Intracellular Thionins of Barley

Plant peptides - redefining an area of ribosomally synthesized and post-translationally modified peptides

Covering 1965 to February 2024Plants are prolific peptide chemists and are known to make thousands of different peptidic molecules. These peptides vary dramatically in their size, chemistry, and bioactivity. Despite their differences, all plant peptides to date are biosynthesized as ribosomally synthesized and post-translationally modified peptides (RiPPs). Decades of research in plant RiPP biosynthesis have extended the definition and scope of RiPPs from microbial sources, establishing paradigms and discovering new families of biosynthetic enzymes. The discovery and elucidation of plant peptide pathways is challenging due to repurposing and evolution of housekeeping genes as both precursor peptides and biosynthetic enzymes and due to the low rates of gene clustering in plants. In this review, we highlight the chemistry, biosynthesis, and function of the known RiPP classes from plants and recommend a nomenclature for the recent addition of BURP-domain-derived RiPPs termed burpitides. Burpitides are an emerging family of cyclic plant RiPPs characterized by macrocyclic crosslinks between tyrosine or tryptophan side chains and other amino acid side chains or their peptide backbone that are formed by copper-dependent BURP-domain-containing proteins termed burpitide cyclases. Finally, we review the discovery of plant RiPPs through bioactivity-guided, structure-guided, and gene-guided approaches.

Spatiotemporal transcriptomic atlas of rhizome formation in Oryza longistaminata

Rhizomes are modified stems that grow underground and produce new individuals genetically identical to the mother plant. Recently, a breakthrough has been made in efforts to convert annual grains into perennial ones by utilizing wild rhizomatous species as donors, yet the developmental biology of this organ is rarely studied. Oryza longistaminata, a wild rice species featuring strong rhizomes, provides a valuable model for exploration of rhizome development. Here, we first assembled a double-haplotype genome of O. longistaminata, which displays a 48-fold improvement in contiguity compared to the previously published assembly. Furthermore, spatiotemporal transcriptomics was performed to obtain the expression profiles of different tissues in O. longistaminata rhizomes and tillers. Two spatially reciprocal cell clusters, the vascular bundle 2 cluster and the parenchyma 2 cluster, were determined to be the primary distinctions between the rhizomes and tillers. We also captured meristem initiation cells in the sunken area of parenchyma located at the base of internodes, which is the starting point for rhizome initiation. Trajectory analysis further indicated that the rhizome is regenerated through de novo generation. Collectively, these analyses revealed a spatiotemporal transcriptional transition underlying the rhizome initiation, providing a valuable resource for future perennial crop breeding.

Defence gene expression and phloem quality contribute to mesophyll and phloem resistance to aphids in wild barley

Aphids, including the bird cherry-oat aphid (Rhopalosiphum padi), are significant agricultural pests. The wild relative of barley, Hordeum spontaneum 5 (Hsp5), has been described to be partially resistant to R. padi, with this resistance proposed to involve higher thionin and lipoxygenase gene expression. However, the specificity of this resistance to aphids and its underlying mechanistic processes are unknown. In this study, we assessed the specificity of Hsp5 resistance to aphids and analysed differences in aphid probing and feeding behaviour on Hsp5 and a susceptible barley cultivar (Concerto). We found that partial resistance in Hsp5 to R. padi extends to two other aphid pests of grasses. Using the electrical penetration graph technique, we show that partial resistance is mediated by phloem- and mesophyll-based resistance factors that limit aphid phloem ingestion. To gain insight into plant traits responsible for partial resistance, we compared non-glandular trichome density, defence gene expression, and phloem composition of Hsp5 with those of the susceptible barley cultivar Concerto. We show that Hsp5 partial resistance involves elevated basal expression of thionin and phytohormone signalling genes, and a reduction in phloem quality. This study highlights plant traits that may contribute to broad-spectrum partial resistance to aphids in barley.

