Cell Wall Flexibility Research Articles

Changes in cell wall composition and ultrastructure related to desiccation during the seed maturation of Paubrasilia echinata (brazilwood).

Paubrasilia echinata (brazilwood) is an endangered native tree from the Brazilian Atlantic Forest whose seeds tolerate maturation drying, but, unlike classic orthodox seeds, they quickly lose viability after shedding. This work analyzed the biochemical and ultrastructural changes during the maturation of brazilwood seeds, with particular attention to the cell walls and organization of the cellular components. The physiological seed maturity was accompanied by increased starch content and decreased soluble sugars. Arabinose increased considerably and was the predominant cell-wall sugar during maturation, suggesting a rise in arabinans that contribute to greater cell wall flexibility. This increase was consistent with the cell wall infolding observed in the hypocotyl axis and cotyledons during the maturation of brazilwood seeds. Ultrastructural analyses showed changes in the number and distribution of protein bodies and amyloplasts and the reorganization of lipid droplets into large drops or masses during seed desiccation. Our findings demonstrate that brazilwood seeds behave like other orthodox seeds during maturation, performing the cell wall and metabolic changes before the major decline in the seed water content. However, the high vacuolization and reorganization of lipid bodies observed at 65 DAA suggest that cell deterioration occurs to some extent at the end of the maturation period and could be responsible for reducing the longevity of the brazilwood dried seeds.

Perennial halophyte Salicornia neei Lag.: Cell wall composition and functional properties of its biopolymers

Salicornia neei halophyte extends in Argentina seashores. To envisage potential applications, cell wall sequential extraction performed on dry plant yielded 1.1, 2.4, 0.3 and 0.9% of pectin fractions respectively extracted by room temperature water, 90 °C-water, CDTA and Na2CO3. They contained 21–33% uronic acids (UA) with low degree of methylation and 0.5–1.2 M ratios of neutral sugars to UA. High arabinose level suggests that long arabinan side-chains maintain cell wall flexibility in water deficit. Fractions also contained 10–36% of proteins. The KOH-soluble fractions (4.3%) were mainly arabinoxylans. At 2.0% w/v, pectin fractions developed “weak gel”-type networks with Ca2+, while arabinoxylans generated “dilute solutions”. Cellulose (28%) and lignin (45.1%) were the main biopolymers in the final residue, which showed low water swelling capacity (3.6 mL/g) due to lignin, increasing when arabinoxylans were also present. Phenolics (9.8%) were mainly water-extractable. Salicornia is a source of biopolymers and antioxidants potentially useful for food applications.

Winter Nights during Summer Time: Stress Physiological Response to Ice and the Facilitation of Freezing Cytorrhysis by Elastic Cell Wall Components in the Leaves of a Nival Species.

Ranunculus glacialis grows and reproduces successfully, although the snow-free time period is short (2–3 months) and night frosts are frequent. At a nival site (3185 m a.s.l.), we disentangled the interplay between the atmospheric temperature, leaf temperatures, and leaf freezing frequency to assess the actual strain. For a comprehensive understanding, the freezing behavior from the whole plant to the leaf and cellular level and its physiological after-effects as well as cell wall chemistry were studied. The atmospheric temperatures did not mirror the leaf temperatures, which could be 9.3 °C lower. Leaf freezing occurred even when the air temperature was above 0 °C. Ice nucleation at on average −2.6 °C started usually independently in each leaf, as the shoot is deep-seated in unfrozen soil. All the mesophyll cells were subjected to freezing cytorrhysis. Huge ice masses formed in the intercellular spaces of the spongy parenchyma. After thawing, photosynthesis was unaffected regardless of whether ice had formed. The cell walls were pectin-rich and triglycerides occurred, particularly in the spongy parenchyma. At high elevations, atmospheric temperatures fail to predict plant freezing. Shoot burial prevents ice spreading, specific tissue architecture enables ice management, and the flexibility of cell walls allows recurrent freezing cytorrhysis. The peculiar patterning of triglycerides close to ice rewards further investigation.

Hemicelluloses and associated compounds determine gall functional traits.

