Distribution of pectic epitopes in cell walls of the sugar beet root

Integrated insights into the cytological, histochemical, and cell wall composition features of Espinosa nothofagi (Hymenoptera) gall tissues: implications for functionality

Many insect-induced galls are considered complex structures due to their tissue compartmentalization and multiple roles performed by them. The current study investigates the complex interaction between Nothofagus obliqua host plant and the hymenopteran gall-inducer Espinosa nothofagi, focusing on cell wall properties and cytological features. The E. nothofagi galls present an inner cortex with nutritive and storage tissues, as well as outer cortex with epidermis, chlorenchyma, and water-storing parenchyma. The water-storing parenchyma cells are rich in pectins, heteromannans, and xyloglucans in their walls, and have large vacuoles. Homogalacturonans contribute to water retention, and periplasmic spaces function as additional water reservoirs. Nutritive storage cell walls support nutrient storage, with plasmodesmata facilitating nutrient mobilization crucial for larval nutrition. Their primary and sometimes thick secondary cell walls support structural integrity and act as a carbon reserve. The absent labeling of non-cellulosic epitopes indicates a predominantly cellulosic nature in nutritive cell walls, facilitating larval access to lipid, protein, and reducing sugar-rich contents. The nutritive tissue, with functional chloroplasts and high metabolism-related organelles, displays signs of self-sufficiency, emphasizing its role in larval nutrition and cellular maintenance. Overall, the intricate cell wall composition in E. nothofagi galls showcases adaptations for water storage, nutrient mobilization, and larval nutrition, contributing significantly to our understanding of plant-insect interactions.

Haplotype-based phylogenetic analysis and population genomics uncover the origin and domestication of sweetpotato

The hexaploid sweetpotato (Ipomoea batatas) is one of the most important root crops worldwide. However, its genetic origin remains controversial, and its domestication history remains unknown. In this study, we used a range of genetic evidence and a newly developed haplotype-based phylogenetic analysis to identify two probable progenitors of sweetpotato. The diploid progenitor was likely closely related to Ipomoea aequatoriensis and contributed the B1 subgenome, IbT-DNA2, and the lineage 1 type of chloroplast genome to sweetpotato. The tetraploid progenitor of sweetpotato was most likely I. batatas 4x, which donated the B2 subgenome, IbT-DNA1, and the lineage 2 type of chloroplast genome. Sweetpotato most likely originated from reciprocal crosses between the diploid and tetraploid progenitors, followed by a subsequent whole-genome duplication. In addition, we detected biased gene exchanges between the subgenomes; the rate of B1 to B2 subgenome conversions was nearly three times higher than that of B2 to B1 subgenome conversions. Our analyses revealed that genes involved in storage root formation, maintenance of genome stability, biotic resistance, sugar transport, and potassium uptake were selected during the speciation and domestication of sweetpotato. This study sheds light on the evolution of sweetpotato and paves the way for improvement of this crop.

Black Poplar (Populus nigra L.) Root Extracellular Trap, Structural and Molecular Remodeling in Response to Osmotic Stress

The root extracellular trap (RET) consists of root-associated, cap-derived cells (root AC-DCs) and their mucilaginous secretions, and forms a structure around the root tip that protects against biotic and abiotic stresses. However, there is little information concerning the changes undergone by the RET during droughts, especially for tree species. Morphological and immunocytochemical approaches were used to study the RET of black poplar (Populus nigra L.) seedlings grown in vitro under optimal conditions (on agar-gelled medium) or when polyethylene glycol-mediated (PEG6000—infused agar-gelled medium) was used to mimic drought conditions through osmotic stress. Under optimal conditions, the root cap released three populations of individual AC-DC morphotypes, with a very low proportion of spherical morphotypes, and equivalent proportions of intermediate and elongated morphotypes. Immunolabeling experiments using anti-glycan antibodies specific to cell wall polysaccharide and arabinogalactan protein (AGP) epitopes revealed the presence of homogalacturonan (HG), galactan chains of rhamnogalacturonan-I (RG-I), and AGPs in root AC-DC cell walls. The data also showed the presence of xylogalacturonan (XGA), xylan, AGPs, and low levels of arabinans in the mucilage. The findings also showed that under osmotic stress conditions, both the number of AC-DCs (spherical and intermediate morphotypes) and the total quantity of mucilage per root tip increased, whereas the mucilage was devoid of the epitopes associated with the polysaccharides RG-I, XGA, xylan, and AGPs. Osmotic stress also led to reduced root growth and increased root expression of the P5CS2 gene, which is involved in proline biosynthesis and cellular osmolarity maintenance (or preservation) in aerial parts. Together, our findings show that the RET is a dynamic structure that undergoes pronounced structural and molecular remodeling, which might contribute to the survival of the root tip under osmotic conditions.

Microcystin-LR and cyanobacterial extracts alter the distribution of cell wall matrix components in rice root cells

Cyanobacterial toxins (known as cyanotoxins) disrupt the plant cytoskeleton (i.e. microtubules and F-actin), which is implicated in the regulation of cell wall architecture. Therefore, cyanotoxins are also expected to affect cell wall structure and composition. However, the effects of cyanobacterial toxicity on plant cell wall have not been yet thoroughly studied. Accordingly, the alterations of cell wall matrix after treatments with pure microcystin-LR (MC-LR), or cell extracts of one MC-producing and one non-MC-producing Microcystis strain were studied in differentiated Oryza sativa (rice) root cells. Semi-thin transverse sections of variously treated LR-White-embedded roots underwent immunostaining for various cell wall epitopes, including homogalacturonans (HGs), arabinogalactan-proteins (AGPs), and hemicelluloses. Homogalacturonan and arabinan distribution patterns were altered in the affected roots, while a pectin methylesterase (PME) activity assay revealed that PMEs were also affected. Elevated intracellular Ca²⁺ levels, along with increased callose and mixed linkage glucans (MLGs) deposition, were also observed after treatment. Xyloglucans appeared unaffected and lignification was not observed. The exact mechanism of cyanobacterial toxicity against the cell wall is to be further investigated.

