Characterization of cDNA for nodulin-75 of soybean: A gene product involved in early stages of root nodule development

An endophytic Fusarium-legume association is partially dependent on the common symbiotic signalling pathway

Legumes interact with a wide range of microbes in their root systems, ranging from beneficial symbionts to pathogens. Symbiotic rhizobia and arbuscular mycorrhizal glomeromycetes trigger a so-called common symbiotic signalling pathway (CSSP), including the induction of nuclear calcium spiking in the root epidermis. By combining gene expression analysis, mutant phenotypic screening and analysis of nuclear calcium elevations, we demonstrate that recognition of an endophytic Fusarium solani strain K (FsK) in model legumes is initiated via perception of chitooligosaccharidic molecules and is, at least partially, CSSP-dependent. FsK induced the expression of Lysin-motif receptors for chitin-based molecules, CSSP members and CSSP-dependent genes in Lotus japonicus. In LysM and CSSP mutant/RNAi lines, root penetration and fungal intraradical progression was either stimulated or limited, whereas FsK exudates triggered CSSP-dependent nuclear calcium spiking, in epidermal cells of Medicago truncatula root organ cultures. Our results corroborate CSSP being involved in the perception of signals from other microbes beyond the restricted group of symbiotic interactions sensu stricto.

PRPs localized to the middle lamellae are required for cortical tissue integrity in Medicago truncatula roots

A family of repetitive proline-rich proteins interact with acidic pectins and play distinct roles in legume root cell walls affecting cortical and vascular structure. A proline-rich protein (PRP) family, composed of tandemly repeated Pro-Hyp-Val-X-Lys pentapeptide motifs, is found primarily in the Leguminosae. Four distinct size classes within this family are encoded by seven tightly linked genes: MtPRP1, MtPRP2 and MtPRP3, and four nearly identical MtPRP4 genes. Promoter fusions to β-glucuronidase showed strong expression in the stele of hairy roots for all 4 PRP genes tested, with additional expression in the cortex for PRP1, PRP2 and PRP4. All except MtPRP4 are strongly expressed in non-tumorous roots, and secreted and ionically bound to root cell walls. These PRPs are absent from root epidermal cell walls, and PRP accumulation is highly localized within the walls of root cortical and vascular tissues. Within xylem tissue, PRPs are deposited in secondary thickenings where it is spatially exclusive to lignin. In newly differentiating xylem, PRPs are deposited in the regularly spaced paired-pits and pit membranes that hydraulically connect neighboring xylem elements. Hairpin-RNA knock-down constructs reducing PRP expression in Medicago truncatula hairy root tumors disrupted cortical and vascular patterning. Immunoblots showed that the knockdown tumors had potentially compensating increases in the non-targeted PRPs, all of which cross-react with the anti-PRP antibodies. However, PRP3 knockdown differed from knockdown of PRP1 and PRP2 in that it greatly reduced viability of hairy root tumors. We hypothesize that repetitive PRPs interact with acidic pectins to form block-copolymer gels that can play distinct roles in legume root cell walls.

Glycine rich proline rich protein from Sorghum bicolor serves as an antimicrobial protein implicated in plant defense response

KEY MESSAGE

Sorghum glycine rich proline rich protein (SbGPRP1) exhibit antimicrobial properties and play a crucial role during biotic stress condition. Several proteins in plants build up the innate immune response system in plants which get triggered during the occurrence of biotic stress. Here we report the functional characterization of a glycine-rich proline-rich protein (SbGPRP1) from Sorghum which was previously demonstrated to be involved in abiotic stresses. Expression studies carried out with SbGPRP1 showed induced expression upon application of phytohormones like salicylic acid which might be the key in fine-tuning the expression level. Upon challenging the Sorghum plants with a compatible pathogen the SbGprp1 transcript was found to be upregulated. SbGPRP1 encodes a 197 amino acid polypeptide which was bacterially-expressed and purified for in vitro assays. Gram-positive bacteria like Bacillus and phytopathogen Rhodococcus fascians showed inhibited growth in the presence of the protein. The NPN assay, electrolytic leakage and SEM analysis showed membrane damage in bacterial cells. Ectopic expression of SbGPRP1 in tobacco plants led to enhanced tolerance towards infection caused by R. fascians. Though the N-terminal part of the protein showed disorderness the C-terminal end was quite capable of forming several α-helices which was correlated with CD spectroscopic analysis. Here, we have tried to determine the structural model for the protein and predicted the association of antimicrobial activity with the C-terminal region of the protein.

Characterization of the Spatial and Temporal Expression of Two Soybean miRNAs Identifies SCL6 as a Novel Regulator of Soybean Nodulation

MicroRNAs (miRNAs) control expression of endogenous target genes through transcript cleavage or translational inhibition. Legume plants can form a specialized organ, the nodule, through interaction with nitrogen fixing soil bacteria. To understand the regulatory roles of miRNAs in the nodulation process, we functionally validated gma-miR171o and gma-miR171q and their target genes in soybean. These two miRNA sequences are identical in sequence but their miRNA genes are divergent and show unique, tissue-specific expression patterns. The expression levels of the two miRNAs are negatively correlated with that of their target genes. Ectopic expression of these miRNAs in transgenic hairy roots resulted in a significant reduction in nodule formation. Both gma-miR171o and gma-miR171q target members of the GRAS transcription factor superfamily, namely GmSCL-6 and GmNSP2. Transient interaction of miRNAs and their target genes in tobacco cells further confirmed their cleavage activity. The results suggest that gma-miR171o and gma-miR171q regulate GmSCL-6 and GmNSP2, which in turn, influence expression of the Nodule inception (NIN), Early Nodulin 40 (ENOD40), and Ethylene Response Factor Required for Nodulation (ERN) genes during the Bradyrhizobium japonicum-soybean nodulation process. Collectively, our data suggest a role for two miRNAs, gma-miR171o and gma-miR171q, in regulating the spatial and temporal aspects of soybean nodulation.

