Tomato extensin and extensin-like cDNAs: structure and expression in response to wounding

Extensins: Self-Assembly, Crosslinking, and the Role of Peroxidases

The extensin (EXT) network is elaborated by the covalent intermolecular crosslinking of EXT glycoprotein monomers, and its proper assembly is important for numerous aspects of basic wall architecture and cellular defense. In this review, we discuss new advances in the secretion of EXT monomers and the molecular drivers of EXT network self-assembly. Many of the functions of EXTs are conferred through covalent crosslinking into the wall, so we also discuss the different types of known intermolecular crosslinks, the enzymes that are involved, as well as the potential for additional crosslinks that are yet to be identified. EXTs also function in wall architecture independent of crosslinking status, and therefore, we explore the role of non-crosslinking EXTs. As EXT crosslinking is upregulated in response to wounding and pathogen infection, we discuss a potential regulatory mechanism to control covalent crosslinking and its relationship to the subcellular localization of the crosslinking enzymes.

Plant Protein O-Arabinosylation

A wide range of proteins with diverse functions in development, defense, and stress responses are O-arabinosylated at hydroxyprolines (Hyps) within distinct amino acid motifs of continuous stretches of Hyps, as found in the structural cell wall extensins, or at non-continuous Hyps as, for example, found in small peptide hormones and a variety of plasma membrane proteins involved in signaling. Plant O-glycosylation relies on hydroxylation of Prolines to Hyps in the protein backbone, mediated by prolyl-4-hydroxylase (P4H) which is followed by O-glycosylation of the Hyp C₄-OH group by either galactosyltransferases (GalTs) or arabinofuranosyltranferases (ArafTs) yielding either Hyp-galactosylation or Hyp-arabinosylation. A subset of the P4H enzymes with putative preference to hydroxylation of continuous prolines and presumably all ArafT enzymes needed for synthesis of the substituted arabinose chains of one to four arabinose units, have been identified and functionally characterized. Truncated root-hair phenotype is one common denominator of mutants of Hyp formation and Hyp-arabinosylation glycogenes, which act on diverse groups of O-glycosylated proteins, e.g., the small peptide hormones and cell wall extensins. Dissection of different substrate derived effects may not be regularly feasible and thus complicate translation from genotype to phenotype. Recently, lack of proper arabinosylation on arabinosylated proteins has been shown to influence their transport/fate in the secretory pathway, hinting to an additional layer of functionality of O-arabinosylation. Here, we provide an update on the prevalence and types of O-arabinosylated proteins and the enzymatic machinery responsible for their modifications.

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.

Characterization of NtREL1, a novel root-specific gene from tobacco, and upstream promoter activity analysis in homologous and heterologous hosts

KEY MESSAGE

A novel root-specific gene and its upstream promoter were cloned and characterized for potential application in root-specific expression of transgenes. The root is an important plant organ subjected to many biotic and abiotic stresses, such as infection by Ralstonia solanacearum. To isolate tobacco root-specific promoters for genetic applications, microarray screening was performed to identify genes highly and specifically expressed in the root. One root-specific gene encoding an extensin-like protein (NtREL1) was isolated, and its expression pattern was further characterized by both microarray analysis and reverse transcription-polymerase chain reaction. NtREL1 was highly expressed only in roots but not in any other organ. NtREL1 expression was affected by hormone treatment (salicylic acid, abscisic acid, and ethephon) as well as low temperature, drought, and R. solanacearum infection. A full-length 849 bp cDNA containing a 657-nucleotide open reading frame was cloned by Rapid Amplification of cDNA Ends. Subsequently, a fragment of 1,574 bp upstream of NtREL1 was isolated by flanking PCR and named pNtREL1. This promoter fragment contains TATA, GATA, and CAAT-boxes, the basic elements of a promoter, and six root-specific expression elements, namely OSE1, OSE2, ROOTMOTIFTAPOX1, SURECOREATSULTR11, P1BS, and WUSATAg. A construct containing the bacterial uidA reporter gene (β-glucuronidase, GUS) driven by the pNtREL1 promoter was transformed into tobacco plants. GUS staining was only detected in the root, but not in leaves and stems. Additionally, transgenic tobacco plants containing peanut resveratrol synthase gene (AhRS) driven by the pNtREL1 promoter produced resveratrol only in the root. Thus, the pNtREL1 promoter can be used to direct root-specific expression of target genes to protect the root from stress or for biological studies.

