Extensin gene expression is induced by mechanical stimuli leading to local cell wall strengthening in Nicotiana plumbaginifolia

Immune Priming Triggers Cell Wall Remodeling and Increased Resistance to Halo Blight Disease in Common Bean

The cell wall (CW) is a dynamic structure extensively remodeled during plant growth and under stress conditions, however little is known about its roles during the immune system priming, especially in crops. In order to shed light on such a process, we used the Phaseolus vulgaris-Pseudomonas syringae (Pph) pathosystem and the immune priming capacity of 2,6-dichloroisonicotinic acid (INA). In the first instance we confirmed that INA-pretreated plants were more resistant to Pph, which was in line with the enhanced production of H₂O₂ of the primed plants after elicitation with the peptide flg22. Thereafter, CWs from plants subjected to the different treatments (non- or Pph-inoculated on non- or INA-pretreated plants) were isolated to study their composition and properties. As a result, the Pph inoculation modified the bean CW to some extent, mostly the pectic component, but the CW was as vulnerable to enzymatic hydrolysis as in the case of non-inoculated plants. By contrast, the INA priming triggered a pronounced CW remodeling, both on the cellulosic and non-cellulosic polysaccharides, and CW proteins, which resulted in a CW that was more resistant to enzymatic hydrolysis. In conclusion, the increased bean resistance against Pph produced by INA priming can be explained, at least partially, by a drastic CW remodeling.

Plasticity via feedback reduces the cost of developmental instability

Costs of plasticity are thought to have important physiological and evolutionary consequences. A commonly predicted cost to plasticity is that plastic genotypes are likely to suffer from developmental instability. Adaptive plasticity requires that the developing organism can in some way sense what environment it is in or how well it is performing in that environment. These two information pathways—an “environmental signal” or a “performance signal” that indicates how well a developing phenotype matches the optimum in the current environment—can differ in their consequences for the organism and its evolution. Here, we consider how developmental instability might emerge as a side‐effect of these two distinct mechanisms. Because a performance cue allows a regulatory feedback loop connecting a trait to a feedback signal, we hypothesized that plastic genotypes using a performance signal would be more developmentally robust compared to those using a purely environmental signal. Using a numerical model of a network of gene interactions, we show that plasticity comes at a cost of developmental instability when the plastic response is mediated via an environmental signal, but not when it is mediated via a performance signal. We also show that a performance signal mechanism can evolve even in a constant environment, leading to genotypes preadapted for plasticity to novel environments even in populations without a history of environmental heterogeneity.

Mutations in the Pectin Methyltransferase QUASIMODO2 Influence Cellulose Biosynthesis and Wall Integrity in Arabidopsis

Pectins are abundant in the cell walls of dicotyledonous plants, but how they interact with other wall polymers and influence wall integrity and cell growth has remained mysterious. Here, we verified that QUASIMODO2 (QUA2) is a pectin methyltransferase and determined that QUA2 is required for normal pectin biosynthesis. To gain further insight into how pectin affects wall assembly and integrity maintenance, we investigated cellulose biosynthesis, cellulose organization, cortical microtubules, and wall integrity signaling in two mutant alleles of Arabidopsis (Arabidopsis thaliana) QUA2, qua2 and tsd2 In both mutants, crystalline cellulose content is reduced, cellulose synthase particles move more slowly, and cellulose organization is aberrant. NMR analysis shows higher mobility of cellulose and matrix polysaccharides in the mutants. Microtubules in mutant hypocotyls have aberrant organization and depolymerize more readily upon treatment with oryzalin or external force. The expression of genes related to wall integrity, wall biosynthesis, and microtubule stability is dysregulated in both mutants. These data provide insights into how homogalacturonan is methylesterified upon its synthesis, the mechanisms by which pectin functionally interacts with cellulose, and how these interactions are translated into intracellular regulation to maintain the structural integrity of the cell wall during plant growth and development.

