Pathogenesis-related functions of plant beta-1,3-glucanases investigated by antisense transformation--a review

Plant Cell Wall as a Key Player During Resistant and Susceptible Plant-Virus Interactions

The cell wall is a complex and integral part of the plant cell. As a structural element it sustains the shape of the cell and mediates contact among internal and external factors. We have been aware of its involvement in both abiotic (like drought or frost) and biotic stresses (like bacteria or fungi) for some time. In contrast to bacterial and fungal pathogens, viruses are not mechanical destructors of host cell walls, but relatively little is known about remodeling of the plant cell wall in response to viral biotic stress. New research results indicate that the cell wall represents a crucial active component during the plant’s response to different viral infections. Apparently, cell wall genes and proteins play key roles during interaction, having a direct influence on the rebuilding of the cell wall architecture. The plant cell wall is involved in both susceptibility as well as resistance reactions. In this review we summarize important progress made in research on plant virus impact on cell wall remodeling. Analyses of essential defensive wall associated proteins in susceptible and resistant responses demonstrate that the components of cell wall metabolism can affect the spread of the virus as well as activate the apoplast- and symplast-based defense mechanisms, thus contributing to the complex network of the plant immune system. Although the cell wall reorganization during the plant-virus interaction remains a challenging task, the use of novel tools and methods to investigate its composition and structure will greatly contribute to our knowledge in the field.

Plasmodesmata-Related Structural and Functional Proteins: The Long Sought-After Secrets of a Cytoplasmic Channel in Plant Cell Walls

Plant cells are separated by cellulose cell walls that impede direct cell-to-cell contact. In order to facilitate intercellular communication, plant cells develop unique cell-wall-spanning structures termed plasmodesmata (PD). PD are membranous channels that link the cytoplasm, plasma membranes, and endoplasmic reticulum of adjacent cells to provide cytoplasmic and membrane continuity for molecular trafficking. PD play important roles for the development and physiology of all plants. The structure and function of PD in the plant cell walls are highly dynamic and tightly regulated. Despite their importance, plasmodesmata are among the few plant cell organelles that remain poorly understood. The molecular properties of PD seem largely elusive or speculative. In this review, we firstly describe the general PD structure and its protein composition. We then discuss the recent progress in identification and characterization of PD-associated plant cell-wall proteins that regulate PD function, with particular emphasis on callose metabolizing and binding proteins, and protein kinases targeted to and around PD.

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.

Differential gene expression in response to Fusarium oxysporum infection in resistant and susceptible genotypes of flax (Linum usitatissimum L.)

BACKGROUND

Flax (Linum usitatissimum L.) is a crop plant used for fiber and oil production. Although potentially high-yielding flax varieties have been developed, environmental stresses markedly decrease flax production. Among biotic stresses, Fusarium oxysporum f. sp. lini is recognized as one of the most devastating flax pathogens. It causes wilt disease that is one of the major limiting factors for flax production worldwide. Breeding and cultivation of flax varieties resistant to F. oxysporum is the most effective method for controlling wilt disease. Although the mechanisms of flax response to Fusarium have been actively studied, data on the plant response to infection and resistance gene candidates are currently very limited.

RESULTS

The transcriptomes of two resistant and two susceptible flax cultivars with respect to Fusarium wilt, as well as two resistant BC₂F₅ populations, which were grown under control conditions or inoculated with F. oxysporum, were sequenced using the Illumina platform. Genes showing changes in expression under F. oxysporum infection were identified in both resistant and susceptible flax genotypes. We observed the predominant overexpression of numerous genes that are involved in defense response. This was more pronounced in resistant cultivars. In susceptible cultivars, significant downregulation of genes involved in cell wall organization or biogenesis was observed in response to F. oxysporum. In the resistant genotypes, upregulation of genes related to NAD(P)H oxidase activity was detected. Upregulation of a number of genes, including that encoding beta-1,3-glucanase, was significantly greater in the cultivars and BC₂F₅ populations resistant to Fusarium wilt than in susceptible cultivars in response to F. oxysporum infection.

CONCLUSIONS

Using high-throughput sequencing, we identified genes involved in the early defense response of L. usitatissimum against the fungus F. oxysporum. In response to F. oxysporum infection, we detected changes in the expression of pathogenesis-related protein-encoding genes and genes involved in ROS production or related to cell wall biogenesis. Furthermore, we identified genes that were upregulated specifically in flax genotypes resistant to Fusarium wilt. We suggest that the identified genes in resistant cultivars and BC₂F₅ populations showing induced expression in response to F. oxysporum infection are the most promising resistance gene candidates.

