Characterization of vacuolar and extracellular beta(1,3)-glucanases of tobacco: Evidence for a strictly compartmentalized plant defense system

Defense signaling pathways in resistance to plant viruses: Crosstalk and finger pointing

Resistance to infection by plant viruses involves proteins encoded by plant resistance (R) genes, viz., nucleotide-binding leucine-rich repeats (NLRs), immune receptors. These sensor NLRs are activated either directly or indirectly by viral protein effectors, in effector-triggered immunity, leading to induction of defense signaling pathways, resulting in the synthesis of numerous downstream plant effector molecules that inhibit different stages of the infection cycle, as well as the induction of cell death responses mediated by helper NLRs. Early events in this process involve recognition of the activation of the R gene response by various chaperones and the transport of these complexes to the sites of subsequent events. These events include activation of several kinase cascade pathways, and the syntheses of two master transcriptional regulators, EDS1 and NPR1, as well as the phytohormones salicylic acid, jasmonic acid, and ethylene. The phytohormones, which transit from a primed, resting states to active states, regulate the remainder of the defense signaling pathways, both directly and by crosstalk with each other. This regulation results in the turnover of various suppressors of downstream events and the synthesis of various transcription factors that cooperate and/or compete to induce or suppress transcription of either other regulatory proteins, or plant effector molecules. This network of interactions results in the production of defense effectors acting alone or together with cell death in the infected region, with or without the further activation of non-specific, long-distance resistance. Here, we review the current state of knowledge regarding these processes and the components of the local responses, their interactions, regulation, and crosstalk.

Basic β-1,3-Glucanase from Drosera binata Exhibits Antifungal Potential in Transgenic Tobacco Plants

The basic β-1,3-glucanase of the carnivorous plant Drosera binata was tested as a purified protein, as well as under the control of a double CaMV35S promoter in transgenic tobacco for its capability to inhibit the growth of Trichoderma viride, Rhizoctonia solani, Alternaria solani, and Fusarium poae in an in-vitro assay. The purified protein inhibited tested phytopathogens but not the saprophytic fungus T. viride. Out of the analysed transgenic plants, lines 13, 16, 19, and 22 exhibited high DbGluc1 transcript abundance normalised to the actin transcript. Because of DbGluc1 transgene expression, lines 13 and 16 showed a 1.7-fold increase and lines 19 and 22 showed more than a 2-fold increase in total β-1,3-glucanase activity compared to the non-transgenic control. In accordance with the purified β-1,3-glucanase in-vitro antifungal assay, crude protein extracts of lines 19 and 22 significantly inhibited the growth of phytopathogens (14–34%). Further analyses revealed that the complementary action of transgenic β-1,3-glucanase and 20% higher activity of endogenous chitinase(s) in these lines were crucial for maximising the antifungal efficiency of crude protein extracts.

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.

Proteome Biomarkers in Xylem Reveal Pierce's Disease Tolerance in Grape

Pierce's disease (PD) is a significant threat to grape cultivation and industry. The disease caused by bacterium Xylella fastidiosa clogs xylem vessels resulting in wilting of the plant. PD-tolerant grape genotypes are believed to produce certain novel components in xylem tissue that help them to combat invading pathogens. Research has been aimed at characterizing the uniquely expressed xylem proteins by PD-tolerant genotypes. The objectives were to i) compare and characterize Vitis xylem proteins differentially expressed in PD-tolerant and PD-susceptible cultivars and, ii) identify xylem proteins uniquely expressed in PD-tolerant genotypes. A high throughput two-dimensional gel electrophoresis of xylem proteins from three Vitis species identified more than 200 proteins with pls 3.0 to 9.0 and molecular weights of 20 to 75 kDa. The differentially expressed proteins were then excised and analyzed with MALDI/TOF mass spectrometer. The mass spectra were collected and protein identification was performed against the Viridiplantae database using Matrix Science algorithm. Proteins were mapped to the universal protein resource to study gene ontology. Comparative analysis of the xylem proteome of three species indicated the highest number of proteins in muscadine grape, followed by Florida hybrid bunch and bunch grape. These proteins were all associated with disease resistance, energy metabolism, protein processing and degradation, biosynthesis, stress related functions, cell wall biogenesis, signal transduction, and ROS detoxification. Furthermore, β-1, 3-glucanase, 10-deacetyl baccatin III-10-O-acetyl transferase-like, COP9, and aspartyl protease nepenthesin precursor proteins were found to be uniquely expressed in PD-tolerant muscadine grape, while they are absent in PD-susceptible bunch grape. Data suggests that muscadine and Florida hybrid bunch grapes express novel proteins in xylem to overcome pathogen attack while bunch grape lacks this capability, making them susceptible to PD.

Hexanoic acid protects tomato plants against Botrytis cinerea by priming defence responses and reducing oxidative stress

Treatment with the resistance priming inducer hexanoic acid (Hx) protects tomato plants from Botrytis cinerea by activating defence responses. To investigate the molecular mechanisms underlying hexanoic acid-induced resistance (Hx-IR), we compared the expression profiles of three different conditions: Botrytis-infected plants (Inf), Hx-treated plants (Hx) and Hx-treated + infected plants (Hx+Inf). The microarray analysis at 24 h post-inoculation showed that Hx and Hx+Inf plants exhibited the differential expression and priming of many Botrytis-induced genes. Interestingly, we found that the activation by Hx of other genes was not altered by the fungus at this time point. These genes may be considered to be specific targets of the Hx priming effect and may help to elucidate its mechanisms of action. It is noteworthy that, in Hx and Hx+Inf plants, there was up-regulation of proteinase inhibitor genes, DNA-binding factors, enzymes involved in plant hormone signalling and synthesis, and, remarkably, the genes involved in oxidative stress. Given the relevance of the oxidative burst occurring in plant-pathogen interactions, the effect of Hx on this process was studied in depth. We showed by specific staining that reactive oxygen species (ROS) accumulation in Hx+Inf plants was reduced and more restricted around infection sites. In addition, these plants showed higher ratios of reduced to oxidized glutathione and ascorbate, and normal levels of antioxidant activities. The results obtained indicate that Hx protects tomato plants from B. cinerea by regulating and priming Botrytis-specific and non-specific genes, preventing the harmful effects of oxidative stress produced by infection.