Bioinformatic Identification and Analysis of Extensins in the Plant Kingdom

Extensins (EXTs) are a family of plant cell wall hydroxyproline-rich glycoproteins (HRGPs) that are implicated to play important roles in plant growth, development, and defense. Structurally, EXTs are characterized by the repeated occurrence of serine (Ser) followed by three to five prolines (Pro) residues, which are hydroxylated as hydroxyproline (Hyp) and glycosylated. Some EXTs have Tyrosine (Tyr)-X-Tyr (where X can be any amino acid) motifs that are responsible for intramolecular or intermolecular cross-linkings. EXTs can be divided into several classes: classical EXTs, short EXTs, leucine-rich repeat extensins (LRXs), proline-rich extensin-like receptor kinases (PERKs), formin-homolog EXTs (FH EXTs), chimeric EXTs, and long chimeric EXTs. To guide future research on the EXTs and understand evolutionary history of EXTs in the plant kingdom, a bioinformatics study was conducted to identify and classify EXTs from 16 fully sequenced plant genomes, including Ostreococcus lucimarinus, Chlamydomonas reinhardtii, Volvox carteri, Klebsormidium flaccidum, Physcomitrella patens, Selaginella moellendorffii, Pinus taeda, Picea abies, Brachypodium distachyon, Zea mays, Oryza sativa, Glycine max, Medicago truncatula, Brassica rapa, Solanum lycopersicum, and Solanum tuberosum, to supplement data previously obtained from Arabidopsis thaliana and Populus trichocarpa. A total of 758 EXTs were newly identified, including 87 classical EXTs, 97 short EXTs, 61 LRXs, 75 PERKs, 54 FH EXTs, 38 long chimeric EXTs, and 346 other chimeric EXTs. Several notable findings were made: (1) classical EXTs were likely derived after the terrestrialization of plants; (2) LRXs, PERKs, and FHs were derived earlier than classical EXTs; (3) monocots have few classical EXTs; (4) Eudicots have the greatest number of classical EXTs and Tyr-X-Tyr cross-linking motifs are predominantly in classical EXTs; (5) green algae have no classical EXTs but have a number of long chimeric EXTs that are absent in embryophytes. Furthermore, phylogenetic analysis was conducted of LRXs, PERKs and FH EXTs, which shed light on the evolution of three EXT classes.

Arabinosylation Plays a Crucial Role in Extensin Cross-linking In Vitro

Extensins (EXTs) are hydroxyproline-rich glycoproteins (HRGPs) that are structural components of the plant primary cell wall. They are basic proteins and are highly glycosylated with carbohydrate accounting for >50% of their dry weight. Carbohydrate occurs as monogalactosyl serine and arabinosyl hydroxyproline, with arabinosides ranging in size from ~1 to 4 or 5 residues. Proposed functions of EXT arabinosylation include stabilizing the polyproline II helix structure and facilitating EXT cross-linking. Here, the involvement of arabinosylation in EXT cross-linking was investigated by assaying the initial cross-linking rate and degree of cross-linking of partially or fully de-arabinosylated EXTs using an in vitro cross-linking assay followed by gel permeation chromatography. Our results indicate that EXT arabinosylation is required for EXT cross-linking in vitro and the fourth arabinosyl residue in the tetraarabinoside chain, which is uniquely α-linked, may determine the initial cross-linking rate. Our results also confirm the conserved structure of the oligoarabinosides across species, indicating an evolutionary significance for EXT arabinosylation.

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.

Localization of hydrogen peroxide accumulation and diamine oxidase activity in pea root nodules under aluminum stress

Aluminum (Al) is one of the environmental stressors that induces formation of reactive oxygen species (ROS) in plants. Hydrogen peroxide (H2O2) and H2O2-generated apoplast diamine oxidase (DAO) activity were detected cytochemically via transmission electron microscopy (TEM), in pea (Pisum sativum L.) root nodules exposed to high (50 μM AlCl3, for 2 and 24h) Al stress. The nodules were shown to respond to Al stress by disturbances in infection thread (IT) growth, bacteria endocytosis, premature degeneration of bacteroidal tissue and generation of H2O2 in nodule apoplast. Large amounts of peroxide were found at the same sites as high DAO activity under Al stress, suggesting that DAO is a major source of Al-induced peroxide accumulation in the nodules. Peroxide distribution and DAO activity in the nodules of both control plants and Al-treated ones were typically found in the plant cell walls, intercellular spaces and infection threads. However, 2 h Al treatment increased DAO activity and peroxide accumulation in the nodule apoplast and bacteria within threads. A prolonged Al treatment (24 h) increased the H2O2 content and DAO activity in the nodule apoplast, especially in the thread walls, matrix and bacteria within infection threads. In addition to ITs, prematurely degenerated bacteroids, which occurred in response to Al, were associated with intense staining for H2O2 and DAO activity. These results suggest the involvement of DAO in the production of a large amount of H2O2 in the nodule apoplast under Al stress. The role of reactive oxygen species in pea-Rhizobium symbiosis under Al stress is discussed.