The Root Hair Development of Pectin Polygalacturonase PGX2 Activation Tagging Line in Response to Phosphate Deficiency

Pectin, cellulose, and hemicellulose constitute the primary cell wall in eudicots and function in multiple developmental processes in plants. Root hairs are outgrowths of specialized epidermal cells that absorb water and nutrients from the soil. Cell wall architecture influences root hair development, but how cell wall remodeling might enable enhanced root hair formation in response to phosphate (P) deficiency remains relatively unclear. Here, we found that POLYGALACTURONASE INVOLVED IN EXPANSION 2 (PGX2) functions in conditional root hair development. Under low P conditions, a PGX2 activation tagged line (PGX2^AT ) displays bubble-like root hairs and abnormal callose deposition and superoxide accumulation in roots. We found that the polar localization and trafficking of PIN2 are altered in PGX2^AT roots in response to P deficiency. We also found that actin filaments were less compact but more stable in PGX2^AT root hair cells and that actin filament skewness in PGX2^AT root hairs was recovered by treatment with 1-N-naphthylphthalamic acid (NPA), an auxin transport inhibitor. These results demonstrate that activation tagging of PGX2 affects cell wall remodeling, auxin signaling, and actin microfilament orientation, which may cooperatively regulate root hair development in response to P starvation.

Sodium bicarbonate reduces the cooked hardness of lotus rhizome via side chain rearrangement and pectin degradation

In this study, 0.1% (W/V) sodium bicarbonate (SB) solution was used to soften lotus rhizome, and the mechanism was characterized by monoclonal antibodies labeling (mAbs) and atomic force microscopy (AFM). The results showed that the cell wall of lotus rhizome was disintegrated under SB treatment. In addition, the mAbs results revealed that low-esterified homogalacturonan (HG) at the tricellular junction was degraded, the rearrangement of Ara and the interaction between Gal and cellulose may be related to the texture changes. Compared with distilled water treatment, SB treatment reduced the relative content of pectin from 34.1% to 19.1% while increased that of cellulose from 65.9% to 80.9%. AFM results revealed that the height of CSF skeleton decreased from about 32 nm to 1.5 nm. These results clearly demonstrate that cooking with 0.1% SB can soften lotus rhizome through degradation of pectin and arrangement of side chains of rhamnogalacturonan-Ⅰ (RG-Ⅰ).

Remodeling of Cell Wall Components in Root Nodules and Flower Abscission Zone under Drought in Yellow Lupine

We recently showed that yellow lupine is highly sensitive to soil water deficits since this stressor disrupts nodule structure and functioning, and at the same time triggers flower separation through abscission zone (AZ) activation in the upper part of the plant. Both processes require specific transformations including cell wall remodeling. However, knowledge about the involvement of particular cell wall elements in nodulation and abscission in agronomically important, nitrogen-fixing crops, especially under stressful conditions, is still scarce. Here, we used immuno-fluorescence techniques to visualize dynamic changes in cell wall compounds taking place in the root nodules and flower AZ of Lupinus luteus following drought. The reaction of nodules and the flower AZ to drought includes the upregulation of extensins, galactans, arabinans, xylogalacturonan, and xyloglucans. Additionally, modifications in the localization of high- and low-methylated homogalacturonans and arabinogalactan proteins were detected in nodules. Collectively, we determined for the first time the drought-associated modification of cell wall components responsible for their remodeling in root nodules and the flower AZ of L. luteus. The involvement of these particular molecules and their possible interaction in response to stress is also deeply discussed herein.

High-resolution 3D mapping of rhizosphere glycan patterning using molecular probes in a transparent soil system

Rhizospheres are microecological zones at the interface of roots and soils. Interactions between bacteria and roots are critical for maintaining plant and soil health but are difficult to study because of constraints inherent in working with underground systems. We have developed an in-situ rhizosphere imaging system based on transparent soils and molecular probes that can be imaged using confocal microscopy. We observed spatial patterning of polysaccharides along roots and on cells deposited into the rhizosphere and also co-localised fluorescently tagged soil bacteria. These studies provide insight into the complex glycan landscape of rhizospheres and suggest a means by which root / rhizobacteria interactions can be non-disruptively studied.

The importance of being edgy: cell geometric edges as an emerging polar domain in plant cells

Polarity is an essential feature of multicellular organisms and underpins growth and development as well as physiological functions. In polyhedral plant cells, polar domains at different faces have been studied in detail. In recent years, cell edges (where two faces meet) have emerged as discrete spatial domains with distinct biochemical identities. Here, we review and discuss recent advances in our understanding of cell edges as functional polar domains in plant cells and other organisms, highlighting conceptual parallels and open questions regarding edge polarity.

Cell Wall Composition and Ultrastructural Immunolocalization of Pectin and Arabinogalactan Protein during Olea europaea L. Fruit Abscission

Cell wall modification is integral to many plant developmental processes where cells need to separate, such as abscission. However, changes in cell wall composition during natural fruit abscission are poorly understood. In olive (Olea europaea L.), some cultivars such as 'Picual' undergo massive natural fruit abscission after fruit ripening. This study investigates the differences in cell wall polysaccharide composition and the localization of pectins and arabinogalactan protein (AGP) in the abscission zone (AZ) during cell separation to understand fruit abscission control in 'Picual' olive. To this end, immunogold labeling employing a suite of monoclonal antibodies to cell wall components (JIM13, LM5, LM6, LM19 and LM20) was investigated in olive fruit AZ. Cell wall polysaccharide extraction revealed that the AZ cell separation is related to the de-esterification and degradation of pectic polysaccharides. Moreover, ultrastructural localization showed that both esterified and unesterified homogalacturonans (HGs) localize mainly in the AZ cell walls, including the middle lamella and tricellular junction zones. Our results indicate that unesterified HGs are likely to contribute to cell separation in the olive fruit AZ. Similarly, immunogold labeling demonstrated a decrease in both galactose-rich and arabinose-rich pectins in AZ cell walls during ripe fruit abscission. In addition, AGPs were localized in the cell wall, plasma membrane and cytoplasm of AZ cells with lower levels of AGPs during ripe fruit abscission. This detailed temporal profile of the cell wall polysaccharide composition, and the pectins and AGP immunolocalization in the olive fruit AZ, offers new insights into cell wall remodeling during ripe fruit abscission.