Pipeline to Identify Hydroxyproline-Rich Glycoproteins

Intrinsically disordered proteins (IDPs) are functional proteins that lack a well-defined three-dimensional structure. The study of IDPs is a rapidly growing area as the crucial biological functions of more of these proteins are uncovered. In plants, IDPs are implicated in plant stress responses, signaling, and regulatory processes. A superfamily of cell wall proteins, the hydroxyproline-rich glycoproteins (HRGPs), have characteristic features of IDPs. Their protein backbones are rich in the disordering amino acid proline, they contain repeated sequence motifs and extensive posttranslational modifications (glycosylation), and they have been implicated in many biological functions. HRGPs are evolutionarily ancient, having been isolated from the protein-rich walls of chlorophyte algae to the cellulose-rich walls of embryophytes. Examination of HRGPs in a range of plant species should provide valuable insights into how they have evolved. Commonly divided into the arabinogalactan proteins, extensins, and proline-rich proteins, in reality, a continuum of structures exists within this diverse and heterogenous superfamily. An inability to accurately classify HRGPs leads to inconsistent gene ontologies limiting the identification of HRGP classes in existing and emerging omics data sets. We present a novel and robust motif and amino acid bias (MAAB) bioinformatics pipeline to classify HRGPs into 23 descriptive subclasses. Validation of MAAB was achieved using available genomic resources and then applied to the 1000 Plants transcriptome project (www.onekp.com) data set. Significant improvement in the detection of HRGPs using multiple-k-mer transcriptome assembly methodology was observed. The MAAB pipeline is readily adaptable and can be modified to optimize the recovery of IDPs from other organisms.

Actinorhizal plant defence-related genes in response to symbiotic Frankia

Actinorhizal plants have become increasingly important as climate changes threaten to remake the global landscape over the next decades. These plants are able to grow in nutrient-poor and disturbed soils, and are important elements in plant communities worldwide. Besides that, most actinorhizal plants are capable of high rates of nitrogen fixation due to their capacity to establish root nodule symbiosis with N2-fixing Frankia strains. Nodulation is a developmental process that requires a sequence of highly coordinated events. One of these mechanisms is the induction of defence-related events, whose precise role in a symbiotic interaction remains to be elucidated. This review summarises what is known about the induction of actinorhizal defence-related genes in response to symbiotic Frankia and their putative function during symbiosis.

Early nodulin genes are induced in alfalfa root outgrowths elicited by auxin transport inhibitors

Rhizobium nod genes are essential for root hair deformation and cortical cell division, early stages in the development of nitrogen-fixing root nodules. Nod(-) mutants are unable to initiate nodules on legume roots. We observed that N-(1-naphthyl)phthalamic acid and 2,3,5-triiodobenzoic acid, compounds known to function as auxin transport inhibitors, induced nodule-like structures on alfalfa roots. The nodule-like structures (pseudonodules) were white, devoid of bacteria, and resembled nodules elicited by Rhizobium meliloti exopolysaccharide (exo) mutants at both the histological and molecular level. Two nodulin genes, ENOD2 and Nms-30, were expressed. RNA isolated from the nodule-like structures hybridized to pGmENOD2, a soybean early nodulin cDNA clone. RNA isolated from roots did not hybridize. We determined by in vitro translations of total RNA that the alfalfa nodulin transcript Nms-30 was also expressed in the nodule-like structures. The late expressed nodulin genes, such as the leghemoglobin genes, were not transcribed. Because N-(1-naphthyl)phthalamic acid and 2,3,5-triiodobenzoic acid induce the development of nodules on alfalfa roots, we suggest that the auxin transport inhibitors mimic the activity of compound(s) made upon the induction of the Rhizobium nod genes.

Expression of LjENOD40 genes in response to symbiotic and non-symbiotic signals: LjENOD40-1 and LjENOD40-2 are differentially regulated in Lotus japonicus

Nitrogen fixation in nodules provides leguminous plants with an ability to grow in nitrogen-starved soil. Infection of the host plants by microsymbionts triggers various physiological and morphological changes during nodule formation. In Lotus japonicus, expression of early nodulin (ENOD) genes is triggered by perception of bacterial signal molecules, nodulation factors (Nod factors). We examined the expression patterns of ENOD40 genes during the nodule formation process. Two ENOD40 genes of L. japonicus were specifically expressed in the nodule formation process, but they showed different expression patterns upon infection. Each ENOD40 gene demonstrates an individual specificity and regulation with regard to rhizobial infection.

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.

Transcriptome profiling in root nodules and arbuscular mycorrhiza identifies a collection of novel genes induced during Medicago truncatula root endosymbioses

Transcriptome profiling based on cDNA array hybridizations and in silico screening was used to identify Medicago truncatula genes induced in both root nodules and arbuscular mycorrhiza (AM). By array hybridizations, we detected several hundred genes that were upregulated in the root nodule and the AM symbiosis, respectively, with a total of 75 genes being induced during both interactions. The second approach based on in silico data mining yielded several hundred additional candidate genes with a predicted symbiosis-enhanced expression. A subset of the genes identified by either expression profiling tool was subjected to quantitative real-time reverse-transcription polymerase chain reaction for a verification of their symbiosis-induced expression. That way, induction in root nodules and AM was confirmed for 26 genes, most of them being reported as symbiosis-induced for the first time. In addition to delivering a number of novel symbiosis-induced genes, our approach identified several genes that were induced in only one of the two root endosymbioses. The spatial expression patterns of two symbiosis-induced genes encoding an annexin and a beta-tubulin were characterized in transgenic roots using promoter-reporter gene fusions.

Short-term metabolic responses of soybean root nodules to nitrate

Soybean (Glycine max L. Merr.) plants exposed to 10 mM KNO(3) for a 4 d period were used to test the correlation between nitrogenase activity, gene expression and sucrose metabolism. Nitrate caused the down-regulation of sucrose synthase (SS) transcripts within 1 d, although a decline in nodule SS activity and an increase in nodule sucrose content only occurred after 3-4 d. In a second experiment, plants were exposed to (15)N-labelled nitrate for 48 h to determine the time period during which nitrate was taken up, and to relate this to the decline in apparent nitrogenase activity (H(2) production in air) and the reduction in SS gene transcript levels. The peak of nitrate uptake appeared to be between 8 h and 14 h whilst apparent nitrogenase activity began to decline at about 17.5 h. The SS mRNA signal declined markedly between 14 h and 24 h. The correlative association of these factors is clear. However, SS activity per se does not appear to be related to the initial decline in apparent nitrogenase activity as a result of nitrate uptake. These findings, therefore, do not support the hypothesis that the regulation of nodule function is mediated by the regulation of SS activity.