Cell separation in kiwifruit without development of a specialised detachment zone

BACKGROUND

Unlike in abscission or dehiscence, fruit of kiwifruit Actinidia eriantha develop the ability for peel detachment when they are ripe and soft in the absence of a morphologically identifiable abscission zone. Two closely-related genotypes with contrasting detachment behaviour have been identified. The 'good-peeling' genotype has detachment with clean debonding of cells, and a peel tissue that does not tear. The 'poor-peeling' genotype has poor detachability, with cells that rupture upon debonding, and peel tissue that fragments easily.

RESULTS

Structural studies indicated that peel detachability in both genotypes occurred in the outer pericarp beneath the hypodermis. Immunolabelling showed differences in methylesterification of pectin, where the interface of labelling coincided with the location of detachment in the good-peeling genotype, whereas in the poor-peeling genotype, no such interface existed. This zone of difference in methylesterification was enhanced by differential cell wall changes between the peel and outer pericarp tissue. Although both genotypes expressed two polygalacturonase genes, no enzyme activity was detected in the good-peeling genotype, suggesting limited pectin breakdown, keeping cell walls strong without tearing or fragmentation of the peel and flesh upon detachment. Differences in location and amounts of wall-stiffening galactan in the peel of the good-peeling genotype possibly contributed to this phenotype. Hemicellulose-acting transglycosylases were more active in the good-peeling genotype, suggesting an influence on peel flexibility by remodelling their substrates during development of detachability. High xyloglucanase activity in the peel of the good-peeling genotype may contribute by having a strengthening effect on the cellulose-xyloglucan network.

CONCLUSIONS

In fruit of A. eriantha, peel detachability is due to the establishment of a zone of discontinuity created by differential cell wall changes in peel and outer pericarp tissues that lead to changes in mechanical properties of the peel. During ripening, the peel becomes flexible and the cells continue to adhere strongly to each other, preventing breakage, whereas the underlying outer pericarp loses cell wall strength as softening proceeds. Together these results reveal a novel and interesting mechanism for enabling cell separation.

EXTENSIN18 is required for full male fertility as well as normal vegetative growth in Arabidopsis

EXTENSINS (EXTs) are a 65-member subfamily of hydroxyproline-rich glycoproteins (HRGPs) of which 20 putatively form crosslinking networks in the cell wall. These 20 classical EXTs are involved at the start of new wall assembly as evidenced by a requirement for EXT3 during cytokinesis, and the ability of some EXTs to polymerize in vitro into dendritic patterns. EXT3 was previously shown to form pulcherosine (three Tyrosines) cross-links. Little direct data exists on the other 19 classical EXTs. Here, we describe the phenotypes of ext18 mutants and rescued progeny as well as associated expression profiles of all 20 classical EXT genes. We found that EXT18 is required for full male fertility, as well as for normal vegetative growth. EXT18 has potential to form crosslinking networks via di-iso-di-tyrosine (four Tyrosines) covalent bonds, and not via pulcherosine due to deficit of lone Tyrosines. This together with ext18 defective pollen grains and pollen tubes, and reduced plant size, suggests that EXT18-type EXTs are important contributors to wall integrity, in pollen and other rapidly extending walls. The data also show that a knockout of EXT18 had a pleiotropic affect on the expression of several EXTs, as did the reintroduction of the native EXT18 gene, thus supporting the thesis that transcription of groups of EXTs are co-regulated and work in different combinations to make distinctive inputs into wall assembly of different cell types. These insights contribute to basic knowledge of cell wall self-assembly in different cell types, and potentially enable biotechnological advances in biomass increase and plant fertility control.

