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

Transcriptome Analysis Reveals Contrasting Plant Responses of Sorghum bicolor upon Colonization by Two Formae Speciales of Sporisorium reilianum

The biotrophic fungus Sporisorium reilianum exists in two host-adapted formae speciales that cause head smut in maize (S. reilianum f. sp. zeae; SRZ) and sorghum (S. reilianum f. sp. reilianum; SRS). In sorghum, the spread of SRZ is limited to the leaves. To understand the plant responses to each forma specialis, we determined the transcriptome of sorghum leaves inoculated either with SRS or SRZ. Fungal inoculation led to gene expression rather than suppression in sorghum. SRZ induced a much greater number of genes than SRS. Each forma specialis induced a distinct set of plant genes. The SRZ-induced genes were involved in plant defense mainly at the plasma membrane and were associated with the Molecular Function Gene Ontology terms chitin binding, abscisic acid binding, protein phosphatase inhibitor activity, terpene synthase activity, chitinase activity, transmembrane transporter activity and signaling receptor activity. Specifically, we found an upregulation of the genes involved in phospholipid degradation and sphingolipid biosynthesis, suggesting that the lipid content of the plant plasma membrane may contribute to preventing the systemic spread of SRZ. In contrast, the colonization of sorghum with SRS increased the expression of the genes involved in the detoxification of cellular oxidants and in the unfolded protein response at the endoplasmic reticulum, as well as of the genes modifying the cuticle wax and lipid composition through the generation of alkanes and phytosterols. These results identified plant compartments that may have a function in resistance against SRZ (plasma membrane) and susceptibility towards SRS (endoplasmic reticulum) that need more attention in the future.

Comparative physiological and transcriptomic analyses illuminate common mechanisms by which silicon alleviates cadmium and arsenic toxicity in rice seedlings

The inessential heavy metal/loids cadmium (Cd) and arsenic (As), which often co-occur in polluted paddy soils, are toxic to rice. Silicon (Si) treatment is known to reduce Cd and As toxicity in rice plants. To better understand the shared mechanisms by which Si alleviates Cd and As stress, rice seedlings were hydroponically exposed to Cd or As, then treated with Si. The addition of Si significantly ameliorated the inhibitory effects of Cd and As on rice seedling growth. Si supplementation decreased Cd and As translocation from roots to shoots, and significantly reduced Cd- and As-induced reactive oxygen species generation in rice seedlings. Transcriptomics analyses were conducted to elucidate molecular mechanisms underlying the Si-mediated response to Cd or As stress in rice. The expression patterns of the differentially expressed genes in Cd- or As-stressed rice roots with and without Si application were compared. The transcriptomes of the Cd- and As-stressed rice roots were similarly and profoundly reshaped by Si application, suggesting that Si may play a fundamental, active role in plant defense against heavy metal/loid stresses by modulating whole genome expression. We also identified two novel genes, Os01g0524500 and Os06g0514800, encoding a myeloblastosis (MYB) transcription factor and a thionin, respectively, which may be candidate targets for Si to alleviate Cd and As stress in rice, as well as for the generation of Cd- and/or As-resistant plants. This study provides valuable resources for further clarification of the shared molecular mechanisms underlying the Si-mediated alleviation of Cd and As toxicity in rice.

Short Peptides Make a Big Difference: The Role of Botany-Derived AMPs in Disease Control and Protection of Human Health

Botany-derived antimicrobial peptides (BAMPs), a class of small, cysteine-rich peptides produced in plants, are an important component of the plant immune system. Both in vivo and in vitro experiments have demonstrated their powerful antimicrobial activity. Besides in plants, BAMPs have cross-kingdom applications in human health, with toxic and/or inhibitory effects against a variety of tumor cells and viruses. With their diverse molecular structures, broad-spectrum antimicrobial activity, multiple mechanisms of action, and low cytotoxicity, BAMPs provide ideal backbones for drug design, and are potential candidates for plant protection and disease treatment. Lots of original research has elucidated the properties and antimicrobial mechanisms of BAMPs, and characterized their surface receptors and in vivo targets in pathogens. In this paper, we review and introduce five kinds of representative BAMPs belonging to the pathogenesis-related protein family, dissect their antifungal, antiviral, and anticancer mechanisms, and forecast their prospects in agriculture and global human health. Through the deeper understanding of BAMPs, we provide novel insights for their applications in broad-spectrum and durable plant disease prevention and control, and an outlook on the use of BAMPs in anticancer and antiviral drug design.

Antimicrobial Peptides from Plants: A cDNA-Library Based Isolation, Purification, Characterization Approach and Elucidating Their Modes of Action

Even in a natural ecosystem, plants are continuously threatened by various microbial diseases. To save themselves from these diverse infections, plants build a robust, multilayered immune system through their natural chemical compounds. Among the several crucial bioactive compounds possessed by plants’ immune systems, antimicrobial peptides (AMPs) rank in the first tier. These AMPs are environmentally friendly, anti-pathogenic, and do not bring harm to humans. Antimicrobial peptides can be isolated in several ways, but recombinant protein production has become increasingly popular in recent years, with the Escherichia coli expression system being the most widely used. However, the efficacy of this expression system is compromised due to the difficulty of removing endotoxin from its system. Therefore, this review suggests a high-throughput cDNA library-based plant-derived AMP isolation technique using the Bacillus subtilis expression system. This method can be performed for large-scale screening of plant sources to classify unique or homologous AMPs for the agronomic and applied field of plant studies. Furthermore, this review also focuses on the efficacy of plant AMPs, which are dependent on their numerous modes of action and exceptional structural stability to function against a wide range of invaders. To conclude, the findings from this study will be useful in investigating how novel AMPs are distributed among plants and provide detailed guidelines for an effective screening strategy of AMPs.

