Stress activation of a bean hydroxyproline-rich glycoprotein promoter is superimposed on a pattern of tissue-specific developmental expression

The Expression of Potato Expansin A3 (StEXPA3) and Extensin4 (StEXT4) Genes with Distribution of StEXPAs and HRGPs-Extensin Changes as an Effect of Cell Wall Rebuilding in Two Types of PVYNTN-Solanum tuberosum Interactions

The plant cell wall acts not only as a physical barrier, but also as a complex and dynamic structure that actively changes under different biotic and abiotic stress conditions. The question is, how are the different cell wall compounds modified during different interactions with exogenous stimuli such as pathogens? Plants exposed to viral pathogens respond to unfavorable conditions on multiple levels. One challenge that plants face under viral stress is the number of processes required for differential cell wall remodeling. The key players in these conditions are the cell wall genes and proteins, which can be regulated in specific ways during the interactions and have direct influences on the rebuilding of the cell wall structure. The cell wall modifications occurring in plants during viral infection remain poorly described. Therefore, this study focuses on cell wall dynamics as an effect of incompatible interactions between the potato virus Y (PVY^NTN) and resistant potatoes (hypersensitive plant), as well as compatible (susceptible plant) interactions. Our analysis describes, for the first time, the expression of the potato expansin A3 (StEXPA3) and potato extensin 4 (StEXT4) genes in PVY^NTN-susceptible and -resistant potato plant interactions. The results indicated a statistically significant induction of the StEXPA3 gene during a susceptible response. By contrast, we demonstrated the predominantly gradual activation of the StEXT4 gene during the hypersensitive response to PVY^NTN inoculation. Moreover, the in situ distributions of expansins (StEXPAs), which are essential cell wall-associated proteins, and the hydroxyproline-rich glycoprotein (HRGP) extensin were investigated in two types of interactions. Furthermore, cell wall loosening was accompanied by an increase in StEXPA deposition in a PVY^NTN-susceptible potato, whereas the HRGP content dynamically increased during the hypersensitive response, when the cell wall was reinforced. Ultrastructural localization and quantification revealed that the HRGP extensin was preferably located in the apoplast, but deposition in the symplast was also observed in resistant plants. Interestingly, during the hypersensitive response, StEXPA proteins were mainly located in the symplast area, in contrast to the susceptible potato where StEXPA proteins were mainly observed in the cell wall. These findings revealed that changes in the intracellular distribution and abundance of StEXPAs and HRGPs can be differentially regulated, depending on different types of PVY^NTN–potato plant interactions, and confirmed the involvement of apoplast and symplast activation as a defense response mechanism.

Plant Cell Wall Dynamics in Compatible and Incompatible Potato Response to Infection Caused by Potato Virus Y (PVYNTN)

The cell wall provides the structure of the plant, and also acts as a barier against biotic stress. The vein necrosis strain of Potato virus Y (PVY^NTN) induces necrotic disease symptoms that affect both plant growth and yield. Virus infection triggers a number of inducible basal defense responses, including defense proteins, especially those involved in cell wall metabolism. This study investigates the comparison of cell wall host dynamics induced in a compatible (potato cv. Irys) and incompatible (potato cv. Sárpo Mira with hypersensitive reaction gene Ny-Smira) PVY^NTN–host–plant interaction. Ultrastructural analyses revealed numerous cell wall changes induced by virus infection. Furthermore, the localization of essential defensive wall-associated proteins in susceptible and resistant potato host to PVY^NTN infection were investigated. The data revealed a higher level of detection of pathogenesis-related protein 2 (PR-2) in a compatible compared to an incompatible (HR) interaction. Immunofluorescence analyses indicated that hydroxyproline-rich glycoproteins (HRGP) (extensin) synthesis was induced, whereas that of cellulose synthase catalytic subunits (CesA4) decreased as a result of PVY^NTN infection. The highest level of extensin localization was found in HR potato plants. Proteins involved in cell wall metabolism play a crucial role in the interaction because they affect the spread of the virus. Analysis of CesA4, PR-2 and HRGP deposition within the apoplast and symplast confirmed the active trafficking of these proteins as a step-in potato cell wall remodeling in response to PVY^NTN infection. Therefore, cell wall reorganization may be regarded as an element of “signWALLing”—involving apoplast and symplast activation as a specific response to viruses.

