Cloning and expression of a novel cDNA encoding a mannose-specific jacalin-related lectin from Oryza sativa

Genome-wide identification and analysis of monocot-specific chimeric jacalins (MCJ) genes in Maize (Zea mays L.)

BACKGROUND

The monocot chimeric jacalins (MCJ) proteins, which contain a jacalin-related lectin (JRL) domain and a dirigent domain (DIR), are specific to Poaceae. MCJ gene family is reported to play an important role in growth, development and stress response. However, their roles in maize have not been thoroughly investigated.

RESULTS

In this study, eight MCJ genes in the maize genome (designated as ZmMCJs) were identified, which displayed unequal distribution across four chromosomes. Phylogenetic relationships between the ZmMCJs were evident through the identification of highly conserved motifs and gene structures. Analysis of transcriptome data revealed distinct expression patterns among the ZmMCJ genes, leading to their classification into four different modules, which were subsequently validated using RT-qPCR. Protein structures of the same module are found to be relatively similar. Subcellular localization experiments indicated that the ZmMCJs are mainly located on the cell membrane. Additionally, hemagglutination and inhibition experiments show that only part of the ZmMCJs protein has lectin activity, which is mediated by the JRL structure, and belongs to the mannose-binding type. The cis-acting elements in the promoter region of ZmMCJ genes predicted their involvement response to phytohormones, such as abscisic acid and jasmonic acid. This suggests that ZmMCJ genes may play a significant role in both biotic and abiotic stress responses.

CONCLUSIONS

Overall, this study adds new insights into our understanding of the gene-protein architecture, evolutionary characteristics, expression profiles, and potential functions of MCJ genes in maize.

OsJRL40, a Jacalin-Related Lectin Gene, Promotes Salt Stress Tolerance in Rice

High salinity is a major stress factor affecting the quality and productivity of rice (Oryza sativa L.). Although numerous salt tolerance-related genes have been identified in rice, their molecular mechanisms remain unknown. Here, we report that OsJRL40, a jacalin-related lectin gene, confers remarkable salt tolerance in rice. The loss of function of OsJRL40 increased sensitivity to salt stress in rice, whereas its overexpression enhanced salt tolerance at the seedling stage and during reproductive growth. β-glucuronidase (GUS) reporter assays indicated that OsJRL40 is expressed to higher levels in roots and internodes than in other tissues, and subcellular localization analysis revealed that the OsJRL40 protein localizes to the cytoplasm. Further molecular analyses showed that OsJRL40 enhances antioxidant enzyme activities and regulates Na⁺-K⁺ homeostasis under salt stress. RNA-seq analysis revealed that OsJRL40 regulates salt tolerance in rice by controlling the expression of genes encoding Na⁺/K⁺ transporters, salt-responsive transcription factors, and other salt response-related proteins. Overall, this study provides a scientific basis for an in-depth investigation of the salt tolerance mechanism in rice and could guide the breeding of salt-tolerant rice cultivars.

The Crystal Structure of the Defense Conferring Rice Protein OsJAC1 Reveals a Carbohydrate Binding Site on the Dirigent-like Domain

Pesticides are routinely used to prevent severe losses in agriculture. This practice is under debate because of its potential negative environmental impact and selection of resistances in pathogens. Therefore, the development of disease resistant plants is mandatory. It was shown that the rice (Oryza sativa) protein OsJAC1 enhances resistance against different bacterial and fungal plant pathogens in rice, barley, and wheat. Recently we reported possible carbohydrate interaction partners for both domains of OsJAC1 (a jacalin-related lectin (JRL) and a dirigent (DIR) domain), however, a mechanistic understanding of its function is still lacking. Here, we report crystal structures for both individual domains and the complex of galactobiose with the DIR domain, which revealed a new carbohydrate binding motif for DIR proteins. Docking studies of the two domains led to a model of the full-length protein. Our findings offer insights into structure and binding properties of OsJAC1 and its possible function in pathogen resistance.