Primary Structure Analysis of Antifungal Peptides from Cultivated and Wild Cereals

Cereal-derived bioactive peptides with antimicrobial activity have been poorly explored compared to those from dicotyledonous plants. Furthermore, there are a few reports addressing the structural differences between antimicrobial peptides (AMPs) from cultivated and wild cereals, which may shed light on significant varieties in the range and level of their antimicrobial activity. We performed a primary structure analysis of some antimicrobial peptides from wild and cultivated cereals to find out the features that are associated with the much higher antimicrobial resistance characteristic of wild plants. In this review, we identified and analyzed the main parameters determining significant antifungal activity. They relate to a high variability level in the sequences of C-terminal fragments and a high content of hydrophobic amino acid residues in the biologically active defensins in wild cereals, in contrast to AMPs from cultivated forms that usually exhibit weak, if any, activity. We analyzed the similarity of various physicochemical parameters between thionins and defensins. The presence of a high divergence on a fixed part of any polypeptide that is close to defensins could be a determining factor. For all of the currently known hevein-like peptides of cereals, we can say that the determining factor in this regard is the structure of the chitin-binding domain, and in particular, amino acid residues that are not directly involved in intermolecular interaction with chitin. The analysis of amino acid sequences of alpha-hairpinins (hairpin-like peptides) demonstrated much higher antifungal activity and more specificity of the peptides from wild cereals compared with those from wheat and corn, which may be associated with the presence of a mini cluster of positively charged amino acid residues. In addition, at least one hydrophobic residue may be responsible for binding to the components of fungal cell membranes.

Overexpression and Down-Regulation of Barley Lipoxygenase LOX2.2 Affects Jasmonate-Regulated Genes and Aphid Fecundity

Aphids are pests on many crops and depend on plant phloem sap as their food source. In an attempt to find factors improving plant resistance against aphids, we studied the effects of overexpression and down-regulation of the lipoxygenase gene LOX2.2 in barley (Hordeum vulgare L.) on the performance of two aphid species. A specialist, bird cherry-oat aphid (Rhopalosiphum padi L.) and a generalist, green peach aphid (Myzus persicae Sulzer) were studied. LOX2.2 overexpressing lines showed up-regulation of some other jasmonic acid (JA)-regulated genes, and antisense lines showed down-regulation of such genes. Overexpression or suppression of LOX2.2 did not affect aphid settling or the life span on the plants, but in short term fecundity tests, overexpressing plants supported lower aphid numbers and antisense plants higher aphid numbers. The amounts and composition of released volatile organic compounds did not differ between control and LOX2.2 overexpressing lines. Up-regulation of genes was similar for both aphid species. The results suggest that LOX2.2 plays a role in the activation of JA-mediated responses and indicates the involvement of LOX2.2 in basic defense responses.

Isolation and Characterization of a Thionin Proprotein-processing Enzyme from Barley

Thionins are plant-specific antimicrobial peptides that have been isolated from the endosperm and leaves of cereals, from the leaves of mistletoes, and from several other plant species. They are generally basic peptides with three or four disulfide bridges and a molecular mass of ~5 kDa. Thionins are produced as preproproteins consisting of a signal peptide, the thionin domain, and an acidic domain. Previously, only mature thionin peptides have been isolated from plants, and in addition to removal of the signal peptide, at least one cleavage processing step between the thionin and the acidic domain is necessary to release the mature thionin. In this work, we identified a thionin proprotein-processing enzyme (TPPE) from barley. Purification of the enzyme was guided by an assay that used a quenched fluorogenic peptide comprising the amino acid sequence between the thionin and the acidic domain of barley leaf-specific thionin. The barley TPPE was identified as a serine protease (BAJ93208) and expressed in Escherichia coli as a strep tag-labeled protein. The barley BTH6 thionin proprotein was produced in E. coli using the vector pETtrx1a and used as a substrate. We isolated and sequenced the BTH6 thionin from barley to confirm the N and C terminus of the peptide in planta. Using an in vitro enzymatic assay, the recombinant TPPE was able to process the quenched fluorogenic peptide and to cleave the acidic domain at least at six sites releasing the mature thionin from the proprotein. Moreover, it was found that the intrinsic three-dimensional structure of the BTH6 thionin domain prevents cleavage of the mature BTH6 thionin by the TPPE.