A critical evaluation of differential display as a tool to identify genes involved in legume nodulation: looking back and looking forward

Screening for differentially expressed genes is a straightforward approach to study the molecular basis of a biological system. In the last 10 years, differential screening technology has evolved rapidly and currently high-throughput tools for genome-wide transcript profiling, such as expressed sequence tags and microarray analysis, are becoming widely available. Here, an overview of this (r)evolution is given with emphasis on the differential display method, which for many years has been the preferred technique of scientists in diverse fields of research. Differential display has also been the method of choice for the identification of genes involved in the symbiotic interaction between Azorhizobium caulinodans and Sesbania rostrata. The advantages with respect to tissue specificity of this particular model system for legume nodulation and the results of a screening for early nodulation-related genes have been considered in the context of transcriptome analyses in other rhizobium-legume interactions.

DcAGP1, a secreted arabinogalactan protein, is related to a family of basic proline-rich proteins

A cDNA corresponding to the core protein of an immunoaffinity-purified arabinogalactan protein (AGP) secreted aucus carota (carrot) cells in liquid culture was isolated. This cDNA, DcAGP1, encodes a new class of non-classical' AGP with strong similarity to a family of basic proline-rich proteins. The protein is rich in proline (17%), alanine (10%) and lysine (11%) and contains four distinct domains: a signal peptide, a proline-rich domain, a histidine-rich basic domain and a cysteine-containing 'PAC' domain that is found in a range of other cell wall proteins. The protein contains several sequence motifs found in otherwise unrelated cell wall proteins, but also displays some unique features. Northern blot analyses show that while the DcAGP1 transcript is abundant in the suspension-culture cells from which the AGP was obtained; in carrot seedlings the gene is only expressed at low levels in the roots and it is neither wound- nor stress-inducible. Furthermore, northern and western blot analyses demonstrate that the core polypeptide of DcAGP1 is differentially glycosylated in two different carrot suspension cultures. The unusual features of the protein sequence suggest that the DcAGP1 protein is a member of a family of basic proline-rich proteins defined by the C-terminal PAC domain, and the possible function(s) of the DcAGP1 protein is considered in the light of current views on AGP structure and function.

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.

Accumulation of ENOD2-like transcripts in non-nodulating woody papilionoid legumes

Japanese pagodatree (Styphnolobium japonicum [L.] Schott) and American yellowwood (Cladrastis kentukea Dum.-Cours.) Rudd are the first woody, non-nodulating papilionoid legumes shown to possess putative early nodulin 2 (ENOD2) genes. ENOD2 cDNAs from Japanese pagodatree (807 bp) and American yellowwood (735 bp) have 75% to 79% sequence identity to ENOD2 sequences and encode deduced proteins that possess conserved ENOD2 pentapeptides (PPHEK and PPEYQ). Lower percentages of glucose and higher percentages of histidine and valine suggest that SjENOD2 and CkENOD2 are different from other ENOD2s. Hybridization analyses indicate the clones represent ENOD2 gene families of two to four genes in Japanese pagodatree and American yellowwood genomes, and ENOD2-like transcripts were detected in stems and flowers, as well as roots. Only roots of control species that nodulate, Maackia amurensis Rupr. & Maxim. and alfalfa (Medicago sativa), produced pseudonodules after treatment with zeatin or 2,3,5-triiodobenzoic acid, an auxin transport inhibitor. Accumulation of MaENOD2 transcripts was enhanced during the first 10 d of treatment, but 2,3,5-triiodobenzoic acid and zeatin enhanced transcript accumulation after 30 d in roots of Japanese pagodatree and American yellowwood. Characteristics that distinguish ENOD2 gene families in basal, non-nodulating woody legumes from other ENOD2 genes may provide new information about the function of these genes during symbiotic and non-symbiotic organ development.

StGCPRP, a potato gene strongly expressed in stomatal guard cells, defines a novel type of repetitive proline-rich proteins

Guard cells represent a highly differentiated cell type within the epidermis of plant leaves and stems. They respond to many endogenous and environmental signals and thereby modify the size of the stomatal pore they surround. We identified a novel gene that is highly expressed in guard cells of potato (Solanum tuberosum). It encodes a repetitive proline (Pro)-rich protein of 54 kD (491 amino acids) and was named StGCPRP (S. tuberosum guard cell Pro-rich protein). StGCPRP has a bipartite structure. The C-terminal part of StGCPRP contains a high percentage (46%) of Pro residues organized in distinct repetitive sequence motifs, whereas its extended N terminus is essentially free of Pros. StGCPRP represents the first member of a novel class of hybrid Pro-rich proteins that we designated NHyPRPs. In young but not in mature leaves, StGCPRP transcripts were also present at high levels in mesophyll cells (in addition to guard cells), indicating developmental regulation of StGCPRP gene expression. In addition, StGCPRP expression is regulated by environmental factors, as shown by a decrease in StGCPRP transcript levels under drought stress. Two proteins similar to StGCPRP were found to be encoded by the Arabidopsis genome, indicating that NHyPRPs are more widely distributed in higher plants.

Characterization and oxidative in vitro cross-linking of an extensin-like protein and a proline-rich protein purified from chickpea cell walls

Two cell wall proteins from chickpea, known to be rapidly insolubilised by an elicitor-stimulated oxidative burst in-vivo, were purified from suspension cells. N-terminal protein sequencing revealed them as a proline-rich protein and an extensin-like protein. Oxidative cross-linking could be modelled in an in vitro system utilising horseradish peroxidase, H2O2 and the substrate proteins.