Periplasmic arabinogalactan glycoproteins act as a calcium capacitor that regulates plant growth and development

Arabinogalactan glycoproteins (AGPs) are implicated in virtually all aspects of plant growth and development, yet their precise role remains unknown. Classical AGPs cover the plasma membrane and are highly glycosylated by numerous acidic arabinogalactan polysaccharides O-linked to hydroxyproline. Their heterogeneity and complexity hindered a structural approach until the recent determination of a highly conserved repetitive consensus structure for a 15-sugar residue arabinogalactan subunit with paired glucuronic carboxyls. Based on NMR data and molecular dynamics simulations, we identify these carboxyls as potential intramolecular Ca(2+)-binding sites. Using rapid ultrafiltration assays and mass spectrometry, we verified that AGPs bind Ca(2+) tightly (K(d) ~ 6.5 μM) and stoichiometrically at pH 5. Ca(2+) binding is reversible in a pH-dependent manner. As typical AGPs contain c. 30 Ca(2+)-binding subunits and are bulk components of the periplasm, they represent a Ca(2+) capacitor discharged at low pH by stretch-activated plasma membrane H(+)-ATPases, hence a substantial source of cytosolic Ca(2+). We propose that these Ca(2+) waves prime the 'calcium oscillator', a signal generator essential to the global Ca(2+) signalling pathway of green plants.

Carbon nanotubes induce growth enhancement of tobacco cells

Carbon nanotubes have shown promise as regulators of seed germination and plant growth. Here, we demonstrate that multiwalled carbon nanotubes (MWCNTs) have the ability to enhance the growth of tobacco cell culture (55-64% increase over control) in a wide range of concentrations (5-500 μg/mL). Activated carbon (AC) stimulated cell growth (16% increase) only at low concentrations (5 μg/mL) while dramatically inhibited the cellular growth at higher concentrations (100-500 μg/mL). We found a correlation between the activation of cells growth exposed to MWCNTs and the upregulation of genes involved in cell division/cell wall formation and water transport. The expression of the tobacco aquaporin (NtPIP1) gene, as well as production of the NtPIP1 protein, significantly increased in cells exposed to MWCNTs compared to control cells or those exposed to AC. The expression of marker genes for cell division (CycB) and cell wall extension (NtLRX1) was also up-regulated in cells exposed to MWCNTs compared to control cells or those exposed to activated carbon only.

Extensin network formation in Vitis vinifera callus cells is an essential and causal event in rapid and H(2)O(2)-induced reduction in primary cell wall hydration

BACKGROUND

Extensin deposition is considered important for the correct assembly and biophysical properties of primary cell walls, with consequences to plant resistance to pathogens, tissue morphology, cell adhesion and extension growth. However, evidence for a direct and causal role for the extensin network formation in changes to cell wall properties has been lacking.

RESULTS

Hydrogen peroxide treatment of grapevine (Vitis vinifera cv. Touriga) callus cell walls was seen to induce a marked reduction in their hydration and thickness. An analysis of matrix proteins demonstrated this occurs with the insolubilisation of an abundant protein, GvP1, which displays a primary structure and post-translational modifications typical of dicotyledon extensins. The hydration of callus cell walls free from saline-soluble proteins did not change in response to H(2)O(2), but fully regained this capacity after addition of extensin-rich saline extracts. To assay the specific contribution of GvP1 cross-linking and other wall matrix proteins to the reduction in hydration, GvP1 levels in cell walls were manipulated in vitro by binding selected fractions of extracellular proteins and their effect on wall hydration during H(2)O(2) incubation assayed.

CONCLUSIONS

This approach allowed us to conclude that a peroxidase-mediated formation of a covalently linked network of GvP1 is essential and causal in the reduction of grapevine callus wall hydration in response to H(2)O(2). Importantly, this approach also indicated that extensin network effects on hydration was only partially irreversible and remained sensitive to changes in matrix charge. We discuss this mechanism and the importance of these changes to primary wall properties in the light of extensin distribution in dicotyledons.