The intriguing questions concerning gall development refer to the processes of the remodelling of the host plant organ. Such processes involve the restructuring of cell walls and can be influenced by phenolics, indole-3-acetic acid (IAA) and reactive oxygen species (ROS). Alterations in cell walls demand the interference in the coupling of cellulose fibrils and hemicelluloses (xyloglucans) at specific stages of gall development. In addition to cell wall remodelling, hemicelluloses, such as the, xyloglucans and heteromannans can act as reserve carbohydrates, while xylans provide rigidity to the secondary cell walls. Developmental traits of the lenticular, fusiform and globoid galls on Inga ingoides (Fabaceae) were analysed using anatomical, cytometric, histochemical and immunocytochemical tools. Phenolics, IAA and ROS accumulated in similar gall tissue compartments, and may have influenced the restructuring of hemicelluloses and pectins. Contrary to expectations, cell wall flexibility regarding the dynamics of xyloglucans and cellulose fibrils does not relate to a temporal scale. The detection of xyloglucans in nutritive cell walls relate to carbohydrate nutritional resources to the galling insect, while xylans were associated to the lignified cell walls. Heteromanans were not detected, either in non-galled or galled tissues. The patterns of cell expansion during gall development relied on the relationship among phenolics, ROS and IAA with the hemicelluloses (xyloglucans and xylans) and cellulose fibrils. Although cell wall dynamics is specific to each gall morphotype in I. ingoides, the xyloglucans function as carbohydrate reserve to the gall inducers, which constitutes a functional trait common to the three morphotypes.

Investigating the mechanisms underlying the hyposaline tolerance of intertidal seaweed, Pyropia haitanensis

Although intertidal macroalgae are highly tolerant to hyposaline stress, the underlying mechanisms remain poorly characterized. In this study, we investigated the effects of a short-term immersion in hyposaline conditions [5‰ (LSS_5) and 0‰ (LSS_0)] on the physiological characteristics and the transcriptome of Pyropia haitanensis. The hyposaline stress treatment caused more severe damages to the thalli than the hypersaline stress conditions. Decreased growth and energy metabolism may benefit the thalli by redirecting energy to specific stress responses under hyposaline stress. The reactive oxygen species production was lower under LSS_5 compared with LSS_0. Although superoxide dismutase activity and glutathione contents increased under the LSS_0 condition, the freshwater induced more severe oxidative damages. Moreover, P. haitanensis thalli rapidly excreted Na+, K+, and proline to achieve osmotic balance. However, K+ content of cells remained high when thalli were exposed to LSS_5, which is advantageous for the high photosynthesis rate of thalli. The mechanisms behind Na+ and K+ export and the osmolyte adjustments under hyposaline stress were distinct from hypersaline stress. Additionally, the upregulation of extensins might increase cell wall flexibility to maintain cell turgor pressure and act as a stress signal under LSS_5. The activation of heat shock proteins maintained the protein homeostasis under LSS_0, which may be a signature of the ability of Pyropia spp. to cope with several abiotic stresses in the intertidal zone. The present study paves the way for more detailed investigations of the mechanisms behind the hyposaline stress tolerance of intertidal seaweed.

Transcriptome analysis of the differential effect of the NADPH oxidase gene RbohB in Phaseolus vulgaris roots following Rhizobium tropici and Rhizophagus irregularis inoculation

BackgroundReactive oxygen species (ROS) are generated by NADPH oxidases known as respiratory burst oxidase homologs (RBOHs) in plants. ROS regulate various cellular processes, including the mutualistic interactions between legumes and nitrogen-fixing bacteria or arbuscular mycorrhizal (AM) fungi. Rboh is a multigene family comprising nine members (RbohA–I) in common bean (Phaseolus vulgaris). The RNA interference-mediated silencing of RbohB (PvRbohB-RNAi) in this species diminished its ROS production and greatly impaired nodulation. By contrast, the PvRbohB-RNAi transgenic roots showed early hyphal root colonization with enlarged fungal hypopodia; therefore, we proposed that PvRbohB positively regulates rhizobial infection (Rhizobium tropici) and inhibits AM colonization by Rhizophagus irregularis in P. vulgaris.ResultsTo corroborate this hypothesis, an RNA-Seq transcriptomic analysis was performed to identify the differentially expressed genes in the PvRbohB-RNAi roots inoculated with Rhizobium tropici or Rhizophagus irregularis. We found that, in the early stages, root nodule symbioses generated larger changes of the transcriptome than did AM symbioses in P. vulgaris. Genes related to ROS homeostasis and cell wall flexibility were markedly upregulated in the early stages of rhizobial colonization, but not during AM colonization. Compared with AM colonization, the rhizobia induced the expression of a greater number of genes encoding enzymes involved in the metabolism of auxins, cytokinins, and ethylene, which were typically repressed in the PvRbohB-RNAi roots.ConclusionsOur research provides substantial insights into the genetic interaction networks in the early stages of rhizobia and AM symbioses with P. vulgaris, as well as the differential roles that RbohB plays in processes related to ROS scavenging, cell wall remodeling, and phytohormone homeostasis during nodulation and mycorrhization in this legume.