Tubby-like Protein 2 regulates homogalacturonan biosynthesis in Arabidopsis seed coat mucilage

A possible transcription factor TLP2 was identified to be involved in the regulation of HG biosynthesis in Arabidopsis seed mucilage. TLP2 can translocate into nucleus from plasma membrane by interacting with NF-YC3. The discovery of TLP2 gene function can further fulfill the regulatory network of pectin biosynthesis in Arabidopsis thaliana. Arabidopsis seed coat mucilage is an excellent model system to study the biosynthesis, function and regulation of pectin. Rhamnogalacturonan I (RG-I) and homogalacturonan (HG) are the major polysaccharides constituent of the Arabidopsis seed coat mucilage. Here, we identified a Tubby-like gene, Tubby-like protein 2 (TLP2), which was up-regulated in developing siliques when mucilage began to be produced. Ruthenium red (RR) staining of the seeds showed defective mucilage of tlp2-1 mutant after vigorous shaking compared to wild type (WT). Monosaccharide composition analysis revealed that the amount of total sugars and galacturonic acid (GalA) decreased significantly in the adherent mucilage (AM) of tlp2-1 mutant. Immunolabelling and dot immunoblotting analysis showed that unesterified HG decreased in the tlp2-1 mutant. Furthermore, TLP2 can translocate into nucleus by interacting with Nuclear Factor Y subunit C3 (NF-YC3) to function as a transcription factor. RNA-sequence and transactivation assays revealed that TLP2 could activate UDP-glucose 4-epimerase 1 (UGE1). In all, it is concluded that TLP2 could regulate the biosynthesis of HG possibly through the positive activation of UGE1.

Expanding roles for pectins in plant development

Pectins are complex cell wall polysaccharides important for many aspects of plant development. Recent studies have discovered extensive physical interactions between pectins and other cell wall components, implicating pectins in new molecular functions. Pectins are often localized in spatially-restricted patterns, and some of these non-uniform pectin distributions contribute to multiple aspects of plant development, including the morphogenesis of cells and organs. Furthermore, a growing number of mutants affecting cell wall composition have begun to reveal the distinct contributions of different pectins to plant development. This review discusses the interactions of pectins with other cell wall components, the functions of pectins in controlling cellular morphology, and how non-uniform pectin composition can be an important determinant of developmental processes.

Dynamic Changes of Pectin Epitopes in Cell Walls during the Development of the Procambium-Cambium Continuum in Poplar

The change of pectin epitopes during procambium-cambium continuum development was investigated by immunolocalization in poplar. The monoclonal antibody JIM5 labels homogalacturonan (HGA) with a low degree of esterification, and the monoclonal antibody JIM7 labels HGA with a high degree of methyl-esterification. Arabinan, rather than galactan, and HGA with low degree of esterification were located in the cell walls of procambial, while HGA with a low degree of esterification was located in the tangential walls, and galactan was located in both the tangential and radial walls of procambial, yet nearly no arabinan was located in the tangential walls of the cambial cells. The changes in pectin distribution took place when periclinal divisions appeared within a procambial trace. The distribution difference of pectin epitopes was also present in procambium-cambium derivatives. The arabinan existed in all cell walls of primary xylem, but was absent from the tangential walls of secondary xylem cells. The galactan existed only in mature primary phloem. Furthermore, 19 pectin methylesterases (PMEs) genes were identified by RNA sequencing, six genes presented highly differentially and were supposed to be involved in the cell wall esterification process. The results provide direct evidence of the dynamic changes of pectin epitopes during the development of the procambium-cambium continuum in poplar.

Initiation of leaf somatic embryogenesis involves high pectin esterification, auxin accumulation and DNA demethylation in Quercus alba

Somatic embryogenesis is considered a convenient tool for investigating the regulating mechanisms of embryo formation; it is also a feasible system for in vitro regeneration procedures, with many advantages in woody species. Nevertheless, trees have shown recalcitrance to somatic embryogenesis, and its efficiency remains very low in many cases. Consequently, despite the clear potential of somatic embryogenesis in tree breeding programs, its application is limited since factors responsible for embryogenesis initiation have not yet been completely elucidated. In the present work, we investigated key cellular factors involved in the change of developmental program during leaf somatic embryogenesis initiation of white oak (Quercus alba), aiming to identify early markers of the process. The results revealed that pectin esterification, auxin accumulation and DNA demethylation were induced during embryogenesis initiation and differentially found in embryogenic cells, while they were not present in leaf cells before induction or in non-embryogenic cells after embryogenesis initiation. These three factors constitute early markers of leaf embryogenesis and represent processes that could be interconnected and involved in the regulation of cell reprogramming and embryogenesis initiation. These findings provide new insights into the mechanisms underlying plant cell reprogramming, totipotency and embryogenic competence acquisition, especially in tree species for which information is scarce, thus opening up the possibility of efficient manipulation of somatic embryogenesis induction.

The middle lamella-more than a glue

In plant tissues, cells are glued to each other by a pectic polysaccharide rich material known as middle lamella (ML). Along with many biological functions, the ML plays a crucial role in maintaining the structural integrity of plant tissues and organs, as it prevents the cells from separating or sliding against each other. The macromolecular organization and the material properties of the ML are different from those of the adjacent primary cell walls that envelop all plant cells and provide them with a stiff casing. Due to its nanoscale dimensions and the extreme challenge to access the structure for material characterization, the ML is poorly characterized in terms of its distinct material properties. This review explores the ML beyond its functionality as a gluing agent. The putative molecular interactions of constituent macromolecules within the ML and at the interface between ML and primary cell wall are discussed. The correlation between the spatiotemporal distribution of pectic polysaccharides in the different portions of the ML and the subcellular distribution of mechanical stresses within the plant tissue are analyzed.

Distribution of some pectic and arabinogalactan protein epitopes during Solanum lycopersicum (L.) adventitious root development

BACKGROUND

The adventitious roots (AR) of plants share the same function as primary and lateral roots (LR), although their development is mainly an adaptive reaction to stress conditions. Regeneration of grafted plants is often accompanied by AR formation thus making the grafting technique a good model for studying AR initiation and development and their means of emergence. Pectins and arabinogalactan proteins (AGP) are helpful markers of particular cellular events, such as programmed cell death (PCD), elongation, proliferation or other differentiation events that accompany AR development. However, little is known about the distribution of pectins and AGPs during AR ontogeny, either in the primordium or stem tissues from which AR arise or their correspondence with these events during LR formation.