NtProRP1, a novel proline-rich protein, is an osmotic stress-responsive factor and specifically functions in pollen tube growth and early embryogenesis in Nicotiana tabacum

Proline-rich proteins (PRPs) are known to play important roles in sexual plant reproduction. Most of the known proteins in the family were found in styles or pollen and modulate pollen tube growth. Here, we identified a novel member of the gene family, NtProRP1, which is preferentially expressed in tobacco pollen grains, pollen tubes and zygotes. NtProRP1 could be secreted into the extracellular space including the cell wall, and the predicted N-terminal signal peptide is crucial for its secretion. In NtProRP1-RNAi plants, pollen germination and pollen tube growth were significantly slower and showed zigzag or swell morphology in vitro. Early embryogenesis also exhibited aberrant development, indicative of its critical role in both pollen tube growth and early embryogenesis. Further investigation revealed that NtProRP1 plays a crucial role in osmotic stress response during pollen tube growth and is likely regulated by Tsi, a stress-responsive gene, suggesting that the regulatory mechanism is also involved in the stress response during sexual plant reproduction. These data provide evidence that NtProRP1 functions as a downstream factor of Tsi1 in the stress response and converges the stress signal into the modulation of pollen tube growth and early embryogenesis.

Comparative transcriptional profiling-based identification of raphanusanin-inducible genes

BACKGROUND

Raphanusanin (Ra) is a light-induced growth inhibitor involved in the inhibition of hypocotyl growth in response to unilateral blue-light illumination in radish seedlings. Knowledge of the roles of Ra still remains elusive. To understand the roles of Ra and its functional coupling to light signalling, we constructed the Ra-induced gene library using the Suppression Subtractive Hybridisation (SSH) technique and present a comparative investigation of gene regulation in radish seedlings in response to short-term Ra and blue-light exposure.

RESULTS

The predicted gene ontology (GO) term revealed that 55% of the clones in the Ra-induced gene library were associated with genes involved in common defence mechanisms, including thirty four genes homologous to Arabidopsis genes implicated in R-gene-triggered resistance in the programmed cell death (PCD) pathway. Overall, the library was enriched with transporters, hydrolases, protein kinases, and signal transducers. The transcriptome analysis revealed that, among the fifty genes from various functional categories selected from 88 independent genes of the Ra-induced library, 44 genes were up-regulated and 4 were down-regulated. The comparative analysis showed that, among the transcriptional profiles of 33 highly Ra-inducible genes, 25 ESTs were commonly regulated by different intensities and duration of blue-light irradiation. The transcriptional profiles, coupled with the transcriptional regulation of early blue light, have provided the functional roles of many genes expected to be involved in the light-mediated defence mechanism.

CONCLUSIONS

This study is the first comprehensive survey of transcriptional regulation in response to Ra. The results described herein suggest a link between Ra and cellular defence and light signalling, and thereby contribute to further our understanding of how Ra is involved in light-mediated mechanisms of plant defence.

Polyploidization as a retraction force in plant genome evolution: sequence rearrangements in triticale

BACKGROUND

Polyploidization is a major evolutionary process in plants where hybridization and chromosome doubling induce enormous genomic stress and can generate genetic and epigenetic modifications. However, proper evaluation of DNA sequence restructuring events and the precise characterization of sequences involved are still sparse.

METHODOLOGY/PRINCIPAL FINDINGS

Inter Retrotransposons Amplified Polymorphism (IRAP), Retrotransposons Microsatellite Amplified Polymorphism (REMAP) and Inter Simple Sequence Repeat (ISSR) largely confirmed the absence of any intraspecific variation in wheat, rye and triticale. The comparative analysis of banding profiles between wheat and rye inbred lines revealed 34% of monomorphic (common to both parental species) bands for the ten different primer combinations used. The analysis of triticale plants uncovered nearly 51% of rearranged bands in the polyploid, being the majority of these modifications, due to the loss of rye bands (83%). Sequence analysis of rye fragments absent in triticale revealed for instance homology with hydroxyproline-rich glycoproteins (HRGP), a protein that belongs to a major family of inducible defence response proteins. Conversely, a wheat-specific band absent in triticale comprises a nested structure of copia-like retrotransposons elements, namely Claudia and Barbara. Sequencing of a polyploid-specific band (absent in both parents) revealed a microsatellite related sequence. Cytological studies using Fluorescent In Situ Hybridization (FISH) with REMAP products revealed a widespread distribution of retrotransposon and/or microsatellite flanking sequences on rye chromosomes, with a preferential accumulation in heterochromatic sub-telomeric domains.