Perspectives on Structural, Physiological, Cellular, and Molecular Responses to Desiccation in Resurrection Plants.

Resurrection plants possess a unique ability to counteract desiccation stress. Desiccation tolerance (DT) is a very complex multigenic and multifactorial process comprising a combination of physiological, morphological, cellular, genomic, transcriptomic, proteomic, and metabolic processes. Modification in the sugar composition of the hemicellulosic fraction of the cell wall is detected during dehydration. An important change is a decrease of glucose in the hemicellulosic fraction during dehydration that can reflect a modification of the xyloglucan structure. The expansins might also be involved in cell wall flexibility during drying and disrupt hydrogen bonds between polymers during rehydration of the cell wall. Cleavages by xyloglucan-modifying enzymes release the tightly bound xyloglucan-cellulose network, thus increasing cell wall flexibility required for cell wall folding upon desiccation. Changes in hydroxyproline-rich glycoproteins (HRGPs) such as arabinogalactan proteins (AGPs) are also observed during desiccation and rehydration processes. It has also been observed that significant alterations in the process of photosynthesis and photosystem (PS) II activity along with changes in the antioxidant enzyme system also increased the cell wall and membrane fluidity resulting in DT. Similarly, recent data show a major role of ABA, LEA proteins, and small regulatory RNA in regulating DT responses. Current progress in “-omic” technologies has enabled quantitative monitoring of the plethora of biological molecules in a high throughput routine, making it possible to compare their levels between desiccation-sensitive and DT species. In this review, we present a comprehensive overview of structural, physiological, cellular, molecular, and global responses involved in desiccation tolerance.

Production of lipid and carbohydrate from microalgae without compromising biomass productivities: Role of Ca and Mg

Abstract We report the cultivation of Chlorella sorokiniana str. SLA-04 in media containing trace amounts of Ca and Mg. The differences in productivities of biomass, lipids and carbohydrates were assessed relative to cultures grown in standard media (BG-11) that contain approximately 8 × higher concentration of Ca and 30 × higher concentration of Mg. Culture performance in N-limited standard media was also investigated. In addition to growth and accumulation of storage products (lipid and carbohydrate), we measured the utilization of N, Ca and Mg and monitored changes in cell size and photosynthetic activity. Our results showed that limitation of Ca or Mg did not inhibit cell replication and culture growth. On the contrary, Ca-limited (CaL) cultures had ∼30% higher biomass productivity relative to the control with excessive nutrients possibly due to improvement in cell wall flexibility and cell division. We also observed that CaL and Mg-limited (MgL) cultures had nearly 3-fold higher lipid concentration (measured as fatty acid methyl ester) and 50% higher carbohydrate concentration than the nutrient excess control cultures. Simultaneous culture growth and lipid accumulation in CaL and MgL cultures suggest that de novo synthesis was the primary mechanism for lipid accumulation in Ca/Mg-limited media. Overall, our study demonstrates that micronutrient optimization, in addition to optimization of macronutrients, could significantly improve microalgal biorefinery yields.

FROST RESISTANCE OF COMPOSITE GAS CONCRETE

Purpose: The aim of this paper is to compare the frost resistance of composite and conventional gas concretes. The comparative analysis includes a theoretical substantiation of the ice formation and the frost resistance of composite and conventional gas concretes in freeze-thaw conditions. Methodology/materials: Cellular and polymer-hydrophobizated concretes are used in the experiment which includes frost-resistance tests of cellular concrete and studies of its structure modification. The obtained results on the abovementioned concretes are compared with those of traditional concretes. It is shown that the ice formation and movement observed in pores and capillaries of polymer-hydrophobizated concretes occur in absolutely different ways. Research findings: Since the frost resistance is one of key factors of the durability of constructional materials, the use of water soluble polymers allows creating a composite structure of cell walls in gas concrete which manifests a high dynamic strength. The flexibility of cell walls increases the strength properties of concrete, its frost resistance and flexure strength. Hydrophobic properties of polymer-hydrophobizated concretes lower the water absorption. Practical implications: The proposed composition of composite gas concrete can be recommended for large-scale implementation in construction industry. Value: Water soluble polymers have a positive effect on the hydration process of the cement binder, namely it retains water in the concrete structure, thereby hardening the material and exerting a synergism effect.