RESULTS

AR were developed from different stem tissues such as parenchyma, xylem rays and the cambium, depending on the stem age and treatment (grafting versus cutting) of the parental tissue. Immunochemical analysis of the presence of pectic (LM8, LM19, LM20) and AGP (JIM8, JIM13, JIM16) epitopes in AR and AR-associated tissues showed differential, tissue-specific distributions of these epitopes. Two pectic epitopes (LM19, LM20) were developmentally regulated and the occurrence of the LM8 xylogalacturonan epitope in the root cap of the AR differed from other species described so far. AGP epitopes were abundantly present in the cytoplasmic compartments (mainly the tonoplast) and were correlated with the degree of cell vacuolisation. JIM8 and JIM13 epitopes were detected in the more advanced stages of primordium development, whereas the JIM16 epitope was present from the earliest division events of the initial AR cells. The comparison between AR and LR showed quantitative (AGP,) and qualitative (pectins) differences.

CONCLUSION

The chemical compositions of adventitious and lateral root cells show differences that correlate with the different origins of these cells. In AR, developmental changes in the distribution of pectins and AGP suggest the turnover of wall compounds. Our data extend the knowledge about the distribution of pectin and AGP during non-embryogenic root development in a species that is important from an agronomic point of view.

A Comparative Study of Sample Preparation for Staining and Immunodetection of Plant Cell Walls by Light Microscopy

Staining and immunodetection by light microscopy are methods widely used to investigate plant cell walls. The two techniques have been crucial to study the cell wall architecture in planta, its deconstruction by chemicals or cell wall-degrading enzymes. They have been instrumental in detecting the presence of cell types, in deciphering plant cell wall evolution and in characterizing plant mutants and transformants. The success of immunolabeling relies on how plant materials are embedded and sectioned. Agarose coating, wax and resin embedding are, respectively, associated with vibratome, microtome and ultramicrotome sectioning. Here, we have systematically carried out a comparative analysis of these three methods of sample preparation when they are applied for cell wall staining and cell wall immunomicroscopy. In order to help the plant community in understanding and selecting adequate methods of embedding and sectioning for cell wall immunodetection, we review in this article the advantages and limitations of these three methods. Moreover, we offer detailed protocols of embedding for studying plant materials through microscopy.

Immunolocalization of pectic polysaccharides during abscission in pea seeds (Pisum sativum L.) and in abscission less def pea mutant seeds

BACKGROUND

In pea seeds (Pisum sativum L.), the presence of the Def locus determines abscission event between its funicle and the seed coat. Cell wall remodeling is a necessary condition for abscission of pea seed. The changes in cell wall components in wild type (WT) pea seed with Def loci showing seed abscission and in abscission less def mutant peas were studied to identify the factors determining abscission and non-abscission event.

METHODS

Changes in pectic polysaccharides components were investigated in WT and def mutant pea seeds using immunolabeling techniques. Pectic monoclonal antibodies (1 → 4)-β-D-galactan (LM5), (1 → 5)-α-L-arabinan(LM6), partially de-methyl esterified homogalacturonan (HG) (JIM5) and methyl esterified HG (JIM7) were used for this study.

RESULTS

Prior to abscission zone (AZ) development, galactan and arabinan reduced in the predestined AZ of the pea seed and disappeared during the abscission process. The AZ cells had partially de-methyl esterified HG while other areas had highly methyl esterified HG. A strong JIM5 labeling in the def mutant may be related to cell wall rigidity in the mature def mutants. In addition, the appearance of pectic epitopes in two F3 populations resulting from cross between WT and def mutant parents was studied. As a result, we identified that homozygous dominant lines (Def/Def) showing abscission and homozygous recessive lines (def/def) showing non-abscission had similar immunolabeling pattern to their parents. However, the heterogeneous lines (Def/def) showed various immunolabeling pattern and the segregation pattern of the Def locus.

CONCLUSIONS

Through the study of the complexity and variability of pectins in plant cell walls as well as understanding the segregation patterns of the Def locus using immunolabeling techniques, we conclude that cell wall remodeling occurs in the abscission process and de-methyl esterification may play a role in the non-abscission event in def mutant. Overall, this study contributes new insights into understanding the structural and architectural organization of the cell walls during abscission.

Colocalization of low-methylesterified pectins and Pb deposits in the apoplast of aspen roots exposed to lead

Low-methylesterified homogalacturonans have been suggested to play a role in the binding and immobilization of Pb in CW. Using root apices of hybrid aspen, a plant with a high phytoremediation potential, as a model, we demonstrated that the in situ distribution pattern of low-methylesterified homogalacturonan, pectin epitope (JIM5-P), reflects the pattern of Pb occurrence. The region which indicated high JIM5-P level corresponded with "Pb accumulation zone". Moreover, JIM5-P was especially abundant in cell junctions, CWs lining the intercellular spaces and the corners of intercellular spaces indicating the highest accumulation of Pb. Furthermore, JIM5-P and Pb commonly co-localized. The observations indicate that low-methylesterified homogalacturonan is the CW polymer that determines the capacity of CW for Pb sequestration. Our results suggest a promising directions for CW modification for enhancing the efficiency of plant roots in Pb accumulation, an important aspect in the phytoremediation of soils contaminated with trace metals.

G-fibre cell wall development in willow stems during tension wood induction

Willows (Salix spp.) are important as a potential feedstock for bioenergy and biofuels. Previous work suggested that reaction wood (RW) formation could be a desirable trait for biofuel production in willows as it is associated with increased glucose yields, but willow RW has not been characterized for cell wall components. Fasciclin-like arabinogalactan (FLA) proteins are highly up-regulated in RW of poplars and are considered to be involved in cell adhesion and cellulose biosynthesis. COBRA genes are involved in anisotropic cell expansion by modulating the orientation of cellulose microfibril deposition. This study determined the temporal and spatial deposition of non-cellulosic polysaccharides in cell walls of the tension wood (TW) component of willow RW and compared it with opposite wood (OW) and normal wood (NW) using specific antibodies and confocal laser scanning microscopy and transmission electron microscopy. In addition, the expression patterns of an FLA gene (SxFLA12) and a COBRA-like gene (SxCOBL4) were compared using RNA in situ hybridization. Deposition of the non-cellulosic polysaccharides (1-4)-β-D-galactan, mannan and de-esterified homogalacturonan was found to be highly associated with TW, often with the G-layer itself. Of particular interest was that the G-layer itself can be highly enriched in (1-4)-β-D-galactan, especially in G-fibres where the G-layer is still thickening, which contrasts with previous studies in poplar. Only xylan showed a similar distribution in TW, OW, and NW, being restricted to the secondary cell wall layers. SxFLA12 and SxCOBL4 transcripts were specifically expressed in developing TW, confirming their importance. A model of polysaccharides distribution in developing willow G-fibre cells is presented.