CONCLUSIONS/SIGNIFICANCE

Here, we used PCR-based molecular marker techniques involving retrotransposons and microsatellites to uncover polyploidization induced genetic restructuring in triticale. Sequence analysis of rearranged genomic fragments either from rye or wheat origin showed these to be retrotransposon-related as well as coding sequences. Further FISH analysis revealed possible chromosome hotspots for sequence rearrangements. The role of chromatin condensation on the origin of genomic rearrangements mediated by polyploidization in triticale is also discussed.

Nitric oxide promotes the wound-healing response of potato leaflets

Nitric oxide (NO) is an essential regulatory molecule in several developmental and (patho) physiological processes. In this work, it is demonstrated that NO participates in the wound-healing response of potato leaves. The experimental approaches showed that the deposition of the cell-wall glucan callose was induced by the application of the NO donor sodium nitroprusside (SNP), and such induction was additive to the wound-induced callose production. Additionally, the expression of wound-related genes as phenylalanine ammonia-lyase (PAL) and extensin showed an accumulation of their transcript levels by SNP treatment. Moreover, the SNP-mediated increase of the PAL transcript level was additive to the induction mediated by wounding. These results indicate that increased levels of NO might potentiate the healing responses in plants leading to a rapid restoration of the damaged tissue.

Extensin over-expression in Arabidopsis limits pathogen invasiveness

SUMMARY The function of the cell wall protein extensin has been the subject of much speculation since it was first isolated over 40 years ago. In order to investigate the role of extensins in plant defence, we used the gain-of-function strategy to generate transgenic Arabidopsis plants over-expressing the EXT1 extensin gene. These were infected with the virulent bacterial pathogen Pseudomonas syringae DC3000 and symptom development was monitored. Lesions on the transgenics were on average five-fold smaller than those on the wild-type, did not increase in area over the time period of infection, accumulated a small bacterial load and showed very little chlorosis outside the lesion boundary. By contrast, lesions on the wild-type were large, spread to over 50% of the leaf area, continued to increase in size over the time course of the infection, accumulated a bacterial load 100-fold higher than that found in the transgenics, and showed a large chlorotic area outside the lesion boundary. SEM of lesions showed no evidence of bacteria at the lesion boundary in the extensin-over-expressing transgenics, whereas bacteria were always seen at the lesion boundary on the wild-type. Analysis of transgenics carrying an EXT1-GUS promoter-reporter fusion showed expression of GUS in a ring around the boundary of the lesion. Basal defences and signal transduction pathways involved in plant defence were not perturbed in the transgenics, as shown by the analysis of the expression of PR1 and PDF1.2 genes. These results show that extensin over-expression limits pathogen invasiveness.

Methyl jasmonate stimulates the de novo biosynthesis of vitamin C in plant cell suspensions

Vitamin C (L-ascorbic acid) is an important primary metabolite of plants that functions as an antioxidant, an enzyme cofactor, and a cell-signalling modulator in a wide array of crucial physiological processes, including biosynthesis of the cell wall, secondary metabolites and phytohormones, stress resistance, photoprotection, cell division, and growth. Plants synthesize ascorbic acid via de novo and salvage pathways, but the regulation of its biosynthesis and the mechanisms behind ascorbate homeostasis are largely unknown. Jasmonic acid and its methyl ester (jasmonates) mediate plant responses to many biotic and abiotic stresses by triggering a transcriptional reprogramming that allows cells to cope with pathogens and stress. By using 14C-mannose radiolabelling combined with HPLC and transcript profiling analysis, it is shown that methyl jasmonate treatment increases the de novo synthesis of ascorbic acid in Arabidopsis and tobacco Bright Yellow-2 (BY-2) suspension cells. In BY-2 cells, this stimulation coincides with enhanced transcription of at least two late methyl jasmonate-responsive genes encoding enzymes for vitamin C biosynthesis: the GDP-mannose 3'',5''-epimerase and a putative L-gulono-1,4-lactone dehydrogenase/oxidase. As far as is known, this is the first report of a hormonal regulation of vitamin C biosynthesis in plants. Finally, the role of ascorbic acid in jasmonate-regulated stress responses is reviewed.