Expression profiles of 22 novel molecular markers for organogenetic pathways acting in alfalfa nodule development

During symbiotic nodule development, a variety of molecular signals of rhizobia and plant origin are likely to be involved in the control of the expression of specific genes in the legume Medicago sativa (alfalfa). Twenty-two new, nodule-associated Expressed Sequence Tags (ESTs, MsNod clones) as well as 16 clones for previously reported alfalfa nodulins were identified by cold-plaque screening. Protein homologs were found for 10 of the 22 MsNod-encoded polypeptides, revealing putative novel functions associated with this symbiosis. Expression of these MsNod genes was investigated in spontaneous nodules (generated in the absence of bacteria), in nodules induced by a Sinorhizobium meliloti wild-type strain and Eps- and Bac- mutant derivatives, as well as in roots inoculated with a Nod- mutant strain. This analysis enabled us to correlate plant gene expression with the different stages of nodule ontogeny and invasion. The effect of phytohormones on MsNod gene expression was analyzed in cytokinin- and auxin-treated alfalfa roots. Cytokinin induced the accumulation of seven MsNod transcripts, four of them were also regulated by the synthetic auxin 2,4-D (2,4-dichlorophenoxyacetic acid). Comparison of MsNod expression profiles in wild-type and transgenic M. truncatula roots overexpressing the early nodulin Enod40 suggested that one clone, the M. sativa L3 ribosomal protein homolog (MsNod377), is a putative component of an Enod40-dependent pathway acting during nodule development. These novel molecular markers may help in the investigation of gene networks and regulatory circuits controlling nodule organogenesis.

Characterization and expression of four proline-rich cell wall protein genes in Arabidopsis encoding two distinct subsets of multiple domain proteins

We have characterized the molecular organization and expression of four proline-rich protein genes from Arabidopsis (AtPRPs). These genes predict two classes of cell wall proteins based on DNA sequence identity, repetitive motifs, and domain organization. AtPRP1 and AtPRP3 encode proteins containing an N-terminal PRP-like domain followed by a C-terminal domain that is biased toward P, T, Y, and K. AtPRP2 and AtPRP4 represent a second, novel group of PRP genes that encode two-domain proteins containing a non-repetitive N-terminal domain followed by a PRP-like region rich in P, V, K, and C. Northern hybridization analysis indicated that AtPRP1 and AtPRP3 are exclusively expressed in roots, while transcripts encoding AtPRP2 and AtPRP4 were most abundant in aerial organs of the plant. Histochemical analyses of promoter/beta-glucuronidase fusions localized AtPRP3 expression to regions of the root containing root hairs. AtPRP2 and AtPRP4 expression was detected in expanding leaves, stems, flowers, and siliques. In addition, AtPRP4 expression was detected in stipules and during the early stages of lateral root formation. These studies support a model for involvement of PRPs in specifying cell-type-specific wall structures, and provide the basis for a genetic approach to dissect the function of PRPs during growth and development.

Novel genes induced during an arbuscular mycorrhizal (AM) symbiosis formed between Medicago truncatula and Glomus versiforme

Many terrestrial plant species are able to form symbiotic associations with arbuscular mycorrhizal fungi. Here we have identified three cDNA clones representing genes whose expression is induced during the arbuscular mycorrhizal symbiosis formed between Medicago truncatula and an arbuscular mycorrhizal fungus, Glomus versiforme. The three clones represent M. truncatula genes and encode novel proteins: a xyloglucan endotransglycosylase-related protein, a putative arabinogalactan protein (AGP), and a putative homologue of the mammalian p110 subunit of initiation factor 3 (eIF3). These genes show little or no expression in M. truncatula roots prior to formation of the symbiosis and are significantly induced following colonization by G. versiforme. The genes are not induced in roots in response to increases in phosphate. This suggests that induction of expression during the symbiosis is due to the interaction with the fungus and is not a secondary effect of improved phosphate nutrition. In situ hybridization revealed that the putative AGP is expressed specifically in cortical cells containing arbuscules. The identification of two mycorrhiza-induced genes encoding proteins predicted to be involved in cell wall structure is consistent with previous electron microscopy data that indicated major alterations in the extracellular matrix of the cortical cells following colonization by mycorrhizal fungi.

Nodule parenchyma-specific expression of the sesbania rostrata early nodulin gene SrEnod2 is mediated by its 3' untranslated region

The early nodulin Enod2 gene encodes a putative hydroxyproline-rich cell wall protein and is expressed exclusively in the nodule parenchyma cell layer. The latter finding suggests that the Enod2 protein may contribute to the special morphological features of the nodule parenchyma and to the creation of an oxygen diffusion barrier. The Enod2 gene of the stem-nodulating legume Sesbania rostrata (SrEnod2) is induced specifically in roots by the plant hormone cytokinin, and this induction occurs at a post-transcriptional level. Here, we characterize the cis determinant(s) in the SrEnod2 locus responsible for nodule parenchyma-specific expression and show that the 3' untranslated region (UTR) of the SrEnod2 gene is both required and sufficient for directing chimeric reporter gene expression in the nodule parenchyma of transgenic Lotus corniculatus plants. Moreover, we show that the SrEnod2 3' UTR does not act as a tissue-specific enhancer element. By conducting a detailed deletion analysis of the 5' and 3' SrEnod2 regions, we delimited the minimal promoter of the SrEnod2 gene, and it appears that the 5' flanking sequences are not essential for nodule parenchyma-specific expression. This finding is in contrast with the report that the 5' upstream region of the soybean Enod2 gene directs nodule parenchyma-specific expression, indicating that different mechanisms may be involved in regulating the expression of these two genes. We definitively demonstrate that the cis element(s) for tissue-specific expression is located within the 3' UTR of a plant nuclear gene.