The salt-responsive transcriptome of chickpea roots and nodules via deepSuperSAGE

BACKGROUND

The combination of high-throughput transcript profiling and next-generation sequencing technologies is a prerequisite for genome-wide comprehensive transcriptome analysis. Our recent innovation of deepSuperSAGE is based on an advanced SuperSAGE protocol and its combination with massively parallel pyrosequencing on Roche's 454 sequencing platform. As a demonstration of the power of this combination, we have chosen the salt stress transcriptomes of roots and nodules of the third most important legume crop chickpea (Cicer arietinum L.). While our report is more technology-oriented, it nevertheless addresses a major world-wide problem for crops generally: high salinity. Together with low temperatures and water stress, high salinity is responsible for crop losses of millions of tons of various legume (and other) crops. Continuously deteriorating environmental conditions will combine with salinity stress to further compromise crop yields. As a good example for such stress-exposed crop plants, we started to characterize salt stress responses of chickpeas on the transcriptome level.

RESULTS

We used deepSuperSAGE to detect early global transcriptome changes in salt-stressed chickpea. The salt stress responses of 86,919 transcripts representing 17,918 unique 26 bp deepSuperSAGE tags (UniTags) from roots of the salt-tolerant variety INRAT-93 two hours after treatment with 25 mM NaCl were characterized. Additionally, the expression of 57,281 transcripts representing 13,115 UniTags was monitored in nodules of the same plants. From a total of 144,200 analyzed 26 bp tags in roots and nodules together, 21,401 unique transcripts were identified. Of these, only 363 and 106 specific transcripts, respectively, were commonly up- or down-regulated (>3.0-fold) under salt stress in both organs, witnessing a differential organ-specific response to stress.Profiting from recent pioneer works on massive cDNA sequencing in chickpea, more than 9,400 UniTags were able to be linked to UniProt entries. Additionally, gene ontology (GO) categories over-representation analysis enabled to filter out enriched biological processes among the differentially expressed UniTags. Subsequently, the gathered information was further cross-checked with stress-related pathways. From several filtered pathways, here we focus exemplarily on transcripts associated with the generation and scavenging of reactive oxygen species (ROS), as well as on transcripts involved in Na+ homeostasis. Although both processes are already very well characterized in other plants, the information generated in the present work is of high value. Information on expression profiles and sequence similarity for several hundreds of transcripts of potential interest is now available.

CONCLUSIONS

This report demonstrates, that the combination of the high-throughput transcriptome profiling technology SuperSAGE with one of the next-generation sequencing platforms allows deep insights into the first molecular reactions of a plant exposed to salinity. Cross validation with recent reports enriched the information about the salt stress dynamics of more than 9,000 chickpea ESTs, and enlarged their pool of alternative transcripts isoforms. As an example for the high resolution of the employed technology that we coin deepSuperSAGE, we demonstrate that ROS-scavenging and -generating pathways undergo strong global transcriptome changes in chickpea roots and nodules already 2 hours after onset of moderate salt stress (25 mM NaCl). Additionally, a set of more than 15 candidate transcripts are proposed to be potential components of the salt overly sensitive (SOS) pathway in chickpea. Newly identified transcript isoforms are potential targets for breeding novel cultivars with high salinity tolerance. We demonstrate that these targets can be integrated into breeding schemes by micro-arrays and RT-PCR assays downstream of the generation of 26 bp tags by SuperSAGE.

SuperSAGE: the drought stress-responsive transcriptome of chickpea roots

BACKGROUND

Drought is the major constraint to increase yield in chickpea (Cicer arietinum). Improving drought tolerance is therefore of outmost importance for breeding. However, the complexity of the trait allowed only marginal progress. A solution to the current stagnation is expected from innovative molecular tools such as transcriptome analyses providing insight into stress-related gene activity, which combined with molecular markers and expression (e)QTL mapping, may accelerate knowledge-based breeding. SuperSAGE, an improved version of the serial analysis of gene expression (SAGE) technique, generating genome-wide, high-quality transcription profiles from any eukaryote, has been employed in the present study. The method produces 26 bp long fragments (26 bp tags) from defined positions in cDNAs, providing sufficient sequence information to unambiguously characterize the mRNAs. Further, SuperSAGE tags may be immediately used to produce microarrays and probes for real-time-PCR, thereby overcoming the lack of genomic tools in non-model organisms.

