Spatial Distribution of Selected Chemical Cell Wall Components in the Embryogenic Callus of Brachypodium distachyon

The Impact of Gold Nanoparticles on Somatic Embryogenesis Using the Example of Arabidopsis thaliana

Although the influence of nanoparticles (NPs) on developmental processes is better understood, little is known about their impact on somatic embryogenesis (SE). This process involves changes in the direction of cell differentiation. Thus, studying the effect of NPs on SE is essential to reveal their impact on cell fate. This study aimed to examine the influence of gold nanoparticles (Au NPs) with different surface charges on the SE of 35S:BBM Arabidopsis thaliana, with particular emphasis on the spatiotemporal localization of pectic arabinogalactan proteins (AGPs) and extensin epitopes in cells changing the direction of their differentiation. The results show that under the influence of nanoparticles, the explant cells of 35S:BBM Arabidopsis thaliana seedling origin did not enter the path of SE. Bulges and the formation of organ-like structures were observed in these explants, in contrast to the control, where somatic embryos developed. Additionally, spatiotemporal changes in the chemical composition of the cell walls during the culture were observed. Under the influence of Au NPs, the following effects were observed: (1) explant cells did not enter the SE pathway, (2) the impacts of Au NPs with different surface charges on the explants were variable, and (3) the compositions of the analyzed pectic AGPs and extensin epitopes were diverse in the cells with different developmental programs: SE (control) and non-SE (treated with Au NPs).

Apoplastic and Symplasmic Markers of Somatic Embryogenesis

Somatic embryogenesis (SE) is a process that scientists have been trying to understand for many years because, on the one hand, it is a manifestation of the totipotency of plant cells, so it enables the study of the mechanisms regulating this process, and, on the other hand, it is an important method of plant propagation. Using SE in basic research and in practice is invaluable. This article describes the latest, but also historical, information on changes in the chemical composition of the cell wall during the transition of cells from the somatic to embryogenic state, and the importance of symplasmic communication during SE. Among wall chemical components, different pectic, AGP, extensin epitopes, and lipid transfer proteins have been discussed as potential apoplastic markers of explant cells during the acquisition of embryogenic competence. The role of symplasmic communication/isolation during SE has also been discussed, paying particular attention to the formation of symplasmic domains within and between cells that carry out different developmental processes. Information about the number and functionality of plasmodesmata (PD) and callose deposition as the main player in symplasmic isolation has also been presented.

Cell Wall Glycan Changes in Different Brachypodium Tissues Give Insights into Monocot Biomass

The annual temperate grass Brachypodium distachyon has become a model system for monocot biomass crops and for understanding lignocellulosic recalcitrance to employ better saccharification and fermentation approaches. It is a monocot plant used to study the grass cell walls that differ from the cell walls of dicot plants such as the eudicot model Arabidopsis. The B. distachyon cell wall is predominantly composed of cellulose, arabinoxylans, and mixed-linkage glucans, and it resembles the cell walls of other field grasses. It has a vascular bundle anatomy similar to C3 grasses. These features make Brachypodium an ideal model to study cell walls. Cell walls are composed of polymers with complex structures that vary between cell types and at different developmental stages. Antibodies that recognize specific cell wall components are currently one of the most effective and specific molecular probes to determine the location and distribution of polymers in plant cell walls in situ. Here, we investigated the glycan distribution in the cell walls of the root and leaf tissues of Brachypodium by employing cell-wall-directed antibodies against diverse glycan epitopes. There are distinct differences in the presence of the epitopes between the root and leaf tissues as well as in the cell type level, which gives insights into monocot biomass.