Bacterial Panicle Blight and Burkholderia glumae: From Pathogen Biology to Disease Control

Bacterial panicle blight (BPB), caused by the bacterium Burkholderia glumae, has affected rice production worldwide. Despite its importance, neither the disease nor the causal agent are well understood. Moreover, methods to manage BPB are still lacking. Nevertheless, the emerging importance of this pathogen has stimulated research to identify the mechanisms of pathogenicity, to gain insight into plant disease resistance, and to develop strategies to manage the disease. In this review, we consolidate current information regarding the virulence factors that have been identified in B. glumae and present a model of the disease and the pathogen. We also provide an update on the current research status to develop methods to control the disease especially through biological control approaches and through the development of resistant cultivars.

Multiple endogenous seed-born bacteria recovered rice growth disruption caused by Burkholderia glumae

Burkholderia glumae is a causal agent of bacterial grain and seedling rot in rice, and is a threat to stable global food supply. The virulence of B. glumae was suppressed when it was inoculated on budding seed rather than on non-budding seed. To clarify the phenomena, pathogen titer inside the rice plant was measured by serial dilution plating of lysates from budding rice seedlings. Surprisingly, morphologically different types of colonies were observed on the plates. These 'contaminated' rice seed-born bacteria (RSB) were identified by sequencing 16S rRNA genes as three strains of Pseudomonas putida (RSB1, RSB10, RSB15) and Stenotrophomonas maltophilia (RSB2). All bacteria and B. glumae were simultaneously inoculated onto rice seeds, and all three P. putida RSBs suppressed the growth disruption caused by B. glumae, whereas RSB2 had no effect. Thus, the virulence was synergistically suppressed when co-treated with RSBs. The effect could be dependent on the high biofilm formation ability of RSB2. By comprehensive microbiota analysis, endogenous rice flora were changed by RSBs treatment. These results suggest the possibility of novel pathogen control through pre-treatment with endogenous beneficial microorganisms. The method would contribute substantially to the implementation of sustainable agriculture stated in Sustainable Development Goals of United Nations.

Enhanced Resistance to Fusarium graminearum in Transgenic Arabidopsis Plants Expressing a Modified Plant Thionin

The fungal pathogen Fusarium graminearum causes Fusarium head blight (FHB) on wheat, barley, and other grains. FHB results in yield reductions and contaminates grain with trichothecene mycotoxins, which threaten food safety and food security. Innovative mechanisms for controlling FHB are urgently needed. We have previously shown that transgenic tobacco and citrus plants expressing a modified thionin (Mthionin) exhibited enhanced resistance toward several bacterial pathogens. The aim of this study was to investigate whether overexpression of Mthionin could be similarly efficacious against F. graminearum, and whether transgenic expression of Mthionin impacts the plant microbiome. Transgenic Arabidopsis plants expressing Mthionin were generated and confirmed. When challenged with F. graminearum, Mthionin-expressing plants showed less disease and fungal biomass in both leaves and inflorescences compared with control plants. When infiltrated into leaves, macroconidia of F. graminearum germinated at lower rates and produced less hyphal growth in Arabidopsis leaves expressing Mthionin. Moreover, marker genes related to defense signaling pathways were expressed at significantly higher levels after F. graminearum infection in Mthionin transgenic Arabidopsis plants. However, Mthionin expression did not appreciably alter the overall microbiome associated with transgenic plants grown under controlled conditions; across leaves and roots of Mthionin-expressing and control transgenic plants, only a few bacterial and fungal taxa differed, and differences between Mthionin transformants were of similar magnitude compared with control plants. In sum, our data indicate that Mthionin is a promising candidate to produce transgenic crops for reducing FHB severity and ultimately mycotoxin contamination.

Antimicrobial Peptides - Small but Mighty Weapons for Plants to Fight Phytopathogens

Antimicrobial Peptides (AMPs) have diverse structures, varied modes of actions, and can inhibit the growth of a wide range of pathogens at low concentrations. Plants are constantly under attack by a wide range of phytopathogens causing massive yield losses worldwide. To combat these pathogens, nature has armed plants with a battery of defense responses including Antimicrobial Peptides (AMPs). These peptides form a vital component of the two-tier plant defense system. They are constitutively expressed as part of the pre-existing first line of defense against pathogen entry. When a pathogen overcomes this barrier, it faces the inducible defense system, which responds to specific molecular or effector patterns by launching an arsenal of defense responses including the production of AMPs. This review emphasizes the structural and functional aspects of different plant-derived AMPs, their homology with AMPs from other organisms, and how their biotechnological potential could generate durable resistance in a wide range of crops against different classes of phytopathogens in an environmentally friendly way without phenotypic cost.

Modulation of defence and iron homeostasis genes in rice roots by the diazotrophic endophyte Herbaspirillum seropedicae

Rice is staple food of nearly half the world’s population. Rice yields must therefore increase to feed ever larger populations. By colonising rice and other plants, Herbaspirillum spp. stimulate plant growth and productivity. However the molecular factors involved are largely unknown. To further explore this interaction, the transcription profiles of Nipponbare rice roots inoculated with Herbaspirillum seropedicae were determined by RNA-seq. Mapping the 104 million reads against the Oryza sativa cv. Nipponbare genome produced 65 million unique mapped reads that represented 13,840 transcripts each with at least two-times coverage. About 7.4% (1,014) genes were differentially regulated and of these 255 changed expression levels more than two times. Several of the repressed genes encoded proteins related to plant defence (e.g. a putative probenazole inducible protein), plant disease resistance as well as enzymes involved in flavonoid and isoprenoid synthesis. Genes related to the synthesis and efflux of phytosiderophores (PS) and transport of PS-iron complexes were induced by the bacteria. These data suggest that the bacterium represses the rice defence system while concomitantly activating iron uptake. Transcripts of H. seropedicae were also detected amongst which transcripts of genes involved in nitrogen fixation, cell motility and cell wall synthesis were the most expressed.