Current overview of allergens of plant pathogenesis related protein families

Pathogenesis related (PR) proteins are one of the major sources of plant derived allergens. These proteins are induced by the plants as a defense response system in stress conditions like microbial and insect infections, wounding, exposure to harsh chemicals, and atmospheric conditions. However, some plant tissues that are more exposed to environmental conditions like UV irradiation and insect or fungal attacks express these proteins constitutively. These proteins are mostly resistant to proteases and most of them show considerable stability at low pH. Many of these plant pathogenesis related proteins are found to act as food allergens, latex allergens, and pollen allergens. Proteins having similar amino acid sequences among the members of PR proteins may be responsible for cross-reactivity among allergens from diverse plants. This review analyzes the different pathogenesis related protein families that have been reported as allergens. Proteins of these families have been characterized in regard to their biological functions, amino acid sequence, and cross-reactivity. The three-dimensional structures of some of these allergens have also been evaluated to elucidate the antigenic determinants of these molecules and to explain the cross-reactivity among the various allergens.

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.

An induced hypersensitive-like response limits expression of foreign peptides via a recombinant TMV-based vector in a susceptible tobacco

BACKGROUND

By using tobacco mosaic virus (TMV)-based vectors, foreign epitopes of the VP1 protein from food-and-month disease virus (FMDV) could be fused near to the C-terminus of the TMV coat protein (CP) and expressed at high levels in susceptible tobacco plants. Previously, we have shown that the recombinant TMV vaccines displaying FMDV VP1 epitopes could generate protection in guinea pigs and swine against the FMDV challenge. Recently, some recombinant TMV, such as TMVFN20 that contains an epitope FN20 from the FMDV VP1, were found to induce local necrotic lesions (LNL) on the inoculated leaves of a susceptible tobacco, Nicotiana tabacum Samsun nn. This hypersensitive-like response (HLR) blocked amplification of recombinant TMVFN20 in tobacco and limited the utility of recombinant TMV vaccines against FMDV.

METHODOLOGY/PRINCIPAL FINDINGS

Here we investigate the molecular mechanism of the HLR in the susceptible Samsun nn. Histochemical staining analyses show that these LNL are similar to those induced in a resistant tobacco Samsun NN inoculated with wild type (wt) TMV. The recombinant CP subunits are specifically related to the HLR. Interestingly, this HLR in Samsun nn (lacking the N/N'-gene) was able to be induced by the recombinant TMV at both 25°C and 33°C, whereas the hypersensitive response (HR) in the resistant tobacco plants induced by wt TMV through the N/N'-gene pathways only at a permissive temperature (below 30°C). Furthermore, we reported for the first time that some of defense response (DR)-related genes in tobacco were transcriptionally upregulated during HLR.

CONCLUSIONS

Unlike HR, HLR is induced in the susceptible tobacco through N/N'-gene independent pathways. Induction of the HLR is associated with the expression of the recombinant CP subunits and upregulation of the DR-related genes.

Identification and characterization of differentially expressed genes from Fagus sylvatica roots after infection with Phytophthora citricola

Phytophthora species are major plant pathogens infecting herbaceous and woody plants including European beech, the dominant or co-dominant tree in temperate Europe and an economically important species. For the analysis of the interaction of Phytophthora citricola with Fagus sylvatica suppression subtractive hybridization was used to isolate transcripts induced during infection and 1,149 sequences were generated. Hybridizations with driver and tester populations demonstrated differential expression in infected roots as compared to controls and verify efficient enrichment of these cDNAs during subtraction. Up regulation of selected genes during pathogenesis demonstrated using RT-PCR is consistent with these results. Pathogenesis-related proteins formed the largest group among functionally categorized transcripts. Cell wall proteins and protein kinases were also frequently found. Several transcription factors were isolated that are reactive to pathogens or wounding in other plants. The library contained a number of jasmonic acid, salicylic acid and ethylene responsive genes as well as genes directly involved in signaling pathways. Besides a mechanistic interconnection among signaling pathways another factor explaining the activation of different pathways could be the hemibiotrophic life style of Phytophthora triggering different signals in both stages.

Tobacco mosaic virus (TMV) replicase and movement protein function synergistically in facilitating TMV spread by lateral diffusion in the plasmodesmal desmotubule of Nicotiana benthamiana

Virus spread through plasmodesmata (Pd) is mediated by virus-encoded movement proteins (MPs) that modify Pd structure and function. The MP of Tobacco mosaic virus ((TMV)MP) is an endoplasmic reticulum (ER) integral membrane protein that binds viral RNA (vRNA), forming a vRNA:MP:ER complex. It has been hypothesized that (TMV)MP causes Pd to dilate, thus potentiating a cytoskeletal mediated sliding of the vRNA:MP:ER complex through Pd; in the absence of MP, by contrast, the ER cannot move through Pd. An alternate model proposes that cell-to-cell spread takes place by diffusion of the MP:vRNA complex in the ER membranes which traverse Pd. To test these models, we measured the effect of (TMV)MP and replicase expression on cell-to-cell spread of several green fluorescent protein-fused probes: a soluble cytoplasmic protein, two ER lumen proteins, and two ER membrane-bound proteins. Our data support the diffusion model in which a complex that includes ER-embedded MP, vRNA, and other components diffuses in the ER membrane within the Pd driven by the concentration gradient between an infected cell and adjacent noninfected cells. The data also suggest that the virus replicase and MP function together in altering Pd conductivity.

The role of plant defence proteins in fungal pathogenesis

SUMMARY It is becoming increasingly evident that a plant-pathogen interaction may be compared to an open warfare, whose major weapons are proteins synthesized by both organisms. These weapons were gradually developed in what must have been a multimillion-year evolutionary game of ping-pong. The outcome of each battle results in the establishment of resistance or pathogenesis. The plethora of resistance mechanisms exhibited by plants may be grouped into constitutive and inducible, and range from morphological to structural and chemical defences. Most of these mechanisms are defensive, exhibiting a passive role, but some are highly active against pathogens, using as major targets the fungal cell wall, the plasma membrane or intracellular targets. A considerable overlap exists between pathogenesis-related (PR) proteins and antifungal proteins. However, many of the now considered 17 families of PR proteins do not present any known role as antipathogen activity, whereas among the 13 classes of antifungal proteins, most are not PR proteins. Discovery of novel antifungal proteins and peptides continues at a rapid pace. In their long coevolution with plants, phytopathogens have evolved ways to avoid or circumvent the plant defence weaponry. These include protection of fungal structures from plant defence reactions, inhibition of elicitor-induced plant defence responses and suppression of plant defences. A detailed understanding of the molecular events that take place during a plant-pathogen interaction is an essential goal for disease control in the future.