Transcriptional profiling of pea ABR17 mediated changes in gene expression in Arabidopsis thaliana

BACKGROUND

Pathogenesis-related proteins belonging to group 10 (PR10) are elevated in response to biotic and abiotic stresses in plants. Previously, we have shown a drastic salinity-induced increase in the levels of ABR17, a member of the PR10 family, in pea. Furthermore, we have also demonstrated that the constitutive expression of pea ABR17 cDNA in Arabidopsis thaliana and Brassica napus enhances their germination and early seedling growth under stress. Although it has been reported that several members of the PR10 family including ABR17 possess RNase activity, the exact mechanism by which the aforementioned characteristics are conferred by ABR17 is unknown at this time. We hypothesized that a study of differences in transcriptome between wild type (WT) and ABR17 transgenic A. thaliana may shed light on this process.

RESULTS

The molecular changes brought about by the expression of pea ABR17 cDNA in A. thaliana in the presence or absence of salt stress were investigated using microarrays consisting of 70-mer oligonucleotide probes representing 23,686 Arabidopsis genes. Statistical analysis identified number of genes which were over represented among up- or down-regulated transcripts in the transgenic line. Our results highlight the important roles of many abscisic acid (ABA) and cytokinin (CK) responsive genes in ABR17 transgenic lines. Although the transcriptional changes followed a general salt response theme in both WT and transgenic seedlings under salt stress, many genes exhibited differential expression patterns when the transgenic and WT lines were compared. These genes include plant defensins, heat shock proteins, other defense related genes, and several transcriptional factors. Our microarray results for selected genes were validated using quantitative real-time PCR.

CONCLUSION

Transcriptional analysis in ABR17 transgenic Arabidopsis plants, both under normal and saline conditions, revealed significant changes in abundance of transcripts for many stress responsive genes, as well as those related to plant growth and development. Our results also suggest that ABR17 may mediate stress tolerance through the modulation of many ABA- and CK-responsive genes and may further our understanding of the role of ABR17 in mediating plant stress responses.

PRGL: A cell wall proline-rich protein containning GASA domain in Gerbera hybrida

PRPs (proline-rich proteins) are a group of cell wall proteins characterized by their proline and hydroproline-rich repetitive peptides. The expression of PRPs in plants is stimulated by wounding and environmental stress. GASA (gibberellic acid stimulated in Arabidopsis) proteins are small peptides sharing a 60 amino acid conserved C-terminal domain containing twelve invariant cysteine residues. Most of GASAs reported are localized to apoplasm or cell wall and their expression was regulated by gibberellins (GAs). It has been reported that, in French bean, these two proteins encoding by two distinct genes formed a two-component chitin-receptor involved in plant-pathogen interactions when plant was infected. We cloned a full-length cDNA of PRGL (proline-rich GASA-like) gene which encodes a protein containing both PRP and GASA-like domains. It is demonstrated that PRGL is a new protein with characteristics of PRP and GASA by analyzing its protein structure and gene expression.

Adult plant resistance-related gene expression in 'Camp Remy' wheat inoculated with Puccinia striiformis

The French wheat variety 'Camp Remy' (CR) possesses a durable, adult plant resistance to yellow rust (YR), caused by the pathogen Puccinia striiformis. Using cDNA-AFLP on different sets of heterogeneous inbred families (HIFs) derived from the cross CR x Récital, we compared gene expression profiles during one seedling and two adult developmental stages following inoculation with P. striiformis. Transcripts differentially expressed in response to YR infection were isolated and cloned. Sequence analysis of the resultant clones revealed several classes of putative genes, including those related to resistance/defence responses, transcription and signal transduction, and primary metabolism. The expression profiles of seven selected genes were obtained using real-time PCR in CR leaves at the same three stages of development. The results confirmed the stage-specific expression of the genes at one or two specific stages in response to P. striiformis infection and demonstrated that CR modifies the expression of some resistance/defence-related genes during its transition from the seedling to adult growth stages. These results provided the first clue to understand the molecular basis of quantitative trait loci for adult plant resistance to YR and connect it with durability.