Characterization, expression, and functional analysis of the pathogenesis-related gene PtDIR11 in transgenic poplar

Lignins and lignans are important for plant resistance to pathogens. Dirigent (DIR) proteins control the regio- and stereo-selectivity of coniferyl alcohol in lignan and lignin biosynthesis. DIR genes have been implicated in defense-related responses in several plant species, but their role in poplar immunity is unclear. We cloned PtDIR11 from Populus trichocarpa; we found that overexpression of PtDIR11 in poplar improved the lignan biosynthesis and enhanced the resistance of poplar to Septotis populiperda. PtDIR11 has a typical DIR domain; it belongs to the DIR-b/d family and is expressed in the cell membrane. Reverse transcription quantitative polymerase chain reaction (RT-qPCR) analysis showed that PtDIR11 expression was highest in stems, followed by leaves and roots. Furthermore, PtDIR11 expression was induced by S. populiperda, salicylic acid (SA), jasmonate (JA), and ethylene (ET) stresses. The recombinant PtDIR11 protein inhibited the growth of S. populiperda in vitro. Overexpressing (OE) PtDIR11 in "Nanlin 895" poplar enhanced growth. The OE lines exhibited minimal changes in lignin content, but their total lignan and flavonoid contents were significantly greater than in the wild-type (WT) lines. Overexpression of PtDIR11 affected multiple biological pathways of poplar, such as phenylpropanoid biosynthesis. The methanol extracts of OE-PtDIR11 lines showed greater anti-S. populiperda activity than did lignin extracts from the WT lines. Furthermore, OE-PtDIR11 lines upregulated genes that were related to phenylpropanoid biosynthesis and genes associated with the JA and ET signal transduction pathways; it downregulated genes that were related to SA signal transduction compared with the WT line under S. populiperda stress. Therefore, the OE transgenic plants analysis revealed that PtDIR11 is a good candidate gene for breeding of disease resistant poplar.

Polyploidization affects the allelic variation of jasmonate-regulated protein Ta-JA1 belonging to the monocot chimeric jacalin (MCJ) family in wild emmer wheat

The jasmonate-regulated protein Ta-JA1 belongs to the monocot chimeric jacalin (MCJ) family and plays a vital role in stress resistance in wheat. However, the impact of wheat polyploidization on Ta-JA1 remains unclear. In this study, 149 members of the MCJ family were identified among members of Triticeae using a genome-wide approach. The genes were resolved into three clades; MCJ genes in each clade were derived from different donor genes during evolution. Segmental duplication may have been the primary driver, compared with tandem duplication, of expansion in the MCJ family of wheat. Gene loss and acquisition occurred during tetraploidization, and the core expansion of the family occurred after tetraploidization. Sequencing data for 2104 accessions of T. aestivum and 99 accessions of T. dicoccoides showed that Ta-JA1-2A and Ta-JA1 were highly conserved in common wheat, and four alleles (TdJA1-Ax2, TdJA1-Ay2, TdJA1-Ax3, and TdJA1-Ay3) were detected in T. dicoccoides. Using gene-specific markers, one AsJA1-B allele was detected in 11 Ae. speltoides accessions and one TuJA1-Ax1 allele was detected in 70 T. urartu accessions. Six alleles were detected on chromosome 2A: TdJA1-Ax1 (13 accessions), TdJA1-Ay1 (57 accessions), TdJA1-Ax2 (23 accessions), TdJA1-Ay2 (42 accessions), TdJA1-Ax3 (29 accessions), and TdJA1-Ay3 (251 accessions). Only one allele (TdJA1-B) on chromosome 2B was detected in 415 T. dicoccoides accessions. A geographical distribution analysis revealed that Israel hosted higher allelic variation than other regions. Quantitative reverse transcription PCR analysis indicated that divergence in expression has occurred among Ta-JA1 alleles and, notably, TdJA1-Ax1 and TdJA1-Ay1 showed significantly higher expression levels than the other four allelic types in T. dicoccoides. The present results contribute to an improved understanding of the effects of polyploidization on the MCJ gene family and the functions of Ta-JA1, and may be useful to enrich common wheat germplasm resources.