Enrichment of 3D-Printed k-Carrageenan Food Gel with Callus Tissue of Narrow-Leaved Lupin Lupinus angustifolius

The aim of the study is to develop and evaluate the printability of k-carrageenan inks enriched with callus tissue of lupin (L. angustifolius) and to determine the effect of two lupin calluses (LA14 and LA16) on the texture and digestibility of 3D-printed gel. The results demonstrated that the enriched ink was successfully 3D printed at concentrations of 33 and 50 g/100 mL of LA14 callus and 33 g/100 mL of LA16 callus. The feasibility of 3D printing is extremely reduced at higher concentrations of callus material in the ink. The hardness, cohesiveness, and gumminess of the 3D-printed gel with LA16 callus were weakened compared to the gel with LA14 callus. The results of rheological measurements showed that an increase in the content of LA16 callus interfered with the formation of a k-carrageenan gel network, while LA14 callus strengthened the k-carrageenan gel with increasing concentration. Gel samples at different concentrations of LA14 and LA16 calluses formed a spongy network structure, but the number of pores decreased, and their size increased, when the volume fraction occupied by LA14 and LA16 calluses increased. Simple polysaccharides, galacturonic acid residues, and phenolic compounds (PCs) were released from A-FP gels after sequential in vivo oral and in vitro gastrointestinal digestion. PCs were released predominantly in the simulated intestinal and colonic fluids. Thus, incorporating lupin callus into the hydrocolloid ink for food 3D printing can be a promising approach to developing a gelling material with new mechanical, rheological, and functional properties.

One-Week Scutellar Somatic Embryogenesis in the Monocot Brachypodium distachyon

Plant somatic embryogenesis (SE) is a natural process of vegetative propagation. It can be induced in tissue cultures to investigate developmental transitions, to create transgenic or edited lines, or to multiply valuable crops. We studied the induction of SE in the scutellum of monocots with Brachypodium distachyon as a model system. Towards the in-depth analysis of SE initiation, we determined the earliest stages at which somatic scutellar cells acquired an embryogenic fate, then switched to a morphogenetic mode in a regeneration sequence involving treatments with exogenous hormones: first an auxin (2,4-D) then a cytokinin (kinetin). Our observations indicated that secondary somatic embryos could already develop in the proliferative calli derived from immature zygotic embryo tissues within one week from the start of in vitro culture. Cell states and tissue identity were deduced from detailed histological examination, and in situ hybridization was performed to map the expression of key developmental genes. The fast SE induction method we describe here facilitates the mechanistic study of the processes involved and may significantly shorten the production of transgenic or gene-edited plants.

3,4-Dehydro-L-proline Induces Programmed Cell Death in the Roots of Brachypodium distachyon

As cell wall proteins, the hydroxyproline-rich glycoproteins (HRGPs) take part in plant growth and various developmental processes. To fulfil their functions, HRGPs, extensins (EXTs) in particular, undergo the hydroxylation of proline by the prolyl-4-hydroxylases. The activity of these enzymes can be inhibited with 3,4-dehydro-L-proline (3,4-DHP), which enables its application to reveal the functions of the HRGPs. Thus, to study the involvement of HRGPs in the development of root hairs and roots, we treated seedlings of Brachypodium distachyon with 250 µM, 500 µM, and 750 µM of 3,4-DHP. The histological observations showed that the root epidermis cells and the cortex cells beneath them ruptured. The immunostaining experiments using the JIM20 antibody, which recognizes the EXT epitopes, demonstrated the higher abundance of this epitope in the control compared to the treated samples. The transmission electron microscopy analyses revealed morphological and ultrastructural features that are typical for the vacuolar-type of cell death. Using the TUNEL test (terminal deoxynucleotidyl transferase dUTP nick end labelling), we showed an increase in the number of nuclei with damaged DNA in the roots that had been treated with 3,4-DHP compared to the control. Finally, an analysis of two metacaspases' gene activity revealed an increase in their expression in the treated roots. Altogether, our results show that inhibiting the prolyl-4-hydroxylases with 3,4-DHP results in a vacuolar-type of cell death in roots, thereby highlighting the important role of HRGPs in root hair development and root growth.