The use of versatile plant antimicrobial peptides in agribusiness and human health

Plant immune responses involve a wide diversity of physiological reactions that are induced by the recognition of pathogens, such as hypersensitive responses, cell wall modifications, and the synthesis of antimicrobial molecules including antimicrobial peptides (AMPs). These proteinaceous molecules have been widely studied, presenting peculiar characteristics such as conserved domains and a conserved disulfide bond pattern. Currently, many AMP classes with diverse modes of action are known, having been isolated from a large number of organisms. Plant AMPs comprise an interesting source of studies nowadays, and among these there are reports of different classes, including defensins, albumins, cyclotides, snakins and several others. These peptides have been widely used in works that pursue human disease control, including nosocomial infections, as well as for agricultural purposes. In this context, this review will focus on the relevance of the structural-function relations of AMPs derived from plants and their proper use in applications for human health and agribusiness.

Four plant defensins from an indigenous South African Brassicaceae species display divergent activities against two test pathogens despite high sequence similarity in the encoding genes

BACKGROUND

Plant defensins are an important component of the innate defence system of plants where they form protective antimicrobial barriers between tissue types of plant organs as well as around seeds. These peptides also have other activities that are important for agricultural applications as well as the medical sector. Amongst the numerous plant peptides isolated from a variety of plant species, a significant number of promising defensins have been isolated from Brassicaceae species. Here we report on the isolation and characterization of four defensins from Heliophila coronopifolia, a native South African Brassicaceae species.

RESULTS

Four defensin genes (Hc-AFP1-4) were isolated with a homology based PCR strategy. Analysis of the deduced amino acid sequences showed that the peptides were 72% similar and grouped closest to defensins isolated from other Brassicaceae species. The Hc-AFP1 and 3 peptides shared high homology (94%) and formed a unique grouping in the Brassicaceae defensins, whereas Hc-AFP2 and 4 formed a second homology grouping with defensins from Arabidopsis and Raphanus. Homology modelling showed that the few amino acids that differed between the four peptides had an effect on the surface properties of the defensins, specifically in the alpha-helix and the loop connecting the second and third beta-strands. These areas are implicated in determining differential activities of defensins. Comparing the activities after recombinant production of the peptides, Hc-AFP2 and 4 had IC50 values of 5-20 μg ml-1 against two test pathogens, whereas Hc-AFP1 and 3 were less active. The activity against Botrytis cinerea was associated with membrane permeabilization, hyper-branching, biomass reduction and even lytic activity. In contrast, only Hc-AFP2 and 4 caused membrane permeabilization and severe hyper-branching against the wilting pathogen Fusarium solani, while Hc-AFP1 and 3 had a mild morphogenetic effect on the fungus, without any indication of membrane activity. The peptides have a tissue-specific expression pattern since differential gene expression was observed in the native host. Hc-AFP1 and 3 expressed in mature leaves, stems and flowers, whereas Hc-AFP2 and 4 exclusively expressed in seedpods and seeds.

CONCLUSIONS

Two novel Brassicaceae defensin sequences were isolated amongst a group of four defensin encoding genes from the indigenous South African plant H. coronopifolia. All four peptides were active against two test pathogens, but displayed differential activities and modes of action. The expression patterns of the peptide encoding genes suggest a role in protecting either vegetative or reproductive structures in the native host against pathogen attack, or roles in unknown developmental and physiological processes in these tissues, as was shown with other defensins.

Developmental and environmental regulation of soybean SE60 gene expression during embryogenesis and germination

Soybean SE60 belongs to the gamma-thionin family of proteins. We recently demonstrated that SE60 plays a role in defense during soybean development. Here, we show that SE60 is expressed in a tissue-specific and developmentally regulated manner. The expression of SE60 is distinct from that of the glycinin (Gy2) and extensin (SbHRGP3) genes of soybean during embryogenesis and germination. A SE60::GUS(-809) transgene, comprising -809 bp of the 5'-flanking region of SE60 fused to the GUS reporter gene, was expressed specifically in developing embryos, but not in the endosperms, from the globular stage of transgenic tobacco and Arabidopsis seeds. Furthermore, light affected the SE60::GUS(-809) expression pattern in germinating seedlings. Electrophoretic mobility shift assay (EMSA) revealed that soybean nuclear proteins as well as E. coli-expressed SB16, a high mobility group protein (HMG), were bound sequence-specifically to the fragment containing AT-rich motifs identified in the SE60 promoter. Interestingly, the soybean nuclear proteins binding to the two G-boxes and RY repeat were prevalent in seeds of 2-4 mm in size. In contrast, the nuclear proteins binding to the AT-rich motif and SE60 RNA expression were more prominent in seeds of 4-6 mm in size. Therefore, we propose that factors binding to the G-boxes or RY repeat initiate SE60 expression during embryogenesis.