Role of extracytoplasmic leucine rich repeat proteins in plant defence mechanisms

Plant-pathogen interactions involve highly complex series of reactions in disease development. Plants are endowed with both, resistance and defence genes. The activation of defence genes after contact with avirulence gene products of pathogens depends on signals transduced by leucine-rich repeats (LRRs) contained in resistance genes. Additionally, LRRs play roles for various actions following ligand recognition. Polygalacturonase inhibiting proteins (PGIPs), the only plant LRR protein with known ligands, are pectinase inhibitors, bound by ionic interactions to the extracellular matrix (ECM) of plant cells. They have a high affinity for fungal endopolygalacturonases (endoPGs). PGIP genes are organised in families encoding proteins with similar physical characteristics but different specificities. They are induced by infection and stress related signals. The molecular basis of PG-PGIP interaction serves as a model to understand the evolution of plant LRR proteins in recognising non-self-molecules. Extensins form a different class of structural proteins with repetitive sequences. They are also regulated by wounding and pathogen infection. Linkage of extensins with LRR motifs is highly significant in defending host tissues against pathogen invasion. Overexpression of PGIPs or expression of several PGIPs in a plant tissue, and perhaps manipulation of extensin expression could be possible strategies for disease management.

The Nicotiana sylvestris extensin gene, Ext 1.2A, is expressed in the root transition zone and upon wounding

The Ext 1.2A gene of Nicotiana sylvestris L. encoding an extensin, a cell wall structural protein, was characterized. Ext 1.2A encodes a polypeptide of 311 amino acids having a highly repetitive structure and showing extensin features such as Ser-(Pro)(4) repeats and a high content of Tyr and Lys. The expression profile of the gene was demonstrated using the reporter GUS (beta-glucuronidase) fused to its promoter region (-630/+124, relative to the transcription start site) and by RNA gel blots. The results show that the (-630/+124) Ext 1.2A/GUS gene fusion is expressed in the root transition zone, where cells undergo an isodiametric growth but have not yet reached the rapid elongation phase, in stem inner and outer phloems and in cortical cells at the stem/petiole junction. The Ext 1.2A gene is also induced after wounding of stems, ribs, leaves or roots. The gene fusion is expressed in stem cortical cells, in ribs and at leaf edges upon wounding. These data suggest that the (-630/+124) promoter region contains regulatory elements responsible for expression in roots and stems, as well as for response to wounding in stems and leaves.

Transgenic analysis of sugar beet xyloglucan endo-transglucosylase/hydrolase Bv-XTH1 and Bv-XTH2 promoters reveals overlapping tissue-specific and wound-inducible expression profiles

The identification and analysis of tissue-specific gene regulatory elements will improve our knowledge of the molecular mechanisms that control the growth and development of different plant tissues and offer potentially useful tools for the genetic engineering of plants. A polymerase chain reaction (PCR)-based 5'-genome walk from sequences of an isolated sugar beet xyloglucan endo-transglucosylase hydrolase (XTH) gene led to the isolation of two independent upstream fragments. They were 1332 and 2163 base pairs upstream of the XTH ATG start site, respectively. In vivo transgenic assays in sugar beet hairy roots and Arabidopsis thaliana revealed that both fragments had promoter function and, in A. thaliana, directed expression in vascular tissues within the root, leaves and petals. Promoter activity was also observed in the leaf trichomes and within rapidly expanding stem internodes. Expression driven by both promoters was found to be wound inducible. Overall, the spatial and temporal expression pattern of these promoters suggested that the corresponding Bv-XTH genes (designated Bv-XTH1 and Bv-XTH2) may be involved in secondary cell wall formation. This work provides new insights on molecular mechanisms that could be exploited for the genetic engineering of sugar beet crops.