Effects of oxygen on nodule physiology and expression of nodulins in alfalfa

Early nodulin 2 (ENOD2) transcripts and protein are specifically found in the inner cortex of legume nodules, a location that coincides with the site of a barrier to O2 diffusion. The extracellular glycoprotein that binds the monoclonal antibody MAC236 has also been localized to this site. Thus, it has been proposed that these proteins function in the regulation of nodule permeability to O2 diffusion. It would then be expected that the levels of ENOD2 mRNA/protein and MAC236 antigen would differ in nodules with different permeabilities to O2. We examined the expression of ENOD2 and other nodule-expressed genes in Rhizobium meliloti-induced alfalfa nodules grown under 8, 20, or 50% O2. Although there was a change in the amount of MAC236 glycoprotein, the levels of ENOD2 mRNA and protein did not differ significantly among nodules grown at the different [O2], suggesting that neither ENOD2 transcription nor synthesis is involved in the long-term regulation of nodule permeability. Moreover, although nodules from all treatments reduced their permeability to O2 as the partial pressure of O2 (pO2) was increased to 100%, the levels of extractable ENOD2 and MAC236 proteins did not differ from those measured at the growth pO2, further suggesting that if these proteins are involved in a short-term regulation of the diffusion barrier, they must be involved in a way that does not require increased transcription or protein synthesis.

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.

The aberrant cell walls of boron-deficient bean root nodules have no covalently bound hydroxyproline-/proline-rich proteins

B-deficient bean (Phaseolus vulgaris L.) nodules examined by light microscopy showed dramatic anatomical changes, mainly in the parenchyma region. Western analysis of total nodule extracts examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that one 116-kD polypeptide was recognized by antibodies raised against hydroxyproline-rich glycoproteins (HRGPs) from the soybean (Glycine max) seed coat. A protein with a comparable molecular mass of 116 kD was purified from the cell walls of soybean root nodules. The amino acid composition of this protein is similar to the early nodulin (ENOD2) gene. Immunoprecipitation of the soybean ENOD2 in vitro translation product showed that the soybean seed coat anti-HRGP antibodies recognized this early nodulin. Furthermore, we used these antibodies to localize the ENOD2 homolog in bean nodules. Immunocytochemistry revealed that in B-deficient nodules ENOD2 was absent in the walls of the nodule parenchyma. The absence of ENOD2 in B-deficient nodules was corroborated by performing hydroxyproline assays. Northern analysis showed that ENOD2 mRNA is present in B-deficient nodules; therefore, the accumulation of ENOD2 is not affected by B deficiency, but its assembly into the cell wall is. B-deficient nodules fix much less N2 than control nodules, probably because the nodule parenchyma is no longer an effective O2 barrier.

Construction of a Lotus japonicus late nodulin expressed sequence tag library and identification of novel nodule-specific genes

A range of novel expressed sequence tags (ESTs) associated with late developmental events during nodule organogenesis in the legume Lotus japonicus were identified using mRNA differential display; 110 differentially displayed polymerase chain reaction products were cloned and analyzed. Of 88 unique cDNAs obtained, 22 shared significant homology to DNA/protein sequences in the respective databases. This group comprises, among others, a nodule-specific homolog of protein phosphatase 2C, a peptide transporter protein, and a nodule-specific form of cytochrome P450. RNA gel-blot analysis of 16 differentially displayed ESTs confirmed their nodule-specific expression pattern. The kinetics of mRNA accumulation of the majority of the ESTs analyzed were found to resemble the expression pattern observed for the L. japonicus leghemoglobin gene. These results indicate that the newly isolated molecular markers correspond to genes induced during late developmental stages of L. japonicus nodule organogenesis and provide important, novel tools for the study of nodulation.

Stress-Induced Declines in Soybean N2 Fixation Are Related to Nodule Sucrose Synthase Activity

Soybean (Glycine max L.) plants were subjected to a number of treatments (drought, 10 mM nitrate, 150 mM NaCl, shoot meristem removal, and removal of approximately 50% of the nodules) to test the hypothesis that metabolic responses contribute to the regulation of N2 fixation. Nitrogenase activity was correlated with the activity of nodule sucrose synthase (SS), but not with that of glutamine oxoglutarate amino transferase. Leghemoglobin levels and other enzyme activities were not significantly or consistently affected by the treatments. SS mRNA was greatly reduced in nodules of drought-, salt-, and nitrate-treated plants; however, this was not correlated with changes in soluble carbohydrate, starch, amino acids, or ureides. Leghemoglobin mRNA was only slightly affected by the treatments. The time course of drought stress showed a decline in the SS transcript level by 1 d, but levels of leghemoglobin, glutamine synthetase, and ascorbate peroxidase mRNA were not markedly affected by 4 d. SS activity at 4 d was reduced by 46%. We propose that N2 fixation in soybean nodules is mediated by both the oxygen-diffusion barrier and the potential to metabolize sucrose via SS. The response to environmental perturbation may involve down-regulation of the nodule SS gene.

Expression of a novel-type small proline-rich protein gene of alfalfa is induced by 2,4-dichlorophenoxiacetic acid in dedifferentiated callus cells

Differential screening of a cDNA library of 2,4-dichlorophenoxiacetic acid (2,4-D)-treated alfalfa (Medicago sativa) callus tissues resulted in the isolation of a 571 bp cDNA clone (MsPRP5) encoding for a proline-rich protein (84 amino acids) with a specific repeat unit of TPVLPPRK/RGRPPPVPP. In addition, a characteristic amino acid block (PPVYK) previously found in other proline-rich proteins also occurs in the C-terminal region of MsPRP5. At the N-terminal, a signal peptide similar to leader sequences of extracellular proteins can be predicted. According to the northern analysis, the corresponding gene is not expressed or is weakly expressed in differentiated vegetative organs and somatic embryos. However the accumulation of MsPRP5 mRNA is auxin concentration-dependent in dedifferentiated callus tissue. An increase in the amount of steady-state mRNA was detected already 20 min after auxin shock (100 microM 2,4-D). Maximum expression was observed at 24-48 h in the presence of 2,4-D. Elevated expression was also found in cells recovering after heat shock and wounding stress. In synchronized alfalfa cells, the transcript level of MsPRP5 gene fluctuated during cell cycle progression with peaks in G1/S phase cells. Considering the structural features and expression properties of MsPRP5, this clone may represents a new type of proline-rich protein gene which responds to hormonal shock and some other stresses as well.