In Silico Analyses of Rice Thionin Genes and the Antimicrobial Activity of OsTHION15 Against Phytopathogens

Thionins are a family of antimicrobial peptides. We performed in silico expression analyses of the 44 rice (Oryza sativa) thionins (OsTHIONs). Modulated expression levels of OsTHIONs under different treatments suggest their involvement in many processes, including biotic, abiotic, and nutritional stress responses, and in hormone signaling. OsTHION15 (LOC_Os06g32600) was selected for further characterization based on several in silico analyses. OsTHION15 in O. sativa subsp. indica 'KDML 105' was expressed in all of the tissues and organs examined, including germinating seed, leaves, and roots of seedlings and mature plants, and inflorescences. To investigate the antimicrobial activity of OsTHION15, we produced a recombinant peptide in Escherichia coli Rosetta-gami (DE3). The recombinant OsTHION15 exhibited inhibitory activities toward rice-pathogenic bacteria such as Xanthomonas oryzae pv. oryzae and Pectobacterium carotovorum pv. atroseptica, with minimum inhibitory concentrations of 112.6 and 14.1 µg ml^-1, respectively. A significant hyphal growth inhibition was also observed toward Fusarium oxysporum f. sp. cubense and Helminthosporium oryzae. In addition, we demonstrated the in planta antibacterial activity of this peptide in Nicotiana benthamiana against X. campestris pv. glycines. These activities suggest the possible application of OsTHION15 in plant disease control.

The role of antimicrobial peptides in plant immunity

Selective pressure imposed by millions of years of relentless biological attack has led to the development of an extraordinary array of defense strategies in plants. Among these, antimicrobial peptides (AMPs) stand out as one of the most prominent components of the plant immune system. These small and usually basic peptides are deployed as a generalist defense strategy that grants direct and durable resistance against biotic stress. Even though their name implies a function against microbes, the range of plant-associated organisms affected by these peptides is much broader. In this review, we highlight the advances in our understanding on the role of AMPs in plant immunity. We demonstrate that the capacity of plant AMPs to act against a large spectrum of enemies relies on their diverse mechanism of action and remarkable structural stability. The efficacy of AMPs as a defense strategy is evidenced by their widespread occurrence in the plant kingdom, an astonishing heterogeneity in host peptide composition, and the extent to which plant enemies have evolved effective counter-measures to evade AMP action. Plant AMPs are becoming an important topic of research due to their significance in allowing plants to thrive and for their enormous potential in agronomical and pharmaceutical fields.

Plant thionins: structure, biological functions and potential use in biotechnology

Antimicrobial peptides (AMPs) are important components of defense system in both plants and animals. They represent an ancient mechanism of innate immunity providing rapid first line of defense against pathogens. Plant AMPs are classified into several families: thionins, defensins, nonspecific lipid-transfer proteins, hevein- and knottin-type peptides, hairpinins and macrocyclic peptides (cyclotides). The review focuses on the thionin family. Thionins comprise a plant-specific AMP family that consists of short (~5 kDA) cysteine-rich peptides containing 6 or 8 cysteine residues with antimicrobial and toxic properties. Based on similarity in amino acid sequences and the arrangement of disulphide bonds, five structural classes of thionins are discriminated. The three-dimensional structures of a number of thionins were determined. The amphipathic thionin molecule resembles the Greek letter Г, in which the long arm is formed by two antiparallel α-helices, while the short one, by two parallel β-strands. The residues responsible for the antimicrobial activity of thionins were identified. Thionins are synthesized as precursor proteins consisting of a signal peptide, the mature peptide region and the C-terminal prodomain. Thionins protect plants from pathogenic bacteria and fungi acting directly on the membranes of microorganisms at micromolar concentrations, although their precise mode of action remains unclear. In addition to plant pathogens, thionins inhibit growth of a number of human pathogens and opportunistic microorganisms, such as Candida spp., Saccharomyces cerevisiae, Fusarium solani, Staphylococcus aureus and Escherichia coli. Thionins are toxic to different types of cells including mammalian cancer cell lines. Transgenic plants expressing thionin genes display enhanced resistance to pathogens. A wide range of biological activities makes thionins promising candidates for practical application in agriculture and medicine.

Heterologous expression and antimicrobial activity of OsGASR3 from rice (Oryza sativa L.)

According to an in silico analysis, OsGASR3 (LOC_Os03g55290) from rice (Oryza sativa L.) was predicted to be involved in plant defense mechanisms. A semi-quantitative reverse transcription polymerase chain reaction assay revealed that OsGASR3 is highly expressed in the inflorescences of Thai jasmine rice (O. sativa L. subsp. indica 'KDML 105'). To characterize the biological activity of OsGASR3, we produced an OsGASR3-glutathione S-transferase fusion protein in Escherichia coli Rosetta-gami (DE3) cells for a final purified recombinant OsGASR3 yield of 0.65 mg/L. The purified OsGASR3 inhibited the hyphal growth of Fusarium oxysporum f.sp. cubense and Helminthosporium oryzae at a relatively low concentration (7.5 μg/mL). Furthermore, OsGASR3 exhibited in planta inhibitory activity against Xanthomonas campestris, suggesting its involvement in defense mechanisms, in addition to its previously reported functions affecting growth and development. These observations indicate that recombinant OsGASR3 may be useful for protecting agriculturally important crops against pathogenic microbes.

