A simple procedure to obtain a medium-size oligogalacturonic acids fraction from orange peel and apple pomace wastes

Pectin oligosaccharides, which can be obtained from fruit wastes, have proven their potential as plant immune-system elicitors. Although the precise size of active species is still under investigation, medium size oligosaccharides have been reported as the most active. Three defined oligogalacturonic acid (OGAs) mixtures were produced from commercial pectin, orange peel and apple pomace residues. The methodology developed involves two sequential acid treatments followed by stepwise ethanol precipitation. Without the need of chromatographic separations, three different fractions were obtained. The fractions were analyzed by high performance anion exchange chromatography (HPAEC) and were completely characterized by mass spectrometry, showing that the small size, medium size and large size fractions contained OGAs of degree of polymerization 3 to 9, 6 to 18, and 16 to 55, respectively.

Pectic oligosacharides from lemon peel wastes: production, purification, and chemical characterization

Lemon peel wastes were extracted with water to remove free sugars and other soluble compounds, and the insoluble solid was employed as a substrate for the manufacture of pectin-derived oligosaccharides by processing with hot, compressed water. When water-extracted lemon peel wastes were treated with water at 160 °C, the oligomer concentration reached the maximum value (31 g/L). Autohydrolysis liquors were subjected to two membrane filtration stages (diafiltration followed by concentration), yielding a refined product containing about 98 wt % of oligomers at a global yield of 14 kg/100 kg oven-dry lemon peel. The concentrate contained oligogalacturonides (with DP in the range of 2-18) and arabinooligosaccharides (with DP in the range of 2-8).

Signaling role of oligogalacturonides derived during cell wall degradation

In addition to the role of the cell wall as a physical barrier against pathogens, some of its constituents, such as pectin-derived oligogalacturonides (OGAs) are essential components to trigger signaling pathways that induce rapid defense responses. Many pathogens directly penetrate the cell wall to access water and nutrients of the plant protoplast, and a rigid cell wall can fend off pathogen attack by forming an impenetrable physical barrier. Thus, cell wall integrity sensing is one mechanism by which plants may detect pathogen attack. Moreover, when the plant-pathogen interaction occurred, OGAs released during cell wall modification can trigger plant defense (e.g., production of reactive oxygen species, production of anti-microbial metabolites and synthesis of pathogenesis-related proteins). This review documents and discusses studies suggesting that OGAs play a dual signaling role during pathogen attack by inducing defense responses and plant architecture adjustment.

Chitosan oligosaccharides modulate the supramolecular conformation and the biological activity of oligogalacturonides in Arabidopsis

Plant cell walls undergo remodeling during growth and development and are the first target of many invading pathogens. Acidic pectin (homogalacturonans) binds calcium and forms chain dimers called egg boxes and even multimers at higher calcium ion concentrations. Chitosan, the deacetylated form of chitin produced by fungi when invading plant tissues, is a cationic polymer that can interact with negatively charged pectin. The interaction between chitosan oligomers (COS) and pectic egg boxes was investigated using 2F4, a monoclonal antibody specific for calcium-associated dimers of pectin. Depending on the size of the pectic molecules, the COS to pectin ratio, the degree of polymerization and the degree of acetylation of COS in the mixture, the calcium-induced egg box conformation of oligogalacturonides (OGA) was strongly stabilized or destroyed. The biological activity of COS-stabilized egg boxes was assayed on Arabidopsis cell suspensions. COS-OGA egg boxes strongly enhanced extracellular alkalinization and decreased potassium fluxes compared to control COS and OGA alone. Furthermore, OGA rescued Arabidopsis from cell death induced by higher concentrations of deacetylated COS. The stabilized COS-OGA egg boxes could constitute a combined emergency signal that informs plant cells on both cell wall degradation and pathogen presence, triggering a much stronger response than individual components alone.

Partial demethylation of oligogalacturonides by pectin methyl esterase 1 is required for eliciting defence responses in wild strawberry (Fragaria vesca)

In addition to the role of the cell wall as a physical barrier against pathogens, some of its constituents, such as pectin-derived oligogalacturonides (OGA), are essential components for elicitation of defence responses. To investigate how modifications of pectin alter defence responses, we expressed the fruit-specific Fragaria x ananassa pectin methyl esterase FaPE1 in the wild strawberry Fragaria vesca. Pectin from transgenic ripe fruits differed from the wild-type with regard to the degree and pattern of methyl esterification, as well as the average size of pectin polymers. Purified oligogalacturonides from the transgenic fruits showed a reduced degree of esterification compared to oligogalacturonides from wild-type fruits. This reduced esterification is necessary to elicit defence responses in strawberry. The transgenic F. vesca lines had constitutively activated pathogen defence responses, resulting in higher resistance to the necrotropic fungus Botrytis cinerea. Further studies in F. vesca and Nicotiana benthamiana leaves showed that the elicitation capacity of the oligogalacturonides is more specific than previously envisaged.