How to let go: pectin and plant cell adhesion

Plant cells do not, in general, migrate. They maintain a fixed position relative to their neighbors, intimately linked through growth and differentiation. The mediator of this connection, the pectin-rich middle lamella, is deposited during cell division and maintained throughout the cell's life to protect tissue integrity. The maintenance of adhesion requires cell wall modification and is dependent on the actin cytoskeleton. There are developmental processes that require cell separation, such as organ abscission, dehiscence, and ripening. In these instances, the pectin-rich middle lamella must be actively altered to allow cell separation, a process which also requires cell wall modification. In this review, we will focus on the role of pectin and its modification in cell adhesion and separation. Recent insights gained in pectin gel mechanics will be discussed in relation to existing knowledge of pectin chemistry as it relates to cell adhesion. As a whole, we hope to begin defining the physical mechanisms behind a cells' ability to hang on, and how it lets go.

Identification of the primary lesion of toxic aluminum in plant roots

Despite the rhizotoxicity of aluminum (Al) being identified over 100 years ago, there is still no consensus regarding the mechanisms whereby root elongation rate is initially reduced in the approximately 40% of arable soils worldwide that are acidic. We used high-resolution kinematic analyses, molecular biology, rheology, and advanced imaging techniques to examine soybean (Glycine max) roots exposed to Al. Using this multidisciplinary approach, we have conclusively shown that the primary lesion of Al is apoplastic. In particular, it was found that 75 µm Al reduced root growth after only 5 min (or 30 min at 30 µm Al), with Al being toxic by binding to the walls of outer cells, which directly inhibited their loosening in the elongation zone. An alteration in the biosynthesis and distribution of ethylene and auxin was a second, slower effect, causing both a transient decrease in the rate of cell elongation after 1.5 h but also a longer term gradual reduction in the length of the elongation zone. These findings show the importance of focusing on traits related to cell wall composition as well as mechanisms involved in wall loosening to overcome the deleterious effects of soluble Al.

Early markers are present in both embryogenesis pathways from microspores and immature zygotic embryos in cork oak, Quercus suber L

BACKGROUND

In Quercus suber, cork oak, a Mediterranean forest tree of economic and social interest, rapid production of isogenic lines and clonal propagation of elite genotypes have been achieved by developing in vitro embryogenesis from microspores and zygotic embryos respectively. Despite its high potential in tree breeding strategies, due to their recalcitrancy, the efficiency of embryogenesis in vitro systems in many woody species is still very low since factors responsible for embryogenesis initiation and embryo development are still largely unknown. The search for molecular and cellular markers during early stages of in vitro embryogenesis constitutes an important goal to distinguish, after induction, responsive from non-responsive cells, and to elucidate the mechanisms involved in embryogenesis initiation for their efficient manipulation. In this work, we have performed a comparative analysis of two embryogenesis pathways derived from microspores and immature zygotic embryos in cork oak in order to characterize early markers of reprogrammed cells in both pathways. Rearrangements of the cell structural organization, changes in epigenetic marks, cell wall polymers modifications and endogenous auxin changes were analyzed at early embryogenesis stages of the two in vitro systems by a multidisciplinary approach.

RESULTS

Results showed that early embryo cells exhibited defined changes of cell components which were similar in both embryogenesis in vitro systems, cellular features that were not found in non-embryogenic cells. DNA methylation level and nuclear pattern, proportion of esterified pectins in cell walls, and endogenous auxin levels were different in embryo cells in comparison with microspores and immature zygotic embryo cells from which embryos originated, constituting early embryogenesis markers.

CONCLUSIONS

These findings suggest that DNA hypomethylation, cell wall remodeling by pectin esterification and auxin increase are involved in early in vitro embryogenesis in woody species, providing new evidences of the developmental pattern similarity between both embryogenesis pathways, from microspores and immature zygotic embryos, in woody species.

Evolving views of pectin biosynthesis

Recent progress in the identification and characterization of pectin biosynthetic proteins and the discovery of pectin domain-containing proteoglycans are changing our view of how pectin, the most complex family of plant cell wall polysaccharides, is synthesized. The functional confirmation of four types of pectin biosynthetic glycosyltransferases, the identification of multiple putative pectin glycosyl- and methyltransferases, and the characteristics of the GAUT1:GAUT7 homogalacturonan biosynthetic complex with its novel mechanism for retaining catalytic subunits in the Golgi apparatus and its 12 putative interacting proteins are beginning to provide a framework for the pectin biosynthetic process. We propose two partially overlapping hypothetical and testable models for pectin synthesis: the consecutive glycosyltransferase model and the domain synthesis model.

An unusual form of reaction wood in Koromiko [Hebe salicifolia G. Forst. (Pennell)], a southern hemisphere angiosperm

Koromiko [Hebe salicifolia G. Forst. (Pennell)] is a woody angiosperm native to New Zealand and Chile. Hebe spp. belong to the otherwise herbaceous family Plantaginaceae in the order Lamiales. Reaction wood exerting expansional forces was found on the lower side of leaning H. salicifolia stems. Such reaction wood is atypical for angiosperms, which commonly form contracting reaction wood on the upper side of leaning stems. Reaction wood typical for angiosperms is formed by species in other families in the order Lamiales. This suggests that the form of reaction wood is specific to the family level. Functionally the reaction wood of H. salicifolia is similar to that found in gymnosperms, which both act by pushing. However, their chemical, anatomical and physical characteristics are different. Typical features of reaction wood present in gymnosperms such as high density, thick-walled rounded cells and the presence of (1 → 4)-β-galactan in the secondary cell wall layer are absent in H. salicifolia reaction wood. Reaction wood of H. salicifolia varies from normal wood in having a higher microfibril angle, which is likely to determine the direction of generated maturation stresses.