Barley transcriptome analyses upon interaction with different aphid species identify thionins contributing to resistance

Aphids are phloem-feeding insects that cause yield loss on a wide range of crops, including cereals such as barley. Whilst most aphid species are limited to one or few host species, some are able to reproduce on many plants belonging to different families. Interestingly, aphid probing behaviour can be observed on both host and non-host species, indicating that interactions take place at the molecular level that may impact host range. Here, we aimed to gain insight into the interaction of barley with aphid species differing in their ability to infest this crop by analysing transcriptional responses. Firstly, we determined colonization efficiency, settlement and probing behaviour for the aphid species Rhopalosiphum padi, Myzus persicae and Myzus cerasi, which defined host, poor-host and non-host interactions, respectively. Analyses of barley transcriptional responses revealed gene sets differentially regulated upon the different barley-aphid interactions and showed that the poor-host interaction with M. persicae resulted in the strongest regulation of genes. Interestingly, we identified several thionin genes strongly up-regulated upon interaction with M. persicae, and to a lesser extent upon R. padi interaction. Ectopic expression of two of these genes in Nicotiana benthamiana reduced host susceptibility to M. persicae, indicating that thionins contribute to defences against aphids.

Two novel antimicrobial defensins from rice identified by gene coexpression network analyses

Defensins form an antimicrobial peptides (AMP) family, and have been widely studied in various plants because of their considerable inhibitory functions. However, their roles in rice (Oryza sativa L.) have not been characterized, even though rice is one of the most important staple crops that is susceptible to damaging infections. Additionally, a previous study identified 598 rice genes encoding cysteine-rich peptides, suggesting there are several uncharacterized AMPs in rice. We performed in silico gene expression and coexpression network analyses of all genes encoding defensin and defensin-like peptides, and determined that OsDEF7 and OsDEF8 are coexpressed with pathogen-responsive genes. Recombinant OsDEF7 and OsDEF8 could form homodimers. They inhibited the growth of the bacteria Xanthomonas oryzae pv. oryzae, X. oryzae pv. oryzicola, and Erwinia carotovora subsp. atroseptica with minimum inhibitory concentration (MIC) ranging from 0.6 to 63μg/mL. However, these OsDEFs are weakly active against the phytopathogenic fungi Helminthosporium oryzae and Fusarium oxysporum f.sp. cubense. This study describes a useful method for identifying potential plant AMPs with biological activities.

Overexpression of a Modified Plant Thionin Enhances Disease Resistance to Citrus Canker and Huanglongbing (HLB)

Huanglongbing (HLB or citrus greening disease) caused by Candidatus Liberibacter asiaticus (Las) is a great threat to the US citrus industry. There are no proven strategies to eliminate HLB disease and no cultivar has been identified with strong HLB resistance. Citrus canker is also an economically important disease associated with a bacterial pathogen (Xanthomonas citri). In this study, we characterized endogenous citrus thionins and investigated their expression in different citrus tissues. Since no HLB-resistant citrus cultivars have been identified, we attempted to develop citrus resistant to both HLB and citrus canker through overexpression of a modified plant thionin. To improve effectiveness for disease resistance, we modified and synthesized the sequence encoding a plant thionin and cloned into the binary vector pBinPlus/ARS. The construct was then introduced into Agrobacterium strain EHA105 for citrus transformation. Transgenic Carrizo plants expressing the modified plant thionin were generated by Agrobacterium-mediated transformation. Successful transformation and transgene gene expression was confirmed by molecular analysis. Transgenic Carrizo plants expressing the modified thionin gene were challenged with X. citri 3213 at a range of concentrations, and a significant reduction in canker symptoms and a decrease in bacterial growth were demonstrated compared to nontransgenic plants. Furthermore, the transgenic citrus plants were challenged with HLB via graft inoculation. Our results showed significant Las titer reduction in roots of transgenic Carrizo compared with control plants and reduced scion Las titer 12 months after graft inoculation. These data provide promise for engineering citrus disease resistance against HLB and canker.

Overexpression of BSR1 confers broad-spectrum resistance against two bacterial diseases and two major fungal diseases in rice

Broad-spectrum disease resistance against two or more types of pathogen species is desirable for crop improvement. In rice, Xanthomonas oryzae pv. oryzae (Xoo), the causal bacteria of rice leaf blight, and Magnaporthe oryzae, the fungal pathogen causing rice blast, are two of the most devastating pathogens. We identified the rice BROAD-SPECTRUM RESISTANCE 1 (BSR1) gene for a BIK1-like receptor-like cytoplasmic kinase using the FOX hunting system, and demonstrated that BSR1-overexpressing (OX) rice showed strong resistance to the bacterial pathogen, Xoo and the fungal pathogen, M. oryzae. Here, we report that BSR1-OX rice showed extended resistance against two other different races of Xoo, and to at least one other race of M. oryzae. In addition, the rice showed resistance to another bacterial species, Burkholderia glumae, which causes bacterial seedling rot and bacterial grain rot, and to Cochliobolus miyabeanus, another fungal species causing brown spot. Furthermore, BSR1-OX rice showed slight resistance to rice stripe disease, a major viral disease caused by rice stripe virus. Thus, we demonstrated that BSR1-OX rice shows remarkable broad-spectrum resistance to at least two major bacterial species and two major fungal species, and slight resistance to one viral pathogen.

The role of thionins in rice defence against root pathogens

Thionins are antimicrobial peptides that are involved in plant defence. Here, we present an in-depth analysis of the role of rice thionin genes in defence responses against two root pathogens: the root-knot nematode Meloidogyne graminicola and the oomycete Pythium graminicola. The expression of rice thionin genes was observed to be differentially regulated by defence-related hormones, whereas all analysed genes were consistently down-regulated in M. graminicola-induced galls, at least until 7 days post-inoculation (dpi). Transgenic lines of Oryza sativa cv. Nipponbare overproducing OsTHI7 revealed decreased susceptibility to M. graminicola infection and P. graminicola colonization. Taken together, these results demonstrate the role of rice thionin genes in defence against two of the most damaging root pathogens attacking rice.