Sugar acts as a regulatory signal on the wound-inducible expression of SbHRGP3::GUS in transgenic plants

SbHRGP3 encodes an HRGP whose expression is correlated with the cessation of root elongation in soybean. The wound-inducible expression of SbHRGP3 interestingly requires sucrose although wounding alone induces the expression of many HRGP genes. To examine whether sugar serves as a specific signal on the wound-inducible expression or whether sugar is required to provide ATP, we examined SbHRGP3:: GUS expression in transgenic tobacco plants. Various oligosaccharides including non-metabolizable sugar induced SbHRGP3:: GUS expression in transgenic plants. The inhibitors of photosynthesis and of cellular respiration did not affect the wound-inducible expression of SbHRGP3:: GUS. However, the induction was significantly affected by PCMBS, an inhibitor of active apoplastic phloem loading of sucrose, suggesting that SbHRGP3:: GUS expression in phloem tissues requires translocated sucrose. We therefore propose that sugar acts as a specific regulatory signal on the wound-inducible expression of SbHRGP3, rather than acting as a simple provider of ATP.

The unusual Arabidopsis extensin gene atExt1 is expressed throughout plant development and is induced by a variety of biotic and abiotic stresses

We detail the expression of the Arabidopsis thaliana (L.) Heynh. atExt1 extensin gene. atExt1 is normally expressed in roots and inflorescences, and is induced by wounding, exogenously supplied salicylic acid, methyl jasmonate, auxins and brassinosteroids. Northern assays and histochemical analysis of transgenics expressing an atExt1:: gus fusion show that this gene is also induced by the brassica pathogen Xanthomonas campestris pv. campestris and that this induction is restricted to tissues close to the site of infection. Expression at regions of abscission and senescence also implicates atExt1 in these important developmental processes.

The cell wall hydroxyproline-rich glycoprotein RSH is essential for normal embryo development in Arabidopsis

Although a large number of embryo mutants have been studied, mostly at the morphological level, the critical molecular and cellular events responsible for embryogenesis are unknown. Here, we report that using an enhancer-trap embryo mutant of Arabidopsis, we identified a gene, ROOT-SHOOT-HYPOCOTYL-DEFECTIVE (RSH), that is essential for the correct positioning of the cell plate during cytokinesis in cells of the developing embryo. We traced the earliest point of influence of RSH to the first asymmetrical division of the zygote. Homozygous rsh embryos were defective morphologically, had irregular cell shape and size, and germinated to form agravitropic-defective seedlings incapable of further development. The RSH gene encodes a Hyp-rich glycoprotein-type cell wall protein. RSH localized to the cell wall throughout the embryo and to a few well-defined postembryonic sites. Although several lines of evidence from previous work suggest that the cell wall is involved in development, the protein(s) involved remained elusive.

Differential and systemic alteration of defence-related gene transcript levels in mycorrhizal bean plants infected with Rhizoctonia solani

The role of arbuscular mycorrhizas in response of plants to soilborne root pathogens is unclear. A time course study was conducted to monitor disease development and expression of mRNA for the defence-related genes phenylalanine ammonia lyase, chalcone synthase, chalcone isomerase, and hydroxyproline-rich glycoprotein in bean (Phasoelus vulgaris L.) plants colonized by the arbuscular mycorrhizal fungus Glomus intraradices Schenck & Smith and postinfected with the soilborne pathogen Rhizoctonia solani Kühn. Precolonization of bean plants by G. intraradices did not significantly reduce the severity of rot symptoms. RNA blot analysis of the defence-related genes revealed a systemic increase in the four defence genes in response to R. solani infections. On the other hand, precolonization of bean plants with G. intraradices elicited no change in phenylalanine ammonia lyase, chalcone synthase, and chalcone isomerase transcripts. A differential and systemic alteration in the expression of all four defence genes was observed in all tissues only during the pathogenic interaction of arbuscular mycorrhizal beans. Depending on the time after infection with R. solani and the tissue examined, varying responses from stimulation to suppression to no change in transcript levels were detected.Key words: induced resistance, defence-related genes, RNA analysis, Rhizoctonia solani, Glomus intraradices.