Genome-Wide Identification of JRL Genes in Moso Bamboo and Their Expression Profiles in Response to Multiple Hormones and Abiotic Stresses

Jacalin-related lectins (JRLs) are a new subfamily of plant lectins that has recently been recognized and plays an important role in plant growth, development, and abiotic stress response. Although moso bamboo (Phyllostachys edulis) is an economically and industrially important bamboo worldwide, there has been no systematic identification of JRLs in this species. Here, we identified 25 JRL genes in moso bamboo, and these genes are unequally distributed among 10 genome scaffolds. Phylogenetic analysis showed that the moso bamboo JRLs were clustered into four JRL subgroups: I, II, V, and VII. Numerous stress-responsive and hormone-regulated cis-elements were detected in the upstream promoter regions of the JRLs. Genome collinearity analyses showed that the JRL genes of moso bamboo are more closely related to those of Brachypodium distachyon than to those of Oryza sativa and Zea mays. Sixty-four percent of the PeJRL genes are present as segmental and tandem duplicates. qRT-PCR expression analysis showed that JRL genes in the same subgroup were significantly downregulated in response to salicylic acid (SA), abscisic acid (ABA), and methyl jasmonate (MeJA) treatments and significantly upregulated under low temperature, drought, and salt stress; they also exhibited tissue-specific expression patterns. Subcellular localization experiments revealed that PeJRL04 and PeJRL13 were localized to the cell membrane, nucleus, and cytoplasm. Three dimensional structure prediction and yeast two-hybrid assays were used to verify that PeJRL13 exists as a self-interacting homodimer in vivo. These findings provide an important reference for understanding the functions of specific moso bamboo JRL genes and for the effective selection of stress-related genes.

Review: The multiple roles of plant lectins

For decades, the biological roles of plant lectins remained obscure and subject to speculation. With the advent of technological and scientific progress, researchers have compiled a vast amount of information regarding the structure, biological activities and functionality of hundreds of plant lectins. Data mining of genomes and transcriptome sequencing and high-throughput analyses have resulted in new insights. This review aims to provide an overview of what is presently known about plant lectins, highlighting their versatility and the importance of plant lectins for a multitude of biological processes, such as plant development, immunity, stress signaling and regulation of gene expression. Though lectins primarily act as readers of the glycocode, the multiple roles of plant lectins suggest that their functionality goes beyond carbohydrate-recognition.

Biochemical and Initial Structural Characterization of the Monocot Chimeric Jacalin OsJAC1

The monocot chimeric jacalin OsJAC1 from Oryza sativa consists of a dirigent and a jacalin-related lectin domain. The corresponding gene is expressed in response to different abiotic and biotic stimuli. However, there is a lack of knowledge about the basic function of the individual domains and their contribution to the physiological role of the entire protein. In this study, we have established a heterologous expression in Escherichia coli with high yields for the full-length protein OsJAC1 as well as its individual domains. Our findings showed that the secondary structure of both domains is dominated by β-strand elements. Under reducing conditions, the native protein displayed clearly visible transition points of thermal unfolding at 59 and 85 °C, which could be attributed to the lectin and the dirigent domain, respectively. Our study identified a single carbohydrate-binding site for each domain with different specificities towards mannose and glucose (jacalin domain), and galactose moieties (dirigent domain), respectively. The recognition of different carbohydrates might explain the ability of OsJAC1 to respond to different abiotic and biotic factors. This is the first report of specific carbohydrate-binding activity of a DIR domain, shedding new light on its function in the context of this monocot chimeric jacalin.

Genome-wide identification and expression analysis of dirigent-jacalin genes from plant chimeric lectins in Moso bamboo (Phyllostachys edulis)

Dirigent-jacalin (D-J) genes belong to the plant chimeric lectin family, and play vital roles in plant growth and resistance to abiotic and biotic stresses. To explore the functions of the D-J family in the growth and development of Moso bamboo (Phyllostachys edulis), their physicochemical properties, phylogenetic relationships, gene and protein structures, and expression patterns were analyzed in detail. Four putative PeD-J genes were identified in the Moso bamboo genome, and microsynteny and phylogenetic analyses indicated that they represent a new branch in the evolution of plant lectins. PeD-J proteins were found to be composed of a dirigent domain and a jacalin-related lectin domain, each of which contained two different motifs. Multiple sequence alignment and homologous modeling analysis indicated that the three-dimensional structure of the PeD-J proteins was significantly different compared to other plant lectins, primarily due to the tandem dirigent and jacalin domains. We surveyed the upstream putative promoter regions of the PeD-Js and found that they mainly contained cis-acting elements related to hormone and abiotic stress response. An analysis of the expression patterns of root, leaf, rhizome and panicle revealed that four PeD-J genes were highly expressed in the panicle, indicating that they may be required during the formation and development of several different tissue types in Moso bamboo. Moreover, PeD-J genes were shown to be involved in the rapid growth and development of bamboo shoots. Quantitative Real-time PCR (qRT PCR) assays further verified that D-J family genes were responsive to hormones and stresses. The results of this study will help to elucidate the biological functions of PeD-Js during bamboo growth, development and stress response.