Identification of a Vitis vinifera endo-β-1,3-glucanase with antimicrobial activity against Plasmopara viticola

Inducible plant defences against pathogens are stimulated by infections and comprise several classes of pathogenesis-related (PR) proteins. Endo-β-1,3-glucanases (EGases) belong to the PR-2 class and their expression is induced by many pathogenic fungi and oomycetes, suggesting that EGases play a role in the hydrolysis of pathogen cell walls. However, reports of a direct effect of EGases on cell walls of plant pathogens are scarce. Here, we characterized three EGases from Vitis vinifera whose expression is induced during infection by Plasmopara viticola, the causal agent of downy mildew. Recombinant proteins were expressed in Escherichia coli. The enzymatic characteristics of these three enzymes were measured in vitro and in planta. A functional assay performed in vitro on germinated P. viticola spores revealed a strong anti-P. viticola activity for EGase3, which strikingly was that with the lowest in vitro catalytic efficiency. To our knowledge, this work shows, for the first time, the direct effect against downy mildew of EGases of the PR-2 family from Vitis.

Comparative Transcriptome and iTRAQ Proteome Analyses of Citrus Root Responses to Candidatus Liberibacter asiaticus Infection

Root samples of 'Sanhu' red tangerine trees infected with and without Candidatus Liberibacter asiaticus (CLas) were collected at 50 days post inoculation and subjected to RNA-sequencing and isobaric tags for relative and absolute quantification (iTRAQ) to profile the differentially expressed genes (DEGs) and proteins (DEPs), respectively. Quantitative real-time PCR was subsequently used to confirm the expression of 16 selected DEGs. Results showed that a total of 3956 genes and 78 proteins were differentially regulated by HLB-infection. Among the most highly up-regulated DEPs were sperm specific protein 411, copper ion binding protein, germin-like proteins, subtilisin-like proteins and serine carboxypeptidase-like 40 proteins whose transcript levels were concomitantly up-regulated as shown by RNA-seq data. Comparison between our results and those of the previously reported showed that known HLB-modulated biological pathways including cell-wall modification, protease-involved protein degradation, carbohydrate metabolism, hormone synthesis and signaling, transcription activities, and stress responses were similarly regulated by HLB infection but different or root-specific changes did exist. The root unique changes included the down-regulation in genes of ubiquitin-dependent protein degradation pathway, secondary metabolism, cytochrome P450s, UDP-glucosyl transferases and pentatricopeptide repeat containing proteins. Notably, nutrient absorption was impaired by HLB-infection as the expression of the genes involved in Fe, Zn, N and P adsorption and transportation were significantly changed. HLB-infection induced some cellular defense responses but simultaneously reduced the biosynthesis of the three major classes of secondary metabolites, many of which are known to have anti-pathogen activities. Genes involved in callose deposition were up-regulated whereas those involved in callose degradation were also up-regulated, indicating that the sieve tube elements in roots were hanging on the balance of life and death at this stage. In addition, signs of carbohydrate starvation were already eminent in roots at this stage. Other interesting genes and pathways that were changed by HLB-infection were also discussed based on our findings.

The impact of heat stress targeting on the hormonal and transcriptomic response in Arabidopsis

Targeting of the heat stress (HS, 40°C) to shoots, roots or whole plants substantially affects Arabidopsis physiological responses. Effective stress targeting was proved by determination of the expression of HS markers, HsfA2 and HSA32, which were quickly stimulated in the targeted organ(s), but remained low in non-stressed tissues for at least 2h. When shoots or whole plants were subjected to HS, a transient decrease in abscisic acid, accompanied by a small increase in active cytokinin levels, was observed in leaves, consistent with stimulation of transpiration, the main cooling mechanism in leaves. HS application targeted to part of plant resulted in a rapid stimulation of expression of components of cytokinin signaling pathway (especially of receptor genes) in the non-exposed tissues, which indicated fast inter-organ communication. Under all HS treatments, shoot apices responded by transient elevation of active cytokinin contents and stimulation of transcription of genes involved in photosynthesis and carbohydrate metabolism. Duration of this stimulation was negatively correlated with stress strength. The impact of targeted HS on the expression of 63 selected genes, including those coding regulatory 14-3-3 proteins, was compared. Stimulation of GRF9 (GRF14μ) in stressed organs after 2-6h may be associated with plant stress adaptation.

Biotechnological production of inducible defense-related proteins in edible radish (raphanus sativus) found in Nepal

BACKGROUND

Fungal infection in plant leads to use of many hazardous antifungal chemicals. Alternative to these chemicals, defense related antifungal proteins can be used in case of fungal diseases.

AIMS

An experiment was done in two varieties of edible radish (Raphanus sativus var. Pyuthane Raato and Raphanus sativus var. all season) with aims to produce defense protein within the plant, to identify and perform molecular characterization of those antifungal proteins. The next aim was to compare the antifungal property of those proteins with commercially available synthetic pesticides.