Defining the Cell Wall, Cell Cycle and Chromatin Landmarks in the Responses of Brachypodium distachyon to Salinity

Excess salinity is a major stress that limits crop yields. Here, we used the model grass Brachypodium distachyon (Brachypodium) reference line Bd21 in order to define the key molecular events in the responses to salt during germination. Salt was applied either throughout the germination period ("salt stress") or only after root emergence ("salt shock"). Germination was affected at ≥100 mM and root elongation at ≥75 mM NaCl. The expression of arabinogalactan proteins (AGPs), FLA1, FLA10, FLA11, AGP20 and AGP26, which regulate cell wall expansion (especially FLA11), were mostly induced by the "salt stress" but to a lesser extent by "salt shock". Cytological assessment using two AGP epitopes, JIM8 and JIM13 indicated that "salt stress" increases the fluorescence signals in rhizodermal and exodermal cell wall. Cell division was suppressed at >75 mM NaCl. The cell cycle genes (CDKB1, CDKB2, CYCA3, CYCB1, WEE1) were induced by "salt stress" in a concentration-dependent manner but not CDKA, CYCA and CYCLIN-D4-1-RELATED. Under "salt shock", the cell cycle genes were optimally expressed at 100 mM NaCl. These changes were consistent with the cell cycle arrest, possibly at the G1 phase. The salt-induced genomic damage was linked with the oxidative events via an increased glutathione accumulation. Histone acetylation and methylation and DNA methylation were visualized by immunofluorescence. Histone H4 acetylation at lysine 5 increased strongly whereas DNA methylation decreased with the application of salt. Taken together, we suggest that salt-induced oxidative stress causes genomic damage but that it also has epigenetic effects, which might modulate the cell cycle and AGP expression gene. Based on these landmarks, we aim to encourage functional genomics studies on the responses of Brachypodium to salt.

Immunodetection of Pectic Epitopes, Arabinogalactan Proteins, and Extensins in Mucilage Cells from the Ovules of Pilosella officinarum Vaill. and Taraxacum officinale Agg. (Asteraceae)

The main aim of this study was to compare the cytological difference between ovular mucilage cells in two Asteraceae species-Pilosella officinarum and Taraxacum officinale-in order to determine whether pectic epitopes, arabinogalactan proteins, or extensins are present. The immunocytochemical technique was used. Both the Taracacum and Pilosella genera have been used recently as models for understanding the mechanisms of apomixis. Knowledge of the presence of signal molecules (pectic epitopes, arabinogalactan proteins, and extensins) can help better understand the developmental processes in these plants during seed growth. The results showed that in Pilosella officinarum, there was an accumulation of pectins in the mucilage, including both weakly and highly esterified pectins, which was in contrast to the mucilage of Taraxacum officinale, which had low amounts of these pectins. However, Taraxacum protoplasts of mucilage cells were rich in weakly methyl-esterified pectins. While the mucilage contained arabinogalactan proteins in both of the studied species, the types of arabinogalactan proteins were different. In both of the studied species, extensins were recorded in the transmitting tissues. Arabinogalactan proteins as well as weakly and highly esterified pectins and extensins occurred in close proximity to calcium oxalate crystals in both Taraxacum and Pilosella cells.

Cell Wall Composition as a Marker of the Reprogramming of the Cell Fate on the Example of a Daucus carota (L.) Hypocotyl in Which Somatic Embryogenesis Was Induced