Di-isodityrosine is the intermolecular cross-link of isodityrosine-rich extensin analogs cross-linked in vitro

Extensins are cell wall hydroxyproline-rich glycoproteins that form covalent networks putatively involving tyrosyl and lysyl residues in cross-links catalyzed by one or more extensin peroxidases. The precise cross-links remain to be chemically identified both as network components in muro and as enzymic products generated in vitro with native extensin monomers as substrates. However, some extensin monomers contain variations within their putative cross-linking motifs that complicate cross-link identification. Other simpler extensins are recalcitrant to isolation including the ubiquitous P3-type extensin whose major repetitive motif, Hyp)(4)-Ser-Hyp-Ser-(Hyp)(4)-Tyr-Tyr-Tyr-Lys, is of particular interest, not least because its Tyr-Tyr-Tyr intramolecular isodityrosine cross-link motifs are also putative candidates for further intermolecular cross-linking to form di-isodityrosine. Therefore, we designed a set of extensin analogs encoding tandem repeats of the P3 motif, including Tyr --> Phe and Lys --> Leu variations. Expression of these P3 analogs in Nicotiana tabacum cells yielded glycoproteins with virtually all Pro residues hydroxylated and subsequently arabinosylated and with likely galactosylated Ser residues. This was consistent with earlier analyses of P3 glycopeptides isolated from cell wall digests and the predictions of the Hyp contiguity hypothesis. The tyrosine-rich P3 analogs also contained isodityrosine, formed in vivo. Significantly, these isodityrosine-containing analogs were further cross-linked in vitro by an extensin peroxidase to form the tetra-tyrosine intermolecular cross-link amino acid di-isodityrosine. This is the first identification of an inter-molecular cross-link amino acid in an extensin module and corroborates earlier suggestions that di-isodityrosine represents one mechanism for cross-linking extensins in muro.

Analysis of regulatory elements of the promoter and the 3' untranslated region of the maize Hrgp gene coding for a cell wall protein

Hydroxyproline-rich glycoproteins (HRGP) are structural components of the plant cell wall. Hrgp genes from maize and related species have a conserved 500 bp sequence in the 5'-flanking region, and all Hrgp genes from monocots have an intron located in the 3' untranslated region. To study the role of these conserved regions, several deletions of the Hrgp gene were fused to the beta-glucuronidase ( GUS) gene and used to transform maize tissues by particle bombardment. The overall pattern of GUS activity directed by sequential deletions of the Hrgp promoter was different in embryos and young shoots. In embryos, the activity of the full-length Hrgp promoter was in the same range as that of the p35SI promoter construct, based on the strong 35S promoter, whereas in the fast-growing young shoots it was 20 times higher. A putative silencer element specific for young shoots was found in the -1,076/-700 promoter region. Other major cis elements for Hrgp expression are probably located in the regions spanning -699/-510 and -297/-160. Sequences close to the initial ATG and mRNA leader were also important since deletion of the region -52/+16 caused a 75% reduction in promoter activity. The presence of the Hrgp intron in the 3' untranslated region changed the levels of GUS activity directed by the Hrgp and the 35S promoters. This pattern of activity was complex, and was dependent on the promoter and cell type analysed.

Glycine-rich proteins encoded by a nodule-specific gene family are implicated in different stages of symbiotic nodule development in Medicago spp

Four genes encoding small proteins with significantly high glycine content have been identified from root nodules of Medicago sativa. All of these proteins as well as their Medicago truncatula homologues carried an amino terminal signal peptide and a glycine-rich carboxy terminal domain. All except nodGRP3 lacked the characteristic repeat structure described for cell wall and stress response-related glycine-rich proteins (GRP). Expression of these GRP genes was undetectable in flower, leaf, stem, and hypocotyl cells, whereas expression was highly induced during root nodule development, suggesting that GRP genes act as nodulins. Moreover, none of these nodule-expressed GRP genes were activated by hormones or stress treatments, which are inducers of many other GRPs. In Rhizobium-free spontaneous nodules and in nodules induced by a noninfective mutant strain of Sinorhizobium meliloti, all these genes were repressed, while they were induced in Fix- nodules, unaffected in bacterial infection, but halted in bacteroid differentiation. These results demonstrated that bacterial infection but not bacteroid differentiation is required for the induction of the nodule-specific GRP genes. Differences in kinetics and localization of gene activation as well as in the primary structure of proteins suggest nonredundant roles for these GRPs in nodule organogenesis.