Crystallization and identification of the glycosylated moieties of two isoforms of the main allergen Hev b 2 and preliminary X-ray analysis of two polymorphs of isoform II

Latex from Hevea brasiliensis contains several allergenic proteins that are involved in type I allergy. One of them is Hev b 2, which is a beta-1,3-glucanase enzyme that exists in different isoforms with variable glycosylation content. Two glucanase isoforms were isolated from trees of the GV-42 clone by gel filtration, affinity and ion-exchange chromatography. Isoform I had a carbohydrate content of about 20%, with N-linked N-acetyl-glucosamine, N-acetyl-galactosamine, fucose and galactose residues as the main sugars, while isoform II showed 6% carbohydrate content consisting of N-acetyl-glucosamine, fucose, mannose and xylose. Both isoforms were crystallized by the hanging-drop vapour-diffusion method. Isoform I crystals were grown using 0.2 M trisodium citrate dihydrate, 0.1 M Na HEPES pH 7.5 and 20%(v/v) 2-propanol, but these crystals were not appropriate for data collection. Isoform II crystals were obtained under two conditions and X-ray diffraction data were collected from both. In the first condition (0.2 M trisodium citrate, 0.1 M sodium cacodylate pH 6.5, 30% 2-propanol), crystals belonging to the tetragonal space group P4(1) with unit-cell parameters a = b = 150.17, c = 77.41 A were obtained. In the second condition [0.2 M ammonium acetate, 0.1 M trisodium citrate dihydrate pH 5.6, 30%(w/v) polyethylene glycol 4000] the isoform II crystals belonged to the monoclinic space group P2(1), with unit-cell parameters a = 85.08, b = 89.67, c = 101.80 A, beta = 113.6 degrees. Preliminary analysis suggests that there are four molecules of isoform II in both asymmetric units.

Isolation and differential expression of β-1,3-glucanase messenger RNAs, SrGLU3 and SrGLU4, following inoculation of Sesbania rostrata

We report here the isolation and characterisation of two new β-1,3-glucanase cDNAs, SrGLU3 and SrGLU4, from a tropical legume Sesbania rostrata Bremek. & Oberm., which form N₂-fixing nodules on the stem after infection by Azorhizobium caulinodans. SrGLU3 was characterised as being grouped in a branch with tobacco class I β-1,3-glucanases, where the isoforms were reported to be induced by either pathogen infection or ethylene treatment. SrGLU4 was characterised as separate from other classes, and we propose this new branch as a new class (Class VI). The SrGLU3 gene was constitutively expressed in normal stem nodules induced by the wild type strain of A. caulinodans (ORS571), and also even in immature stem nodules induced by a mutant (ORS571-C1), which could not form mature stem-nodules. In contrast, the transcript accumulation of SrGLU4 was hardly detectable in immature nodules inoculated by the ORS571-C1 mutant. We suggest that S. rostrata makes use of SrGLU4 to discriminate between symbionts and non-symbionts (mutants) in developing nodules. We propose the SrGLU4 gene as a new nodulin during nodulation.

Arabidopsis cell wall proteome defined using multidimensional protein identification technology

With the completion of the sequencing of the Arabidopsis genome and the recent advances in proteomic technology, the identification of proteins from highly complex mixtures is now possible. Rather than using gel electrophoresis and peptide mass fingerprinting, we have used multidimensional protein identification technology (MudPIT) to analyse the "tightly-bound" proteome for purified cell walls from Arabidopsis cell suspension cultures. Using bioinformatics for the prediction of signal peptides for targeting to the secretory pathway and for the absence of ER retention signal, 89 proteins were selected as potential extracellular proteins. Only 33% of these were identified in previous proteomic analyses of Arabidopsis cell walls. A functional classification revealed that a large proportion of the proteins were enzymes, notably carbohydrate active enzymes, peroxidases and proteases. Comparison of all the published proteomic analyses for the Arabidopsis cell wall identified 268 non-redundant genes encoding wall proteins. Sixty of these (22%) were derived from our analysis of tightly-bound wall proteins.

Functional analysis of a beta-1,3-glucanase gene (Tag1) with anther-specific RNA and protein accumulation using antisense RNA inhibition

A critical stage in pollen development is the dissolution of tetrads into free microspores. Tetrads are surrounded by a wall composed primarily of beta-1,3-glucan. At the completion of meiosis, tetrads are released into the anther locule after hydrolysis of the callose by a beta-1,3-glucanase complex. The cDNA corresponding to a beta-1,3-glucanase cloned from tobacco (Tag 1) represents a gene that is highly similar to other beta-1,3-glucanases and is expressed exclusively in anthers from the tetrad to free microspore stage of pollen development. Tag 1 protein was overexpressed in E. coli, accumulating in insoluble inclusion bodies. Polyclonal antibodies against Tag 1 recombinant protein identify a single 33 kD protein accumulating only in anthers at tetrad and free microspore stages where beta-1,3-glucanase activity is present. Transgenic plants expressing Tag 1 antisense RNA were produced. Although Tag 1 RNA and protein levels were greatly reduced, tetrad dissolution and pollen development were normal. These data indicate that under the conditions these tobacco plants were grown, wild type levels of Tag 1 protein are not necessary for male fertility.

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.

Characterization of a tissue-specific and developmentally regulated beta-1,3-glucanase gene in pea (Pisum sativum)

As part of a search for seed coat-specific expressed genes in Pisum sativum cv. Finale by PCR-based methods, we identified and isolated a cDNA encoding a beta- 1,3-glucanase, designated PsGNS2. The deduced peptide sequence of PsGNS2 is similar to a subfamily of beta-1,3-glucanases, which is characterized by the presence of a long amino acid extension at the C-terminal end compared to the other beta-1,3-glucanases. PsGNS2 is expressed in young flowers and in the seed coat and is weakly expressed in vegetative tissues (roots and stems) during seedling development. It is not inducible by environmental stress or in response to fungal infection. In developing pea flowers the transcript is detectable in all four whirls. In the seed coat the expression is temporally and spatially regulated. High abundance of the transcript became visible in the seed coat when the embryo reached the late heart stage and remained until the mid seed-filling stage. In situ hybridization data demonstrated that the expression of PsGNS2 is restricted to a strip of the inner parenchyma tissue of the seed coat, which is involved in temporary starch accumulation and embryo nutrition. This tissue showed also less callose deposits than the other ones. The 5' genomic region of PsGNS2 was isolated and promoter activity studies in transgenic Medicago truncatula showed a seed-specific expression. Highest activity of the promoter was found in the seed coat and in the endosperm part of the seed.