Overexpression of rice jacalin-related mannose-binding lectin (OsJAC1) enhances resistance to ionizing radiation in Arabidopsis

BACKGROUND

Jacalin-related lectins in plants are important in defense signaling and regulate growth, development, and response to abiotic stress. We characterized the function of a rice mannose-binding jacalin-related lectin (OsJAC1) in the response to DNA damage from gamma radiation.

RESULTS

Time- and dose-dependent changes of OsJAC1 expression in rice were detected in response to gamma radiation. To identify OsJAC1 function, OsJAC1-overexpressing transgenic Arabidopsis plants were generated. Interestingly, OsJAC1 overexpression conferred hyper-resistance to gamma radiation in these plants. Using comparative transcriptome analysis, genes related to pathogen defense were identified among 22 differentially expressed genes in OsJAC1-overexpressing Arabidopsis lines following gamma irradiation. Furthermore, expression profiles of genes associated with the plant response to DNA damage were determined in these transgenic lines, revealing expression changes of important DNA damage checkpoint and perception regulatory components, namely MCMs, RPA, ATM, and MRE11.

CONCLUSIONS

OsJAC1 overexpression may confer hyper-resistance to gamma radiation via activation of DNA damage perception and DNA damage checkpoints in Arabidopsis, implicating OsJAC1 as a key player in DNA damage response in plants. This study is the first report of a role for mannose-binding jacalin-related lectin in DNA damage.

Biomass Accumulation and Cell Wall Structure of Rice Plants Overexpressing a Dirigent-Jacalin of Sugarcane (ShDJ) Under Varying Conditions of Water Availability

A sugarcane gene encoding a dirigent-jacalin, ShDJ, was induced under drought stress. To elucidate its biological function, we integrated a ShDJ-overexpression construction into the rice Nipponbare genome via Agrobacterium-mediated transformation. Two transgenic lines with a single copy gene in T₀ were selected and evaluated in both the T₁ and T₄ generations. Transgenic lines had drastically improved survival rate under water deficit conditions, at rates close to 100%, while WT did not survive. Besides, transgenic lines had improved biomass production and higher tillering under water deficit conditions compared with WT plants. Reduced pectin and hemicellulose contents were observed in transgenic lines compared with wild-type plants under both well-watered and water deficit conditions, whereas cellulose content was unchanged in line #17 and reduced in line #29 under conditions of low water availability. Changes in lignin content under water deficit were only observed in line #17. However, improvements in saccharification were found in both transgenic lines along with changes in the expression of OsNTS1/2 and OsMYB58/63 secondary cell wall biosynthesis genes. ShDJ-overexpression up-regulated the expression of the OsbZIP23, OsGRAS23, OsP5CS, and OsLea3 genes in rice stems under well-watered conditions. Taken together, our data suggest that ShDJ has the potential for improving drought tolerance, plant biomass accumulation, and saccharification efficiency.

Lectins as Promising Therapeutics for the Prevention and Treatment of HIV and Other Potential Coinfections

Human immunodeficiency virus-acquired immunodeficiency syndrome (HIV/AIDS) remains a global health problem. Current therapeutics specifically target the viral pathogen at various stages of its life cycle, although complex interactions between HIV and other pathogenic organisms are evident. Targeting HIV and concomitant infectious pathogens simultaneously, both by therapeutic regimens and in prevention strategies, would help contain the AIDS pandemic. Lectins, a ubiquitous group of proteins that specifically bind glycosylated molecules, are interesting compounds that could be used for this purpose, with demonstrated anti-HIV properties. In addition, potential coinfecting pathogens, including other enveloped viruses, bacteria, yeasts and fungi, and protozoa, display sugar-coated macromolecules on their surfaces, making them potential targets of lectins. This review summarizes the currently available findings suggesting that lectins should be further developed to simultaneously fight the AIDS pandemic and concomitant infections in HIV infected individuals.