Rapid deposition of extensin during the elicitation of grapevine callus cultures is specifically catalyzed by a 40-kilodalton peroxidase

Elicitation or peroxide stimulation of grape (Vitis vinifera L. cv Touriga) vine callus cultures results in the rapid and selective in situ insolubilization of an abundant and ionically bound cell wall protein-denominated GvP1. Surface-enhanced laser desorption/ionization/time of flight-mass spectrometry analysis, the amino acid composition, and the N-terminal sequence of purified GvP1 identified it as an 89.9-kD extensin. Analysis of cell walls following the in situ insolubilization of GvP1 indicates large and specific increases in the major amino acids of GvP1 as compared with the amino acids present in salt-eluted cell walls. We calculate that following deposition, covalently bound GvP1 contributes up to 4% to 5% of the cell wall dry weight. The deposition of GvP1 in situ requires peroxide and endogenous peroxidase activity. Isoelectric focusing of saline eluates of callus revealed only a few basic peroxidases that were all isolated or purified to electrophoretic homogeneity. In vitro and in situ assays of extensin cross-linking activity using GvP1 and peroxidases showed that a 40-kD peroxidase cross-linked GvP1 within minutes, whereas other grapevine peroxidases had no significant activity with GvP1. Internal peptide sequences indicated this extensin peroxidase (EP) is a member of the class III peroxidases. We conclude that we have identified and purified an EP from grapevine callus that is responsible for the catalysis of GvP1 deposition in situ during elicitation. Our results suggest that GvP1 and this EP play an important combined role in grapevine cell wall defense.

Peptides isolated from cell walls of Medicago truncatula nodules and uninfected root

The hydroxyproline-rich root nodules of legumes provide a microaerobic niche for symbiotic nitrogen-fixing Rhizobacteria. The contributions of the cell wall and associated structural proteins, particularly the hydroxyproline-rich glycoproteins (HRGPs), are therefore of interest. Our approach involved identification of the protein components by direct chemical analysis of the insoluble wall. Chymotryptic peptide mapping showed a "P3-type" extensin containing the highly arabinosylated Ser-Hyp4-Ser-Hyp-Ser-Hyp4-Tyr3-Lys motif as a major component. Cell wall amino acid analyses and quantitative hydroxyproline arabinoside profiles, predominantly of tri- and tetraarabinosides, confirmed this extensin as the major structural protein in the cell walls of both root nodules and uninfected roots. On the other hand, judging from the Pro, Glu and non-glycosylated Hyp content, the nodule-specific proline-rich glycoproteins, such as the early nodulins (ENOD-PRPs), are present in much lesser amounts. Although we isolated no PRP peptides from nodule cell walls, a single PRP peptide from root cell walls confirmed the presence of a PRP in roots and represented the first direct evidence for a crosslinked PRP in muro. Compared with root cell walls (approximately 7% protein dry weight) nodule cell walls contained significantly more protein (approximately 13% dry weight) with an overall amino acid and peptide composition indicating the presence of structural protein unrelated to the HRGPs.

Plant glycine-rich proteins: a family or just proteins with a common motif?

Twelve years ago a set of glycine-rich proteins (GRP) of plants were characterized and since then a wealth of new GRPs have been identified. The highly specific but diverse expression pattern of grp genes, taken together with the distinct sub-cellular localisation of some GRP groups, clearly indicate that these proteins are implicated in several independent physiological processes. Notwithstanding the absence of a clear definition of the role of GRPs in plant cells, studies conducted with these proteins have provided new and interesting insights on the molecular and cell biology of plants. Complex regulated promoters and distinct mechanisms of gene expression regulation have been demonstrated. New protein targeting pathways, as well as the exportation of GRPs from different cell types have been discovered. These data show that GRPs can be useful as markers and/or models to understand distinct aspects of plant biology. In this review, the structural and functional features of this family of plant proteins will be summarised. Special emphasis will be given to the gene expression regulation of GRPs isolated from different plant species, as it can help to unravel their possible biological functions.

Pulcherosine, an oxidatively coupled trimer of tyrosine in plant cell walls: its role in cross-link formation

An oxidatively coupled trimer of tyrosine has been isolated from hydrolysates of primary cell walls of a tomato cell culture. UV-absorption, fluorescence and 1H NMR spectra showed that the trimer was pulcherosine, composed of isodityrosine and tyrosine oxidatively coupled via a biphenyl linkage such that the aromatic core is 2,2'-dihydroxy-3-phenoxybiphenyl. Pulcherosine could act as an intermediate in the conversion of isodityrosine to the tetramer, di-isodityrosine. Steric considerations show that the three tyrosine units of pulcherosine could not be near-neighbour residues within a single polypeptide chain. Pulcherosine therefore forms inter-polypeptide cross-links and/or wide intra-polypeptide loops.