Gene-encoded antimicrobial peptides from plants

On the basis of an extensive screening of seeds from various plant species, we have isolated and characterized several different antimicrobial peptides. They were all typified by having a broad antifungal activity spectrum, a relatively low molecular weight (3-14 kDa), a high cysteine content and a high isoelectric point (pI > 10). With respect to their amino acid sequence, these peptides can be classified into six structural classes. Synergistic enhancement (up to 73-fold) of antimicrobial activity was demonstrated in some combinations of peptides belonging to different classes. cDNA clones corresponding to different antifungal peptides were isolated and used to transform tobacco plants. Extracts of these transgenic plants showed higher (up to 16-fold) antifungal activity than untransformed control plants. Such antimicrobial peptides may find applications in molecular breeding of plants with increased disease resistance.

Sulfur-enhanced defence: effects of sulfur metabolism, nitrogen supply, and pathogen lifestyle

Evidence from field experiments indicates differential roles of sulfur and nitrogen supply for plant resistance against pathogens. Dissection of these observations in defined pathosystems and controlled nutritional conditions indicates an activation of plant sulfur metabolism in several incompatible and compatible interactions. Contents of cysteine and glutathione as markers of primary sulfate assimilation and stress response show increases in ARABIDOPSIS THALIANA upon infection, coinciding with the synthesis of sulfur-containing defence compounds. Similar increases of thiols were observed with necrotrophic, biotrophic, and hemibiotrophic pathogens. Sulfate supply was found to be neutral or beneficial for tolerance against fungal but neutral for bacterial pathogens under IN VITRO conditions. According to various reports and own observations the effects of nitrogen supply appeared to be neutral or harmful, depending on the pathogen. The activation of sulfur metabolism was a consequence of activation of gene expression as revealed by macroarray analysis of an A. THALIANA/ALTERNARIA BRASSICICOLA pathosystem. This activation appeared to be largely independent from sufficient or optimal sulfate supply and from the established sulfate deficiency response. The data suggest that plant-pathogen interactions and sulfur metabolism are linked by jasmonic acid as signal.

Expression of the antimicrobial peptides in plants to control phytopathogenic bacteria and fungi

Three antimicrobial peptides exhibiting in vitro antifungal activity were expressed in Arabidopsis to compare their in planta activity. Beta-Purothionin, cecropin B, and phor21 were expressed under an endogenous promoter with moderate-level activity and excreted extracellularly. Expression of beta-purothionin rendered the greatest antibacterial and antifungal resistance while cecropin B enhanced only antibacterial activity and phor21 did not improve antimicrobial resistance. The transgenic beta-purothionin arrested fungal growth on leaf surfaces and infection of stomata. Leaf extracts from plants producing beta-purothionin and cecropin B displayed membrane permeabilizing activity. The in planta antimicrobial activity of the tested peptides was consistent with previously reported in vitro experiments. The expression strategy allowed enhanced antifungal resistance without high-level transgene expression.

Enhanced resistance to seed-transmitted bacterial diseases in transgenic rice plants overproducing an oat cell-wall-bound thionin

Bacterial attack is a serious agricultural problem for growth of rice seedlings in the nursery and field. The thionins purified from seed and etiolated seedlings of barley are known to have antimicrobial activity against necrotrophic pathogens; however, we found that no endogenous rice thionin genes alone are enough for resistance to two major seed-transmitted phytopathogenic bacteria, Burkholderia plantarii and B. glumae, although rice thionin genes constitutively expressed in coleoptile, the target organ of the bacteria. Thus, we isolated thionin genes from oat, one of which was overexpressed in rice. When wild-type rice seed were germinated with these bacteria, all seedlings were wilted with severe blight. In the seedling infected with B. plantarii, bacterial staining was intensively marked around stomata and intercellular spaces. However, transgenic rice seedlings accumulating a high level of oat thionin in cell walls grew almost normally with bacterial staining only on the surface of stomata. These results indicate that the oat thionin effectively works in rice plants against bacterial attack.