The chimeric leucine-rich repeat/extensin cell wall protein LRX1 is required for root hair morphogenesis in Arabidopsis thaliana

In plants, the cell wall is a major determinant of cell morphogenesis. Cell enlargement depends on the tightly regulated expansion of the wall, which surrounds each cell. However, the qualitative and quantitative mechanisms controlling cell wall enlargement are still poorly understood. Here, we report the molecular and functional characterization of LRX1, a new Arabidopsis gene that encodes a chimeric leucine-rich repeat/extensin protein. LRX1 is expressed in root hair cells and the protein is specifically localized in the wall of the hair proper, where it becomes insolubilized during development. lrx1-null mutants, isolated by a reverse-genetic approach, develop root hairs that frequently abort, swell, or branch. Complementation and overexpression experiments using modified LRX1 proteins indicate that the interaction with the cell wall is important for LRX1 function. These results suggest that LRX1 is an extracellular component of a mechanism regulating root hair morphogenesis and elongation by controlling either polarized growth or cell wall formation and assembly.

Induction of mRNA accumulation corresponding to a gene encoding a cell wall hydroxyproline-rich glycoprotein by fungal elicitors

The Hrgp (hydroxyproline-rich glycoprotein) gene codes in maize for one of the most abundant proteins of the cell wall. HRGPs may contribute to the structural support of the wall and they have also been involved in plant defense mechanisms. This second aspect has been tested for the Hrgp gene in maize where, in contrast with the situation in dicot species, the gene is encoded by a single-copy sequence. Hrgp mRNA accumulation is induced in maize suspension-cultured cells by elicitors, isolated either from maize pathogenic or non-pathogenic fungi. The induction of Hrgp mRNA accumulation by elicitor extracted from Fusarium moniliforme has been studied in detail. The level of induction depends on elicitor concentration and remains high until at least 24 h. Ethylene and protein phosphorylation appear to be involved in the transduction pathway of Hrgp gene activation by the F. moniliforme elicitor but not by 5 microM methyl jasmonate or 1 mM salycilic acid. Different compounds known to participate in plant stress responses such as ascorbic acid or reduced glutathione have also a positive effect on Hrgp mRNA accumulation.

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

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

Functional analysis of the promoter region of a maize (Zea mays L.) H3 histone gene in transgenic Arabidopsis thaliana

A 1023 bp fragment and truncated derivatives of the maize (Zea mays L.) histone H3C4 gene promoter were fused to the beta-glucuronidase (GUS) gene and introduced via Agrobacterium tumefaciens into the genome of Arabidopsis thaliana. GUS activity was found in various meristems of transgenic plants as for other plant histone promoters, but unexplained activity also occurred at branching points of both stems and roots. Deletion of the upstream 558 bp of the promoter reduced its activity to an almost basal expression. Internal deletion of a downstream fragment containing plant histone-specific sequence motifs reduced the promoter activity in all tissues and abolished the expression in meristems. Thus, both the proximal and distal regions of the promoter appear necessary to achieve the final expression pattern in dicotyledonous plant tissues. In mesophyll protoplasts isolated from the transformed Arabidopsis plants, the full-length promoter showed both S phase-dependent and -independent activity, like other plant histone gene promoters. Neither of the 5'-truncated nor the internal-deleted promoters were able to direct S phase-dependent activity, thus revealing necessary cooperation between the proximal and distal parts of the promoter to achieve cell cycle-regulated expression. The involvement of the different regions of the promoter in the different types of expression is discussed.

Expression of a Soybean Hydroxyproline-Rich Glycoprotein Gene Is Correlated with Maturation of Roots

A novel extensin gene has been identified in soybean (Glycine max L.) that encodes a hydroxyproline-rich glycoprotein (SbHRGP3) with two different domains. In this study expression of SbHRGP3 was investigated during soybean root development. SbHRGP was expressed in roots of mature plants, as well as seedlings, and showed a distinct pattern of expression during root development. The expression of SbHRGP3 increased gradually during root development of seedlings and reached a maximum while the secondary roots were maturing. The maximum expression level was contributed mainly by the secondary roots rather than by the primary root. Furthermore, expression of SbHRGP3 was preferentially detected in the regions undergoing maturation of the primary and secondary roots. These results imply that the expression of SbHRGP3 is regulated in an organ- and development-specific manner and that in soybean SbHRGP3 expression may be required for root maturation, presumably for the cessation of root elongation. Wounding and sucrose in combination enhanced expression of SbHRGP3 in roots, whereas both wounding and sucrose were required for the expression of SbHRGP3 in leaves.