RESULTS

We applied SuperSAGE to the analysis of gene expression in chickpea roots in response to drought. To this end, we sequenced 80,238 26 bp tags representing 17,493 unique transcripts (UniTags) from drought-stressed and non-stressed control roots. A total of 7,532 (43%) UniTags were more than 2.7-fold differentially expressed, and 880 (5.0%) were regulated more than 8-fold upon stress. Their large size enabled the unambiguous annotation of 3,858 (22%) UniTags to genes or proteins in public data bases and thus to stress-response processes. We designed a microarray carrying 3,000 of these 26 bp tags. The chip data confirmed 79% of the tag-based results, whereas RT-PCR confirmed the SuperSAGE data in all cases.

CONCLUSION

This study represents the most comprehensive analysis of the drought-response transcriptome of chickpea available to date. It demonstrates that--inter alias--signal transduction, transcription regulation, osmolyte accumulation, and ROS scavenging undergo strong transcriptional remodelling in chickpea roots already 6 h after drought stress. Certain transcript isoforms characterizing these processes are potential targets for breeding for drought tolerance. We demonstrate that these can be easily accessed by micro-arrays and RT-PCR assays readily produced downstream of SuperSAGE. Our study proves that SuperSAGE owns potential for molecular breeding also in non-model crops.

Isolation of a cotton reversibly glycosylated polypeptide (GhRGP1) promoter and its expression activity in transgenic tobacco

Reversibly glycosylated polypeptides (RGPs) are thought to be involved in polysaccharide metabolism. A cDNA of the cotton (Gossypium hirsutum) RGP gene, designated GhRGP1, has previously been characterized, and is preferentially expressed in fiber cells. In order to investigate its temporal and spatial control, we isolated a 624bp fragment upstream of the GhRGP1 coding sequence using a polymerase chain reaction (PCR)-based genomic walking method, transcriptionally fused the 624bp promoter sequence to the beta-glucuronidase (GUS) gene, and analyzed the stable gene expression in tobacco (Nicotiana tabacum). In 4-week-old transgenic tobacco plants, the highest expression level was observed in roots, and the GUS activity was 1.13- and 6.65-fold higher than that in stems and leaves, respectively. In the reproductive growth stage, the GUS expression level was highest in the pistils and the GUS activity in the stigmas and styles were 17.6-fold higher than that in the ovaries. High GUS activity was also detected in the anthers. In addition, histochemical staining for GUS activity on transgenic tobacco plants further indicated a higher expression in the trichomes, seeds and vascular tissues of stems. Abiotic stress treatments on transgenic tobacco plants showed that wounding and dehydration induced GUS expression. These results demonstrated the spatial and temporal regulation of a cotton RGP promoter in a model plant, and provided an important insight into the factors that control the fiber development and stress responses of the gene.

Wound-induced vascular occlusions in tissues of the reed Phragmites australis: their development and chemical nature

This work focuses on the development of vascular occlusions, which are gels resealing the wounded vascular systems of injured organs, in the common reed Phragmites australis. Their formation seems to be crucial in keeping the internal environment of the plant stable. Histochemical tests, combined with an extraction series, were used to follow changes in the chemical nature of gels during their development. It was found that the first gel material was secreted by living cells in the vicinity of the incision within 1 or 2 d after wounding. Early gels were colourless and mainly composed of acidic polysaccharides interlinked by Ca2+ bridges. The properties of the gel material gradually changed during maturation. The matrix of polysaccharides in the early gels was later modified and interlinked by other components, resulting in a highly resistant material. Structural proteins were identified as the principal interlocking components of the material, and were responsible for its high resistance.