METHODS

Both varieties of radish were infected with fungi (Alternaria alternata and Fusarium oxysporum). Protein samples were isolated from leaves following the standard protocol as described for β-glucuronidase (GUS) assay and were run along with the standard protein marker of 10-250kDa in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) to identify and molecularly characterize them.

RESULTS

An additional band in the range of 37-50kDa was observed in the fungal infected samples, which was not seen on uninfected samples. The antifungal assay was carried out for every sample in 96 wells microtitre plate. The extracted protein samples from fungal inoculated plants showed the significant inhibition of fungal growth compared to other samples. On the basis of molecular weight and their antifungal properties, the protein samples from the fungal infected plant were found to be PR2 (Glucanase) and PR3 (Chitinase).

CONCLUSION

Defense related proteins were successfully produced in two varieties of radish found in Nepal. The use of such biologically produced proteins may reduce the use of biologically harmful synthetic pesticides.

Invasion of shoot apical meristems by Chrysanthemum stunt viroid differs among Argyranthemum cultivars

Chrysanthemum stunt viroid (CSVd) is a damaging pathogen attacking Argyranthemum plants. Our study attempted to reveal distribution patterns of CSVd in shoot apical meristems (SAM) and to explore reasons for differential ability of CSVd to invade SAM of selected Argyranthemum cultivars. Symptom development was also observed on greenhouse-grown Argyranthemum plants. Viroid localization using in situ hybridization revealed that the ability of CSVd to invade SAM differed among cultivars. In diseased 'Yellow Empire' and 'Butterfly', CSVd was found in all tissues including the uppermost cell layers in the apical dome (AD) and the youngest leaf primordia 1 and 2. In diseased 'Border Dark Red' and 'Border Pink', CSVd was detected in the lower part of the AD and elder leaf primordia, leaving the upper part of the AD, and leaf primordia 1 and 2 free of viroid. Histological observations and transmission electron microscopy showed similar developmental patterns of vascular tissues and plasmodesmata (PD) in the SAM of 'Yellow Empire' and 'Border Dark Red', while immunolocalization studies revealed a major difference in the number of callose (β-1, 3-glucan) particles deposited at PD in SAM. A lower number of callose particles were found deposited at PD of SAM of 'Yellow Empire' than 'Border Dark Red'. This difference is most likely responsible for the differences in ability of CSVd to invade SAM among Argyranthemum cultivars.

Callose homeostasis at plasmodesmata: molecular regulators and developmental relevance

Plasmodesmata are membrane-lined channels that are located in the plant cell wall and that physically interconnect the cytoplasm and the endoplasmic reticulum (ER) of adjacent cells. Operating as controllable gates, plasmodesmata regulate the symplastic trafficking of micro- and macromolecules, such as endogenous proteins [transcription factors (TFs)] and RNA-based signals (mRNA, siRNA, etc.), hence mediating direct cell-to-cell communication and long distance signaling. Besides this physiological role, plasmodesmata also form gateways through which viral genomes can pass, largely facilitating the pernicious spread of viral infections. Plasmodesmatal trafficking is either passive (e.g., diffusion) or active and responses both to developmental and environmental stimuli. In general, plasmodesmatal conductivity is regulated by the controlled build-up of callose at the plasmodesmatal neck, largely mediated by the antagonistic action of callose synthases (CalSs) and β-1,3-glucanases. Here, in this theory and hypothesis paper, we outline the importance of callose metabolism in PD SEL control, and highlight the main molecular factors involved. In addition, we also review other proteins that regulate symplastic PD transport, both in a developmental and stress-responsive framework, and discuss on their putative role in the modulation of PD callose turn-over. Finally, we hypothesize on the role of structural sterols in the regulation of (PD) callose deposition and outline putative mechanisms by which this regulation may occur.

A novel role of PR2 in abscisic acid (ABA) mediated, pathogen-induced callose deposition in Arabidopsis thaliana

Pathogenesis-related protein 2 (PR2) is known to play a major role in plant defense and general stress responses. Resistance against the fungal pathogen Leptosphaeria maculans in Arabidopsis requires abscisic acid (ABA), which promotes the deposition of callose, a β-1,3-glucan polymer. Here, we examined the role of PR2 in callose deposition in relation to ABA treatment and challenge with L. maculans and Pseudomonas syringae. Characterization of PR2-overexpressing plants and the knockout line indicated that PR2 negatively affects callose deposition. Recombinant PR2 purified from Pichia pastoris showed callose-degrading activity, and a considerable reduction in the callose-degrading activity was observed in the leaf extract of the PR2 knockout line compared with the wild-type. ABA pretreatment before challenge with L. maculans concomitantly repressed PR2 and enhanced callose accumulation. Likewise, overexpression of an ABA biosynthesis gene NCED3 resulted in reduced PR2 expression and increased callose deposition. We propose that ABA promotes callose deposition through the transcriptional repression of PR2 in Arabidopsis challenged by L. maculans and P. syringae. Callose by itself is likely to act antagonistically on salicylic acid (SA) defense signaling, suggesting that PR2 may function as a modulator of callose- and SA-dependent defense responses.