Heterogeneous distribution of pectin epitopes and calcium in different pit types of four angiosperm species

Intervessel pits act as safety valves that prevent the spread of xylem embolism. Pectin-calcium crosslinks within the pit membrane have been proposed to affect xylem vulnerability to cavitation. However, as the chemical composition of pit membranes is poorly understood, this hypothesis has not been verified. Using electron microscopy, immunolabeling, an antimonate precipitation technique, and ruthenium red staining, we studied the distribution of selected polysaccharides and calcium in the pit membranes of four angiosperm tree species. We tested whether shifts in xylem vulnerability resulting from perfusion of stems with a calcium chelating agent corresponded with the distribution of pectic homogalacturonans (HG) and/or calcium within interconduit pit membranes. No HG were detected in the main part of intervessel pit membranes, but were consistently found in the marginal membrane region known as the annulus. Calcium colocalized with HG in the annulus. In contrast to intervessel pits, the membrane of vessel-ray pits showed a high pectin content. The presence of two distinct chemical domains, the annulus and the actual pit membrane, can have substantial implications for pit membrane functioning. We propose that the annulus could affect the observed shift in xylem vulnerability after calcium removal by allowing increased pit membrane deflection.

Linking irradiance-induced changes in pit membrane ultrastructure with xylem vulnerability to cavitation

The effect of shading on xylem hydraulic traits and xylem anatomy was studied in hybrid poplar (Populus trichocarpa x deltoides, clone H11-11). Hydraulic measurements conducted on stem segments of 3-month-old saplings grown in shaded (SH) or control light (C) conditions indicated that shading resulted in more vulnerable and less efficient xylem. Air is thought to enter vessels through pores in inter-vessel pit membranes, thereby nucleating cavitation. Therefore, we tested if the ultrastructure and/or chemistry of pit membranes differed in SH and C plants. Transmission electron micrographs revealed that pit membranes were thinner in SH, which was paralleled by lower compound middle lamella thickness. Immunolabelling with JIM5 and JIM7 monoclonal antibodies surprisingly indicated that pectic homogalacturonans were not present in the mature pit membrane regardless of the light treatment. Porosity measurements conducted with scanning electron microscopy were significantly affected by the method used for sample dehydration. Drying through a gradual ethanol series seems to be a better alternative to drying directly from a hydrated state for pit membrane observations in poplar. Scanning electron microscopy based estimates of pit membrane porosity probably overestimated real porosity as suggested by the results from the 'rare pit' model.

Analyses and localization of pectin-like carbohydrates in cell wall and mucilage of the green alga Netrium digitus

The unicellular, simply shaped desmid Netrium digitus inhabiting acid bog ponds grows in two phases. Prior to division, the cell elongates at its central zone, whereas in a second phase, polar tip growth occurs. Electron microscopy demonstrates that Netrium is surrounded by a morphologically homogeneous cell wall, which lacks pores. Immunocytochemical and biochemical analyses give insight into physical wall properties and, thus, into adaptation to the extreme environment. The monoclonal antibodies JIM5 and JIM7 directed against pectic epitopes with different degrees of esterification label preferentially growing wall zones in Netrium. In contrast, 2F4 marks the cell wall only after experimental de-esterification. Electron energy loss spectroscopy reveals Ca-binding capacities of pectins and gives indirect evidence for the degree of their esterification. An antibody raised against Netrium mucilage is not only specific to mucilage but also recognizes wall components in transmission electron microscopy and dot blots. These results indicate a smooth transition between mucilage and the cell wall in Netrium.

Recovery of heat shock-triggered released apoplastic Ca2+ accompanied by pectin methylesterase activity is required for thermotolerance in soybean seedlings

Synthesis of heat shock proteins (HSPs) in response to heat shock (HS) is essential for thermotolerance. The effect of a Ca(2+) chelator, EGTA, was investigated before a lethal HS treatment in soybean (Glycine max) seedlings with acquired thermotolerance induced by preheating. Such seedlings became non-thermotolerant with EGTA treatment. The addition of Ca(2+), Sr(2+) or Ba(2+) to the EGTA-treated samples rescued the seedlings from death by preventing the increased cellular leakage of electrolytes, amino acids, and sugars caused by EGTA. It was confirmed that EGTA did not affect HSP accumulation and physiological functions but interfered with the recovery of HS-released Ca(2+) concentration which was required for thermotolerance. Pectin methylesterase (PME, EC 3.1.1.11), a cell wall remodelling enzyme, was activated in response to HS, and its elevated activity caused an increased level of demethylesterified pectin which was related to the recovery of the HS-released Ca(2+) concentration. Thus, the recovery of HS-released Ca(2+) in Ca(2+)-pectate reconstitution through PME activity is required for cell wall remodelling during HS in soybean which, in turn, retains plasma membrane integrity and co-ordinates with HSPs to confer thermotolerance.

Monoclonal antibodies to rhamnogalacturonan I backbone

Monoclonal antibodies were raised against rhamnogalacturonan I backbone, a pectin domain, using Arabidopsis thaliana seed mucilage-derived rhamnogalacturonan I oligosaccharides--BSA conjugates. Two monoclonal antibodies, designated INRA-RU1 and INRA-RU2, selected for further characterization, were specific for the backbone of rhamnogalacturonan I, displaying no binding activity against the other pectin domains i.e. homogalacturonans, galactans or arabinans. A range of oligosaccharides was prepared by enzymatic digestion of rhamnogalacturonan I isolated from Arabidopsis thaliana seed mucilage and from sugar beet pectin, purified by low-pressure chromatography and characterized by high-performance anion-exchange chromatography and mass spectrometry. These rhamnogalacturonan I oligomers were used to characterize the binding site of the two monoclonal antibodies by competitive inhibition. Both INRA-RU1 and INRA-RU2 showed maximal binding to the [-->2)-alpha-L-rhamnosep-(1-->4)-alpha-D-galacturonic acid p-(1-->](7) structural motif but differed in their minimum binding requirement. INRA-RU2 required at least two disaccharide (rhamnose-galacturonic acid) repeats for the antibody to bind, while INRA-RU1 required a minimum of six disaccharide repeats. Furthermore, the binding capacity of INRA-RU1 decreased steeply as the number of disaccharide repeats go beyond seven. Each of these antibodies reacted with hairy regions isolated from sugar beet pectin. Immunofluorescence microscopy indicated that both antibodies can be readily used to detect rhamnogalacturonan I epitopes in various cell wall samples.