Processing of thionin precursors in barley leaves by a vacuolar proteinase

Thionins are synthesized as precursors with a signal peptide and a long C-terminal acidic peptide that is post-translationally processed. A fusion protein including the maltose-binding protein from Escherichia coli (MalE), thionin DG3 from barley leaves, and its acidic C-terminal peptide has been used to obtain antibodies that recognize both domains of the precursor. In barley leaf sections, mature thionins accumulated in the vacuolar content, while the acidic peptide was not detected in any cell fraction. Brefeldin A and monensin inhibited processing of the precursor but its export from the microsomal fraction was not inhibited. Both purified vacuoles and an acid (pH 5.5) extract from leaves processed the fusion protein into a MalE-thionin and an acidic peptide fragment. A 70-kDa proteinase that effected this cleavage was purified from the acid extract. Processing of the fusion protein by both lysed vacuoles and the purified proteinase was inhibited by Zn2+ and by Cu2+, but not by inhibitors of the previously described vacuolar processing thiol or aspartic proteinases. In vivo processing of the thionin precursor in leaf sections was also inhibited by Zn2+ and Cu2+. Variants of the fusion protein with altered processing sites that represented those of thionin precursors from different taxa were readily processed by the proteinase, whereas changing the polarity of either the C-terminal or N-terminal residues of the processing site prevented cleavage by the proteinase.

Primary structure of omega-hordothionin, a member of a novel family of thionins from barley endosperm, and its inhibition of protein synthesis in eukaryotic and prokaryotic cell-free systems

A new sulfur-rich basic polypeptide, so called omega-hordothionin, has been isolated from barley endosperm by extractions with NaCl and ammonium bicarbonate followed by reverse-phase high performance liquid chromatography. Purified omega-hordothionin was found to be homogeneous by SDS/polyacrylamide gel electrophoresis, N-terminal amino-acid sequencing and electrospray-ionization mass spectrometric analysis. The complete primary structure of omega-hordothionin was determined by automatic degradation of the intact molecule and peptides obtained by proteolytic cleavage. Omega-hordothionin consists of a single polypeptide chain of 48 amino acids with a molecular mass of 5508 Da deduced from its amino acid sequence, which fully coincides with the 5508.2 Da determined by electrospray-ionization mass spectrometry. The isolated polypeptide showed a characteristic composition with a high content of basic amino acids (five arginine residues, two lysine residues and six histidine residues) and eight cysteine residues, and has strong sequence identity (66%) with the sorghum SI alpha 1 alpha-amylase inhibitor. Omega-hordothionin, like gamma-hordothionin, exhibited translation inhibitory activity on both eukaryotic cell-free systems from mammalian (rat liver and rabbit reticulocyte lysates) and prokaryotic cell-free systems (Escherichia coli). However, in contrast to gamma-hordothionin, omega-hordothionin did not inhibit plant systems such as Triticum aestivum, Cucumis sativus, Vicia sativa and Hordeum vulgare. Gamma-hordothionin also inhibited the alpha-amylase activity from human saliva, while omega-hordothionin and the other different genetic variants of thionins, alpha-hordothionin and beta-hordothionin, failed to show any inhibitory effect.