Subcellular dynamics and role of Arabidopsis β-1,3-glucanases in cell-to-cell movement of tobamoviruses

β-1,3-Glucanases (BG) have been implicated in enhancing virus spread by degrading callose at plasmodesmata (Pd). Here, we investigate the role of Arabidopsis BG in tobamovirus spread. During Turnip vein clearing virus infection, the transcription of two pathogenesis-related (PR)-BG AtBG2 and AtBG3 increased but that of Pd-associated BG AtBG_pap did not change. In transgenic plants, AtBG2 was retained in the endoplasmic reticulum (ER) network and was not secreted. As a stress response mediated by salicylic acid, AtBG2 was secreted and appeared as a free extracellular protein localized in the entire apoplast but did not accumulate at Pd sites. At the leading edge of Tobacco mosaic virus spread, AtBG2 co-localized with the viral movement protein in the ER-derived bodies, similarly to other ER proteins, but was not secreted to the cell wall. In atbg2 mutants, callose levels at Pd and virus spread were unaffected. Likewise, AtBG2 overexpression had no effect on virus spread. However, in atbg_pap mutants, callose at Pd was increased and virus spread was reduced. Our results demonstrate that the constitutive Pd-associated BG but not the stress-regulated extracellular PR-BG are directly involved in regulation of callose at Pd and cell-to-cell transport in Arabidopsis, including the spread of viruses.

Co-evolution between Grapevine rupestris stem pitting-associated virus and Vitis vinifera L. leads to decreased defence responses and increased transcription of genes related to photosynthesis

Grapevine rupestris stem pitting-associated virus (GRSPaV) is a widespread virus infecting Vitis spp. Although it has established a compatible viral interaction in Vitis vinifera without the development of phenotypic alterations, it can occur as distinct variants that show different symptoms in diverse Vitis species. The changes induced by GRSPaV in V. vinifera cv 'Bosco', an Italian white grape variety, were investigated by combining agronomic, physiological, and molecular approaches, in order to provide comprehensive information about the global effects of GRSPaV. In two years, this virus caused a moderate decrease in physiological efficiency, yield performance, and sugar content in berries associated with several transcriptomic alterations. Transcript profiles were analysed by a microarray technique in petiole, leaf, and berry samples collected at véraison and by real-time RT-PCR in a time course carried out at five grapevine developmental stages. Global gene expression analyses showed that transcriptomic changes were highly variable among the different organs and the different phenological phases. GRSPaV triggers some unique responses in the grapevine at véraison, never reported before for other plant-virus interactions. These responses include an increase in transcripts involved in photosynthesis and CO(2) fixation, a moderate reduction in the photosynthesis rate and some defence mechanisms, and an overlap with responses to water and salinity stresses. It is hypothesized that the long co-existence of grapevine and GRSPaV has resulted in the evolution of a form of mutual adaptation between the virus and its host. This study contributes to elucidating alternative mechanisms used by infected plants to contend with viruses.

Biology of callose (β-1,3-glucan) turnover at plasmodesmata

The turnover of callose (β-1,3-glucan) within cell walls is an essential process affecting many developmental, physiological and stress related processes in plants. The deposition and degradation of callose at the neck region of plasmodesmata (Pd) is one of the cellular control mechanisms regulating Pd permeability during both abiotic and biotic stresses. Callose accumulation at Pd is controlled by callose synthases (CalS; EC 2.4.1.34), endogenous enzymes mediating callose synthesis, and by β-1,3-glucanases (BG; EC 3.2.1.39), hydrolytic enzymes which specifically degrade callose. Transcriptional and posttranslational regulation of some CalSs and BGs are strongly controlled by stress signaling, such as that resulting from pathogen invasion. We review the role of Pd-associated callose in the regulation of intercellular communication during developmental, physiological, and stress response processes. Special emphasis is placed on the involvement of Pd-callose in viral pathogenicity. Callose accumulation at Pd restricts virus movement in both compatible and incompatible interactions, while its degradation promotes pathogen spread. Hence, studies on mechanisms of callose turnover at Pd during viral cell-to-cell spread are of importance for our understanding of host mechanisms exploited by viruses in order to successfully spread within the infected plant.