Mechanisms of desiccation tolerance in the bromeliad Pitcairnia burchellii Mez: biochemical adjustments and structural changes

Rocky outcrops represent the diversity center of vascular desiccation tolerant (DT) plants. Vegetation in this environment is exposed to an extended dry season and extreme conditions due to rocky soils and high sun exposure. In this study, we demonstrated that Pitcairnia burchellii, a bromeliad from rocky outcrops, tolerates intense desiccation for about 90 days due to strategies as accumulation of compatible osmolytes and antioxidant substances together with leaf morphological changes. In dehydrated plants, an increase in antioxidant activity was observed and the vacuolization of parenchyma cells was accompanied by proline accumulation in leaves and rhizomes. Precursors related to phenylpropanoid pathway increased significantly during plant dehydration. Accordingly, increases in anthocyanin and phenolic contents as well as lignin deposition were observed in leaves of dehydrated plants. Cell divisions and a decrease in stored starch were observed in the rhizomes indicating starch mobilization. Anatomical analyses revealed the presence of a more developed water-storage tissue in dehydrated leaves. During desiccation, leaves curl upwards and the adaxial V deep water-storage tissue is supported by two larger lateral vascular bundles. Cell wall folding and an increased proportion of arabinose-containing polymers was observed in leaves under dehydration, suggesting increasing of cell wall flexibility during desiccation. Such biochemical and morphological changes are consistent with the ability of P. burchellii to tolerate intense desiccation and behave as a resurrection species.

Physico-chemical and viscoelastic properties of high pressure homogenized lemon peel fiber fraction suspensions obtained after sequential pectin extraction

The viscoelastic properties of high pressure homogenized lemon peel cell wall fiber suspensions, obtained after sequential selective pectin extraction, were investigated in the current study. For comparison, a general pectin extraction was additionally performed on lemon peel under acid thermal conditions. The obtained insoluble residues and solubilized pectin fractions were characterized based on their neutral sugar and galacturonic acid content, degree of methoxylation and molar mass distribution. Subsequently, microstructure and viscoelastic characteristics of the diverse insoluble residue suspensions were investigated by means of microscopic visualization, determination of the particle size distribution and rheological properties after high pressure homogenization.Due to the use of different extraction conditions, the insoluble cell wall residues were found to have diverse pectin characteristics and contents. While homogenization of non- or moderate-depleted cell wall suspensions resulted in small viscoelastic moduli due to the disruption of the stiff microfibrillar cell wall network, extensive pectin removal was found to correlate to large viscoelastic moduli of the cell wall suspension upon high pressure homogenization. It is suggested that an extensive pectin extraction resulted in the creation of interfibrillar voids within the cellulose-hemicellulose network, increasing the cell wall flexibility and hydration of the fibers upon suspension. Due to the increased flexibility of this fiber network, the interfibrillar voids in the pectin depleted cell wall material were proposed to expand with increasing pressure level during high pressure homogenization of the generated cell wall suspensions, thereby increasing its viscoelastic properties.

PECTIN METHYLESTERASE34 Contributes to Heat Tolerance through Its Role in Promoting Stomatal Movement.

Pectin, a major component of the primary cell wall, is synthesized in the Golgi apparatus and exported to the cell wall in a highly methylesterified form, then is partially demethylesterified by pectin methylesterases (PMEs; EC 3.1.1.11). PME activity on the status of pectin methylesterification profoundly affects the properties of pectin and, thereby, is critical for plant development and the plant defense response, although the roles of PMEs under heat stress (HS) are poorly understood. Functional genome annotation predicts that at least 66 potential PME genes are contained in Arabidopsis (Arabidopsis thaliana). Thermotolerance assays of PME gene T-DNA insertion lines revealed two null mutant alleles of PME34 (At3g49220) that both consistently showed reduced thermotolerance. Nevertheless, their impairment was independently associated with the expression of HS-responsive genes. It was also observed that PME34 transcription was induced by abscisic acid and highly expressed in guard cells. We showed that the PME34 mutation has a defect in the control of stomatal movement and greatly altered PME and polygalacturonase (EC 3.2.1.15) activity, resulting in a heat-sensitive phenotype. PME34 has a role in the regulation of transpiration through the control of the stomatal aperture due to its cell wall-modifying enzyme activity during the HS response. Hence, PME34 is required for regulating guard cell wall flexibility to mediate the heat response in Arabidopsis.

