the intensity of the colour of the sludge eggs. Knowledge of the biochemical structural properties of egg surfaces would be useful to understand the egg adhesion phenomena on vegetables contaminated by reused wastewater.

A cyclic model for bimodal activation of calcium activated potassium channels in radish vacuoles

This paper presents the mathematical framework of a cyclic model proposed for describing the transition between a fast and a slow mode (fast-slow effect) induced by the application of step membrane potentials to ion channels from radish vacuoles. A voltage stimulation pulse with frequency in the range of 2 Hz or higher increased the activation time (slow mode) of the recorded currents. When the frequency of the stimulation pattern was restored to 0.1 Hz the activation time decreased twofold (fast mode). This experimental result cannot be explained by classical kinetic theory. The model, based on a simple extension of the Hodgkin and Huxley chain, describes the whole current experimental data and provides hints on the structural conformation of ion channels.

Postharvest changes in white asparagus cell wall during refrigerated storage

The postharvest changes of the white asparagus cell wall have been studied in relation to the toughening process along the length of a spear that was divided into three sections: apical, middle, and basal. Polysaccharides underwent significant turnover during storage. Uronic acid concentration decreased in all sections and in almost all polysaccharide fractions, while neutral sugars increased very slightly in the apical section, decreased in the middle, and increased in the basal one. Xylose, glucose, and galactose are the main neutral sugars implicated in the turnover; xylose accumulated in the hemicellulose fractions of the middle and basal sections, glucose decreased in the hemicellulose and cellulose fractions of the middle section and increased in the cellulose fraction of the basal one, and galactose disappeared mostly from the cellulose fractions of the three sections. Lignin increased most in the middle section and least in the basal one. No increase was detected in the apical one. No important increases of wall phenolics were detected in any part of the spear. The hardening process was limited only to the basal section.

delta-Tonoplast intrinsic protein defines unique plant vacuole functions

Plant cell vacuoles may have either storage or degradative functions. Vegetative storage proteins (VSPs) are synthesized in response to wounding and to developmental switches that affect carbon and nitrogen sinks. Here we show that VSPs are stored in a unique type of vacuole that is derived from degradative central vacuoles coincident with insertion of a new tonoplast intrinsic protein (TIP), delta-TIP, into their membranes. This finding demonstrates a tight coupling between the presence of delta-TIP and acquisition of a specialized storage function and indicates that TIP isoforms may determine vacuole identity.

Enterohemorrhagic Escherichia coli O157:H7 present in radish sprouts

Using cultivation, immunofluorescence microscopy, and scanning electron microscopy, we demonstrated the presence of viable enterohemorrhagic Escherichia coli O157:H7 not only on the outer surfaces but also in the inner tissues and stomata of cotyledons of radish sprouts grown from seeds experimentally contaminated with the bacterium. HgCl2 treatment of the outer surface of the hypocotyl did not kill the contaminating bacteria, which emphasized the importance of either using seeds free from E. coli O157:H7 in the production of radish sprouts or heating the sprouts before they are eaten.

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.

A rapid increase in the level of binding protein (BiP) is accompanied by synthesis and degradation of storage proteins in pumpkin cotyledons

The binding protein (BiP) has been implicated in cotranslational folding of nascent polypeptides, and in the recognition and disposal of aberrant polypeptides. To elucidate the involvement of BiP in the biosynthesis of vacuolar proteins, we have characterized the protein in pumpkin cotyledons during seed maturation and seedling growth. Isolated microsomes from maturing pumpkin cotyledons contained a significant amount of BiP, protein-disulfide isomerase and calreticulin. We have purified a 70-kDa protein; sequences of the N-terminus and internal fragments of this protein exhibited a high identity to the sequence of soybean. Immunoblot analysis with specific antibodies raised against the purified BiP showed that the amount of BiP in a cotyledon increased markedly at the middle stages and then decreased. The increase was accompanied by the synthesis of storage proteins and the development of the endoplasmic reticulum in the cotyledons at the middle stage of seed maturation. Most of these storage proteins degraded dramatically between 2 and 5 days after seed germination, and the degradation was also accompanied by a rapid increase in the level of BiP. Subcellular fractionation of the 4-day-old cotyledons showed a high accumulation of BiP in the endoplasmic reticulum. It is possible that BiP might be involved in the synthesis of seed storage proteins during maturation and in the synthesis of hydrolytic enzymes responsible for the degradation of the storage proteins during seed germination.