Negative regulation of gene expression of a novel proline-, threonine-, and glycine-rich protein by water stress in Lycopersicon chilense

We have isolated a full length cDNA clone (designated PTGRP) encoding a proline-rich protein from leaves of Lycopersicon chilense. Sequence analysis of the 552-bp insert revealed that the open reading frame encodes a 12.6-kDa protein. The deduced amino acid sequence of PTGRP consists of a C-terminal proline-rich domain with two identical repeat motifs Phe-Pro-Met-Pro-Thr-Thr-Pro-Ser-Thr-Gly-Gly-Gly-Phe-Pro-Ser. The N terminus lacks proline and is hydrophobic. Unlike other proline-rich proteins this protein contains five glycine-rich repeat motifs (Gly-X)n representative of glycine-rich proteins. Southern blot analysis showed that PTGRP is a member of a small gene family within the L. chilense genome. Northern blot experiments revealed that the PTGRP gene is significantly down regulated by water stress. PTGRP mRNA transcription decreased 5- to 10-fold in leaves and stems after 4-8 days of water stress. The mRNA reaccumulated when the drought-stressed plants were rewatered. The in situ hybridization experiments also revealed that PTGRP mRNAs were more abundant in leaf sections of plants watered regularly compared with those of plants submitted to water stress. Down regulation of the PTGRP gene was also observed in desiccated cell suspensions of L. chilense and in those treated with abscisic acid, mannitol, and NaCl. Based on the common features of proline-rich proteins (high proline content, repeated motifs, and a putative signal peptide) and their involvement in the cell wall, it is likely that the PTGRP protein is targeted to the cell wall. Its down regulation by drought could be correlated with the remodeling of the plant cell wall in response to water stress.

Plant genes induced in the Rhizobium-legume symbiosis

Rhizobium, Bradyrhizobium and Azorhizobium can elicit the formation of N2-fixing nodules on the roots or stems of their leguminous host plants. The nodule formation involves several developmental steps determined by different sets of genes from both partners, the gene expression being temporally and spatially coordinated. The plant proteins that are specifically synthesised during the formation and function of the nodule are called nodulins. The nodulins that are expressed before the onset of N2 fixation are termed early nodulins. These proteins are probably involved in the infection process as well as in nodule morphogenesis rather than in nodule function. The nodulins expressed just before or during N2 fixation are termed late nodulins and they participate in the function of the nodule by creating the physiological conditions required for nitrogen fixation, ammonium assimilation and transport. In this review we will describe nodulins, nodulin genes and the relationship between nodulin gene expression and nodule development. The study of nodulin gene expression may provide insight into root-nodule development and the mechanism of communication between bacteria and host plant.

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.

Morphological and Molecular Characteristics of Host-Conditioned Ineffective Root Nodules in Cowpea

In cowpea (Vigna unguiculata [L.] Walp.) a recessive allele, designated cpi, elicits the formation of non-N2-fixing nodules with all bacterial isolates tested. Comparisons of mutant and wild-type nodules demonstrated that the ineffective nodules were anatomically similar to the wild type and contained both infection threads and bacteroids. Ineffective nodules were smaller, however, largely because of the reduced size of the infected cells. Additionally, the number of bacteroids was reduced and senescence occurred prematurely in the infected cells. Grafting studies demonstrated that the defect in nodule development was controlled by the root rather than the shoot. Northern analysis of four nodulin genes indicated that in the ineffective nodules transcript levels of the early nodulin VuENOD2 were initially reduced but were equivalent to wild-type nodules by 21 d. In contrast, transcript levels of the early nodulin VuB were initially similar in both genotypes but as the nodules matured the mRNA levels declined more slowly in the ineffective nodules. The late nodulins leghemoglobin and uricase were expressed in the ineffective nodules but at greatly reduced levels. Thus, the cpi-conditioned defect in nodulation is associated with impaired bacteroid maturation and maintenance, altered nodulin expression, and accelerated senescence.

gsa1 is a universal tetrapyrrole synthesis gene in soybean and is regulated by a GAGA element

Expression of plant tetrapyrroles is high in photosynthetic tissues and in legume root nodules in the form of chlorophyll and heme, respectively. The universal tetrapyrrole precursor delta-aminolevulinic acid (ALA) is synthesized from glutamate 1-semialdehyde (GSA) by GSA aminotransferase in plants, which is encoded by gsa. Immunoblot analysis showed that GSA aminotransferase was expressed in soybean leaves and nodules, but not in roots, and that protein correlated with enzyme activity. These observations indicate that GSA aminotransferase expression is controlled in tetrapyrrole formation and argue against significant activity of an enzyme other than the well described aminotransferase for GSA-dependent ALA formation. gas mRNA and protein were induced in soybean nodules, and their activation was temporally intermediate between those of the respective early and late genes endo2 and lb. A GSA aminotransferase gene, designated gsa1, was isolated and appears to be one of two gsa genes in the soybean genome. gsa1 mRNA accumulated to high levels in leaves and nodules, but not in uninfected roots as discerned with a gsa1-specific probe. Message levels were higher in leaves from etiolated plantlets than in mature plants, and expression in the former was slightly elevated by light. The expression pattern of gsa1 mRNA was qualitatively similar to that of total gsa. The data strongly suggest that gsa1 is a universal tetrapyrrole synthesis gene and that a gsa gene specific for a tissue, tetrapyrrole, or light condition is unlikely. The gsa1 promoter contained a genetic element found in numerous Drosophila melanogaster genes; the so-called GAGA element displayed single-stranded character in vitro and formed a complex with nuclear factors from nodules and leaves but not from roots. From these observations we infer that the GAGA element is involved in the transcriptional control of gsa1.