Changes in the composition of the cell walls are postulated to accompany changes in the cell's fate. We check whether there is a relationship between the presence of selected pectic, arabinogalactan proteins (AGPs), and extensins epitopes and changes in cell reprogramming in order to answer the question of whether they can be markers accompanying changes of cell fate. Selected antibodies were used for spatio-temporal immunolocalization of wall components during the induction of somatic embryogenesis. Based on the obtained results, it can be concluded that (1) the LM6 (pectic), LM2 (AGPs) epitopes are positive markers, but the LM5, LM19 (pectic), JIM8, JIM13 (AGPs) epitopes are negative markers of cells reprogramming to the meristematic/pluripotent state; (2) the LM8 (pectic), JIM8, JIM13, LM2 (AGPs) and JIM11 (extensin) epitopes are positive markers, but LM6 (pectic) epitope is negative marker of cells undergoing detachment; (3) JIM4 (AGPs) is a positive marker, but LM5 (pectic), JIM8, JIM13, LM2 (AGPs) are negative markers for pericycle cells on the xylem pole; (4) LM19, LM20 (pectic), JIM13, LM2 (AGPs) are constitutive wall components, but LM6, LM8 (pectic), JIM4, JIM8, JIM16 (AGPs), JIM11, JIM12 and JIM20 (extensins) are not constitutive wall components; (5) the extensins do not contribute to the cell reprogramming.

From Single Cell to Plants: Mesophyll Protoplasts as a Versatile System for Investigating Plant Cell Reprogramming

Plants are sessile organisms that have a remarkable developmental plasticity, which ensures their optimal adaptation to environmental stresses. Plant cell totipotency is an extreme example of such plasticity, whereby somatic cells have the potential to form plants via direct shoot organogenesis or somatic embryogenesis in response to various exogenous and/or endogenous signals. Protoplasts provide one of the most suitable systems for investigating molecular mechanisms of totipotency, because they are effectively single cell populations. In this review, we consider the current state of knowledge of the mechanisms that induce cell proliferation from individual, differentiated somatic plant cells. We highlight initial explant metabolic status, ploidy level and isolation procedure as determinants of successful cell reprogramming. We also discuss the importance of auxin signalling and its interaction with stress-regulated pathways in governing cell cycle induction and further stages of plant cell totipotency.

Extracellular matrix and wall composition are diverse in the organogenic and non-organogenic calli of Actinidia arguta

Differences in the composition and the structural organisation of the extracellular matrix correlate with the morphogenic competence of the callus tissue that originated from the isolated endosperm of kiwifruit. The chemical composition and structural organisation of the extracellular matrix, including the cell wall and the layer on its surface, may correspond with the morphogenic competence of a tissue. In the presented study, this relationship was found in the callus tissue that had been differentiated from the isolated endosperm of the kiwiberry, Actinidia arguta. The experimental system was based on callus samples of exactly the same age that had originated from an isolated endosperm but were cultured under controlled conditions promoting either an organogenic or a non-organogenic pathway. The analyses which were performed using bright field, fluorescence and scanning electron microscopy techniques showed significant differences between the two types of calli. The organogenic tissue was compact and the outer walls of the peripheral cells were covered with granular structures. The non-organogenic tissue was composed of loosely attached cells, which were connected via a net-like structure. The extracellular matrices from both the non- and organogenic tissues were abundant in pectic homogalacturonan and extensins (LM19, LM20, JIM11, JIM12 and JIM20 epitopes), but the epitopes that are characteristic for rhamnogalacturonan I (LM5 and LM6), hemicellulose (LM25) and the arabinogalactan protein (LM2) were detected only in the non-organogenic callus. Moreover, we report the epitopes, which presence is characteristic for the Actinidia endosperm (LM21 and LM25, heteromannan and xyloglucan) and for the endosperm-derived cells that undergo dedifferentiation (loss of LM21 and LM25; appearance or increase in the content of LM5, LM6, LM19, JIM11, JIM12, JIM20, JIM8 and JIM16 epitopes).

In Vitro Tissue Culture in Brachypodium: Applications and Challenges

Brachypodium distachyon has become an excellent model for plant breeding and bioenergy grasses that permits many fundamental questions in grass biology to be addressed. One of the constraints to performing research in many grasses has been the difficulty with which they can be genetically transformed and the generally low frequency of such transformations. In this review, we discuss the contribution that transformation techniques have made in Brachypodium biology as well as how Brachypodium could be used to determine the factors that might contribute to transformation efficiency. In particular, we highlight the latest research on the mechanisms that govern the gradual loss of embryogenic potential in a tissue culture and propose using B. distachyon as a model for other recalcitrant monocots.