Cloning and developmental/stress-regulated expression of a gene encoding a tomato arabinogalactan protein

Arabinogalactan proteins (AGPs) represent a major class of plant hydroxyproline-rich glycoproteins (HRGPs) and are components of cell walls and plasma membranes. AGPs are thought to play roles in cell differentiation, development, and cell-cell interactions. Using a synthetic DNA oligonucleotide based upon an amino acid sequence motif common to AGPs from Lolium, rose, and carrot (i.e., Hyp-Ala-Hyp-Ala-Hyp), we have isolated and sequenced the first AGP gene from a partial Sau3A tomato genomic library packaged in bacteriophage charon 35. The deduced 215 amino acid protein contains 20% Ala, 22% Pro, 10% Gly, and 11% Ser and consists of two Pro-Ala-Pro-Ala-Pro pentapeptide repeats and 16 Ala-Pro dipeptide repeats, consistent with known AGP amino acid compositions and sequences. Comparison of the genomic sequence to a reverse transcribed PCR product and tomato cDNA confirmed the AGP gene is expressed and contains one large intervening sequence. RNA blot hybridization analysis in tomato indicates this AGP gene is strongly expressed in stem and flower, moderately expressed in root and green fruit, and weakly expressed in leaves and red fruit as a 980 nucleotide transcript. Five-day-old seedlings also express this transcript; however, this expression is not regulated by light. More significantly, a gradient of AGP gene expression is observed in tomato stems, ranging from high levels of expression in young internodes to low levels of expression in old internodes. Wounding serves to down-regulate expression in young and old internodes. Heat shock also affects AGP gene expression in stems by transiently down-regulating mRNA levels.

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.

Common occurrence of homologues of petunia glycine-rich protein-1 among plants

The presence of specific glycine-rich proteins (GRP) related to petunia GRP1 (ptGRP1) was examined in three species of monocots (wheat, barley and maize) and five species of dicots (rape, turnip, soybean, crabapple and tomato). Protein blot analysis showed that anti-ptGRP1 antibody cross-reacted with a single different polypeptide in all species except maize. The molecular mass of these polypeptides ranged from 14 to 55 kDa. Tissue-print immunoblots of rape petioles and stems showed that the rape ptGRP1 homologue, like ptGRP1, is primarily located in the vascular tissue, and that its expression decreases with developmental age of the tissue. In barley, the ptGRP1 homologue is found in leaf vascular bundles, and may also be present in the surrounding bundle sheaths. Unlike the dicots examined, expression of the protein did not appear to decrease significantly with developmental age.

Molecular characterization of the gene coding for GPRP, a class of proteins rich in glycine and proline interacting with membranes in Arabidopsis thaliana

The gene coding for a new class of proteins rich in glycine and proline (GPRP) was cloned in Arabidopsis thaliana. In the protein sequence, five amino acids - glycine, proline, alanine, tyrosine and histidine - account for 79.4% of the total composition. The protein has two different glycine-rich domains interrupted by a hydrophobic segment having a high probability of helix formation. The protein synthesized in vitro interacts with microsomes possibly through the hydrophobic domain. The gene in Arabidopsis has two introns, one in the coding region and the other one in the 5' non-coding region. The later one is 778 bp long. Homologous sequences are found in carrot, tomato and tobacco. GPRP mRNA is found in the different organs of the plant analyzed except in mature seeds and anthers, and mostly in epidermal and vascular tissues. Possible hypotheses about the function of GPRP are discussed.

A novel wound-inducible extensin gene is expressed early in newly isolated protoplasts of Nicotiana sylvestris

A cDNA clone (6PExt 1.2) encoding a novel extensin was isolated from a cDNA library made from 6 h old mesophyll protoplasts of Nicotiana sylvestris. The screening was performed with a heterologous probe from carrot. The encoded polypeptide showed features characteristic of hydroxyproline-rich glycoproteins such as Ser-(Pro)4 repeats and a high content in Tyr and Lys residues. The presence of four Tyr-X-Tyr-Lys motifs suggests the possibility for intramolecular isodityrosine cross-links whereas three Val-Tyr-Lys motifs may participate in intermolecular cross-links. The analysis of genomic DNA gel blots using both the N. sylvestris and the carrot clones as probes showed that the 6PExt 1.2 gene belongs to a complex multigene family encoding extensin and extensin-related polypeptides in N. sylvestris as well as in related Nicotianeae including a laboratory hybrid. This was confirmed by the analysis of RNA gel blots: a set of mRNAs ranging in size from 0.3 kb to 3.5 kb was found by the carrot extensin probe. The 6PExt 1.2 probe found a 1.2 kb mRNA in protoplasts and in wounded tissues as well as a 0.9 kb mRNA which seemed to be stem-specific. The gene encoding 6PExt 1.2 was induced by wounding in protoplasts, in leaf strips and after Agrobacterium tumefaciens infection of stems.