Spatial and temporal analysis of the local response to wounding in Arabidopsis leaves

We studied the local response to wounding in Arabidopsis thaliana leaves using a two-step microarray analysis. A microarray containing 3500 cDNA clones was first screened to enrich for genes affected by wounding in the immediate vicinity of the wound (4 h post wounding). 359 non-redundant putative wound responsive genes were then spotted on a smaller wound-response array for detailed analysis of spatial expression (local, adjacent and systemic), timing of expression (0.5, 4, 8, 17 h), and effect of hormone treatments (methyl jasmonate, ethylene and abscisic acid). Our results show that genes that respond early at the site of the wound also respond throughout the plant, with similar kinetics. Early-induced genes which respond systemically encode predominantly signal transduction and regulatory factors (36%), and the expression of many of them is also controlled by methyl jasmonate (about 35% of the 36%). Genes specific to the wound site and the wounded leaf have a slower response to wounding and are mainly metabolic genes. At the wound, many genes of the lignin biosynthesis pathway were induced. In silico analysis of the 5' promoter regions of genes affected by wounding revealed G-box-related motifs in a significant proportion of the promoters. These results show that the establishment of a systemic response to wounding is a priority for the plant, and that the local response at the wound site is established later. Ethylene and abscisic acid are involved in the local response, regulating repression of photosynthetic genes and expression of drought responsive genes respectively.

A biochemical and molecular characterization of LEP1, an extensin peroxidase from lupin

An analysis of apoplastic extensin cross-linking activity in vegetative organs of Lupinus albus indicated that leaves contained the highest specific activity. Assays of peroxidases fractionated from this material demonstrated that this activity could be largely attributed to a soluble and apoplastic 51-kDa peroxidase, denoted LEP1. Relative to other purified peroxidases, LEP1 demonstrates high extensin cross-linking activity and can be classified as an extensin peroxidase (EP). Optimal conditions for the in vitro oxidation of other phenolic substrates included 1.5-3.0 mm peroxide at pH 5.0. EP activity of LEP1 was low under these conditions but optimal and substantially higher with 100 microm peroxide and neutral pH, suggesting that physiological changes in pH and peroxide in muro could heavily influence the extensin cross-linking activity of LEP1 in vivo. Analysis of LEP1 glycans indicated 11-12 N-linked glycans, predominantly the heptasaccharide Man3XylFucGlcNAc2, but also larger structures showing substantial heterogeneity. Comparative assays with horseradish peroxidase isoform C and peanut peroxidases suggested the high level of glycosylation in LEP1 may be responsible for the high solubility of this EP in the apoplastic space. A full-length cDNA corresponding to LEP1 was cloned. Quantitative reverse transcriptase-PCR demonstrated LEP1 induction in apical portions of etiolated hypocotyls 30-60 min after exposure to white light, prior to the onset of growth inhibition. Comparative modeling of the translated sequence indicated an unusually unobstructed equatorial cleft across the substrate access channel, which might facilitate interaction with extensin and confer higher EP activity.

The expression of an extensin-like protein correlates with cellular tip growth in tomato

Extensins are abundant proteins presumed to determine physical characteristics of the plant cell wall. We have cloned a cDNA encoding LeExt1 from a tomato (Lycopersicon esculentum Mill.) root hair cDNA library. The deduced sequence of the LeExt1 polypeptide defined a novel type of extensin-like proteins in tomato. Patterns of mRNA distribution indicated that expression of the LeExt1 gene was initiated in the root hair differentiation zone of the tomato rhizodermis. Cloning of the corresponding promoter and fusion to the -glucuronidase (GUS) reporter gene allowed detailed examination of LeExt1 expression in transgenic tomato plants. Evidence is presented for a direct correlation between LeExt1 expression and cellular tip growth. LeExt1/GUS expression was detectable in trichoblasts (=root hair-bearing cells), but not in atrichoblasts of the tomato rhizodermis. Both hair formation and LeExt1 expression was inducible by the plant hormone ethylene. Comparative analysis of the LeExt1/GUS expression was performed in transgenic tomato, potato (Solanum tuberosum), tobacco (Nicotiana tabacum), and Arabidopsis plants. In the apical/basal dimension, GUS staining was absent from the root cap and undifferentiated cells at the root tip in all species investigated. It was induced at the distal end of the differentiation zone and remained high proximally to the root/hypocotyl boundary. In the radial dimension, GUS expression was root hair specific in the solanaceous species. Whereas LeExt1 mRNA was exclusively detectable in the rhizodermis, root hair-specific expression correlated with GUS expression in germinating pollen tubes. This is correlative evidence for a role of LeExt1 in root hair tip growth [corrected].