Composition of the Reconstituted Cell Wall in Protoplast-Derived Cells of Daucus is Affected by Phytosulfokine (PSK)

Phytosulfokine-α (PSK), a peptidyl plant growth factor, has been recognized as a promising intercellular signaling molecule involved in cellular proliferation and dedifferentiation. It was shown that PSK stimulated and enhanced cell divisions in protoplast cultures of several species leading to callus and proembryogenic mass formation. Since PSK had been shown to cause an increase in efficiency of somatic embryogenesis, it was reasonable to check the distribution of selected chemical components of the cell walls during the protoplast regeneration process. So far, especially for the carrot, a model species for in vitro cultures, it has not been specified what pectic, arabinogalactan protein (AGP) and extensin epitopes are involved in the reconstruction of the wall in protoplast-derived cells. Even less is known about the correlation between wall regeneration and the presence of PSK during the protoplast culture. Three Daucus taxa, including the cultivated carrot, were analyzed during protoplast regeneration. Several antibodies directed against wall components (anti-pectin: LM19, LM20, anti-AGP: JIM4, JIM8, JIM13 and anti-extensin: JIM12) were used. The obtained results indicate a diverse response of the used Daucus taxa to PSK in terms of protoplast-derived cell development, and diversity in the chemical composition of the cell walls in the control and the PSK-treated cultures.

Hydroxyproline-Rich Glycoproteins as Markers of Temperature Stress in the Leaves of Brachypodium distachyon

Plants frequently encounter diverse abiotic stresses, one of which is environmental thermal stress. To cope with these stresses, plants have developed a range of mechanisms, including altering the cell wall architecture, which is facilitated by the arabinogalactan proteins (AGP) and extensins (EXT). In order to characterise the localisation of the epitopes of the AGP and EXT, which are induced by the stress connected with a low (4 °C) or a high (40 °C) temperature, in the leaves of Brachypodium distachyon, we performed immunohistochemical analyses using the antibodies that bind to selected AGP (JIM8, JIM13, JIM16, LM2 and MAC207), pectin/AGP (LM6) as well as EXT (JIM11, JIM12 and JIM20). The analyses of the epitopes of the AGP indicated their presence in the phloem and in the inner bundle sheath (JIM8, JIM13, JIM16 and LM2). The JIM16 epitope was less abundant in the leaves from the low or high temperature compared to the control leaves. The LM2 epitope was more abundant in the leaves that had been subjected to the high temperatures. In the case of JIM13 and MAC207, no changes were observed at the different temperatures. The epitopes of the EXT were primarily observed in the mesophyll and xylem cells of the major vascular bundle (JIM11, JIM12 and JIM20) and no correlation was observed between the presence of the epitopes and the temperature stress. We also analysed changes in the level of transcript accumulation of some of the genes encoding EXT, EXT-like receptor kinases and AGP in the response to the temperature stress. In both cases, although we observed the upregulation of the genes encoding AGP in stressed plants, the changes were more pronounced at the high temperature. Similar changes were observed in the expression profiles of the EXT and EXT-like receptor kinase genes. Our findings may be relevant for genetic engineering of plants with increased resistance to the temperature stress.

Unmethyl-esterified homogalacturonan and extensins seal Arabidopsis graft union

BACKGROUND

Grafting is a technique widely used in horticulture. The processes involved in grafting are diverse, and the technique is commonly employed in studies focusing on the mechanisms that regulate cell differentiation or response of plants to abiotic stress. Information on the changes in the composition of the cell wall that occur during the grafting process is scarce. Therefore, this study was carried out for analyzing the composition of the cell wall using Arabidopsis hypocotyls as an example. During the study, the formation of a layer that covers the surface of the graft union was observed. So, this study also aimed to describe the histological and cellular changes that accompany autografting of Arabidopsis hypocotyls and to perform preliminary chemical and structural analyses of extracellular material that seals the graft union.