A calcium-binding protein with similarity to serum albumin localized to the ER-Golgi network and cell walls of spinach (Spinacia oleracea)

Using polyclonal antibodies raised against human serum albumin (HSA), a 70-kDa microsomal protein with an isoelectric point of approximately 6.5 was detected in spinach (Spinacia oleracea L.). The protein was purified by selective ammonium sulfate precipitation and anion exchange HPLC. The protein shared 100% identity with the first 15 amino acids at the NH2 terminus of HSA, including the X-X-H amino acid region, which was identified in HSA as being responsible for binding of copper, zinc, indole derivatives and calcium. Blue staining of the protein with the cationic carbocyanine dye 'Stains-all' and 45Ca overlay following SDS-PAGE also suggest that the 70-kDa plant protein binds calcium. The protein reacted positively with carbohydrate specific thymol stain, and the carbohydrates associated with the protein were identified by gas chromatography-mass spectrometry (GC-MS) as galactose and galacturonic acid. The 70-kDa plant protein was present in the detergent-poor phase following Triton X-114 extraction of the microsomal proteins. Cell fractionation using continuous sucrose gradients showed that the protein is present in membrane fractions with high activity of endoplasmic reticulum (ER) and Golgi marker enzymes. Using nitrocellulose tissue prints probed with anti-HSA antibodies, we demonstrated that the protein is present in the apoplastic space of petioles, suggesting that the protein is secreted to the apoplast of cortex cells in plants. Localization and binding properties suggest that the plant protein identified in the present study may participate in secretion processes, possibly involved with the transport of precursors required for cell-wall synthesis.

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.

Analysis and mapping of gene families encoding beta-1,3-glucanases of soybean

Oligonucleotide primers designed for conserved sequences from coding regions of beta-1,3-glucanase genes from different species were used to amplify related sequences from soybean [Glycine max (L.) Merr.]. Sequencing and cross-hybridization of amplification products indicated that at least 12 classes of beta-1,3-glucanase genes exist in the soybean. Members of classes mapped to 34 loci on five different linkage groups using an F(2) population of 56 individuals. beta-1,3-Glucanase genes are clustered onto regions of five linkage groups. Data suggest that more closely related genes are clustered together on one linkage group or on duplicated regions of linkage groups. Northern blot analyses performed on total RNA from root, stem, leaf, pod, flower bud, and hypocotyl using DNA probes for the different classes of beta-1,3-glucanase genes revealed that the mRNA levels of all classes were low in young leaves. SGlu2, SGlu4, SGlu7, and SGlu12 mRNA were highly accumulated in young roots and hypocotyls. SGlu7 mRNA also accumulated in pods and flower buds.

The Gas1 glycoprotein, a putative wall polymer cross-linker

The yeast cell wall, which for years has been regarded as a static cellular component, has been revealed to be dynamic in its structure and composition and complex in its enzymatic activity. The S. cerevisiae cell wall is composed of beta-1,3/beta-1,6-glucans, mannoproteins, and chitin, which are assembled into an extracellular matrix essential for maintenance of cell integrity. Gas1p, a glycoprotein anchored to the outer leaflet of the plasma membrane through a glycosylphosphatidylinositol, plays a key role in cell wall assembly. Loss of Gas1p leads to several morphogenetic defects and to a decrease in the amount of cross-links between the cell wall glucans. These defects in turn trigger a compensatory response that guarantees cell viability. Several Gas1p homologs have been isolated from Candida species and S. pombe. The Gas1p family also includes two plant proteins with endo-beta-1,3-glucanase activity. Sequence comparisons reveal that Gas1p family proteins have a modular organization of domains. The genetic and molecular analyses reviewed here suggest that Gas1p could play a role as a polymer cross-linker, presumably by catalyzing a transglycosylation reaction.

Arabidopsis thaliana contains a large family of germin-like proteins: characterization of cDNA and genomic sequences encoding 12 unique family members

We have identified 39 Arabidopsis thaliana ESTs encoding germin-like proteins (GLPs) and have completely sequenced 25 of these cDNAs. Our analysis demonstrates that the Arabidopsis genome contains a gene family with at least 12 GLP genes. Comparisons with other known germins and germin-like proteins indicate that these Arabidopsis GLP subfamilies are unique from wheat germin. All other known GLPs fall into one of these subfamilies. The translated GLPs show approximately 35% amino acid identity with other GLPs outside of their subfamily and significantly higher levels of identity within their respective subfamily. The 3' ends of many of the GLP cDNAs are heterogeneous and several sites of polyadenylation are used. Ten of the GLPs have N-terminal signal sequences and most appear to be exported from the cell. Structurally, the GLPs are predicted to have a high content of beta-pleated sheet. Seven conserved regions of beta-sheet were found in each of the GLP proteins along with alpha-helices located at both N- and C-termini. These same structural elements are also conserved in wheat germin. With one exception, all GLP family members contain at least one N-glycosylation site. All of these sites are conserved in an unstructured loop between beta-1 and beta-2. Genes for two of these GLPs were identified in genomic sequences previously deposited in the GenBank. The GLP3b gene is physically linked to the polyubiquitin 4 gene. The 3' end of the GLP3b mRNA is only 0.5 kb from the ubq4 start of transcription. Analysis of the GLP3b promoter shows the presence of a single putative auxin-response sequence located at -124 to -111 upstream from the 5' end of the GLP3b mRNA. The GLP9 gene was identified in an Arabidopsis contig from Chromosome 4.

Virus and fungal resistance: from laboratory to field

Virus and fungal resistance traits are important targets in the genetic engineering of agricultural and horticultural crops. We have engineered resistance against potato virus X in important commercial potato cultivars. Four years of field trials with resistant potatoes have demonstrated the commercial feasibility of improving potato cultivars by selectively adding new traits while preserving intrinsic properties. In our pursuit for a broad resistance against fungi we have focused on the exploitation of genes encoding antifungal proteins. We present results demonstrating the antifungal effect of some of these proteins in vitro , as well as the synergy between specific chitinases and β-1,3-glucanases. We also report high level resistance against Fusarium oxysporum in transgenic tomato plants expressing a specific combination of genes encoding these enzymes.