Cell wall components and pectin esterification levels as markers of proliferation and differentiation events during pollen development and pollen embryogenesis in Capsicum annuum L

Plant cell walls and their polymers are regulated during plant development, but the specific roles of their molecular components are still unclear, as well as the functional meaning of wall changes in different cell types and processes. In this work the in situ analysis of the distribution of different cell wall components was performed during two developmental programmes, gametophytic pollen development, which is a differentiation process, and stress-induced pollen embryogenesis, which involves proliferation followed by differentiation processes. The changes in cell wall polymers were compared with a system of plant cell proliferation and differentiation, the root apical meristem. The analysis was also carried out during the first stages of zygotic embryogenesis. Specific antibodies recognizing the major cell wall polymers, xyloglucan (XG) and the rhamnogalacturonan II (RGII) pectin domain, and antibodies against high- and low-methyl-esterified pectins were used for both dot-blot and immunolocalization with light and electron microscopy. The results showed differences in the distribution pattern of these molecular complexes, as well as in the proportion of esterified and non-esterified pectins in the two pollen developmental pathways. Highly esterified pectins were characteristics of proliferation, whereas high levels of the non-esterified pectins, XG and RGII were abundant in walls of differentiating cells. Distribution patterns similar to those of pollen embryos were found in zygotic embryos. The wall changes reported are characteristic of proliferation and differentiation events as markers of these processes that take place during pollen development and embryogenesis.

GDP-D-mannose 3,5-epimerase (GME) plays a key role at the intersection of ascorbate and non-cellulosic cell-wall biosynthesis in tomato

The GDP-D-mannose 3,5-epimerase (GME, EC 5.1.3.18), which converts GDP-d-mannose to GDP-l-galactose, is generally considered to be a central enzyme of the major ascorbate biosynthesis pathway in higher plants, but experimental evidence for its role in planta is lacking. Using transgenic tomato lines that were RNAi-silenced for GME, we confirmed that GME does indeed play a key role in the regulation of ascorbate biosynthesis in plants. In addition, the transgenic tomato lines exhibited growth defects affecting both cell division and cell expansion. A further remarkable feature of the transgenic plants was their fragility and loss of fruit firmness. Analysis of the cell-wall composition of leaves and developing fruit revealed that the cell-wall monosaccharide content was altered in the transgenic lines, especially those directly linked to GME activity, such as mannose and galactose. In agreement with this, immunocytochemical analyses showed an increase of mannan labelling in stem and fruit walls and of rhamnogalacturonan labelling in the stem alone. The results of MALDI-TOF fingerprinting of mannanase cleavage products of the cell wall suggested synthesis of specific mannan structures with modified degrees of substitution by acetate in the transgenic lines. When considered together, these findings indicate an intimate linkage between ascorbate and non-cellulosic cell-wall polysaccharide biosynthesis in plants, a fact that helps to explain the common factors in seemingly unrelated traits such as fruit firmness and ascorbate content.

The ERECTA Receptor-Like Kinase Regulates Cell Wall-Mediated Resistance to Pathogens in Arabidopsis thaliana

Some receptor-like kinases (RLK) control plant development while others regulate immunity. The Arabidopsis ERECTA (ER) RLK regulates both biological processes. To discover specific components of ER-mediated immunity, a genetic screen was conducted to identify suppressors of erecta (ser) susceptibility to Plectosphaerella cucumerina fungus. The ser1 and ser2 mutations restored disease resistance to this pathogen to wild-type levels in the er-1 background but failed to suppress er-associated developmental phenotypes. The deposition of callose upon P. cucumerina inoculation, which was impaired in the er-1 plants, was also restored to near wild-type levels in the ser er-1 mutants. Analyses of er cell walls revealed that total neutral sugars were reduced and uronic acids increased relative to those of wild-type walls. Interestingly, in the ser er-1 walls, neutral sugars were elevated and uronic acids were reduced relative to both er-1 and wild-type plants. The cell-wall changes found in er-1 and the ser er-1 mutants are unlikely to contribute to their developmental alterations. However, they may influence disease resistance, as a positive correlation was found between uronic acids content and resistance to P. cucumerina. We propose a specific function for ER in regulating cell wall-mediated disease resistance that is distinct from its role in development.

Pectin-like carbohydrates in the green alga Micrasterias characterized by cytochemical analysis and energy filtering TEM

Pectins are the major matrix polysaccharides of plant cell walls and are important for controlling growth, wall porosity and regulation of the ionic environment in plant cells. Pectic epitopes recognized by the monoclonal antibodies JIM5, JIM7 and 2F4 could be localized in the primary wall during development of the green alga Micrasterias. As the degree of pectin esterification determines the calcium-binding capacity and thus the physical properties of the cell wall, chemical and enzymatic in situ de-esterification was performed. This resulted in displacement of epitopes recognized by JIM5, JIM7 and 2F4, respectively, in changes in the intensity of the antibody labelling as visualized in CLSM. In addition, calcium-binding capacities of cell walls and components of the secretory apparatus were determined in transmission electron microscopy by electron energy loss spectroscopy and electron spectroscopic imaging. These analyses revealed that pectic polysaccharides are transported to the cell wall in a de-esterified form. At the primary wall, pectins get methyl-esterified at the inner side, thus allowing flexibility of the wall. At the outer side of the wall they become again de-esterified and bind high amounts of calcium which leads to cell wall stiffening. Mucilage vesicles possess the highest calcium-binding capacity of all structures observed in Micrasterias, indicating that the pectic polysaccharides of mucilage are secreted in a de-esterified, compact form. When mucilage is excreted through the cell wall, it loses its ability to bind calcium. The esterification of pectins involved is obviously required for swelling of mucilage by water uptake, which generates the motive force for orientation of this unicellular organism in respect to light. Incubation of Micrasterias in pectin methylesterase (PME), which de-esterifies pectic polymers in higher plants, resulted in growth inhibition, cell shape malformation and primary wall thickening. A PME-like enzyme could be found in Micrasterias by PME activity assays.