The role of arabidopsis WDR protein in plant growth and defense strategies

ABSTRACTEvidence indicates that the mechanisms controlling photosynthesis efficiency also regulate plant response to biotic and abiotic stress. Light-induced cell death is genetically maintained for the control of innate immunity. In a recent study we showed that the expression of AtWDR26 was induced by light, multiple plant hormones, and abiotic stress; increased AtWDR26 strongly upregulated gene groups related to chloroplast metabolism, disease resistance, and abiotic stress tolerance. Gain- and loss-of-function analyses in transgenic plants demonstrated the involvement of AtWDR26 in signaling pathways; these controls were osmotic as well as salt stress tolerance. More detailed transcriptome evidence suggested that AtWDR26 was a powerful inducer of gene expression associated with chloroplast metabolism. This included the electron transport chain of the photosystem, carbohydrate synthesis, and enzymatic activity involved in photorespiration. Moreover, genes in auxin synthesis (and perception) constituted a significant portion of those that were upregulated. Gene expression involved in disease resistance, control of cell wall flexibility, Zn uptake, and AP2/ERF transcription factors was also be upregulated. We concluded that AtWDR26 is one component in the regulatory network between light-regulated plant growth and the adaptation response to disease resistance and abiotic stress. Auxin signal acts downstream for AtWDR26 regulation and the adaptation response to biotic and abiotic stress: this occurs through modulating cell wall flexibility, Zn homeostasis, and controlling stress-related transcription factors.

Tubulin perturbation leads to unexpected cell wall modifications and affects stomatal behaviour in Populus.

Cortical microtubules are integral to plant morphogenesis, cell wall synthesis, and stomatal behaviour, presumably by governing cellulose microfibril orientation. Genetic manipulation of tubulins often leads to abnormal plant development, making it difficult to probe additional roles of cortical microtubules in cell wall biogenesis. Here, it is shown that expressing post-translational C-terminal modification mimics of α-tubulin altered cell wall characteristics and guard cell dynamics in transgenic Populus tremula x alba that otherwise appear normal. 35S promoter-driven transgene expression was high in leaves but unusually low in xylem, suggesting high levels of tubulin transgene expression were not tolerated in wood-forming tissues during regeneration of transformants. Cellulose, hemicellulose, and lignin contents were unaffected in transgenic wood, but expression of cell wall-modifying enzymes, and extractability of lignin-bound pectin and xylan polysaccharides were increased in developing xylem. The results suggest that pectin and xylan polysaccharides deposited early during cell wall biogenesis are more sensitive to subtle tubulin perturbation than cellulose and matrix polysaccharides deposited later. Tubulin perturbation also affected guard cell behaviour, delaying drought-induced stomatal closure as well as light-induced stomatal opening in leaves. Pectins have been shown to confer cell wall flexibility critical for reversible stomatal movement, and results presented here are consistent with microtubule involvement in this process. Taken together, the data show the value of growth-compatible tubulin perturbations for discerning microtubule functions, and add to the growing body of evidence for microtubule involvement in non-cellulosic polysaccharide assembly during cell wall biogenesis.

Developmental anatomy and immunocytochemistry reveal the neo-ontogenesis of the leaf tissues of Psidium myrtoides (Myrtaceae) towards the globoid galls of Nothotrioza myrtoidis (Triozidae).

The temporal balance between hyperplasia and hypertrophy, and the new functions of different cell lineages led to cell transformations in a centrifugal gradient that determines the gall globoid shape. Plant galls develop by the redifferentiation of new cell types originated from those of the host plants, with new functional and structural designs related to the composition of cell walls and cell contents. Variations in cell wall composition have just started to be explored with the perspective of gall development, and are herein related to the histochemical gradients previously detected on Psidium myrtoides galls. Young and mature leaves of P. myrtoides and galls of Nothotrioza myrtoidis at different developmental stages were analysed using anatomical, cytometrical and immunocytochemical approaches. The gall parenchyma presents transformations in the size and shape of the cells in distinct tissue layers, and variations of pectin and protein domains in cell walls. The temporal balance between tissue hyperplasia and cell hypertrophy, and the new functions of different cell lineages led to cell transformations in a centrifugal gradient, which determines the globoid shape of the gall. The distribution of cell wall epitopes affected cell wall flexibility and rigidity, towards gall maturation. By senescence, it provided functional stability for the outer cortical parenchyma. The detection of the demethylesterified homogalacturonans (HGAs) denoted the activity of the pectin methylesterases (PMEs) during the senescent phase, and was a novel time-based detection linked to the increased rigidity of the cell walls, and to the gall opening. Current investigation firstly reports the influence of immunocytochemistry of plant cell walls over the development of leaf tissues, determining their neo-ontogenesis towards a new phenotype, i.e., the globoid gall morphotype.