Differential Gene Expression in Response to Auxin Treatment in the Wild Type and rac, an Adventitious Rooting-Incompetent Mutant of Tobacco

Histological analyses of auxin-treated cuttings from the wild type and the rac mutant of tobacco (Nicotiana tabacum cv Xanthii) previously revealed that some rac phloem parenchyma or inner cortical parenchyma cells form callus in response to exogenous auxin treatment but these cells never undergo the organized divisions associated with adventitious root initiation in the wild type. Here we report the effect of the rac mutation on the temporal and spatial expression patterns of three genes previously shown to be associated with adventitious root meristems, HRGPnt3, iaa4/5, and gh3. Using histochemical staining analyses of HRGPnt3-GUS transformant cuttings, we determined that the rac mutation blocks auxin activation of the HRGPnt3 promoter. Thus, activation of the HRGPnt3 promoter occurs specifically during adventitious root initiation in tobacco cuttings. Histochemical staining analyses of iaa4/5-GUS and gh3-GUS transformant cuttings revealed that the rac mutation does not repress the auxin activation of the iaa4/5 and gh3 promoters. Based on our histochemical staining analyses, we conclude that differential gene expression occurs in response to auxin treatment during adventitious root initiation in the wild type compared with callus formation in rac cuttings. We also determined that HRGPnt3 mRNA accumulation occurs in response to components of our root-induction protocol other than auxin, indicating that HRGPnt3 expression is regulated both developmentally and environmentally.

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.

Rhizobia modulate root-hair-specific expression of extensin genes

Three cDNAs (ext3, ext127, and ext26), originally isolated by differential screening from a root-hair cDNA library of Vigna unguiculata, were found to encode extensin-like cell wall proteins. Transcripts homologous to these cDNAs were only detected in root hairs where mRNA levels decreased 1 day after inoculation with rhizobia. This coincided with the onset of root-hair deformation, the first morphological step in the Rhizobium-legume interaction. Decreases in transcript levels following inoculation with wild-type Rhizobium sp. NGR234 were more pronounced than with NGR delta nodABC, a mutant deficient in Nod-factor production. Inoculation with a rhizobial strain carrying a mutation in a gene encoding a transcriptional activator for nod genes (NGR delta nodD1) did not repress mRNA levels, indicating that a second nodulation signal may be present that is nodD dependent. Application of purified NodNGR factors only affected transcript levels of ext3. The genomic locus of the gene homologous to ext26 (Ext26G) was cloned. In the 5' flanking region, several potential TATA boxes and CAP signals were identified. Part of the promoter region shares homology with the Pisum sativum seed lectin promoter and the Nicotiana tabacum nitrate reductase promoter region. Nonetheless, the function of these homologous regions in gene regulation is unknown.

A novel extensin gene encoding a hydroxyproline-rich glycoprotein requires sucrose for its wound-inducible expression in transgenic plants

A novel hydroxyproline-rich glycoprotein (SbHRGP3) that consists of two different domains is encoded by an extensin gene from soybean. The first domain (domain 1) located at the N terminus is composed of 11 repeats of Ser-Pro4-Lys-His-Ser-Pro4-Tyr3-His, whereas the second domain (domain 2) at the C terminus contains five repeats of Ser-Pro4-Val-Tyr-Lys-Tyr-Lys-Ser-Pro4-Tyr-Lys-Tyr-Pro-Ser-Pro5-Tyr-Lys-T yr- Pro-Ser-Pro4-Val-Tyr-Lys-Tyr-Lys. These two repeat motifs are organized in an extremely well-ordered pattern in each domain, which suggests that SbHRGP3 belongs to a new group of proteins having the repeat motifs of two distinct groups of dicot extensins. The expression of the SbHRGP3 gene increased with seedling maturation, and its expression was relatively high in the mature regions of the hypocotyl and in the root of soybean seedlings. An SbHRGP3-beta-glucuronidase (SbHRGP3-GUS) chimeric gene was constructed and expressed in transgenic tobacco plants. The expression of the SbHRGP3-GUS gene was not induced by wounding alone in transgenic tobacco plants; sucrose was also required. Expression was specific to phloem tissues and cambium cells of leaves and stems. In transgenic tobacco seedlings, SbHRGP3-GUS gene expression was activated by the maturation of the primary root and then inactivated; however, reactivation was specifically at the epidermis of the zone from which the lateral root was to be initiated. Its reactivation occurred just before the lateral root initiation. These results indicate that the SbHRGP3 gene in different tissues responds to different signals.