Maize EMBRYO SAC family peptides interact differentially with pollen tubes and fungal cells

EMBRYO SAC1-4 (ES1-4) peptides belong to the defensin subgroup of cysteine-rich peptides known to mediate pollen tube burst in Zea mays (maize). ES1-4 are reported here to also be capable of inhibiting germination and growth of the maize fungal pathogens Fusarium graminearum and Ustilago maydis at higher concentrations. Dividing the peptides into smaller pieces showed that a 15-amino-acid peptide located in a highly variable loop region lacking similarity to other defensins or defensin-like peptides binds to maize pollen tube surfaces, causing swelling prior to burst. This peptide fragment and a second conserved neighbouring fragment showed suppression of fungal germination and growth. The two peptides caused swelling of fungal cells, production of reactive oxygen species, and finally the formation of big vacuoles prior to burst at high peptide concentration. Furthermore, peptide fragments were found to bind differently to fungal cells. In necrotrophic F. graminearum, a peptide fragment named ES-d bound only at cell surfaces whereas the peptide ES-c bound at cell surfaces and also accumulated inside cells. Conversely, in biotrophic U. maydis, both peptide fragments accumulated inside cells, but, if applied at higher concentration, ES-c but not ES-d accumulated mainly in vacuoles. Mapping of peptide interaction sites identified amino acids differing in pollen tube burst and fungal response reactions. In summary, these findings indicate that residues targeting pollen tube burst in maize are specific to the ES family, while residues targeting fungal growth are conserved within defensins and defensin-like peptides.

Isolation and Characterization of a Novel Pathogenesis-Related Protein Gene (GmPRP) with Induced Expression in Soybean (Glycine max) during Infection with Phytophthora sojae

Pathogenesis-related proteins (PR proteins) play crucial roles in the plant defense system. A novel PRP gene was isolated from highly resistant soybean infected with Phytophthora sojae (P. sojae) and was named GmPRP (GenBank accession number: KM506762). The amino acid sequences of GmPRP showed identities of 74%, 73%, 72% and 69% with PRP proteins from Vitis vinifera, Populus trichocarpa, Citrus sinensis and Theobroma cacao, respectively. Quantitative real-time reverse transcription PCR (qRT-PCR) data showed that the expression of GmPRP was highest in roots, followed by the stems and leaves. GmPRP expression was upregulated in soybean leaves infected with P. sojae. Similarly, GmPRP expression also responded to defense/stress signaling molecules, including salicylic acid (SA), ethylene (ET), abscisic acid (ABA) and jasmonic acid (JA). GmPRP was localized in the cell plasma membrane and cytoplasm. Recombinant GmPRP protein exhibited ribonuclease activity and significant inhibition of hyphal growth of P. sojae 1 in vitro. Overexpression of the GmPRP gene in T2 transgenic tobacco and T2 soybean plants resulted in enhanced resistance to Phytophthora nicotianae (P. nicotianae) and P. sojae race 1, respectively. These results indicated that the GmPRP protein played an important role in the defense of soybean against P. sojae infection.

Multitasking antimicrobial peptides in plant development and host defense against biotic/abiotic stress

Crop losses due to pathogens are a major threat to global food security. Plants employ a multilayer defense against a pathogen including the use of physical barriers (cell wall), induction of hypersensitive defense response (HR), resistance (R) proteins, and synthesis of antimicrobial peptides (AMPs). Unlike a complex R gene-mediated immunity, AMPs directly target diverse microbial pathogens. Many a times, R-mediated immunity breaks down and plant defense is compromised. Although R-gene dependent pathogen resistance has been well studied, comparatively little is known about the interactions of AMPs with host defense and physiology. AMPs are ubiquitous, low molecular weight peptides that display broad spectrum resistance against bacteria, fungi and viruses. In plants, AMPs are mainly classified into cyclotides, defensins, thionins, lipid transfer proteins, snakins, and hevein-like vicilin-like and knottins. Genetic distance lineages suggest their conservation with minimal effect of speciation events during evolution. AMPs provide durable resistance in plants through a combination of membrane lysis and cellular toxicity of the pathogen. Plant hormones - gibberellins, ethylene, jasmonates, and salicylic acid, are among the physiological regulators that regulate the expression of AMPs. Transgenically produced AMP-plants have become a means showing that AMPs are able to mitigate host defense responses while providing durable resistance against pathogens.

The secreted antifungal protein thionin 2.4 in Arabidopsis thaliana suppresses the toxicity of a fungal fruit body lectin from Fusarium graminearum

Plants possess active defense systems and can protect themselves from pathogenic invasion by secretion of a variety of small antimicrobial or antifungal proteins such as thionins. The antibacterial and antifungal properties of thionins are derived from their ability to induce open pore formation on cell membranes of phytopathogens, resulting in release of potassium and calcium ions from the cell. Wheat thionin also accumulates in the cell walls of Fusarium-inoculated plants, suggesting that it may have a role in blocking pathogen infection at the plant cell walls. Here we developed an anti-thionin 2.4 (Thi2.4) antibody and used it to show that Thi2.4 is localized in the cell walls of Arabidopsis and cell membranes of F. graminearum, when flowers are inoculated with F. graminearum. The Thi2.4 protein had an antifungal effect on F. graminearum. Next, we purified the Thi2.4 protein, conjugated it with glutathione-S-transferase (GST) and coupled the proteins to an NHS-activated column. Total protein from F. graminearum was applied to GST-Thi2.4 or Thi2.4-binding columns, and the fungal fruit body lectin (FFBL) of F. graminearum was identified as a Thi2.4-interacting protein. This interaction was confirmed by a yeast two-hybrid analysis. To investigate the biological function of FFBL, we infiltrated the lectin into Arabidopsis leaves and observed that it induced cell death in the leaves. Application of FFBL at the same time as inoculation with F. graminearum significantly enhanced the virulence of the pathogen. By contrast, FFBL-induced host cell death was effectively suppressed in transgenic plants that overexpressed Thi2.4. We found that a 15 kD Thi2.4 protein was specifically expressed in flowers and flower buds and suggest that it acts not only as an antifungal peptide, but also as a suppressor of the FFBL toxicity. Secreted thionin proteins are involved in this dual defense mechanism against pathogen invasion at the plant-pathogen interface.