Seven members of the (1→3)-β-glucanase gene family in barley (Hordeum vulgare) are clustered on the long arm of chromosome 3 (3HL)

Members of the (1→3)-β-glucan glucanohydrolase (EC 3.2.1.39) gene family have been mapped on the barley genome using three doubled haploid populations and seven wheat-barley addition lines. Specific probes or polymerase chain reaction (PCR) primers were generated for the seven barley (1→3)-β-glucanase genes for which cDNA or genomic clones are currently available. The seven genes are all located on the long arm of chromosome 3 (3HL), and genes encoding isoenzymes GI, GII, GIII, GIV, GV and GVII (ABG2) are clustered in a region less than 20 cM in length. The region is flanked by the RFLP marker MWG2099 on the proximal side and the Barley Yellow Mosaic Virus (BYMV) resistance gene ym4 at the distal end. The gene encoding isoenzyme GVI lies approximately 50 cM outside this cluster, towards the centromere. With the exception of the gene encoding isoenzyme GIV, all of the (1→3)-β-glucanase genes are represented by single copies on the barley genome. The probe for the isoenzyme GIV gene hybridized with four DNA bands during Southern blot analysis, only one of which could be incorporated into the consensus linkage map.

Export of β-1,3-glucanase from mutant rice cells rechallenged and stressed with lysine plus threonine

Mutant rice cells (Oryza sativa L.) grown in liquid suspension cultures exported greater quantities of protein and β-glucanases than controls. These mutants were isolated from anther calli resistant to 1 mM lysine plus threonine (LT), regenerated and reestablished as cell suspension cultures from seeds. Cellular protein levels are genetically conditioned, and the levels of extracellular proteins and enzyme activities are inversely related to that of the cellular portions. The rechallenge of cells with 1 mM LT inhibited the expression of both β-1,3-glucanases and β-1,4-glucosidases but had no significant effect upon the levels of chitinase activity. Mutant cells were more sensitive than controls to stress caused by exogenous LT. In general, under exogenous LT stress the mutant/control ratio for extracellular glucanases increased as the assay conditions were changed from a basic to an acidic pH. The specific activity of βglucanases was highest in media and lowest in cells. Both the mutant and control cells exported β-glucanases into the suspension medium, but the level of activity in media was greater in that in which the mutant was suspended. The export was probably modulated by the internal protein levels which were highest in mutant cells without LT. Seedlings from mutants with enhanced lysine also had enhanced acidic β-glucanase activity.

beta-1,3-Glucanase is highly-expressed in laticifers of Hevea brasiliensis

Clones encoding beta-1,3-glucanase have been isolated from a Hevea cDNA library prepared from the latex of Hevea brasiliensis using a probe Nicotiana plumbaginifolia cDNA encoding beta-1,3-glucanase, gnl. Nucleotide sequence analysis showed that a 1.2 kb Hevea cDNA encoding a basic beta-1,3-glucanase showed 68% nucleotide homology to gnl cDNA. Northern blot analysis using the Hevea cDNA as probe detected a mRNA of 1.3 kb which was expressed at higher levels in latex than in leaf. In situ hybridization analysis using petiole sections from Hevea localized the beta-1,3-glucanase mRNA to the laticifer cells. Genomic Southern analysis suggested the presence of a low-copy gene family encoding beta-1,3-glucanases in H. brasiliensis.

Dark induction and subcellular localization of the pathogenesis-related PRB-1b protein

The PRB-1b gene codes for a basic-type pathogenesis-related protein of the PR-1 family of tobacco. PRB-1b mRNA accumulation is induced in response to biotic and abiotic elicitors, such as TMV, ethylene, salicylic acid, alpha-amino butyric acid and darkness. In order to determine the location of elements that control dark-regulated PRB-1b gene expression, we tested promoter, transcribed regions and 3'-downstream regions of the gene for their ability to respond to dark induction in transgenic tobacco plants. An ethylene-inducible promoter region of 863 bp was not able to confer dark induction to a beta-glucuronidase reporter gene, while a construct containing the transcribed region of the gene and 3'-downstream sequences, driven by the cauliflower mosaic virus 35S promoter, was correctly dark-regulated. The results indicate that dark-induction of the PRB-1b gene can be controlled by 3'-downstream elements at the transcriptional level or by transcribed sequences at the post-transcriptional level. A circadian clock regulation of the PRB-1b gene was excluded, as fluctuations of PRB-1b transcript levels were not observed in plants placed in constant light or darkness. Subcellular localization of the PRB-1b protein was also determined, in tobacco protoplasts preparations and in cell cultures. The PRB-1b polypeptide was predominantly detected in protoplast vacuoles and was not secreted to the media in cell cultures. These results support an intracellular localization for the PRB-1b protein, as reported for other basic-type components of the pathogenesis-related proteins family.

Heterologous expression of cDNAs encoding barley (Hordeum vulgare) (1-->3)-beta-glucanase isoenzyme GV

Two cDNAs have been isolated from libraries generated from poly(A)+RNA of young barley roots and leaves, using a cDNA encoding barley (1-->3)-beta-glucanase isoenzyme GII as a probe. Nucleotide sequence analyses and ribonuclease protection assays show that the two cDNAs differ only in the length of their 3'-untranslated regions; the corresponding mRNAs are likely to originate from a single gene by tissue-specific processing at separate polyadenylation sites. When the coding region of the cDNA is expressed in E. coli, the resultant protein catalyses the hydrolysis of (1-->3)-beta-glucan with an action pattern characteristic of a (1-->3)-beta-glucan endohydrolase (EC 3.2.1.39). The enzyme has been designated isoenzyme GV of the barley (1-->3)-beta-glucanase family).