[Dietary fiber and pectin fractions of Beta vulgaris var. conditiva]

The alcohol-insoluble substance (AIS) from red beet (Beta vulgaris var. conditiva Alef.) (3.31% of the edible substance) was extracted sequentially with water, ammonium oxalate, 0.05 N HCl and 0.05 N NaOH. Accordingly 3.93 g, about 0.8 g, 2.96 g resp. 3.80 g galacturonan/100 g AIS were extracted with this procedure. These pectin extracts were purified as Cu2 +-salts and fractionated into a water-soluble and a water-insoluble part. The composition of neutral monosaccharide units was estimated in the fractions. Gal, Ara and Glc dominated; Xyl, Rha and Man were also present but in smaller amounts. A higher GalA content was found in the soluble fractions (with the exception of the alkali extract). Pectins from red beet are middle- or low-esterified and partially acetylated. The composition of the AIS and of the residue after pectin extraction (RE) was determined (14.6 resp. 9.5% pectin; 10,6 resp. 17.6% protein; 58.7 resp. 64.9% total polysaccharides). In the AIS 23.3% soluble and 54.7% insoluble dietary fiber were estimated, whereas in RE 15.3 resp. 54.7% were found (enzymatic method). Following dietary fiber fractions were determined by the detergent method for both preparations: 39.0 resp. 52.7% NDF; 6.3 resp. 4.5% NDF filtrate; 23.6 resp. 41.8% ADF; 1.2 resp. 1.8% lignin. The water binding capacity decreased from 19.85 g water (AIS) to 11. 53 g water (RE) related to 1 g AUS. From these just 50% were found in the NDF fractions and about 13% in the ADF fractions. Alterations of the grown biological structures during pectin extraction and dietary fiber analysis (detergent method) were investigated by scanning electron microscopy.

Changes in physical properties of vacuolar membrane during transformation of protein bodies into vacuoles in germinating pumpkin seeds

Changes in membrane molecular dynamics associated with the transformation of protein body membranes into vacuolar membranes during pumpkin seed germination, were monitored by EPR-spin probe technique. Using highly purified membrane preparations as well as 5-SASL and 16-SASL spin labels, parameters like general membrane lipid fluidity, order parameter, semicone angle, rotational correlation times tau 2B and tau 2C, ratio of immobilized to mobile lipids were determined and the activation energy for rotational diffusion of 16-SASL was calculated. Analysis of these parameters at different temperatures indicated a more rigid nature of protein body membrane comparing to vacuolar membrane, as a result of a more restricted motional freedom of lipids. These differences are discussed in terms of protein composition and various functional specialization of both types of membranes.

Specific, high affinity binding of chitin fragments to tomato cells and membranes. Competitive inhibition of binding by derivatives of chitooligosaccharides and a Nod factor of Rhizobium

Suspension-cultured tomato cells have a sensitive perception system for chitin fragments with a degree of polymerization (DP) > or = 4 and react to these compounds with a transient alkalinization of their culture medium (Felix, G., Regenass, M., and Boller, T. (1993) Plant. J. 4, 307-316). A chitin fragment with DP 5 was aminated at the reducing end and coupled to t-butoxycarbonyl-L-[35S]methionine via an amidoglycine spacer. The radiolabeled chitin fragment (approximately 1000 Ci/mmol) exhibited specific, saturable, reversible binding to whole tomato cells as well as to tomato microsomal membranes with dissociation constants of 1.4 and 23 nM, respectively. Binding of the radioligand was competed by chitin fragments of different DP with IC50 values (50% inhibition of binding) that closely paralleled the concentrations inducing the alkalinization response half-maximally. Deacetylated chitooligosaccharides and N-propanoyl chitooligosaccharides were weak elicitors of the alkalinization response as well as weak competitors of radioligand binding. A lipochitooligosaccharide (Nod factor) from Rhizobium leguminosarum stimulated the alkalinization response in tomato cells half-maximally at 3 nM and competed radioligand binding to the cells with an IC50 of 8 nM. These results demonstrate the presence of a high affinity binding site for chitin fragments on the tomato cell membrane that may function as a receptor.