RESULTS

During grafting, polyphenolic and lipid compounds were detected, along with extracellular deposition of carbohydrate/protein material. The spatiotemporal changes observed in the structure of the extracellular material included the formation of a fibrillar network, polymerization of the fibrillar network into a membranous layer, and the presence of bead-like structures on the surface of cells in established graft union. These bead-like structures appeared either "closed" or "open". Only three cell wall epitopes, namely: LM19 (un/low-methyl-esterified homogalacturonan), JIM11, and JIM20 (extensins), were detected abundantly on the cut surfaces that made the adhesion plane, as well as in the structure that covered the graft union and in the bead-like structures, during the subsequent stages of regeneration.

CONCLUSIONS

To the best of our knowledge, this is the first report on the composition and structure of the extracellular material that gets deposited on the surface of graft union during Arabidopsis grafting. The results showed that unmethyl-esterified homogalacturonan and extensins are together involved in the adhesion of scion and stock, as well as taking part in sealing the graft union. The extracellular material is of importance not only due to the potential pectin-extensin interaction but also due to its origin. The findings presented here implicate a need for studies with biochemical approach for a detailed analysis of the composition and structure of the extracellular material.

Cell Wall Epitopes and Endoploidy as Reporters of Embryogenic Potential in Brachypodium Distachyon Callus Culture

Effective regeneration of callus tissue into embryos and then into whole plants is essential for plant biotechnology. The embryonic potential is often low and can further decrease with time in culture, which limits the utilisation of calli for transformation procedures and in vitro propagation. In this study, we show that the loss of embryogenic potential in callus cultures of Brachypodium distachyon is progressive over time. Flow cytometry analyses indicated endoploidy levels increased in 60- and 90-day-old calli with effective loss of the 2C DNA content peak in the latter. Analysis of indolic compounds content revealed a decrease in 60- and 90-day-old calli compared to either freshly isolated explants or 30-day-old calli. Immunohistochemical analysis revealed a decrease in arabinogalactan proteins (AGP) signal with the time of culture, but extensin (EXT) epitopes either increased (JIM12 epitopes) or decreased (JIM11 epitopes). The transcript accumulation levels of AGPs and EXTs confirmed these results, with most of AGP and EXT transcripts gradually decreasing. Some chimeric EXT transcripts significantly increased on the 30th day of culture, perhaps because of an increased embryogenic potential. Selected somatic embryogenesis-related genes and cyclins demonstrated a gradual decrease of transcript accumulation for YUCCA (YUC), AINTEGUMENTA-LIKE (AIL), BABY BOOM (BBM), and CLAVATA (CLV3) genes, as well as for most of the cyclins, starting from the 30th day of culture. Notably, WUSCHEL (WUS) transcript was detectable only on the 30th and 60th day and was not detectable in the zygotic embryos and in 90-day-old calli.

Cell wall extensins in root-microbe interactions and root secretions

Extensins are cell wall glycoproteins, belonging to the hydroxyproline-rich glycoprotein (HRGP) family, which are involved in many biological functions, including plant growth and defence. Several reviews have described the involvement of HRGPs in plant immunity but little focus has been given specifically to cell wall extensins. Yet, a large set of recently published data indicates that extensins play an important role in plant protection, especially in root-microbe interactions. Here, we summarise the current knowledge on this topic and discuss the importance of extensins in root defence. We first provide an overview of the distribution of extensin epitopes recognised by different monoclonal antibodies among plants and discuss the relevance of some of these epitopes as markers of the root defence response. We also highlight the implication of extensins in different types of plant interactions elicited by either pathogenic or beneficial micro-organisms. We then present and discuss the specific importance of extensins in root secretions, as these glycoproteins are not only found in the cell walls but are also released into the root mucilage. Finally, we propose a model to illustrate the impact of cell wall extensin on root secretions.