Gibberellin-repressible gene expression in the barley aleurone layer

Gibberellins are noted for their ability to induce expression of genes, such as alpha-amylase, in the aleurone layers of cereals. However, a number of mRNA species in the mature imbibed aleurone cell of barley, such as a storage globulin (Heck et al., Mol Gen Genet 239: 209-218 1993), are simultaneously and specifically repressed by gibberellin. In a continuing effort to understand this effect, we report cloning and characterization of two additional cDNAs from barley designated pHvGS-1 and pcHth3 that have high corresponding mRNA levels in the mature imbibed aleurone but are repressed 10-fold or more within 24 h of treatment with gibberellic acid (GA3). The extent of repression was concentration dependent and maximally effective at 10(-6) M GA3. Repression was also noted in the constitutive gibberellin response mutant, slender, in the absence of exogenous GA3. The antagonistic phytohormone, abscisic acid, had no effect or was weakly inductive of the steady-state levels of these mRNAs. During development of the seed, repressible mRNAs are present to different degrees in the maturing aleurone layer and embryo, but not in the starchy endosperm. Some repressible mRNA persists in the mature dry aleurone layer, but is degraded during imbibition, replenished by de novo transcription, and maintained at high steady-state levels until GA3 is perceived. Preliminary investigation suggests that repression is at least partly due to destabilization of the mRNAs which have estimated half-lives of 12 h or greater in the absence of GA3. pcHth3 encodes a member of the gamma-thionin gene family located on chromosome 7. pHvGS-1 corresponds to a gene on chromosome 3 of unknown function.

Selective disulphide linkage of plant thionins with other proteins

Thionins are shown to form disulphide linkages with other proteins. The reaction with bacterial enzymes beta-glucuronidase and neomycin phosphotransferase II could be prevented and reversed with dithiothreitol and blocked with N-ethylmaleimide. Other cysteine-rich low-molecular-weight toxic peptides from plants (LTP-3 from barley and P19 from potato) did not react as the thionins. Certain cysteine-containing proteins, such bovine serum albumin, ovalbumin and cytochrome c, reacted with thionins, while others, including carbonic anhydrase, soybean trypsin inhibitor, bovine-lung trypsin inhibitor and phosphorylase B did not. Selectivity of the reaction with a periplasmic component of the phytopathogenic bacterium Pseudomonas solanacearum was also shown.

Organ-specific expression of highly divergent thionin variants that are distinct from the seed-specific crambin in the crucifer Crambe abyssinica

Most thionins of higher plants are toxic to various bacteria, fungi, and animal and plant cells. The only known exception is the seed-specific thionin, crambin, of the crucifer Crambe abyssinica. Crambin has no net charge, is very hydrophobic and exhibits no toxicity. In the present work, the organization of the crambin precursor polypeptide was deduced from cD-NA sequences. The precursor shows a domain structure similar to that of the preproprotein of other thionins, which contains a signal peptide, a thionin domain and a C-terminal amino acid extension. Unlike the thionin precursors studied thus far, both the thionin domain and the C-terminal amino acid extension of the crambin precursor have no net charge and are hydrophobic, thus facilitating their interaction, by analogy to that proposed for the corresponding domains of other thionin precursors that have positive and negative charges. The existence of a large number of novel and highly variable thionin variants in Crambe abyssinica has been deduced from cDNA sequences that were amplified by the polymerase chain reaction (PCR) from RNA of seeds, leaves and cotyledons. While the deduced amino acid sequences of the thionin domains of most of these thionin precursor molecules are highly divergent, the two other domains are conserved. Most of the predicted thionin variants are positively charged. The presence of positively charged residues in the thionin domains consistently correlates with the presence of a negatively charged residue in the C-terminal amino acid extension of the various thionin precursors. The different thionin variants are encoded by distinct sets of genes and are expressed in an organ-specific manner.

Thionins: properties, possible biological roles and mechanisms of action

Thionins are low-molecular-weight proteins (M(r) ca. 5000) occurring in seeds, stems, roots and leaves of a number of plant species. The different members of this family of plant proteins show both sequence and structural homology, and are toxic to bacteria, fungi, yeasts and various naked cells in vitro. Toxicity requires an electrostatic interaction of the positively charged thionin with the negatively charged phospholipids making up the membrane, followed by either pore formation or a specific interaction with a certain lipid domain. This domain might be composed of phosphoinositides, which mediate transduction of environmental signals in eukaryotes. Their in vitro toxicity to plant pathogenic bacteria and fungi could reflect a direct role in plant defence, although, in view of the many divergent activities displayed by thionins both in vitro and in vivo, a biological role other than inhibition of microbial growth is equally plausible.