Detection of exo-beta-1,3-glucanase activity in polyacrylamide gels after electrophoresis under denaturing or nondenaturing conditions

A method for the visualization of exo-beta-1,3-glucanase activity in polyacrylamide gels is presented. The procedure consists of the enzyme reaction in the gel with the substrate alpha-naphthylglucopyranoside, and a subsequent staining of the obtained alpha-naphthol with dyes Fast Red B, or Fast Blue BB, respectively. A mixture of exoglucanases produced by the fungus Polyporus squamosus was used for the optimization of the method. The procedure is applicable for the standard Laemmli discontinuous electrophoresis system, even in the presence of sodium dodecyl sulfate, as well as for electrophoresis in linear gradients of the polyacrylamide concentration. The staining method was used for the analysis of exoglucanases secreted by several yeast genera. All yeasts tested produced two types of exoglucanases, a high molecular mass species heterogeneous in size, and one or two smaller homogeneous enzymes.

Triticum aestivum puroindolines, two basic cystine-rich seed proteins: cDNA sequence analysis and developmental gene expression

From a mid-maturation seed cDNA library we have isolated cDNA clones encoding two Triticum aestivum puroindolines. Puroindoline-a and puroindoline-b, which are 55% similar, are basic, cystine-rich and tryptophan-rich proteins. Puroindolines are synthesized as preproproteins which include N- and C-terminal propeptides which could be involved in their vacuolar localization. The mature proteins have a molecular mass of 13 kDa and a calculated isoelectric point greater than 10. A notable feature of the primary structure of puroindolines is the presence of a tryptophan-rich domain which also contains basic residues. A similar tryptophan-rich domain was found within an oat seed protein and a mammalian antimicrobial peptide. The ten cysteine residues of puroindolines are organized in a cysteine skeleton which shows similarity to the cysteine skeleton of other wheat seed cystine-rich proteins. Northern blot analysis showed that puroindoline genes are specifically expressed in T. aestivum developing seeds. No puroindoline transcripts as well as no related genes were detected in Triticum durum. The identity of puroindolines to wheat starch-granule associated proteins is discussed as well as the potential role of puroindolines in the plant defence mechanism.

Cloning and characterization of Tag 1, a tobacco anther beta-1,3-glucanase expressed during tetrad dissolution

A critical stage in pollen development is the dissolution of the four products of meiosis, the tetrads, into free microspores. The tetrads are surrounded by a thick callose wall composed of beta-1,3-glucan. At the completion of meiosis, the tetrads are released into the anther locule after hydrolysis of the callose by a beta-1,3-glucanase. Using the polymerase chain reaction, we have amplified and subsequently cloned a cDNA corresponding to a beta-1,3-glucanase, tobacco (Nicotiana tabacum cv. Samsun) anther glucanase (Tag 1), which is expressed exclusively in anthers from meiosis to the free microspore stage of pollen development. The identity of the clone was determined by DNA and deduced protein sequence similarity to other known beta-1,3-glucanases. Several regions strictly conserved among four classes of glucanases are also conserved in the Tag 1 protein. Tag 1 represents a novel class of beta-1,3-glucanase based on phylogenetic analysis and RNA expression pattern. Tag 1 RNA was detected in situ only in the tapetum, with maximal expression just prior to tetrad dissolution. Due to its expression pattern and sequence similarity to other beta-1,3-glucanases, we believe Tag 1 may be involved in tetrad dissolution.

Genes encoding acidic and basic class III beta-1,3-glucanases are expressed in tomato plants upon viroid infection

beta-1,3-glucanases are hydrolytic enzymes considered to constitute part of the general array of defense genes induced by pathogen infection in higher plants. We have isolated and characterized two complementary DNA clones, corresponding to new beta-1,3-glucanases from tomato plants (Lycopersicon esculentum) which are expressed upon challenge with citrus exocortis viroid. Amino acid sequence comparison revealed that they are most similar to beta-1,3-glucanases from tobacco, particularly to PR-Q', the unique component of the class III beta-1,3-glucanase. The deduced amino acid sequences of the two tomato beta-1,3-glucanases indicate that, although being highly similar in amino acid sequence, they have different isoelectric points: pI 10.5 for the basic isoform (Tom PR-Q'b) and pI 5.2 for the acidic one (Tom PR-Q'a). The expression of these two beta-1,3-glucanase messenger RNAs (mRNAs) in response to viroid infection and ethephon treatments was examined. mRNAs for these two isoforms are coordinately expressed and induced similarly to mRNAs for other PR proteins, indicating that they are part of a general and coordinate mechanism of response of tomato plants susceptible to viroid infection.

Physiological compensation in antisense transformants: specific induction of an "ersatz" glucan endo-1,3-beta-glucosidase in plants infected with necrotizing viruses

Plant class I glucan endo-1,3-beta-glucosidases (beta-1,3-glucanase; 1,3-beta-D-glucan glucanohydrolase, EC 3.2.1.39) have been implicated in development and defense against pathogen attack. Nevertheless, beta-1,3-glucanase deficiencies generated by antisense transformation of Nicotiana sylvestris and tobacco have little biological effect. We report here that another beta-1,3-glucanase activity is induced in these deficient mutants after infection with necrotizing viruses. Induction of class I beta-1,3-glucanase was markedly inhibited in leaves of N. sylvestris and tobacco antisense transformants infected with tobacco necrosis virus and tobacco mosaic virus, respectively. A serologically distinct beta-1,3-glucanase activity was present in the infected antisense transformants but was absent in both healthy and infected control plants and in antisense transformants treated with the stress hormone ethylene. Immunoblot analyses, localization studies, and measurements of antibody specificity indicate that this compensatory beta-1,3-glucanase activity is an intracellular enzyme different from known tobacco beta-1,3-glucanases. Therefore, plants can compensate for a deficiency in enzyme activity by producing a functionally equivalent replacement--i.e., "ersatz"--protein or proteins. The fact that compensation for beta-1,3-glucanase activity occurs in response to infection argues strongly for an important role of these enzymes in pathogenesis.

Structure and expression of a barley acidic beta-glucanase gene

A barley acidic beta-1,3-glucanase gene was recovered from a barley genomic library by homology with a partial cDNA of barley basic beta-1,3-glucanase isoenzyme GII. The gene, Abg2, is homologous to the PR2 family of pathogenesis-related beta-1,3-glucanase genes. The ABG2 protein has 81% amino acid similarity to barley basic beta-1,3-glucanase GII. The ABG2 protein is encoded as a preprotein of 336 amino acids including a 28 amino acid signal peptide. A 299 bp intron occurs within codon 25. The mature ABG2 protein has a predicted mass of 32,642 Da and a calculated isoelectric point of 4.9. The second exon of the Abg2 gene shows a strong preference for G + C in the third position of degenerate codons. The Abg2 gene was functionally expressed in Escherichia coli. Abg2 mRNA is constitutively expressed in barley root; leaf expression of Abg2 mRNA is induced by mercuric chloride and infection by Erysiphe graminis f. sp. hordei. Southern blot analysis indicates that Abg2 is a member of a small gene family.