5-Azacitidine Induces Cell Death in a Tissue Culture of Brachypodium distachyon

Morphological and histological observations revealed that, at a concentration of 50 µM, 5-azacitidine (5-azaC) totally inhibited the induction of embryogenic masses (EM), while the cultivation of explants (zygotic embryos; ZEs) in the presence of 5 µM of 5-azaC led to the formation of a callus with EM in 10% of the cases. Transmission electron microscopy (TEM) analyzes revealed the presence of the morphological and ultrastructural features that are typical for the vacuolar type of cell death in the callus cells that were treated. A TUNEL assay confirmed the presence of DNA double-strand breaks for the callus cells that had been treated with both 5 and 50 µM 5-azaC concentrations. Analysis of the gene expression of selected cell death markers demonstrated a reduced expression of metacaspase, protein executer 1 (EX1), and thioredoxin (TRX) in the callus cells that had been treated compared to the control culture. The strongest increase in the gene activity was characteristic for glutathione S-transferase (GST). Our studies also included an analysis of the distribution of some arabinogalactan proteins (AGPs) and extensin epitopes, which can be used as markers of cells that are undergoing death in a Brachypodium distachyon tissue culture.

Spatio-temporal localization of selected pectic and arabinogalactan protein epitopes and the ultrastructural characteristics of explant cells that accompany the changes in the cell fate during somatic embryogenesis in Arabidopsis thaliana

During somatic embryogenesis (SE), explant cells undergo changes in the direction of their differentiation, which lead to diverse cell phenotypes. Although the genetic bases of the SE have been extensively studied in Arabidopsis thaliana, little is known about the chemical characteristics of the wall of the explant cells, which undergo changes in the direction of differentiation. Thus, we examined the occurrence of selected pectic and AGP epitopes in explant cells that display different phenotypes during SE. Explants examinations have been supplemented with an analysis of the ultrastructure. The deposition of selected pectic and AGP epitopes in somatic embryos was determined. Compared to an explant at the initial stage, a/embryogenic/totipotent and meristematic/pluripotent cells were characterized by a decrease in the presence of AGP epitopes, b/the presence of AGP epitopes in differentiated cells was similar, and c/an increase of analyzed epitopes was detected in the callus cells. Totipotent cells could be distinguished from pluripotent cells by: 1/the presence of the LM2 epitope in the latest one, 2/the appearance of the JIM16 epitope in totipotent cells, and 3/the more abundant presence of the JIM7 epitope in the totipotent cells. The LM5 epitope characterized the wall of the cells that were localized within the mass of embryogenic domain. The JIM8, JIM13 and JIM16 AGP epitopes appeared to be the most specific for the callus cells. The results indicate a relationship between the developmental state of the explant cells and the chemical composition of the cell walls.

Organ and Tissue-Specific Localisation of Selected Cell Wall Epitopes in the Zygotic Embryo of Brachypodium distachyon

The plant cell wall shows a great diversity regarding its chemical composition, which may vary significantly even during different developmental stages. In this study, we analysed the distribution of several cell wall epitopes in embryos of Brachypodium distachyon (Brachypodium). We also described the variations in the nucleus shape and the number of nucleoli that occurred in some embryo cells. The use of transmission electron microscopy, and histological and immunolocalisation techniques permitted the distribution of selected arabinogalactan proteins, extensins, pectins, and hemicelluloses on the embryo surface, internal cell compartments, and in the context of the cell wall ultrastructure to be demonstrated. We revealed that the majority of arabinogalactan proteins and extensins were distributed on the cell surface and that pectins were the main component of the seed coat and other parts, such as the mesocotyl cell walls and the radicula. Hemicelluloses were localised in the cell wall and outside of the radicula protodermis, respectively. The specific arrangement of those components may indicate their significance during embryo development and seed germination, thus suggesting the importance of their protective functions. Despite the differences in the cell wall composition, we found that some of the antibodies can be used as markers to identify specific cells and the parts of the developing Brachypodium embryo.