Cell wall pectins and xyloglucans are internalized into dividing root cells and accumulate within cell plates during cytokinesis

Recently, we have reported that cell wall pectins are internalized into apical meristem root cells. In cells exposed to the fungal metabolite brefeldin A, all secretory pathways were inhibited, while endocytic pathways remained intact, resulting in accumulation of internalized cell wall pectins within brefeldin A-induced compartments. Here we report that, in addition to the already published cell wall epitopes, rhamnogalacturonan I and xyloglucans also undergo large-scale internalization into dividing root cells. Interestingly, multilamellar endosomes were identified as compartments internalizing arabinan cell wall pectins reactive to the 6D7 antibody, while large vacuole-like endosomes internalized homogalacturonans reactive to the 2F4 antibody. As all endosomes belong topographically to the exocellular space, cell wall pectins deposited in these "cell wall islands", enclosed by the plasma-membrane-derived membrane, are ideally suited to act as temporary stores for rapid formation of cell wall and generation of new plasma membrane. In accordance with this notion, we report that all cell wall pectins and xyloglucans that internalize into endosomes are highly enriched within cytokinetic cell plates and accumulate within brefeldin A compartments. On the other hand, only small amounts of the pectins reactive to the JIM7 antibody, which are produced in the Golgi apparatus, localize to cell plates and they do not accumulate within brefeldin A compartments. In conclusion, meristematic root cells have developed pathways for internalization and recycling of cell wall molecules which are relevant for plant-specific cytokinesis.

F-actin-dependent endocytosis of cell wall pectins in meristematic root cells. Insights from brefeldin A-induced compartments

Brefeldin A (BFA) inhibits exocytosis but allows endocytosis, making it a valuable agent to identify molecules that recycle at cell peripheries. In plants, formation of large intracellular compartments in response to BFA treatment is a unique feature of some, but not all, cells. Here, we have analyzed assembly and distribution of BFA compartments in development- and tissue-specific contexts of growing maize (Zea mays) root apices. Surprisingly, these unique compartments formed only in meristematic cells of the root body. On the other hand, BFA compartments were absent from secretory cells of root cap periphery, metaxylem cells, and most elongating cells, all of which are active in exocytosis. We report that cell wall pectin epitopes counting rhamnogalacturonan II dimers cross-linked by borate diol diester, partially esterified (up to 40%) homogalacturonan pectins, and (1-->4)-beta-D-galactan side chains of rhamnogalacturonan I were internalized into BFA compartments. In contrast, Golgi-derived secretory (esterified up to 80%) homogalacturonan pectins localized to the cytoplasm in control cells and did not accumulate within characteristic BFA compartments. Latrunculin B-mediated depolymerization of F-actin inhibited internalization and accumulation of cell wall pectins within intracellular BFA compartments. Importantly, cold treatment and protoplasting prevented internalization of wall pectins into root cells upon BFA treatment. These observations suggest that cell wall pectins of meristematic maize root cells undergo rapid endocytosis in an F-actin-dependent manner.

Isolation of oligogalacturonic acids up to DP 20 by preparative high-performance anion-exchange chromatography and pulsed amperometric detection

Milligram quantities of oligogalacturonic acids up to a degree of polymerization (DP) of 20 were purified by high-performance anion-exchange chromatography utilizing a preparative-scale (21-mm i.d.) CarboPac PA1 column and a nonlinear potassium acetate (pH 7.5) gradient. Detection was accomplished by pulsed amperometry without post-column addition of hydroxide. Pulsed amperometry at near-neutral pH is an excellent detection method for preparative separations of carbohydrates because it avoids base-catalyzed degradation reactions that can occur at high pH. This method was simpler, faster, had higher sample loading capacity and allowed for the isolation of higher DP oligogalacturonic acids than other methods reported previously. With this improved method, multi-milligram quantities of valuable oligogalacturonic acids (up to DP 20) can be readily isolated.

Signal transduction in the wound response of tomato plants

The wound response of tomato plants has been extensively studied, and provides a useful model to understand signal transduction events leading from injury to marker gene expression. The principal markers that have been used in these studies are genes encoding proteinase inhibitor (pin) proteins. Activation of pin genes occurs in the wounded leaf and in distant unwounded leaves of the plant. This paper reviews current understanding of signalling pathways in the wounded leaf, and in the systemically responding unwounded leaves. First, the nature of known elicitors and their potential roles in planta are discussed, in particular, oligogalacturonides, jasmonates and the peptide signal, systemin. Inhibitors of wound-induced proteinase inhibitor (pin) expression are also reviewed, with particular reference to phenolics, sulphydryl reagents and fusicoccin. In each section, results obtained from the bioassay are considered within the wider context of data from mutants and from transgenic plants with altered levels of putative signalling components. Following this introduction, current models for pin gene regulation are described and discussed, together with a summary for the involvement of phosphorylation-dephosphorylation in wound signalling. Finally, a new model for wound-induced pin gene expression is presented, arising from recent data from the author's laboratory.