Hydrophobic interactions of the structural protein GRP1.8 in the cell wall of protoxylem elements

The glycine-rich structural protein GRP1.8 of French bean (Phaseolus vulgaris) is specifically localized in the modified primary cell walls of protoxylem elements. Continuous deposition of GRP1.8 into the cell walls during elongation growth of the plant suggests that GRP1.8 is part of a repair mechanism to strengthen the protoxylem. In this work, a reporter-protein system was developed to study the interaction of GRP1.8 with the extracellular matrix. Fusion proteins of a highly soluble chitinase with different domains of GRP1.8 were expressed in the vascular tissue of tobacco (Nicotiana tabacum), and the chemical nature of the interaction of these fusion proteins in the cell wall compartment was analyzed. In contrast with chitinase that required only low-salt conditions for complete extraction, the different chitinase/GRP1.8 fusion proteins were completely extracted only by a nonionic or ionic detergent, indicating hydrophobic interactions of GRP1.8. The same interactions were found with the endogenous GRP1.8 in bean hypocotyls. In addition, in vitro experiments indicate that oxidative cross-linking of tyrosines might account for the insolubilization of GRP1.8 observed in later stages of protoxylem development. Our data suggest that GRP1.8 forms a hydrophobic protein-layer in the cell wall of protoxylem vessels.

Accurate and high resolution in situ hybridization analysis of gene expression in secondary stem tissues

Accurate in situ hybridization analysis in secondary stem tissues of plants has been hindered by specific characteristics of these tissues. First, secondary cell walls non-specifically bind probes used for in situ hybridization thus preventing gene expression analysis in the lignified regions of the stem, such as the xylem. Second, the mRNA in the cambial meristem and its recent derivatives are prone to inadequate fixation when conventional techniques are used. Here we describe an in situ hybridization technique which uses fast freezing and freeze substitution to cryoimmobilize the mRNA followed by embedding in a methacrylate resin for high-resolution analysis of gene expression. By using a transgenic poplar line harbouring rolC:uidA, rolC:iaaM, the gene expression pattern could be compared with histochemical GUS staining. This in situ hybridization technique results in superior preservation of cellular contents, retention of mRNA in all cell types in the poplar stem, a significant reduction of non-specific binding to secondary cell walls and a resolution not previously possible in secondary tissues. This technique will be particularly valuable for the expression analysis of genes involved in xylogenesis and wood formation.

Control of expression of a gene encoding an extensin by phytochrome and a blue light receptor in spores of Adiantum capillus-veneris L

In the present study, using a newly developed fluorescent differential display technique, we have carried out large-scale screening for genes whose expression was regulated by phytochrome and antagonistically by a blue light receptor in the spores of the fern Adiantum capillus-veneris L. Spores after imbibition were briefly irradiated with red, red/blue or blue light and collected 8 h after the irradiation. Total RNA was isolated from each sample and used to make cDNA with an oligo-dT primer. The cDNA was then used as a template for PCR with the oligo-dT primer and 80 arbitrary primers. The resulting PCR products were analyzed by an automated fluorescent DNA sequencer. Among 8000 displayed bands, we identified 15 up-regulated and four down-regulated bands by red light, and this red light effect was irreversibly reversed by blue light. We cloned one of the up-regulated cDNA fragments and used it to screen a cDNA library prepared from the spores. The isolated insert is predicted to encode Ser-(Pro)n repeats and showed homology with cell wall-associated extensins. The expression of this cDNA was induced 8 h after a red light treatment and the red light induction was photoreversibly prevented by far-red light and photoirreversibly by blue light. The mRNA of this gene was detectable 4 h after red light irradiation and gradually increased in germinating spores.