ANK, a host cytoplasmic receptor for the Tobacco mosaic virus cell-to-cell movement protein, facilitates intercellular transport through plasmodesmata

Plasmodesma (PD) is a channel structure that spans the cell wall and provides symplastic connection between adjacent cells. Various macromolecules are known to be transported through PD in a highly regulated manner, and plant viruses utilize their movement proteins (MPs) to gate the PD to spread cell-to-cell. The mechanism by which MP modifies PD to enable intercelluar traffic remains obscure, due to the lack of knowledge about the host factors that mediate the process. Here, we describe the functional interaction between Tobacco mosaic virus (TMV) MP and a plant factor, an ankyrin repeat containing protein (ANK), during the viral cell-to-cell movement. We utilized a reverse genetics approach to gain insight into the possible involvement of ANK in viral movement. To this end, ANK overexpressor and suppressor lines were generated, and the movement of MP was tested. MP movement was facilitated in the ANK-overexpressing plants, and reduced in the ANK-suppressing plants, demonstrating that ANK is a host factor that facilitates MP cell-to-cell movement. Also, the TMV local infection was largely delayed in the ANK-suppressing lines, while enhanced in the ANK-overexpressing lines, showing that ANK is crucially involved in the infection process. Importantly, MP interacted with ANK at PD. Finally, simultaneous expression of MP and ANK markedly decreased the PD levels of callose, β-1,3-glucan, which is known to act as a molecular sphincter for PD. Thus, the MP-ANK interaction results in the downregulation of callose and increased cell-to-cell movement of the viral protein. These findings suggest that ANK represents a host cellular receptor exploited by MP to aid viral movement by gating PD through relaxation of their callose sphincters.

An Arabidopsis GPI-anchor plasmodesmal neck protein with callose binding activity and potential to regulate cell-to-cell trafficking

Plasmodesmata (Pds) traverse the cell wall to establish a symplastic continuum through most of the plant. Rapid and reversible deposition of callose in the cell wall surrounding the Pd apertures is proposed to provide a regulatory process through physical constriction of the symplastic channel. We identified members within a larger family of X8 domain-containing proteins that targeted to Pds. This subgroup of proteins contains signal sequences for a glycosylphosphatidylinositol linkage to the extracellular face of the plasma membrane. We focused our attention on three closely related members of this family, two of which specifically bind to 1,3-beta-glucans (callose) in vitro. We named this family of proteins Pd callose binding proteins (PDCBs). Yellow fluorescent protein-PDCB1 was found to localize to the neck region of Pds with potential to provide a structural anchor between the plasma membrane component of Pds and the cell wall. PDCB1, PDCB2, and PDCB3 had overlapping and widespread patterns of expression, but neither single nor combined insertional mutants for PDCB2 and PDCB3 showed any visible phenotype. However, increased expression of PDCB1 led to an increase in callose accumulation and a reduction of green fluorescent protein (GFP) movement in a GFP diffusion assay, identifying a potential association between PDCB-mediated callose deposition and plant cell-to-cell communication.

A plasmodesmata-associated beta-1,3-glucanase in Arabidopsis

Plasmodesmal conductivity is regulated in part by callose turnover, which is hypothesized to be determined by beta-1,3-glucan synthase versus glucanase activities. A proteomic analysis of an Arabidopsis thaliana plasmodesmata (Pd)-rich fraction identified a beta-1,3-glucanase as present in this fraction. The protein encoded by the putative plasmodesmal associated protein (ppap) gene, termed AtBG_ppap, had previously been found to be a post-translationally modified glycosylphosphatidylinositol (GPI) lipid-anchored protein. When fused to green fluorescent protein (GFP) and expressed in tobacco (Nicotiana tabacum) or Nicotiana benthamiana epidermal cells, this protein displays fluorescence patterns in the endoplasmic reticulum (ER) membrane system, along the cell periphery and in a punctate pattern that co-localizes with aniline blue-stained callose present around the Pd. Plasma membrane localization was verified by co-localization of AtBG_ppap:GFP together with a plasma membrane marker N-[3-triethylammoniumpropyl]-4-[p-diethylaminophenylhexatrienyl] pyridinium dibromide (FM4-64) in plasmolysed cells. In Arabidopsis T-DNA insertion mutants that do not transcribe AtBG_ppap, functional studies showed that GFP cell-to-cell movement between epidermal cells is reduced, and the conductivity coefficient of Pd is lower. Measurements of callose levels around Pd after wounding revealed that callose accumulation in the mutant plants was higher. Taken together, we suggest that AtBG_ppap is a Pd-associated membrane protein involved in plasmodesmal callose degradation, and functions in the gating of Pd.