A new proline-rich early nodulin from Medicago truncatula is highly expressed in nodule meristematic cells

We cloned and characterized MtPRP4, a new member of the repetitive proline-rich protein gene family in Medicago truncatula. The sequence of MtPRP4 predicts a 62-kD protein consisting of a 22-amino acid N-terminal signal peptide and a 527-amino acid repetitive proline-rich domain composed of three repetitive pentapeptide motifs arranged into two decapeptide repeats: PPVEKPPVHK and PPVEKPPVYK. MtPRP4 is the largest PRP described to date and contains repeated motifs that have not previously been found together in a single polypeptide. RNA gel blot experiments detected MtPRP4 transcripts in symbiotic root nodules, but not in roots, hypocotyls, or leaves. Accumulation of MtPRP4 transcript was an early response to Rhizobium inoculation and did not depend on nodule infection. In situ hybridization experiments demonstrated that MtPRP4 was expressed early in the development of the nodule meristem and that expression was highest in the meristematic cells of mature indeterminate nodules. These data support the proposition that an important early response of legume host roots to Rhizobium involves remodeling the host extracellular matrix and that proline-rich wall proteins play an important role in this architectural modification.

ENOD12, an early nodulin gene, is not required for nodule formation and efficient nitrogen fixation in alfalfa

To demonstrate the importance of an extensively studied early nodulin gene ENOD12 in symbiotic nodule development, plants of different Medicago sativa subspecies were tested for the presence or absence of ENOD12 alleles. In M. s. ssp coerulea w2 (Mcw2), two ENOD12 genes were detected, whereas in M. s. ssp quasifalcata k93 (Mqk93) only one gene was present. In both plants, the ENOD12 genes were expressed in nodules induced by Rhizobium meliloti. The nucleotide sequence of the ENOD12 genes showed that the two Mcw2-specific genes were similar to the ENOD12A and ENOD12B genes of the tetraploid M. s. ssp sativa. ENOD12 from Mqk93 was similar to the corresponding gene found in M. truncatula. From the aligned ENOD12 sequences, an evolutionary tree was constructed. Genetic analysis of the progenies of a cross between Mqk93 and Mcw2 showed that several offspring in F1 carried a null allele originating from Mcw2, and among the F2 progenies, plants with the null allele only lacking the ENOD12 gene appeared. Surprisingly, the ENOD12-deficient plants were similar to their wild-type parents in viability, nodule development, nodule structure, and nitrogen fixation efficiency. Therefore, we concluded that in Medicago the ENOD12 gene is not required for symbiotic nitrogen fixation. Furthermore, we proposed that the heterozygous nature of these legumes can be exploited for the identification of mutated alleles of other known nodulin genes; this will permit the construction of plant mutants deficient in these genes.

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.

A survey of transcripts expressed specifically in root nodules of broadbean (Vicia faba L.)

More than 600 potentially nodule-specific clones have been detected by differential hybridization of a broadbean cDNA library constructed from root nodule poly(A)+ RNA. These isolated cDNAs belong to at least 28 different clone groups containing cross-hybridizing sequences. The number of clones within a clone group varies from about 200 to only one single clone. Northern hybridization experiments revealed nodule-specific transcripts for 14 clone groups and markedly nodule-enhanced transcripts for another 7 clone groups. Sequence homologies indicate that three transcript sequences code for different leghemoglobins. Two other transcripts encode a nodule-specific sucrose synthase and a nodule-enhanced asparagine synthetase, respectively. Four deduced gene products are proline-rich, two of them being the homologues of PsENOD2 and PsENOD12. The third proline-rich protein (PRP) is composed of similar amino acid repeats as the nodule-specific PsENOD12 but is expressed in nodules and roots in comparable amounts. The fourth PRP is a nodule-enhanced extensin-type protein built up by Ser-Pro4 repeats. Two further nodule-specific transcripts encode gene products showing some similarity to structural glycine-rich proteins. Additionally, transcripts could be identified for broadbean homologues of the nodulins MsNOD25, PsENOD3 and PsENOD5 and transcripts specifying a nodule-enhanced lipoxygenase and a translation elongation factor EF-1 alpha, which is expressed in all broadbean tissues tested.

Characterization of the soybean early nodulin cDNA clone GmENOD55

Two cDNA clones of the soybean early nodulin GmENOD55 were characterized. These clones may represent two members of the soybean early nodulin gene family GmENOD55. GmENOD55 has an N-terminal signal peptide and it contains an internal domain consisting of proline and serine residues. Analyses of nodules lacking infection threads and intracellular bacteria suggest that the GmENOD55 gene is first expressed after release of Bradyrhizobium japonicum in plant cells. This conclusion is supported by in situ hybridization studies showing that the expression is restricted to the infected cell type.

A gene that encodes a proline-rich nodulin with limited homology to PsENOD12 is expressed in the invasion zone of Rhizobium meliloti-induced alfalfa root nodules

To define the early stages of the interaction between Rhizobium and host legumes, we have cloned and characterized three early nodulin-encoding sequences from an alfalfa (Medicago sativa L.) cDNA library by probing with a fragment of a cDNA clone for PsENOD12, an infection-related nodulin from pea (Pisum sativum L.). Although the coding regions of the three clones are 95 to 98% homologous to each other, they are only 43% homologous to the pea clone. However, the putative signal peptide encoded by the alfalfa cDNA clones is 100% homologous to the PsENDO12 signal peptide. The spatial and temporal expression patterns of PsENOD12 and the alfalfa clones were compared. In situ hybridization experiments detected RNA transcripts in the invasion zone of mature nitrogen-fixing nodules, the same site where PsENOD12 mRNAs are found. Transcripts were also found by in situ hybridization in cells of Rhizobium meliloti exoH mutant-induced nodules penetrated by infection threads, but northern analysis did not detect transcripts in inf- (infection thread minus) nodules elicited by R. meliloti exoB nodules or in pseudonodules elicited by treatment with the auxin transport inhibitor N-1-(naphthyl)phthalamic acid. In addition, the alfalfa gene represented by these cDNA clones exhibited a temporal expression pattern that differed from that of PsENOD12, which is transiently expressed. These data, plus information derived from Southern blot analysis, indicate that we have isolated cDNA clones for a novel early nodulin, which we have designated MsENOD10 (Medicago sativa Early Nodulin 10).