Hordothionins inhibit protein synthesis at the level of initiation in the wheat-germ system

The inhibitory effect of pure alpha and beta hordothionins on protein synthesis directed by pea mRNA has been studied in the wheat-germ translation system. It is demonstrated that a component of the wheat germ counteracts the thionin effect. Formation of polysomes in vitro in the presence of thionin was inhibited to the same extent as the total translation system while run-off translation of isolated polysomes from pea plants was not affected by thionin. These data are consistent with an effect of thionin on the initiation reaction. Analyses of the formation of initiation complexes in the presence and absence of mRNA support this view and show that thionin interferes with the formation of the 43S complex. In accordance with this observation and in contrast to earlier studies no evidence has been obtained for a direct interaction between mRNAs and thionins. The analysis of the translation products also gave no indication for preferential translation of individual mRNAs by the thionin-inhibited translation system. Compared to translation in vitro, exposure of barley protoplasts to thionins showed a less dramatic effect on protein synthesis as measured by incorporation of [35S]methionine into proteins. These data are discussed with respect to the effects of thionins on the plasma membranes as shown previously with animal cell cultures. It is concluded that at least in barley such effects would need higher concentrations of thionins than are required for the inhibition of protein synthesis.

Expression of biologically active hordothionins in tobacco. Effects of pre- and pro-sequences at the amino and carboxyl termini of the hordothionin precursor on mature protein expression and sorting

Hordothionins (HTHs) are small anti-bacterial proteins present in barley endosperm which are processed from larger precursor proteins, consisting of an amino-terminal signal peptide (SP), the mature highly basic HTH and a carboxy-terminal acidic peptide (AP). Different HTH precursor proteins were expressed in tobacco to study the effects of the pre-sequences (SP) and pro-sequences (AP) on expression, processing, sorting and biological activity and hence the feasibility of engineering bacterial disease resistance into crops which lack these proteins. Maximum HTH expression levels of approximately 0.7% (11 mumol/kg) of total soluble protein in young tobacco leaves were obtained using a semi-synthetic gene construct encoding a complete chimaeric HTH precursor protein. Tenfold lower HTH expression levels (maximum 1.3 mumol/kg) were obtained using synthetic gene constructs without the AP-coding sequence and no expression was found in plants containing synthetic HTH gene constructs without SP- and AP-coding sequences. In both cases where expression was found, the precursors were apparently correctly processed, although the HTH produced in plants containing a construct without AP sequence appeared to be slightly modified. No effect on plant phenotype was observed. Localization studies indicated that the HTH was in identical fractions of plants expressing the two different precursors, albeit at a different ratio, and was not secreted into the intercellular spaces of leaves or culture medium by protoplasts. Our results indicated that the AP is not involved in sorting and suggested that it might facilitate transport through membranes. The in vitro toxicity of HTH isolated from transgenic tobacco plants expressing the two different precursor proteins for the bacterial plant pathogen Clavibacter michiganensis subsp. michiganensis appeared similar to that of the HTH purified from barley endosperm.

cDNA-derived identification of novel thionin precursors in Viscum album that contain highly divergent thionin domains but conserved signal and acidic polypeptide domains

The existence of new thionin variants in Viscum album has been deduced from cDNA sequences. Unlike the viscotoxins and related thionins previously found in different members of the Viscaceae, these novel thionins contain eight rather than six cysteine residues. In this respect they resemble thionins described previously from various cereals and from Pyrularia pubera, which also contain eight cysteine residues at identical positions. All of the new thionins of V. album are encoded as higher-molecular-weight precursors consisting of a signal peptide, a thionin domain and an acidic polypeptide domain. While the deduced amino acid sequences of the thionin domains of different precursor molecules are highly divergent, the two other domains are conserved among all of the variants and are distinct from the corresponding domains of thionin precursors of other plant species.