Plant-derived antifungal proteins and peptides

Plants produce potent constitutive and induced antifungal compounds to complement the structural barriers to microbial infection. Approximately 250 000 – 500 000 plant species exist, but only a few of these have been investigated for antimicrobial activity. Nevertheless, a wide spectrum of compound classes have been purified and found to have antifungal properties. The commercial potential of effective plant-produced antifungal compounds remains largely unexplored. This review article presents examples of these compounds and discusses their properties.Key words: antifungal, peptides, phytopathogenic, plants, proteins.

Tobacco plants respond to the constitutive expression of the tospovirus movement protein NS(M) with a heat-reversible sealing of plasmodesmata that impairs development

Viral infection often results in typical symptoms, the biological background of which has remained elusive. We show that constitutive expression of the NSM viral movement protein (MP) of tomato spotted wilt virus in Nicotiana tabacum is sufficient to induce severe, infection-like symptoms, including pronounced deficiencies in root and shoot development. Leaves failed to expand and were arranged in a rosette due to the absence of internode elongation. Following the sink-source transition they accumulated excessive amounts of starch and developed fusing chlorotic patches in the mesophyll, resembling virus-induced chlorotic lesions. Eventually, the leaves became entirely white and brittle. With a combination of techniques, including photosystem II quantum-yield measurements, iontophoresis of symplasmic tracers, bombardment with pPVX.GFP and double immunolabelling it was shown that these symptoms correlated with the obstruction of NSM-targeted mesophyll plasmodesmata (Pd) in source tissues by depositions of 1,3-beta-D-glucan (GLU) or callose. Temperature-shift treatments (TST; 22-->32 degrees C), known to abolish chlorotic local lesions, also abolished the chlorotic 'superlesions' of transgenic plants and rescued plant development, by restoring the transport capacity of Pd through the action of 1,3-beta-D-glucanase (GLU-h) or callase. Return of these elongated, TST-recovered plants to 22 degrees C reintroduced superlesions and arrested shoot elongation, resulting in the formation of a rosette of clustered leaves at the shoot tip. Collectively, this indicates that the symptoms of NSM plants are self-inflicted and due to a basal defence response that counteracts prolonged interference of the MP with Pd functioning. This type of defence may also play a role in the formation of symptoms during viral infection.

Identification of an interactor of cadmium ion-induced glycine-rich protein involved in regulation of callose levels in plant vasculature

Cadmium-induced glycine-rich protein (cdiGRP) is a cell wall-associated factor that increases callose levels in plant vasculature. To better understand the cdiGRP/callose regulation system, we identified a tobacco protein, GrIP (cdiGRP-interacting protein, GrIP), that associates with cdiGRP and localizes at the plant cell wall. Constitutive overexpression of GrIP enhanced the accumulation of the cdiGRP protein and callose in vasculature-associated cells with or without treatment with cadmium ions. That GrIP gene expression was not affected by cadmium ions indicated that GrIP does not directly modulate the callose levels induced by the treatment. Instead, GrIP most likely functions by further elevating the accumulated amount of cdiGRP, the expression of which is up-regulated by the cadmium ions. Interestingly, the levels of cdiGRP mRNA were not affected by constitutive expression of GrIP, demonstrating that the enhancement in cdiGRP protein accumulation by GrIP overexpression occurs posttranslationally. Collectively, these observations suggest that GrIP interacts with cdiGRP and increases its level of accumulation; in turn, the elevated amounts of cdiGRP induce callose deposits in the plant cell walls. Therefore, GrIP and cdiGRP represent sequentially acting factors in a biochemical pathway that regulates callose accumulation in the plant vasculature.

Plasmodesmata at the crossroads between development, dormancy, and defense

Plants are frequently exposed to environmental stress and organisms that seek to benefit from their autotrophic nature. To cope with these challenges plants have developed stress-resistance mechanisms, which involve sensing, activation of signal transduction cascades, changes in gene expression, and physiological adjustment. Exposure to one kind of stress often leads to cross-tolerance, that is, resistance to different kinds of stresses. The search for a common underlying mechanism concentrates mostly on changes in cellular physiology and gene expression. We focus on the cross-protective measures that are taken at the level above the single cell. We argue that the controlled alterations in symplasmic permeability that underlie development also play a role in survival and defense strategies. In development, most of the alterations are transient and dynamic, whereas the more persistent alterations function predominantly in dormancy and defense and are under the control of two key enzymes: 1,3-β-D-glucan synthase and 1,3-β-D-glucanase. 1,3-β-D-Glucan synthase functions in the narrowing or closing of plasmodesmata, whereas 1,3-β-D-glucanase counteracts this process. We propose that the closing of symplasmic paths constitutes an unspecific but effective early measure in adaptation and defense, which is accompanied by specific strategies tailored to the various challenges plants face.Key words: cross-adaptation, dormancy sphincter, 1,3-β-D-glucanase, 1,3-β-D-glucan synthase, meristem, overwintering, plasmodesmata, virus movement.