Isolation and characterization of novel nodulin cDNAs representing genes expressed at early stages of soybean nodule development

We took advantage of a subtractive hybridization procedure to isolate a set of cDNA clones of nodule-specific genes (nodulin genes) from developing soybean root nodules. Single-stranded 32P-labelled cDNA synthesized from nodule poly(A)+ RNA was hybridized with a large excess of uninfected root poly(A)+ RNA. Unhybridized cDNA was selected and used to screen nodule cDNA libraries. By this procedure we isolated several novel nodulin cDNA clones together with most of the nodulin cDNAs previously described. Four novel nodulin genes, which were expressed long before the onset of nitrogen fixation, were further characterized. GmN#36 and GmN#93 transcripts appeared in the roots less than 3 days after sowing and inoculation with Bradyrhizobium, but GmN#36 transcripts were also detected at very low levels in the stems of uninfected plants. Transcripts of GmN#315 and GmN#70 first appeared at 6-7 days, just before nodule emergence. Amino acid sequences of the predicted products of GmN#36, GmN#93 and GmN#70 exhibited no significant homology to proteins identified so far. The GmN#315 encoded protein has a limited but significant homology to some plant cyanins, suggesting that it is a metal-binding glycoprotein. In situ hybridization studies revealed that GmN#36 transcripts first appeared in the pericycle cells of the root stele near the infected site. During nodule emergence they were found in a few cell layers surrounding the vascular strands connecting the nodule meristem with the root stele, and in mature nodules they were present specifically in the pericycle cells in vascular bundles. These observations led us to hypothesize that GmN#36 gene products play a role in the transport and/or degradation of photosynthate. On the other hand, GmN#93 transcripts first appeared in the primary nodule meristem just below the root epidermis. In mature nodules they were only present in the infected cells.

Conserved regulation of the soybean early nodulin ENOD2 gene promoter in determine and indeterminate transgenic root nodules

The beta-glucuronidase (GUS) activity expressed from the soybean early nodulin ENOD2(B) gene promoter was localized histochemically in nodules of Lotus corniculatus and Trifolium repens. In both the determinate Lotus nodules and the indeterminate Trifolium nodules, activity was found in the parenchyma cells and especially in cells close to the vascular tissue of nodules. The characteristic cell-specific expression of the soybean ENOD2 gene was therefore maintained by the ENOD2(B) promoter in the two developmentally different nodule types. Important DNA elements recognized in transgenic nodules were identified by deletion and hybrid promoter analysis in Lotus corniculatus. An indispensable positive element (PE) and a possible tissue specific element was defined between positions -1792 and -1582 from the transcription start site. Another qualitative control element located between -380 and -53 conferred the ENOD2 characteristic cell type expression on hybrid promoters. This element contains the conserved nodulin gene sequences CTCTT and AAAGAT. In contrast to the ENOD2(B) promoter a chimeric leghemoglobin Ibc3-GUS gene was expressed in the infected cells of both types of nodules. In the indeterminate nodules expression was restricted to the interzone II-III and the active nitrogen-fixing zone III. Interchange of the distal strong positive element (SPE) of Ibc3 and the ENOD2 positive element resulted in an expression pattern different from that observed for the Ibc3 and ENOD2 genes, indicating that different interactions of trans-acting factors are required for regulation of early as well as late nodulin genes.

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.

Deletion analysis and localization of SbPRP1, a soybean cell wall protein gene, in roots of transgenic tobacco and cowpea

SbPRP1 is a member of the soybean (Glycine max L. Merr) proline-rich cell wall protein family and is expressed at high levels in root tissue. To characterize the sequences required for this expression, we have fused 1.1 kb of upstream flanking DNA sequence from an SbPRP1 genomic clone to a gene encoding beta-glucuronidase (GUS). This construct was introduced into tobacco using Agrobacterium tumefaciens-mediated transformation. Histochemical staining of GUS activity in transgenic tobacco indicated that SbPRP1 is expressed in the apical and elongating region of both primary and lateral roots, most strongly in the epidermis. A similar localization pattern was found in transformed hairy roots when this construct was introduced into cowpea (Vigna aconitifolia) using Agrobacterium rhizogenes-mediated transformation. Nested 5'-deletion analysis of the SbPRP1 promoter indicated that a minimal promoter for SbPRP1 expression in roots is located within the first 262 bases of upstream flanking DNA and that the region between -1080 and -262 is required for maximal expression of this gene. Gel retardation assays showed that nuclear factors can be detected in soybean roots which specifically bind to sequences located between -1080 and -623, a region which is needed for maximal expression of the SbPRP1 promoter. Northern hybridization analysis was also used to show that little SbPRP1 mRNA was present in roots during the first 24 h after imbibition. These studies indicate that SbPRP1 expression is localized to the actively growing region of the root and that this expression is temporally regulated during very early stages of seedling growth.

Expression of DcPRP1 is linked to carrot storage root formation and is induced by wounding and auxin treatment

A carrot (Daucus carota, L.) genomic clone (DcPRP1) was isolated on the basis of its homology to previously described cDNAs encoding a wound-inducible, proline-rich cell wall protein. DNA sequence analysis showed that DcPRP1 contains a single open-reading frame encoding a 235-amino acid protein that is colinear with that predicted from the cDNA sequence with the exception of four amino acids at the N terminus and a 60-nucleotide insertion present within the genomic clone. Genomic Southern hybridization analysis showed that the cloned sequence hybridized with a single restriction enzyme fragment using several restriction enzymes. Primer extension and northern hybridization analysis indicated that the expression of DcPRP1 is developmentally regulated and linked to the formation of storage roots, where this gene is expressed at high levels after wounding. The level of DcPRP1 mRNA was greatest in tissue immediately adjacent to the wound site. Treatment of unwounded carrot storage roots with 10 microM 2,4-dichlorophenoxy-acetic acid, indoleacetic acid, or naphthalene-1-acetic acid also resulted in the accumulation of DcPRP1 transcripts to a level equal to that seen in wounded tissue.