Preparation of low-density polyethylene foams with high rebound resilience by blending with polyethylene-octylene elastomer

Low-density polyethylene (LDPE)/polyethylene–octylene elastomer (POE) foams with different composition ratios (POE, cross-linking agent, and blowing agent) were produced by using the continuous cross-linking and foaming process to improve the rebound resilience of chemical cross-linked LDPE foams. The effects of POE, cross-linking agent, and foaming agent on rebound resilience of LDPE/POE foams were investigated by using a rebound test, cross-linking degree experiment, differential scanning calorimetry, and scanning electron microscopy. Results show that the rebound resilience of LDPE was improved by increasing the flexibility of cell walls and the cell density and decreasing the foam density. Compared with the rebound resilience of pure LDPE (33%), the proposed LDPE/POE foams could meet the requirements of gymnastics mats for high rebound resilience (55%). POLYM. ENG. SCI., 53:2527–2534, 2013. © 2013 Society of Plastics Engineers

Plasticity of Xyloglucan Composition in Bean (Phaseolus vulgaris)-Cultured Cells during Habituation and Dehabituation to Lethal Concentrations of Dichlobenil

Bean cells that have been habituated to grow in a lethal concentration (12 μM) of 2,6-dichlorobenzonitrile (dichlobenil or DCB, a cellulose biosynthesis inhibitor) are known to have decreased cellulose content in their cell walls. Xyloglucan, which is bound to cellulose and together with it forms the main loading network of plant cell walls, has also been described to decrease in habituated cells, but whether the change on cellulose affects the xyloglucan structure besides its abundance has not been analyzed. Fragmentation analysis with xyloglucan-specific endoglucanase (XEG) and endocellulase revealed that habituation to DCB caused a change in the fine structure of xyloglucan, namely a decrease in fucosyl residues attached to the galactosyl-xylosyl residues along the glucan backbone. After the removal of herbicide from the medium (dehabituated cells), xyloglucan recovered its fucosyl residues. In addition, some cello-oligosaccharides could be detected only in habituated cells' xyloglucan digested by XEG and endocellulase, corresponding to a glucan covalently bound or co-precipitated with the hemicelluloses. These results show that structural flexibility of cell walls relies in part on the plasticity of xyloglucan composition and opens up new perspectives to further research in this field.

Cell wall components affect mechanical properties: studies with thistle flowers

Pollen presentation of Cirsium horridulum depends partially on the thigmonastic contraction of staminal filaments. Although the elastic cuticle is a major component in filament elasticity, it is not clear how the cell wall copes with the shape change. Based on mechanical studies, FT-IR spectroscopy and biochemical analyses we investigated the relationship between cell wall composition and elastic properties using thistle floral tissues as a model. EDTA-extractable pectin correlated with the increased elasticity of the filament and the basal style, suggesting that pectin plays a major role in the elastic behavior of soft tissues. In contrast, covalently linked pectin contributes to the stiffness of the upper style and corolla. Mechanical tests contrasting the soft basal and rigid apical parts of the style after incubation in solutions designed to alter the pectin network confirmed these results. The rigid corolla contained more cellulose than the softer style and filaments. The cellulose-associated xyloglucan of the style and filament cell walls increase the flexibility of cell walls.

Characterization of somatic embryogenesis in sandalwood (Santalum album L.)

In the synchronous embryogenesis system of sandalwood developed in our laboratory, we observed that the early events of differentiation from freshly induced callus (stage 0) are accomplished in three distinct stages viz., preglobular masses (stage 1), globular embryos (stage 2), and bipolar embryos (stage 3). Transition from stage 0 to 1 was accomplished using 2,4-D and involves a stage specific appearance of two polypeptides of 15 and 30 kDa molecular weight. A 24 kDa polypeptide that was detected as a marked band in extracts of primary callus was not detected in stages 1, 2, and 3. Further, the tissue level of a 50 kDa glycoprotein decreased during transition from stage 2 to stage 3. However, the levels of glycoproteins in the medium were markedly higher in stage 0 cultures compared to those in stage 1. The activities of a protein kinase, glycosidase, and xylanase increased markedly with progressing embryogenesis. Our observations suggest that in addition to being controlled at the level of stage-specific gene expression, somatic embryogenesis in sandalwood is also regulated at the level of controls on cell wall flexibility and posttranslational changes in the pool of preexisting proteins.