Proteome analysis. Novel proteins identified at the peribacteroid membrane from Lotus japonicus root nodules

The peribacteroid membrane (PBM) forms the structural and functional interface between the legume plant and the rhizobia. The model legume Lotus japonicus was chosen to study the proteins present at the PBM by proteome analysis. PBM was purified from root nodules by an aqueous polymer two-phase system. Extracted proteins were subjected to a global trypsin digest. The peptides were separated by nanoscale liquid chromatography and analyzed by tandem mass spectrometry. Searching the nonredundant protein database and the green plant expressed sequence tag database using the tandem mass spectrometry data identified approximately 94 proteins, a number far exceeding the number of proteins reported for the PBM hitherto. In particular, a number of membrane proteins like transporters for sugars and sulfate; endomembrane-associated proteins such as GTP-binding proteins and vesicle receptors; and proteins involved in signaling, for example, receptor kinases, calmodulin, 14-3-3 proteins, and pathogen response-related proteins, including a so-called HIR protein, were detected. Several ATPases and aquaporins were present, indicating a more complex situation than previously thought. In addition, the unexpected presence of a number of proteins known to be located in other compartments was observed. Two characteristic protein complexes obtained from native gel electrophoresis of total PBM proteins were also analyzed. Together, the results identified specific proteins at the PBM involved in important physiological processes and localized proteins known from nodule-specific expressed sequence tag databases to the PBM.

TIP, a novel host factor linking callose degradation with the cell-to-cell movement of Potato virus X

The cell-to-cell movement of Potato virus X (PVX) requires four virus-encoded proteins, the triple gene block (TGB) proteins (TGB25K, TGB12K, and TGB8K) and the coat protein. TGB12K increases the plasmodesmal size exclusion limit (SEL) and may, therefore, interact directly with components of the cell wall or with plant proteins associated with bringing about this change. A yeast two-hybrid screen using TGB12K as bait identified three TGB12K-interacting proteins (TIP1, TIP2, and TIP3). All three TIPs interacted specifically with TGB12K but not with TGB25K or TGB8K. Similarly, all three TIPs interacted with beta-1,3-glucanase, the enzyme that may regulate plasmodesmal SEL through callose degradation. Sequence analyses revealed that the TIPs encode very similar proteins and that TIP1 corresponds to the tobacco ankyrin repeat-containing protein HBP1. A TIP1::GFP fusion protein localized to the cytoplasm. Coexpression of this fusion protein with TGB12K induced cellular changes manifested as deposits of additional cytoplasm at the cell periphery. This work reports a direct link between a viral movement protein required to increase plasmodesmal SEL and a host factor that has been implicated as a key regulator of plasmodesmal SEL. We propose that the TIPs are susceptibility factors that modulate the plasmodesmal SEL.

Prediction of glycosylphosphatidylinositol-anchored proteins in Arabidopsis. A genomic analysis

Glycosylphosphatidylinositol (GPI) anchoring of proteins provides a potential mechanism for targeting to the plant plasma membrane and cell wall. However, relatively few such proteins have been identified. Here, we develop a procedure for database analysis to identify GPI-anchored proteins (GAP) based on their possession of common features. In a comprehensive search of the annotated Arabidopsis genome, we identified 167 novel putative GAP in addition to the 43 previously described candidates. Many of these 210 proteins show similarity to characterized cell surface proteins. The predicted GAP include homologs of beta-1,3-glucanases (16), metallo- and aspartyl proteases (13), glycerophosphodiesterases (6), phytocyanins (25), multi-copper oxidases (2), extensins (6), plasma membrane receptors (19), and lipid-transfer-proteins (18). Classical arabinogalactan (AG) proteins (13), AG peptides (9), fasciclin-like proteins (20), COBRA and 10 homologs, and novel potential signaling peptides that we name GAPEPs (8) were also identified. A further 34 proteins of unknown function were predicted to be GPI anchored. A surprising finding was that over 40% of the proteins identified here have probable AG glycosylation modules, suggesting that AG glycosylation of cell surface proteins is widespread. This analysis shows that GPI anchoring is likely to be a major modification in plants that is used to target a specific subset of proteins to the cell surface for extracellular matrix remodeling and signaling.