Ultrastructure and histochemical analysis of extracellular matrix surface network in kiwifruit endosperm-derived callus culture

The study used Actinidia deliciosa endosperm-derived callus to investigate aspects of the morphology, histology and chemistry of extracellular matrix (ECM) structures in morphogenically stable tissue from long-term culture. SEM showed ECM as a membranous layer or reticulated fibrillar and granular structure linking the peripheral cells of callus domains. TEM confirmed that ECM is a distinct heterogeneous layer, up to 4 mum thick and consisting of amorphous dark-staining material, osmiophilic granules and reticulated fibres present outside the outer callus cell wall. ECM covered the surface of cells forming morphogenic domains and was reduced during organ growth. This structure may be linked to acquisition of morphogenic competence and thus may serve as a structural marker of it in endosperm-derived callus. ECM was also observed on senescent cells in contact with the morphogenic area. Treatment of living calluses with chloroform and washing with ether-methanol led to partial destruction of the extracellular layer. Digestion with pectinase removed the membranous layer almost completely and exposed thick fibrillar strands and granular remnants. Digestion with protease did not visibly affect the surface layer. Indirect immunofluorescence showed low-methylesterified pectic epitopes labelled by JIM5 monoclonal antibody. Immunolabelling, histochemistry, and solvent and enzyme treatments suggested pectins and lipids as components of the surface layer. These compounds may indicate protective, water retention and/or cell communication functions for this external layer.

Investigation of ferulate deposition in endosperm cell walls of mature and developing wheat grains by using a polyclonal antibody

A polyclonal antibody has been raised against ferulic acid ester linked to arabinoxylans (AX). 5-O-feruloyl-alpha-L-arabinofuranosyl(1-->4)-beta-D-xylopyranosyl was obtained by chemical synthesis, and was coupled to bovine serum albumin for the immunization of rabbit. The polyclonal antibody designated 5-O-Fer-Ara was highly specific for 5-O-(trans-feruloyl)-L-arabinose (5-O-Fer-Ara) structure that is a structural feature of cell wall AX of plants belonging to the family of Gramineae. The antibody has been used to study the location and deposition of feruloylated AX in walls of aleurone and starchy endosperm of wheat grain. 5-O-Fer-Ara began to accumulate early in aleurone cell wall development (beginning of grain filling, 13 days after anthesis, DAA) and continued to accumulate until the aleurone cells were firmly fixed between the starchy endosperm and the nucelus epidermis (19 DAA). From 26 DAA to maturity, the aleurone cell walls changed little in appearance. The concentration of 5-O-Fer-Ara is high in both peri- and anticlinal aleurone cell walls with the highest accumulation of 5-O-Fer-Ara at the cell junctions at the seed coat interface. The situation is quite different in the starchy endosperm: whatever the stage of development, a low amount of 5-O-Fer-Ara epitope was detected. Contrary to what was observed for aleurone cell walls, no peak of accumulation of feruloylated AX was noticed between 13 and 19 DAA. Visualization of labelled Golgi vesicles suggested that the feruloylation of AX is intracellular. The distribution of (5-O-Fer-Ara) epitope is further discussed in relation to the role of ferulic acid and its dehydrodimers in cell wall structure and tissue organization of wheat grain.

Versatile derivatives of carbohydrate-binding modules for imaging of complex carbohydrates approaching the molecular level of resolution

The innate binding specificity of different carbohydrate-binding modules (CBMs) offers a versatile approach for mapping the chemistry and structure of surfaces that contain complex carbohydrates. We have employed the distinct recognition properties of a double His-tagged recombinant CBM tagged with semiconductor quantum dots for direct imaging of crystalline cellulose at the molecular level of resolution, using transmission and scanning transmission electron microscopy. In addition, three different types of CBMs from families 3, 6, and 20 that exhibit different carbohydrate specificities were each fused with either green fluorescent protein (GFP) or red fluorescent protein (RFP) and employed for double-labeling fluorescence microscopy studies of primary cell walls and various mixtures of complex carbohydrate target molecules. CBM probes can be used for characterizing both native complex carbohydrates and engineered biomaterials.

Biochemical and immunohistochemical analysis of pectic polysaccharides in the cell walls of Arabidopsis mutant QUASIMODO 1 suspension-cultured cells: implications for cell adhesion

Mutation in the Arabidopsis thaliana QUASIMODO 1 gene (QUA1), which encodes a putative glycosyltransferase, reduces cell wall pectin content and cell adhesion. Suspension-cultured calli were generated from roots of wild-type (wt) and qua1-1 A. thaliana plants. The altered cell adhesion phenotype of the qua1-1 plant was also found with its suspension-cultured calli. Cell walls of both wt and qua1-1 calli were analysed by chemical, enzymatic and immunohistochemical techniques in order to assess the role of pectic polysaccharides in the mutant phenotype. Compared with the wt, qua1-1 calli cell walls contained more arabinose (23.6 versus 21.6 mol%), rhamnose (3.1 versus 2.7 mol%), and fucose (1.4 versus 1.2 mol%) and less uronic acid (24.2 versus 27.6 mol%), and they were less methyl-esterified (DM: 22.9% versus 30.3%). When sequential pectin extraction of calli cell walls was performed, qua1-1 water-soluble and chelator-soluble extracts contained more arabinose and less uronic acid than wt. Water-soluble pectins were less methyl-esterified in qua1-1 than in wt. Chelator-soluble pectins were more acetyl-esterified in qua1-1. Differences in the cell wall chemistry of wt and mutant calli were supported by a reduction in JIM7 labelling (methyl-esterified homogalacturonan) of the whole wall in small cells and particularly by a reduced labelling with 2F4 (calcium-associated homogalacturonan) in the middle lamella at tricellular junctions of large qua1-1 cells. Differences in the oligosaccharide profile obtained after endopolygalacturonase degradation of alkali extracts from qua1-1 and wt calli indicated variations in the structure of covalently bonded homogalacturonan. About 29% more extracellular polymers rich in pectins were recovered from the calli culture medium of qua1-1 compared with wt. These results show that perturbation of QUASIMODO 1-1 gene expression in calli resulted in alterations of homogalacturonan content and cell wall location. The consequences of these structural variations are discussed with regard to plant cell adhesion.