A wound-induced promoter driving npt-II expression limited to dedifferentiated cells at wound sites is sufficient to allow selection of transgenic shoots

There is much data to indicate that only a small number of cells in plant explants are competent for stable transformation by Agrobacterium. Circumstantial evidence suggests that certain cells reentering cell division at wound sites are competent for transformation by Agrobacterium. We have discovered a member of the intracellular PR gene family from asparagus (AoPR1) which is strongly expressed upon wounding and during the reactivation of the cell cycle in cultured asparagus cells, but which shows very little expression in intact plant tissues. The promoter from the AoPR1 gene was fused to an intron-containing GUS reporter gene and shown to be more strongly expressed than the commonly used CaMV 35S constitutive promoter in target cells for plant transformation. A transcriptional fusion of the AoPR1 promoter with an NPT-II gene was found to be a very efficient marker for the selection of transgenic tobacco callus. Expression of the AoPR1-NPT-II gene allowed efficient shoot formation on transgenic callus and efficient adventitious root formation on transgenic shoots. These latter observations provided firm evidence that transformation selection marker gene expression is most crucial at the early stages of the transformation process, during the establishment of transformed micro-calli.

Extracellular targeting of the vacuolar tobacco proteins AP24, chitinase and beta-1,3-glucanase in transgenic plants

The Nicotiana tabacum ap24 gene encoding a protein with antifungal activity toward Phytophthora infestans has been characterized. Analysis of cDNA clones revealed that at least three ap24-like genes are induced in tobacco upon infection with tobacco mosaic virus. Amino acid sequencing of the purified protein showed that AP24 is synthesized as a preproprotein from which an amino-terminal signal peptide and a carboxyl-terminal propeptide (CTPP) are cleaved off during post-translational processing. The functional role of the CTPP was investigated by expressing chimeric genes encoding either wild-type AP24 or a mutant protein lacking the CTPP. Plants expressing the wild-type construct resulted in proteins properly sorted to the vacuole. In contrast, the proteins produced in plants expressing the mutant construct were secreted extracellularly, indicating that the CTPP is necessary for targeting of AP24 to the vacuoles. Similar results were obtained for vacuolar chitinases and beta-1,3-glucanases of tobacco. The extracellularly targeted mutant proteins were shown to have retained their biological activity. Together, these results suggest that within all vacuolar pathogenesis-related proteins the targeting information resides in a short carboxyl-terminal propeptide which is removed during or after transport to the plant vacuole.

Purification and properties of three (1-->3)-beta-D-glucanase isoenzymes from young leaves of barley (Hordeum vulgare)

Three (1-->3)-beta-D-glucan glucanohydrolase (EC 3.2.1.39) isoenzymes GI, GII and GIII were purified from young leaves of barley (Hordeum vulgare) using (NH4)2SO4 fractional precipitation, ion-exchange chromatography, chromatofocusing and gel-filtration chromatography. The three (1-->3)-beta-D-glucanases are monomeric proteins of apparent M(r)32,000 with pI values in the range 8.8-10.3. N-terminal amino-acid-sequence analyses confirmed that the three isoenzymes represent the products of separate genes. Isoenzymes GI and GII are less stable at elevated temperatures and are active over a narrower pH range than is isoenzyme GIII, which is a glycoprotein containing 20-30 mol of hexose equivalents/mol of enzyme. The preferred substrate for the enzymes is laminarin from the brown alga Laminaria digitata, an essentially linear (1-->3)-beta-D-glucan with a low degree of glucosyl substitution at 0-6 and a degree of polymerization of approx. 25. The three enzymes are classified as endohydrolases, because they yield (1-->3)-beta-D-oligoglucosides with degrees of polymerization of 3-8 in the initial stages of hydrolysis of laminarin. Kinetic analyses indicate apparent Km values in the range 172-208 microM, kcat. constants of 36-155 s-1 and pH optima of 4.8. Substrate specificity studies show that the three isoenzymes hydrolyse substituted (1-->3)-beta-D-glucans with degrees of polymerization of 25-31 and various high-M(r), substituted and side-branched fungal (1-->3;1-->6)-beta-D-glucans. However, the isoenzymes differ in their rates of hydrolysis of a (1-->3;1-->6)-beta-D-glucan from baker's yeast and their specific activities against laminarin vary significantly. The enzymes do not hydrolyse (1-->3;1-->4)-beta-D-glucans, (1-->6)-beta-D-glucan, CM-cellulose, insoluble (1-->3)-beta-D-glucans or aryl beta-D-glycosides.

A 61 bp enhancer element of the tobacco beta-1,3-glucanase B gene interacts with one or more regulated nuclear proteins

We show that a 61 bp fragment derived from the promoter region of the tobacco class I beta-1,3-glucanase GLB gene enhances transcription in Nicotiana plumbaginifolia protoplasts independent of orientation relative to the start of transcription. This fragment leads to a cooperative stimulation of transcription when combined with the cauliflower mosaic virus 35S as-1 enhancer element. The GLB enhancer contains two copies of the sequence AGCCGCC, which is conserved in several genes showing expression patterns similar to the GLB gene, as well as a sequence identical at 6 of 7 bp. Point mutations in these three sequences eliminate the enhancer activity of the 61 bp fragment. Nuclear extracts prepared from leaves of tobacco plants contain one or more putative transcription factors that interact specifically with the GLB enhancer. This factor was much less abundant in nuclear extracts prepared from upper leaves of untreated tobacco plants than in nuclear extracts prepared from upper leaves of ethylene-treated plants or from lower leaves. Since beta-1,3-glucanase genes are expressed at very low levels in upper leaves of tobacco plants, at higher levels in lower leaves, and are induced in all leaves after treatment of plants with the stress hormone ethylene, we conclude that the enhancer element interacts with one or more transcription factors whose binding activity is correlated with gene expression in vivo.

Purification, characterization and differential hormonal regulation of a beta-1,3-glucanase and two chitinases from chickpea (Cicer arietinum L.)