Expression of the rice Osgrp1 promoter-Gus reporter gene is specifically associated with cell elongation/expansion and differentiation

To study the expression and regulation of a rice glycine-rich cell wall protein gene, Osgrp1, transgenic rice plants were regenerated that contain the Osgrp1 promoter or its 5' deletions fused with the bacterial beta-glucuronidase (GUS) reporter gene. We report here a detailed histochemical analysis of the Osgrp1-Gus expression patterns in transgenic rice plants. In roots of transgenic rice plants, GUS expression was specifically located in cell elongation and differentiation regions, and no GUS expression was detectable in the apical meristem and the mature region. In shoots, GUS activity was expressed only in young leaves or in the growing basal parts of developing leaves, and little GUS activity was expressed in mature leaves or mature parts of developing leaves. In shoot apices, GUS activity was detected only in those leaf cells which were starting to expand and differentiate, and GUS expression was not detected in the apical meristem and the young meristematic leaf primordia. GUS activity was highly expressed in the young stem tissue, particularly in the developing vascular bundles and epidermis. Thus, the expression of the Osgrp1 gene is closely associated with cell elongation/expansion during the post-mitotic cell differentiation process. The Osgrp1-Gus gene was also expressed in response to wounding and down-regulated by water-stress conditions in the elongation region of roots. Promoter deletion analysis indicates that both positive and negative mechanisms are involved in regulating the specific expression patterns. We propose a simple model for the developmental regulation of the Osgrp1 gene expression.

Two pathogen-responsive genes in parsley encode a tyrosine-rich hydroxyproline-rich glycoprotein (hrgp) and an anionic peroxidase

Two recently isolated cDNAs representing genes that are transcriptionally activated in fungus-infected parsley leaves or elicitor-treated, cultured parsley cells are shown to encode a hydroxyproline-rich glycoprotein (HRGP) and an anionic peroxidase. The deduced HRGP protein is rich in tyrosine residues, a feature also found in other pathogen- and wound-induced plant HRGPs. Expression of the peroxidase gene(s) is induced rapidly upon elicitation and precedes that of the HRGP gene. In situ hybridization experiments demonstrate the presence of HRGP and peroxidase mRNAs in parsley tissue around fungal infection sites. Peroxidase mRNA accumulation is particularly sharply restricted to plant cells directly adjacent to fungal hyphae. These results provide further evidence for an important role of specific cell wall modifications in plant defense.

Cell wall proteins in seedling cotyledons of Prosopis chilensis

Four cell wall proteins of cotyledons of Prosopis chilensis seedlings were characterized by PAGE and Western analyses using a polyclonal antibody, generated against soybean seed coat extensin. These proteins had M(r)s of 180,000, 126,000, 107,000 and 63,000, as determined by SDS-PAGE. The proteins exhibited a fluorescent positive reaction with dansylhydrazine suggesting that they are glycoproteins; they did not show peroxidase activity. The cell wall proteins were also characterized by their amino acid composition and by their amino-terminal sequence. These analyses revealed that there are two groups of related cell wall proteins in the cotyledons. The first group comprises the proteins of M(r)s 180,000, 126,000, 107,000 which are rich in glutamic acid/glutamine and aspartic acid/asparagine and they have almost identical NH2-terminal sequences. The second group comprises the M(r) 63,000 protein which is rich in proline, glycine, valine and tyrosine, with an NH2-terminal sequence which was very similar to that of soybean proline-rich proteins.

Structure and expression of genes coding for structural proteins of the plant cell wall

The best-known protein components of the plant cell wall have highly repetitive, proline-rich sequences. The use of recombinant DNA approaches has enabled complete sequences of these proteins to be determined and features of the expression of the corresponding genes to be examined. These results, coupled with the use of immunological techniques, have shown that proline-rich proteins are interesting probes to study developmental and defence processes in plants. In this review, the sequence and expression of different groups of proline-rich proteins in plants are presented. These groups include hydroxyproline-rich glycoproteins (HRGP) or extensins, proline-rich proteins (PRP) and glycine-rich proteins (GRP). The specific features of each group and the possible functions of these proteins are discussed, as well as the data available on the mechanisms controlling the expression of their corresponding genes. Contents Summary 259 I. Introduction 259 II. Hydroxypioline-rich glycoproteins (HRGPs) 261 III. Proline-rich proteins (PRPs) 270 IV. Glycine-rich proteins (GRPs) 274 V. Concluding remarks 277 References 279.