A novel fingerprint method for analyzing the expression of complex multigene families of very low transcript abundance

A novel RNA fingerprint method has been developed based on the polymerase chain reaction to identify individual mRNA species derived from different members of a complex multigene family of very low transcript abundance. Using this method, changes in the composition of complex thionin mRNA populations and in the appearance of individual thionin mRNA species in different organs and in response to external stimuli were detected in Hordeum murinum. A single nucleotide exchange within the transcribed DNA may be sufficient to allow the distinction between otherwise identical transcripts.

Expression of the alpha-thionin gene from barley in tobacco confers enhanced resistance to bacterial pathogens

Thionins are cysteine-rich, 5 kDa polypeptides which are toxic to plant pathogens in vitro. Expression of the gene encoding alpha-thionin from barley endosperm, under the 35S promoter from cauliflower mosaic virus, conferred to transgenic tobacco enhanced resistance to the bacterial plant pathogens Pseudomonas syringae pv. tabaci 153 and P. syringae pv. syringae. The barley alpha-thionin gene, which has two introns, was correctly spliced in tobacco. The alpha-thionin in transgenic plants had the expected mobility in the gradient, when separated by high-performance liquid chromatography, reacted with monospecific antibodies and showed the expected antibiotic properties in vitro.

A comparison of leaf thionin sequences of barley cultivars and wild barley species

Leaf thionins of several barley cultivars and wild barley species were analysed. We found large differences in the numbers of leaf thionin genes in different Hordeum species. While, for instance, cultivars of Hordeum vulgare (Section Hordeum) contain more than 50 copies of thionin genes per haploid genome, the numbers are much lower in Hordeum species belonging to the sections Critesion and Stenostachys. The apparent number of genes correlates with the concentration of leaf thionin and its mRNA, which differs more than 100-fold among various Hordeum species. Leaf thionins are synthesized as high molecular weight precursor proteins that contain a signal peptide domain, a thionin domain and an acidic polypeptide domain. Analysis of cDNA clones of leaf thionins revealed a family of related transcripts. When the predicted amino acid sequences of the precursor molecules of wild barley species were compared, differences in the sequence variability of the three domains became apparent. The frequency of amino acid exchanges is much higher within the thionin domain than in the signal peptide and acidic polypeptide domains. The amino acid exchanges within the thionin domain do not occur at random but are confined to variable regions that alternate with highly conserved areas. Conserved regions comprise mostly cysteine residues and adjacent amino acids and may be important for the correct formation of the specific disulphide configuration of thionins.

Isolation and characterization of cDNAs encoding viscotoxins of mistletoe (Viscum album)

Viscotoxins have been isolated from leaf homogenates of European mistletoe (Viscum album L.) and purified to apparent homogeneity. Antisera raised against these polypeptides were used to screen a cDNA expression library in lambda gt11. Two positive clones have been isolated, one encoding a full-length preprotein of viscotoxin A3 and the other encoding the precursor of viscotoxin B. Besides the viscotoxin domain the precursor contained a signal sequence and an acidic polypeptide domain. Similar higher molecular mass precursor proteins have been described for thionins of leaves and seeds of barley. Even though the acidic part of the viscotoxin precursor is much shorter than the corresponding domain of the precursors of the leaf and seed thionins of barley, both the negative charge and the number and the relative position of cysteine residues have been conserved within the acidic domain. This result is consistent with our proposal that the acidic domain of the thionin precursor may play an important role in keeping the thionin inactive within the plant cell.

Cultivar-related differences in the distribution of cell-wall-bound thionins in compatible and incompatible interactions between barley and powdery mildew

Leaf-specific thionins of barley (Hordeum vulgare L.) have been identified as a novel class of cell-wall proteins toxic to plant-pathogenic fungi and possibly involved in the defence mechanism of plants. The distribution of these polypeptides has been studied in the host-pathogen system of barley and Erisyphe graminis DC.f.sp. hordei Marchal (powdery mildew). Immunogold-labelling of thionins in several barley cultivars indicates that resistance or susceptibility may be attributed to the presence or absence of thionins at the penetration site in walls and papillae of epidermal leaf cells.All of the leaf-specific thionin genes are confined to the distal end of the short arm of chromosome 6 of barley. None of the genes for cultivarspecific resistance to powdery mildew which have previously been mapped on barley chromosomes are found close to this locus.