Chickpea (Cicer arietinum L.) cell-suspension cultures were used to isolate one beta-1,3-glucanase (EC 3.2.1.29) and two chitinases (EC 3.2.1.14). The beta-1,3-glucanase (M(r) = 36 kDa) and one of the chitinases (M(r) = 32 kDa) belong to class I hydrolases with basic isoelectric points (10.5 and 8.5, respectively) and were located intracellularly. The basic chitinase (BC) was also found in the culture medium. The second chitinase (M(r) = 28 kDa), with an acidic isoelectric point of 5.7, showed homology to N-terminal sequences of class III chitinases and represented the main protein accumulating in the culture medium. Polyclonal antibodies raised against the basic beta-1,3-glucanase (BG) and the acidic chitinase (AC) were shown to be monospecific. The anti-AC antiserum failed to recognize the BC on immune blots, confirming the structural diversity between class I and class III chitinases. Neither chitinase exhibited lysozyme activity. All hydrolases were endo in action on appropriate substrates. The BC inhibited the hyphal growth of several test fungi, whereas the AC failed to show any inhibitory activity. Expression of BG activity appeared to be regulated by auxin in the cell culture and in the intact plant. In contrast, the expression of neither chitinase was apparently influenced by auxin, indicating a differential hormonal regulation of beta-1,3-glucanase and chitinase activities in chickpea. After elicitation of cell cultures or infection of chickpea plants with Ascochyta rabiei, both system were found to have hydrolase patterns which were qualitatively and quantitatively comparable.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular transport and processing of a tobacco vacuolar β-1,3-glucanase

The class I β-1,3-glucanases are basic, vacuolar enzymes implicated in the defense of plants against pathogen infection. The tobacco (Nicotiana tabacum L.) enzyme is synthesized as a preproprotein with an N-terminal signal peptide for targeting to the lumen of the endoplasmic reticulum and an N-glycosylated C-terminal extension which is lost during protein maturation. The transport and processing of β-1,3-glucanase in cellsuspension cultures of the tobacco cultivar Havana 425 was investigated by pulse-chase labelling and cell fractionation. We verified that mature β-1,3-glucanase is localized in the vacuole of the suspension-cultured cells. Comparison of the time course of processing in homogenates, the soluble fraction, and membrane fractions indicates that proglucanase is transported from the endoplasmic reticulum via the Golgi compartment to the vacuole. Processing to the mature form occurs in the vacuole. Treatment of cells with tunicamycin, which inhibits N-glycosylation, and digestion of the (35)S-labelled processing intermediates with endoglycosidase H indicate that β-1,3-glucanase has a single N-glycan attached to the C-terminal extension. Glycosylation is not required for proteolytic processing or correct targeting to the vacuole.

The function of vacuolar beta-1,3-glucanase investigated by antisense transformation. Susceptibility of transgenic Nicotiana sylvestris plants to Cercospora nicotianae infection

Vacuolar class I beta-1,3-glucanases (EC 3.2.1.39) are believed to be important in the induced defense reaction of plants to fungal infection. We used antisense transformation to test this hypothesis and to identify other possible physiological functions of this enzyme. Nicotiana sylvestris plants were transformed with antisense constructions containing the region from position 27 to 608 of the coding sequence of the basic, vacuolar beta-1,3-glucanase gene GLA of tobacco regulated by cauliflower mosaic virus 35S RNA expression signals. Plants homozygous for this transgene showed a marked, ca. 20-fold reduction in the constitutive expression of class I beta-1,3-glucanase antigen in their leaves. RNA blot analysis indicated that the antisense plants expressed low levels of the sense transcript of the host beta-1,3-glucanase gene and the antisense transcript of the transgene. Immune blot analysis of plant extracts indicated that only expression of the N. sylvestris homologue of class I tobacco beta-1,3-glucanase and not the acidic, class II isoforms of the enzyme was blocked in the antisense plants. Class I isoforms of beta-1,3-glucanase and chitinase were coordinately induced in leaves of untransformed and empty-vector-transformed N. sylvestris plants treated with ethylene or infected with the fungal leaf pathogen Cercospora nicotianae. In antisense plants, chitinase but not beta-1,3-glucanase was induced under these conditions indicating that antisense transformation effectively blocks constitutive as well as induced expression of class I beta-1,3-glucanase. Under greenhouse conditions, antisense plants developed normally and were fertile. The plants did not exhibit increased susceptibility to C. nicotianae infection. These results suggest that expression of the beta-1,3-glucanase isoform blocked by antisense transformation is not necessary for 'housekeeping' functions of N. sylvestris nor defense against the fungal pathogen tested.

Suppression of beta-1,3-glucanase transgene expression in homozygous plants

A chimeric construct containing the Nicotiana plumbaginifolia beta-1,3-glucanase gn1 gene was introduced into Nicotiana tabacum SR1 to produce high levels of the enzyme constitutively. We determined that the GN1 protein represents a basic beta-1,3-glucanase isoform which accumulates into the vacuoles of the transgenic plants. Analysis of the progeny of the transgenic plant with the highest levels of gn1 expression revealed an unexpected phenomenon of gene suppression. Plants hemizygous for the T-DNA locus contained high levels of gn1 mRNA and exhibited a 14-fold higher beta-1,3-glucanase activity than untransformed plants. However, the expression of gn1 was completely suppressed in the homozygous plants: no corresponding mRNA or protein could be detected. This suppression mechanism occurs at a post-transcriptional level and is under developmental control. In addition, by generating haploid plants we found that this silencing phenomenon is not dependent on allelic interaction between T-DNA copies present at the same locus of homologous chromosomes, but rather is correlated with the transgene dose in the plant genome. We postulate that high doses of GN1 protein relative to the level(s) of other still unknown plant products could trigger the cellular processes directed to suppress gn1 expression.

Sorting of proteins to the vacuoles of plant cells

The secretory system of plant cells sorts a large number of soluble proteins that either are secreted or accumulate in vacuoles. Secretion is a bulk-flow process that requires no information beyond the presence of a signal peptide necessary to enter the endoplasmic reticulum. Many vacuolar proteins are glycoproteins and the glycans are often modified as the proteins pass through the Golgi complex. Vacuolar targeting information is not contained in glycans as it is in animal cells; rather, targeting information is in polypeptide domains as it is in yeast cells. Several such domains have now been identified, but these show little or no amino acid sequence homology. We discuss the possibilities that targeting of protein to plant vacuoles may involve receptors as well as aggregation of protein at low pH.