Transgenic sweet potato expressing thionin from barley gives resistance to black rot disease caused by Ceratocystis fimbriata in leaves and storage roots

Black rot of sweet potato caused by pathogenic fungus Ceratocystis fimbriata severely deteriorates both growth of plants and post-harvest storage. Antimicrobial peptides from various organisms have broad range activities of killing bacteria, mycobacteria, and fungi. Plant thionin peptide exhibited anti-fungal activity against C. fimbriata. A gene for barley α-hordothionin (αHT) was placed downstream of a strong constitutive promoter of E12Ω or the promoter of a sweet potato gene for β-amylase of storage roots, and introduced into sweet potato commercial cultivar Kokei No. 14. Transgenic E12Ω:αHT plants showed high-level expression of αHT mRNA in both leaves and storage roots. Transgenic β-Amy:αHT plants showed sucrose-inducible expression of αHT mRNA in leaves, in addition to expression in storage roots. Leaves of E12Ω:αHT plants exhibited reduced yellowing upon infection by C. fimbriata compared to leaves of non-transgenic Kokei No. 14, although the level of resistance was weaker than resistance cultivar Tamayutaka. Storage roots of both E12Ω:αHT and β-Amy:αHT plants exhibited reduced lesion areas around the site inoculated with C. fimbriata spores compared to Kokei No. 14, and some of the transgenic lines showed resistance level similar to Tamayutaka. Growth of plants and production of storage roots of these transgenic plants were not significantly different from non-transgenic plants. These results highlight the usefulness of transgenic sweet potato expressing antimicrobial peptide to reduce damages of sweet potato from the black rot disease and to reduce the use of agricultural chemicals.

The Medicago truncatula N5 gene encoding a root-specific lipid transfer protein is required for the symbiotic interaction with Sinorhizobium meliloti

The Medicago truncatula N5 gene is induced in roots after Sinorhizobium meliloti infection and it codes for a putative lipid transfer protein (LTP), a family of plant small proteins capable of binding and transferring lipids between membranes in vitro. Various biological roles for plant LTP in vivo have been proposed, including defense against pathogens and modulation of plant development. The aim of this study was to shed light on the role of MtN5 in the symbiotic interaction between M. truncatula and S. meliloti. MtN5 cDNA was cloned and the mature MtN5 protein expressed in Escherichia coli. The lipid binding capacity and antimicrobial activity of the recombinant MtN5 protein were tested in vitro. MtN5 showed the capacity to bind lysophospholipids and to inhibit M. truncatula pathogens and symbiont growth in vitro. Furthermore, MtN5 was upregulated in roots after infection with either the fungal pathogen Fusarium semitectum or the symbiont S. meliloti. Upon S. meliloti infection, MtN5 was induced starting from 1 day after inoculation (dpi). It reached the highest concentration at 3 dpi and it was localized in the mature nodules. MtN5-silenced roots were impaired in nodulation, showing a 50% of reduction in the number of nodules compared with control roots. On the other hand, transgenic roots overexpressing MtN5 developed threefold more nodules with respect to control roots. Here, we demonstrate that MtN5 possesses biochemical features typical of LTP and that it is required for the successful symbiotic association between M. truncatula and S. meliloti.

Plant pathogenesis-related (PR) proteins: a focus on PR peptides

The novel classes of plant pathogenesis-related (PR) proteins identified during the last decade also include novel peptide families. This review specifically focuses on these pathogenesis-related peptides, including proteinase inhibitors (PR-6 family), plant defensins (PR-12 family), thionins (PR-13 family) and lipid transfer proteins (PR-14 family). For each family of PR peptides, the general features concerning occurrence, expression and possible functions of their members are described. Next, more specifically the occurrence of each PR peptide family in the model plant Arabidopsis thaliana is discussed. Single-gene studies performed on particular gene members of a PR peptide family are reported. In addition, expression data of yet undescribed gene members of that particular PR peptide family are presented by consultation of publicly available micro-array databases. Finally an update is provided on the potential role of these PR peptides in A. thaliana, with a focus on their possible involvement in plant defense.

PR-13/Thionin but not PR-1 mediates bacterial resistance in Nicotiana attenuata in nature, and neither influences herbivore resistance

Increases in pathogenesis-related (PR) transcripts are commonly interpreted as evidence of plants' resistance responses to pathogens; however, few studies have examined whether increases in PR proteins protect plants growing under natural conditions. Pseudomonas syringae pv. tomato DC3,000, which is virulent and causes disease in Arabidopsis, is also pathogenic to the native tobacco Nicotiana attenuata. N. attenuata responds to P. syringae pv. tomato DC3,000's challenges with increases in salicylic acid and transcripts of at least two PR genes, PR-1 and PR13/Thionin. To determine if either of these PR proteins functions in bacterial resistance, we independently silenced both genes by RNAi and found that only PR-13/Thionin mediates resistance to P. syringae pv. tomato DC3,000 in glasshouse experiments. When NaPR-1- and NaThionin-silenced plants were transplanted into the plant's native habitat in the Great Basin Desert of Utah, opportunistic Pseudomonas spp. performed better on NaThionin-silenced plants compared with NaPR-1-silenced and wild-type (WT) plants, and accounted for increased plant mortality. The native herbivore community of N. attenuata attacked both NaPR-1- and PR-13/NaThionin-silenced plants to the same degree as it did in WT plants, indicating that neither PR protein provides resistance to herbivores. Although PR-1 is generally considered a marker gene of disease resistance, we found no evidence that it has an antimicrobial function. In contrast, PR-13/NaThionin is clearly an ecologically relevant defense protein involved in resisting pathogens in N. attenuata.