PLANT CELL WALL PROTEINS

The nature of cell wall proteins is as varied as the many functions of plant cell walls. With the exception of glycine-rich proteins, all are glycosylated and contain hydroxyproline (Hyp). Again excepting glycine-rich proteins, they also contain highly repetitive sequences that can be shared between them. The majority of cell wall proteins are cross-linked into the wall and probably have structural functions, although they may also participate in morphogenesis. On the other hand, arabinogalactan proteins are readily soluble and possibly play a major role in cell-cell interactions during development. The interactions of these proteins between themselves and with other wall components is still unknown, as is how wall components are assembled. The possible functions of cell wall proteins are suggested based on repetitive sequence, localization in the plant body, and the general morphogenetic pattern in plants.

Assembly and enlargement of the primary cell wall in plants

Growing plant cells are shaped by an extensible wall that is a complex amalgam of cellulose microfibrils bonded noncovalently to a matrix of hemicelluloses, pectins, and structural proteins. Cellulose is synthesized by complexes in the plasma membrane and is extruded as a self-assembling microfibril, whereas the matrix polymers are secreted by the Golgi apparatus and become integrated into the wall network by poorly understood mechanisms. The growing wall is under high tensile stress from cell turgor and is able to enlarge by a combination of stress relaxation and polymer creep. A pH-dependent mechanism of wall loosening, known as acid growth, is characteristic of growing walls and is mediated by a group of unusual wall proteins called expansins. Expansins appear to disrupt the noncovalent bonding of matrix hemicelluloses to the microfibril, thereby allowing the wall to yield to the mechanical forces generated by cell turgor. Other wall enzymes, such as (1-->4) beta-glucanases and pectinases, may make the wall more responsive to expansin-mediated wall creep whereas pectin methylesterases and peroxidases may alter the wall so as to make it resistant to expansin-mediated creep.

Di-isodityrosine, a novel tetrametric derivative of tyrosine in plant cell wall proteins: a new potential cross-link

A novel amino acid, di-isodityrosine, has been isolated from hydrolysates of cell walls of tomato cell culture. Analysis by UV spectrometry, partial derivatization with 2,4-dinitrofluorobenzene and mass and NMR spectrometry show that the compound is composed to two molecules of isodityrosine, joined by a biphenyl linkage. The possible reactions involved in the formation of this molecule in vivo are discussed, as is the possibility that it could form an interpolypeptide linkage between cell wall proteins such as extensin, and hence aid in the insolubilization of the protein in the wall.

A gene for Brassica napus extensin is differentially expressed on wounding

We have analysed the expression of the endogenous extensin genes in Brassica napus, using northern hybridisation and dot blotting. In the unstressed plant, the extA gene is only expressed in the root, expression in the leaf, petiole and stem being absent. We have found that wounding dramatically alters this normal pattern of expression. Expression in wounded leaf is seen after 36 h, in wounded petioles after 11 h and in wounded stem after 17 h. Differences in the amount of extensin mRNA accumulated are also seen: wounded petiole accumulating extensin message to a level higher than the leaf or the stem. Inhibitors of ethylene biosynthesis greatly delay the onset of accumulation of extensin mRNA in wounded tissues. Wounding the root causes the level of extensin message to decline with time, until levels below the limit of non-specific hybridisation are reached 11 h after wounding. Thus, application of the wounding stimulus results in the accumulation of extensin gene transcripts to different degrees and at different times in the aerial parts of the plant, and results in a decline in the same transcripts in the roots. Extensin transcript accumulation as a result of wounding is also dependent on the age of the tissue; high levels of message are seen in old wounded leaves, while expression in young wounded leaves is absent.