Developmental regulation and phytochrome-mediated induction of mRNAs encoding a proline-rich protein, glycine-rich proteins, and hydroxyproline-rich glycoproteins in Phaseolus vulgaris L

We have studied developmental and light regulation of mRNAs encoding a putative cell wall proline-rich protein (PvPRP1), cell wall glycine-rich proteins (GRPs), and cell wall hydroxyproline-rich glycoproteins (HRGPs) in bean (Phaseolus vulgaris). Light increases the levels of these mRNAs 2- to 150-fold in highly spatially regulated patterns during seedling development. These mRNA changes include differential regulation of transcripts derived from the GRP and HRGP multigene families. In 6-day-old light-grown seedlings, the PvPRP1 and GRP1.0 mRNAs were most abundant in the apical region of hypocotyls, epicotyls, and roots. In contrast, several HRGP transcripts were most abundant in the mature region of hypocotyls and roots in light-grown seedlings. When etiolated 6-day-old seedlings were illuminated with white light for 8 hr, maximal accumulation of PvPRP1 and GRP1.0 mRNAs occurred in the apical hook, whereas HRGP and GRP1.8 mRNAs accumulated in the mature region of hypocotyls. Etiolated seedlings subjected to a pulse of red light accumulated PvPRP1, GRP, and HRGP mRNAs in the hypocotyls. Far-red light inhibited red light induction of these mRNAs, indicating a phytochrome-mediated process. The possible roles of PRPs, GRPs, and HRGPs in cell differentiation and photomorphogenesis are discussed.

Characterization of cDNAs for stylar transmitting tissue-specific proline-rich proteins in tobacco

The pistil of flowers is a specialized organ which contains the female gametophytes and provides the structures necessary for pollination and fertilization. Pollen deposited on the stigmatic surface of a compatible plant germinates a pollen tube which penetrates the stigmatic papillae and grows intercellularly through the style towards the ovules in the ovary. Pollen tube growth is largely restricted to the transmitting tissue in the style. Therefore the stylar transmitting tissue is extremely important for the migration of the pollen cell towards the ovary. We have isolated two related cDNAs, transmitting tissue-specific (TTS)-1 and TTS-2, derived from two proline-rich protein (PRP)-encoding mRNAs that accumulate specifically in the transmitting tissue of tobacco. The deduced PRP sequences share similarities with proline-rich cell wall glycoproteins found in a variety of plants. TTS-1 and TTS-2 mRNAs are induced in very young floral buds, accumulate most abundantly during the later stages of flower development when style elongation is the most rapid, and remain at relatively high levels at anthesis. These mRNAs become undetectable in maturing green fruits. In situ hybridization shows that TTS-1 and TTS-2 mRNA accumulation is restricted to the transmitting tissue of the style. The possible roles that these transmitting tissue-specific PRPs may play in maintaining the structural integrity of the style or in the function of this organ is discussed.

Isolation and characterization of two wound-regulated tomato extensin genes

Extensins comprise a family of structural cell wall hydroxyproline-rich glycoproteins in plants. Two tomato genomic clones, Tom J-10 and Tom L-4, were isolated from a tomato genomic DNA library by in situ plaque hybridization with extensin DNA probes. Tom J-10 encoded an extensin with 388 amino acid residues and a predicted molecular mass of 43 kDa. The Tom J-10 encoded extensin lacked a typical signal peptide sequence, but contained two distinct protein domains consisting of 19 tandem repeats of Ser-Pro4-Ser-Pro-Lys-Tyr-Val-Tyr-Lys at the amino terminus which were directly followed by 8 tandem repeats of the consensus sequence Ser-Pro4-Tyr3-Lys-Ser-Pro4-Ser-Pro at the carboxy terminus. RNA blot hybridization analysis with the Tom J-10 extensin probe demonstrated the presence of a 4.0 kb tomato stem mRNA which accumulated markedly in response to wounding. Tom L-4 encoded an extensin with 322 amino acid residues and a predicted molecular mass of 35 kDa. The Tom L-4 encoded extensin contained a typical signal peptide sequence at the amino terminus and was followed by at least 3 distinct domains. These domains consisted of an amino terminal domain containing several Lys-Pro and Ser-Pro4 repeat units, a central domain with repeats of the consensus sequence Ser-Pro2-5-Thr-Pro-Ser-Tyr-Glu-His-Pro-Lys-Thr-Pro, and a carboxy terminal domain containing repeats of the consensus sequence Ser-Ser-Pro4-Ser-Pro-Ser-Pro4-Thr-Tyr1-3. RNA blot hybridization analysis with the Tom L-4 extensin probe demonstrated the presence of a 2.6 kb tomato stem mRNA which accumulated in response to wounding.

Plant responses to environmental stress

Considerable progress is being made in identifying genes that are important for tolerance to abiotic stress and in defining stress-responsive gene promoters and signal-transduction pathways. Although genetically engineered crop plants with greater resistance to environmental stress have not yet been produced, research is at a turning point where correlative changes can now be tested for effectiveness in conferring stress tolerance.