Regenerative potential, metabolic profile, and genetic stability of Brachypodium distachyon embryogenic calli as affected by successive subcultures

Brachypodium distachyon, a model species for forage grasses and cereal crops, has been used in studies seeking improved biomass production and increased crop yield for biofuel production purposes. Somatic embryogenesis (SE) is the morphogenetic pathway that supports in vitro regeneration of such species. However, there are gaps in terms of studies on the metabolic profile and genetic stability along successive subcultures. The physiological variables and the metabolic profile of embryogenic callus (EC) and embryogenic structures (ES) from successive subcultures (30, 60, 90, 120, 150, 180, 210, 240, and 360-day-old subcultures) were analyzed. Canonical discriminant analysis separated EC into three groups: 60, 90, and 120 to 240 days. EC with 60 and 90 days showed the highest regenerative potential. EC grown for 90 days and submitted to SE induction in 2 mg L^-1 of kinetin-supplemented medium was the highest ES producer. The metabolite profiles of non-embryogenic callus (NEC), EC, and ES submitted to principal component analysis (PCA) separated into two groups: 30 to 240- and 360-day-old calli. The most abundant metabolites for these groups were malonic acid, tryptophan, asparagine, and erythrose. PCA of ES also separated ages into groups and ranked 60- and 90-day-old calli as the best for use due to their high levels of various metabolites. The key metabolites that distinguished the ES groups were galactinol, oxaloacetate, tryptophan, and valine. In addition, significant secondary metabolites (e.g., caffeoylquinic, cinnamic, and ferulic acids) were important in the EC phase. Ferulic, cinnamic, and phenylacetic acids marked the decreases in the regenerative capacity of ES in B. distachyon. Decreased accumulations of the amino acids aspartic acid, asparagine, tryptophan, and glycine characterized NEC, suggesting that these metabolites are indispensable for the embryogenic competence in B. distachyon. The genetic stability of the regenerated plants was evaluated by flow cytometry, showing that ploidy instability in regenerated plants from B. distachyon calli is not correlated with callus age. Taken together, our data indicated that the loss of regenerative capacity in B. distachyon EC occurs after 120 days of subcultures, demonstrating that the use of EC can be extended to 90 days.

Morpho-histological, histochemical, and molecular evidences related to cellular reprogramming during somatic embryogenesis of the model grass Brachypodium distachyon

The wild grass species Brachypodium distachyon (L.) has been proposed as a new model for temperate grasses. Among the biotechnological tools already developed for the species, an efficient induction protocol of somatic embryogenesis (SE) using immature zygotic embryos has provided the basis for genetic transformation studies. However, a systematic work to better understanding the basic cellular and molecular mechanisms that underlie the SE process of this grass species is still missing. Here, we present new insights at the morpho-histological, histochemical, and molecular aspects of B. distachyon SE pathway. Somatic embryos arose from embryogenic callus formed by cells derived from the protodermal-dividing cells of the scutellum. These protodermal cells showed typical meristematic features and high protein accumulation which were interpreted as the first observable steps towards the acquisition of a competent state. Starch content decreased along embryogenic callus differentiation supporting the idea that carbohydrate reserves are essential to morphogenetic processes. Interestingly, starch accumulation was also observed at late stages of SE process. Searches in databanks revealed three sequences available annotated as BdSERK, being two copies corresponding to SERK1 and one showing greater identity to SERK2. In silico analysis confirmed the presence of characteristic domains in a B. distachyon Somatic Embryogenesis Receptor Kinase genes candidates (BdSERKs), which suggests SERK functions are conserved in B. distachyon. In situ hybridization demonstrated the presence of transcripts of BdSERK1 in all development since globular until scutellar stages. The results reported in this study convey important information about the morphogenetic events in the embryogenic pathway which has been lacking in B. distachyon. This study also demonstrates that B. distachyon provides a useful model system for investigating the genetic regulation of SE in grass species.