Ultrastructural rRNA localization in plant cell nucleoli. RNA/RNA in situ hybridization, autoradiography and cytochemistry

The distribution of ribosomal transcripts in the plant nucleolus has been studied by non-isotopic in situ hybridization in ultrathin Lowicryl K4M sections and by high-resolution autoradiography after labelling with tritiated uridine. In parallel, cytochemical techniques were applied to localize RNA on different plant nucleolar components of Allium cepa L. root meristematic cells and Capsicum annuum L. pollen grains. For RNA/RNA in situ hybridization, several biotinylated single-stranded ribosomal RNA probes were used for mapping different fragments of the 18 S and the 25 S rRNA gene transcribed regions. Ribosomal RNAs (from pre-rRNAs to mature 18 and 25 S RNAs) were found in the nucleolus, in the dense fibrillar (DFC) and granular components (GC). Hybridization signal was found at the periphery of some fibrillar centres (FCs) with probes recognizing both 18 and 25 S rRNA sequences. A quantitative study was performed to analyze the significance of this labelling. Incorporation of tritiated uridine into roots was carried out and, later, after a long time-exposure, autoradiography revealed the presence of newly synthesized RNA mainly in the DFC and at the periphery of the FCs. The presence of RNA in these areas was also confirmed by the cytochemical techniques used in this study. Taken together, these data favour the hypothesis that transcription can begin at the periphery of the FCs, although we cannot exclude the possibility that the DFC plays a role in this process.

Orientation of macromolecules in the walls of elongating carrot cells

When round cells from a carrot cell suspension culture are diluted into fresh medium without auxin, the cells elongate to almost 50 times their original diameter within three days. This process of elongation is accompanied by changes in both the composition and the orientation of cell wall polymers. We have obtained information on the orientation of wall polymers in elongating cells by two complementary techniques, one using microscopy and one spectroscopy. Images obtained by the fast-freeze, deep-etch, rotary-shadowed replica technique show that walls of round carrot cells have no net orientation of cellulose microfibrils, and that many thin fibres can be seen cross-linking microfibrils. Walls of elongated carrot cells, in contrast, show a marked net orientation of microfibrils at right angles to the axis of elongation. Fourier Transform Infrared (FTIR) spectra obtained from defined areas of single cell walls show that walls of round carrot cells contain more protein, esters and phenolics in a given area (10 microns × 10 microns) than walls of elongated carrot cells, that contain proportionally more carbohydrate. The orientation of particular functional groups, with respect to the direction of elongation of the cell, can be determined by inserting a polariser into the path of the infrared beam, before it passes through a cell wall sample mounted on the stage of the microscope accessory. In the walls of elongated cells, ester bands, amide bands characteristic of proteins, and stretching frequencies in the carbohydrate region of the spectrum all show a net orientation transverse to the long axis of the cells. In the walls of round carrot cells, however, there is no such net orientation of polymers. Spectra obtained from 25 microns-thick fresh sections of the etiolated stem of a carrot seedling show that different wall components are polarised in different tissue types. These techniques have therefore enabled us to define differences in both the composition and the architecture of walls of elongating cells at the level of a single cell, and to suggest that polymers not previously thought to be ordered, such as pectin and protein, are strictly oriented in some wall types.

Complementary immunolocalization patterns of cell wall hydroxyproline-rich glycoproteins studied with the use of antibodies directed against different carbohydrate epitopes

Antisera raised against the major hydroxyproline-rich glycoprotein (HRGP) in carrot (Daucus carota L.) taproot, extensin-1, and a minor HRGP, extensin-2, were characterized by western blot analysis, enzyme-linked immunosorbent assay, and periodate oxidation and found to be directed against carbohydrate epitopes shared by both glycoproteins. The anti-extensin-1 antibodies (gE1) target periodate-sensitive epitopes and may recognize the terminal alpha-1,3-arabinoside of extensin-1. The anti-extensin-2 antibodies (gE2) recognize periodate-insensitive epitopes, possibly binding the reducing, internal beta-1,2-arabinosides on the carbohydrate side chains. Despite the cross-reactivity of these antibodies, immunolocalization studies of carrot taproot and green bean (Phaseolus vulgaris L.) leaf tissues reveal a spatial segregation of gE1- and gE2-labeling patterns. The gE1 antibodies bind only to the cellulose-rich region of the cell wall (J.P. Staehelin and L.A. Stafstrom [1988] Planta 174: 321-332), whereas gE2 labeling is restricted to the expanded middle lamella at three cell junctions. Periodate oxidation of nonosmicated, thin-sectioned tissue abolishes gE1 labeling but leads to labeling of the entire cell wall by gE2, presumably as a result of unmasking cryptic epitopes on extensin-1 in the cellulose layer. Purified extensin-2 protein is more efficient than extensin-1 protein at agglutinating avirulent Pseudomonas strains lacking extracellular polysaccharide. Our results indicate that extensin-2 does not form a heterologous HRGP network with extensin-1 and that, in contrast to extensin-1, which appears to serve a structural role, extensin-2 could participate in passive defense responses against phytopathogenic bacteria.