Pepper gene encoding a basic beta-1,3-glucanase is differentially expressed in pepper tissues upon pathogen infection and ethephon or methyl jasmonate treatment

A basic beta-1,3-glucanase cDNA clone (CABGLU) was isolated from the cDNA library constructed from hypersensitive response lesions of pepper leaves infected with avirulent strain of Xanthomonas campestris pv. vesicatoria. The deduced polypeptide of CABGLU which contains a C-terminal extension N-glycosylated at a single site characterized as typical structure of class I beta-1,3-glucanase has a high level of identity with tobacco basic beta-1,3-glucanase (77.4%), but only a moderate level of identity with tomato acidic beta-1,3-glucanase (42.6%). Genomic DNA gel blot analysis indicates that the pepper genome contains one or two beta-1,3-glucanase copy genes. Transcripts of the CABGLU gene were more induced in incompatible interactions than in compatible interactions, when inoculated with X. campestris pv. vesicatoria or Phytophthora capsici. Accumulation of CABGLU mRNA was strongly induced in pepper leaves by both ethephon and methyl jasmonate. The CABGLU mRNA was constitutively expressed only in the roots of all the plant organs. These data indicate that the basic beta-1,3-glucanase gene may be induced by pathogen attack and abiotic stresses.

Riboflavin induces disease resistance in plants by activating a novel signal transduction pathway

ABSTRACT The role of riboflavin as an elicitor of systemic resistance and an activator of a novel signaling process in plants was demonstrated. Following treatment with riboflavin, Arabidopsis thaliana developed systemic resistance to Peronospora parasitica and Pseudomonas syringae pv. Tomato, and tobacco developed systemic resistance to Tobacco mosaic virus (TMV) and Alternaria alternata. Riboflavin, at concentrations necessary for resistance induction, did not cause cell death in plants or directly affect growth of the culturable pathogens. Riboflavin induced expression of pathogenesis-related (PR) genes in the plants, suggesting its ability to trigger a signal transduction pathway that leads to systemic resistance. Both the protein kinase inhibitor K252a and mutation in the NIM1/NPR1 gene which controls transcription of defense genes, impaired responsiveness to riboflavin. In contrast, riboflavin induced resistance and PR gene expression in NahG plants, which fail to accumulate salicylic acid (SA). Thus, riboflavin-induced resistance requires protein kinase signaling mechanisms and a functional NIM1/NPR1 gene, but not accumulation of SA. Riboflavin is an elicitor of systemic resistance, and it triggers resistance signal transduction in a distinct manner.

Movement of plant viruses is delayed in a beta-1,3-glucanase-deficient mutant showing a reduced plasmodesmatal size exclusion limit and enhanced callose deposition

Susceptibility to virus infection is decreased in a class I beta-1,3-glucanase (GLU I)-deficient mutant (TAG4.4) of tobacco generated by antisense transformation. TAG4.4 exhibited delayed intercellular trafficking via plasmodesmata of a tobamovirus (tobacco mosaic virus), of a potexvirus (recombinant potato virus X expressing GFP), and of the movement protein (MP) 3a of a cucumovirus (cucumber mosaic virus). Monitoring the cell-to-cell movement of dextrans and peptides by a novel biolistic method revealed that the plasmodesmatal size exclusion limit (SEL) of TAG4.4 was also reduced from 1.0 to 0.85 nm. Therefore, GLU I-deficiency has a broad effect on plasmodesmatal movement, which is not limited to a particular virus type. Deposition of callose, a substrate for beta-1,3-glucanases, was increased in TAG4.4 in response to 32 degrees C treatment, treatment with the fungal elicitor xylanase, and wounding, suggesting that GLU I has an important function in regulating callose metabolism. Callose turnover is thought to regulate plasmodesmatal SEL. We propose that GLU I induction in response to infection may help promote MP-driven virus spread by degrading callose.

Harpin induces disease resistance in Arabidopsis through the systemic acquired resistance pathway mediated by salicylic acid and the NIM1 gene

Harpin, the product of the hrpN gene of Erwinia amylovora, elicits the hypersensitive response and disease resistance in many plants. Harpin and known inducers of systemic acquired resistance (SAR) were tested on five genotypes of Arabidopsis thaliana to assess the role of SAR in harpin-induced resistance. In wild-type plants, harpin elicited systemic resistance to Peronospora parasitica and Pseudomonas syringae pv. tomato, accompanied by induction of the SAR genes PR-1 and PR-2. However, in experiments with transgenic Arabidopsis plants containing the nahG gene which prevents accumulation of salicylic acid (SA), harpin neither elicited resistance nor activated SAR gene expression. Harpin also failed to activate SAR when applied to nim1 (non-inducible immunity) mutants, which are defective in responding to SA and regulation of SAR. In contrast, mutants compromised in responsiveness to methyl jasmonate and ethylene developed the same resistance as did wild-type plants. Thus, harpin elicits disease resistance through the NIM1-mediated SAR signal transduction pathway in an SA-dependent fashion. The site of action of harpin in the SAR regulatory pathway is upstream of SA.