Plant peptides and peptidomics

Extracellular plant peptides perform a large variety of functions, including signalling and defence. Intracellular peptides often have physiological functions or may merely be the products of general proteolysis. Plant peptides have been identified and, in part, functionally characterized through biochemical and genetic studies, which are lengthy and in some cases impractical. Peptidomics is a branch of proteomics that has been developed over the last 5 years, and has been used mainly to study neuropeptides in animals and the degradome of proteases. Peptidomics is a fast, efficient methodology that can detect minute and transient amounts of peptides and identify their post-translational modifications. This review describes known plant peptides and introduces the use of peptidomics for the detection of novel plant peptides.

Sucrose-mediated priming of plant defense responses and broad-spectrum disease resistance by overexpression of the maize pathogenesis-related PRms protein in rice plants

Expression of pathogenesis-related (PR) genes is part of the plant's natural defense response against pathogen attack. The PRms gene encodes a fungal-inducible PR protein from maize. Here, we demonstrate that expression of PRms in transgenic rice confers broad-spectrum protection against pathogens, including fungal (Magnaporthe oryzae, Fusarium verticillioides, and Helminthosporium oryzae) and bacterial (Erwinia chrysanthemi) pathogens. The PRms-mediated disease resistance in rice plants is associated with an enhanced capacity to express and activate the natural plant defense mechanisms. Thus, PRms rice plants display a basal level of expression of endogenous defense genes in the absence of the pathogen. PRms plants also exhibit stronger and quicker defense responses during pathogen infection. We also have found that sucrose accumulates at higher levels in leaves of PRms plants. Sucrose responsiveness of rice defense genes correlates with the pathogen-responsive priming of their expression in PRms rice plants. Moreover, pretreatment of rice plants with sucrose enhances resistance to M. oryzae infection. Together, these results support a sucrose-mediated priming of defense responses in PRms rice plants which results in broad-spectrum disease resistance.

Interference of quorum sensing and virulence of the rice pathogen Burkholderia glumae by an engineered endophytic bacterium

Many bacterial species are known to thrive within plants. Among these bacteria, a group referred to as endophytes provide beneficial effects to the host plants by the promotion of plant growth and the suppression of plant pathogens. Among 44 putative endophytic isolates isolated from surface-sterilized rice roots, Burkholderia sp. KJ006 was selected for further study because of a lack of pathogenicity to rice, a broad spectrum of antifungal properties, and the presence of the nifH gene, which is an indicator for nitrogen fixation. In an attempt to control Burkholderia glumae, a casual pathogen of seedling rot and grain rot of rice, an N-acyl-homoserine lactonase (aiiA) gene from Bacillus thuringiensis was introduced into Burkholderia sp. KJ006 given that the major virulence factor of Burkholderia glumae is controlled in a population-dependent manner (quorum sensing). The engineered strain KJ006 (pKPE-aiiA) inhibited production of quorum-sensing signals by Burkholderia glumae in vitro and reduced the disease incidence of rice seedling rot caused by Burkholderia glumae in situ. Our results indicate the possibility that a bacterial endophyte transformed with the aiiA gene can be used as a novel biological control agent against pathogenic Burkholderia glumae that are known to occupy the same ecological niche.

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.

Role of defense/stress-related marker genes, proteins and secondary metabolites in defining rice self-defense mechanisms

Rice, a first cereal crop whose draft genome sequence from two subspecies (japonica-type cv. Nipponbare and indica-type 93-11) was available in 2002, along with its almost complete genome sequence in 2005, has drawn the attention of researchers worldwide because of its immense impact on human existence. One of the most critical research areas in rice is to discern the self-defense mechanism(s), an innate property of all living organisms. The last few decades have seen scattered research into rice responses to diverse environmental stimuli and stress factors. Our understanding on rice self-defense mechanism has increased considerably with accelerated research during recent years mainly due to identification and characterization of several defense/stress-related components, genes, proteins and secondary metabolites. As these identified components have been used to study the defense/stress pathways, their compilation in this review will undoubtedly help rice (and others) researchers to effectively use them as a potential marker for better understanding, and ultimately, in defining rice (and plant) self-defense response pathways.

Plant gamma-thionins: novel insights on the mechanism of action of a multi-functional class of defense proteins

This review focuses on the first plant defense protein class described in literature, with growth inhibition activity toward pathogens. These peptides were named gamma-thionins or defensins, which are small proteins that can be classified into four main subtypes according to their specific functions. Gamma-thionins are small cationic peptides with different and special abilities. They are able to inhibit digestive enzymes or act against bacteria and/or fungi. Current research in this area focuses particularly these two last targets, being the natural crop plant defenses improved through the use of transgenic technology. Here, we will compare primary and tertiary structures of gamma-thionins and also will analyze their similarities to scorpion toxins and insect defensins. This last comparison offers some hypothesis for gamma-thionins mechanisms of action against certain pathogens. This specific area has benefited from the recent determination of many gamma-thionin structures. Furthermore, we also summarize molecular interactions between plant gamma-thionins and fungi receptors, which include membrane proteins and lipids, shedding some light over pathogen resistance. Researches on gamma-thionins targets could help on plant genetic improvement for production of increased resistance toward pathogens. Thus, positive results recently obtained for transgenic plants and future prospects in the area are also approached. Finally, gamma-thionins activity has also been studied for future drug development, capable of inhibit tumor cell growth in human beings.