[Effect of single exposure of the seeds of Lactuca sativa to heavy ions of galactic cosmic radiation at orbital manned space stations "Saliut-6" and "Saliut-7"]

This paper presents the results of measuring single heavy ions (HI) of galactic cosmic rays during 40- to 457-day flights of manned orbital stations Salyut-6 and Salyut-7. Cytogenetic analysis of Lactuca sativa seeds showed a significant increase of aberrant cells and cells with multiple chromosome aberrations in seeds hit with HI compared to non-hit++ seeds during flights of 123 to 457 days in duration. There was a good correlation between curves showing aberrant cells as a function of flight of the dose absorbed, viz. 16.0 to 63.4 mGy. These findings suggest a high biological effectiveness of heavy ions of galactic cosmic rays.

Specific photoaffinity labeling of two plasma membrane polypeptides with an azido auxin

Plasma membrane vesicles were isolated from zucchini (Cucurbita pepo) hypocotyl tissue by aqueous phase partitioning and assessed for homogeneity by the use of membrane-specific enzyme assays. The highly pure (ca. 95%) plasma membrane vesicles maintained a pH differential across the membrane and accumulated a tritiated azido analogue of 3-indoleacetic acid (IAA), 5-azido-[7-3H]IAA ([3H]N3IAA), in a manner similar to the accumulation of [3H]IAA. The association of the [3H]N3IAA with membrane vesicles was saturable and subject to competition by IAA and auxin analogues. Auxin-binding proteins were photoaffinity labeled by addition of [3H]N3IAA to plasma membrane vesicles prior to exposure to UV light (15 sec; 300 nm) and detected by subsequent NaDodSO4/PAGE and fluorography. When the reaction temperature was lowered to -196 degrees C, high-specific-activity labeling of a 40-kDa and a 42-kDa polypeptide was observed. Triton X-100 (0.1%) increased the specific activity of labeling and reduced the background, which suggests that the labeled polypeptides are intrinsic membrane proteins. The labeled polypeptides are of low abundance, as expected for auxin receptors. Further, the addition of IAA and auxin analogues to the photoaffinity reaction mixture resulted in reduced labeling that was qualitatively similar to their effects on the accumulation of radiolabeled IAA in membrane vesicles. Collectively, these results suggest that the radiolabeled polypeptides are auxin receptors. The covalent nature of the label should facilitate purification and further characterization of the receptors.

A method for staining pollen tubes in pistil

A quadruple staining procedure has been developed for staining pollen tubes in pistil. The staining mixture is made by adding the following in the order given: lactic acid, 80 ml; 1% aqueous malachite green, 4 ml; 1% aqueous acid fuchsin, 6 ml; 1% aqueous aniline blue, 4 ml; 1% orange G in 50% alcohol, 2 ml; and chloral hydrate, 5 g. Pistils are fixed for 6 hr in modified Carnoy's fluid (absolute alcohol:chloroform:glacial acetic acid 6:4:1), hydrated in descending alcohols, transferred to stain and held there for 24 hr at 45 +/- 2 C. They were then transferred to a clearing and softening fluid containing 78 ml lactic acid, 10 g phenol, 10 g chloral hydrate and 2 ml 1% orange G. The pistils were held there for 24 hr at 45 +/- 2 C, hydrolyzed in the clearing and softening fluid at 58 +/- 1 C for 30 min, then stored in lactic acid for later use or immediately mounted in a drop of medium containing equal parts of lactic acid and glycerol for examination. Pollen tubes are stained dark blue to bluish red and stylar tissue light green to light greenish blue. This stain permits pollen tubes to be traced even up to their entry into the micropyle.