Short chain oligogalacturonides induce ethylene production and expression of the gene encoding aminocyclopropane 1-carboxylic acid oxidase in tomato plants

Oligogalacturonic acids (OGAs), derived from plant cell wall pectin, have been implicated in a number of signal transduction pathways involved in growth, development and defense responses of higher plants. This study investigates the size range of OGAs capable of inducing ethylene synthesis in tomato plants, and demonstrates that in contrast with many other effects, only short chain OGAs are active. Oligomers across a range of DP from 2-15 were separated and purified to homogeneity by QAE-Sephadex anion exchange chromatography using a novel elution system. The OGAs were applied to tomato plants and assayed for their ability to induce ethylene gas release and changes in steady state levels of mRNA encoding the ethylene forming enzyme aminocyclopropane-1-carboxylic acid oxidase (ACO). The study demonstrated that only OGAs in the size range of DP4-6 were active both in eliciting ACO expression and in the production of ethylene.

Rhamnogalacturonan alpha-d-galactopyranosyluronohydrolase. An enzyme that specifically removes the terminal nonreducing galacturonosyl residue in rhamnogalacturonan regions of pectin

A new enzyme, rhamnogalacturonan (RG) alpha-d-galactopyranosyluronohydrolase (RG-galacturonohydrolase), able to release a galacturonic acid residue from the nonreducing end of RG chains but not from homogalacturonan, was purified from an Aspergillus aculeatus enzyme preparation. RG-galacturonohydrolase acted with inversion of anomeric configuration, initially releasing beta-d-galactopyranosyluronic acid. The enzyme cleaved smaller RG substrates with the highest catalytic efficiency. A Michaelis constant of 85 &mgr;m and a maximum reaction rate of 160 units mg-1 was found toward a linear RG fragment with a degree of polymerization of 6. RG-galacturonohydrolase had a molecular mass of 66 kD, an isoelectric point of 5.12, a pH optimum of 4.0, and a temperature optimum of 50 degreesC. The enzyme was most stable between pH 3.0 and 6.0 (for 24 h at 40 degreesC) and up to 60 degreesC (for 3 h).

Analysis of partially methyl-esterified galacturonic acid oligomers by high-performance anion-exchange chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Two methods were developed to detect partially methyl-esterified galacturonic acid oligomers, generated by endopolygalacturonase treatment of a 30% methyl-esterified pectin. The enzyme digest was shown, by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, to contain sodiated galacturonic acid oligomers with a degree of polymerization of 2-12, containing 0-6 methyl esters. Galacturonic acid (monomer) could not be detected because of matrix ions interference in the low mass region. Using high-performance anion-exchange chromatography, with a sodium acetate gradient at pH 5.0 and postcolumn sodium hydroxide addition to allow pulsed amplified detection, a complex elution profile was obtained with the endopolygalacturonase-treated 30% methyl-esterified pectin. All the components eluted before nonesterified tetragalacturonic acid. The partially methyl-esterified oligogalacturonic acids eluted in a discernible series of oligomers with an identical number of nonesterified carboxylic acid groups; the large, more esterified oligomers eluted before small, less esterified oligomers. The methyl esters may hinder the interaction of the neighboring carboxylic acid groups with the anion-exchange resin, thereby giving the components an apparent lower overall negative charge.

Functional interactions among cytoskeleton, membranes, and cell wall in the pollen tube of flowering plants

The pollen tube is a cellular system that plays a fundamental role during the process of fertilization in higher plants. Because it is so important, the pollen tube has been subjected to intensive studies with the aim of understanding its biology. The pollen tube represents a fascinating model for studying interactions between the internal cytoskeletal machinery, the membrane system, and the cell wall. These compartments, often studied as independent units, show several molecular interactions and can influence the structure and organization of each other. The way the cell wall is constructed, the dynamics of the endomembrane system, and functions of the cytoskeleton suggest that these compartments are a molecular "continuum," which represents a link between the extracellular environment and the pollen tube cytoplasm. Several experimental approaches have been used to understand how these interactions may translate the pollen-pistil interactions into differential processes of pollen tube growth.

Analysis of linear oligogalacturonic acids by negative-ion electrospray ionization mass spectrometry

Linear oligogalacturonic acids (1,4-linked alpha-D-galacturonic acid oligomers), obtained by partial acid hydrolysis of orange polygalacturonides, were studied by negative-ion electrospray ionization mass spectrometry, without prior sample derivatization. After preparative separation using high-resolution anion-exchange chromatography, some fractions enriched in uronic acids were desalted, transferred into a methanol+water solution adjusted to pH 10, and directly submitted to electrospray ionization mass spectrometry in the negative-ion recording mode. Clear molecular mass assignments of the oligomers, covering a degree of polymerization between 4 and 7, were obtained.