Recent advances in rice biotechnology--towards genetically superior transgenic rice

Rice biotechnology has made rapid advances since the first transgenic rice plants were produced 15 years ago. Over the past decade, this progress has resulted in the development of high frequency, routine and reproducible genetic transformation protocols for rice. This technology has been applied to produce rice plants that withstand several abiotic stresses, as well as to gain tolerance against various pests and diseases. In addition, quality improving and increased nutritional value traits have also been introduced into rice. Most of these gains were not possible through conventional breeding technologies. Transgenic rice system has been used to understand the process of transformation itself, the integration pattern of transgene as well as to modulate gene expression. Field trials of transgenic rice, especially insect-resistant rice, have recently been performed and several other studies that are prerequisite for safe release of transgenic crops have been initiated. New molecular improvisations such as inducible expression of transgene and selectable marker-free technology will help in producing superior transgenic product. It is also a step towards alleviating public concerns relating to issues of transgenic technology and to gain regulatory approval. Knowledge gained from rice can also be applied to improve other cereals. The completion of the rice genome sequencing together with a rich collection of full-length cDNA resources has opened up a plethora of opportunities, paving the way to integrate data from the large-scale projects to solve specific biological problems.

Constitutive expression of human lactoferrin and its N-lobe in rice plants to confer disease resistance

The milk protein, lactoferrin, is known to have antibacterial, antiviral, and antifungal activities. To explore the possibility of conferring disease resistance in plants by expressing this protein, the gene for the full-length human lactoferrin (HLF), as well as the N-lobe, the N-terminal half molecule (HLFN), was introduced into rice plants and expressed constitutively under the control of the cauliflower mosaic virus 35S promotor. Western blot analysis of leaves from HLF-transgenic rice plants showed an 80 kDa-band, which was about 1–2 kDa less than human milk lactoferrin. HLFN was expressed as a 45-kDa protein and retained its heparin-binding property. Deglycosylation experiments suggested that both proteins produced by the plants had plant-type oligosaccharide chains. The transgenic rice plants were assessed for resistance against disease-causing bacteria, virus, and fungi. Of the pathogens tested, significant resistance against Burkholderia (Pseudomonas) plantarii, the causative agent of bacterial seedling blight disease, was observed in the transgenic plants expressing HLF or HLFN.Key words: expression of domains; glycosylation; lactoferrin; plant disease resistance; transgenic rice.

Plant disease: a threat to global food security

A vast number of plant pathogens from viroids of a few hundred nucleotides to higher plants cause diseases in our crops. Their effects range from mild symptoms to catastrophes in which large areas planted to food crops are destroyed. Catastrophic plant disease exacerbates the current deficit of food supply in which at least 800 million people are inadequately fed. Plant pathogens are difficult to control because their populations are variable in time, space, and genotype. Most insidiously, they evolve, often overcoming the resistance that may have been the hard-won achievement of the plant breeder. In order to combat the losses they cause, it is necessary to define the problem and seek remedies. At the biological level, the requirements are for the speedy and accurate identification of the causal organism, accurate estimates of the severity of disease and its effect on yield, and identification of its virulence mechanisms. Disease may then be minimized by the reduction of the pathogen's inoculum, inhibition of its virulence mechanisms, and promotion of genetic diversity in the crop. Conventional plant breeding for resistance has an important role to play that can now be facilitated by marker-assisted selection. There is also a role for transgenic modification with genes that confer resistance. At the political level, there is a need to acknowledge that plant diseases threaten our food supplies and to devote adequate resources to their control.

Characterization of antimicrobial peptides against a US strain of the rice pathogen Rhizoctonia solani

AIM

To identify antimicrobial peptides with high lytic activity against Rhizoctonia solani strain LR172, causal agent of rice sheath blight and aerial blight of soyabeans in the US.

METHODS AND RESULTS

Among 12 natural and synthetic antimicrobial peptides tested in vitro, the wheat-seed peptide, purothionin, showed the strongest inhibitory activity that was similar to the antifungal antibiotics, nystatin and nikkomycin Z. Cecropin B, a natural peptide from cecropia moth, and synthetic peptide D4E1 produced the highest inhibitory activity against R. solani among linear peptides. Membrane permeabilization levels strongly correlated with antifungal activity of the peptides. Noticeable changes in membrane integrity were observed at concentrations of >/=0.5 micromol l(-1) for purothionin, 2 micromol l(-1) for cecropin B, D4E1, D2A21, melittin, and phor21, and 8 micromol l(-1) for magainin II and phor14. An increase of nuclear membrane permeabilization was observed in fungal cells treated with cecropin B, but not with purothionin. Diffusion of nuclear content was observed by fluorescent microscopy 10 min after adding a lethal concentration of cecropin B. Evaluation by electron microscopy confirmed severe cytoplasmic degradation and plasma membrane vesiculation. Purothionin and cecropin B were the most stable against proteolytic degradation when added to liquid cultures of R. solani.

CONCLUSIONS

Purothionin, cecropin B, D4E1 and phor21 were shown to exhibit high in vitro lytic activity against R. solani strain LR172 for rice and soyabean. These peptides are greater than 16 amino acids long and rapidly increase fungal membrane permeabilization. Resistance to proteolysis is important for sufficient antifungal activity of antimicrobial peptides.

SIGNIFICANCE AND IMPACT OF THE STUDY

Selected antimicrobial peptides offer an attractive alternative to traditional chemicals that could be utilized in molecular breeding to develop crops resistant to rice sheath blight and aerial blight of soyabean.