Transcriptomic Analysis of Responses to Imbalanced Carbon: Nitrogen Availabilities in Rice Seedlings

The internal C:N balance must be tightly controlled for the normal growth and development of plants. However, the underlying mechanisms, by which plants sense and balance the intracellular C:N status correspondingly to exogenous C:N availabilities remain elusive. In this study, we use comparative gene expression analysis to identify genes that are responsive to imbalanced C:N treatments in the aerial parts of rice seedlings. Transcripts of rice seedlings treated with four C:N availabilities (1:1, 1:60, 60:1 and 60:60) were compared and two groups of genes were classified: high C:low N responsive genes and low C:high N responsive genes. Our analysis identified several functional correlated genes including chalcone synthase (CHS), chlorophyll a-b binding protein (CAB) and other genes that are implicated in C:N balancing mechanism, such as alternative oxidase 1B (OsAOX1B), malate dehydrogenase (OsMDH) and lysine and histidine specific transporter 1 (OsLHT1). Additionally, six jasmonate synthetic genes and key regulatory genes involved in abiotic and biotic stresses, such as OsMYB4, autoinhibited calcium ATPase 3 (OsACA3) and pleiotropic drug resistance 9 (OsPDR9), were differentially expressed under high C:low N treatment. Gene ontology analysis showed that high C:low N up-regulated genes were primarily enriched in fatty acid biosynthesis and defense responses. Coexpression network analysis of these genes identified eight jasmonate ZIM domain protein (OsJAZ) genes and several defense response related regulators, suggesting that high C:low N status may act as a stress condition, which induces defense responses mediated by jasmonate signaling pathway. Our transcriptome analysis shed new light on the C:N balancing mechanisms and revealed several important regulators of C:N status in rice seedlings.

Evolutionary analysis of the jacalin-related lectin family genes in 11 fishes

Jacalin-related lectins are a type of carbohydrate-binding proteins, which are distributed across a wide variety of organisms and involved in some important biological processes. The evolution of this gene family in fishes is unknown. Here, 47 putative jacalin genes in 11 fish species were identified and divided into 4 groups through phylogenetic analysis. Conserved gene organization and motif distribution existed in each group, suggesting their functional conservation. Some fishes have eleven jacalin genes, while others have only one or zero gene in their genomes, suggesting dynamic changes in the number of jacalin genes during the evolution of fishes. Intragenic recombination played a key role in the evolution of jacalin genes. Synteny analyses of jacalin genes in some fishes implied conserved and dynamic evolution characteristics of this gene family and related genome segments. Moreover, a few functional divergence sites were identified within each group pairs. Divergent expression profiles of the zebra fish jacalin genes were further investigated in different stresses. The results provided a foundation for exploring the characterization of the jacalin genes in fishes and will offer insights for additional functional studies.

Polarized Defense Against Fungal Pathogens Is Mediated by the Jacalin-Related Lectin Domain of Modular Poaceae-Specific Proteins

Modular proteins are an evolutionary answer to optimize performance of proteins that physically interact with each other for functionality. Using a combination of genetic and biochemical experiments, we characterized the rice protein OsJAC1, which consists of a jacalin-related lectin (JRL) domain predicted to bind mannose-containing oligosaccharides, and a dirigent domain which might function in stereoselective coupling of monolignols. Transgenic overexpression of OsJAC1 in rice resulted in quantitative broad-spectrum resistance against different pathogens including bacteria, oomycetes, and fungi. Overexpression of this gene or its wheat ortholog TAJA1 in barley enhanced resistance against the powdery mildew fungus. Both protein domains of OsJAC1 are required to establish resistance as indicated by single or combined transient expression of individual domains. Expression of artificially separated and fluorescence-tagged protein domains showed that the JRL domain is sufficient for targeting the powdery mildew penetration site. Nevertheless, co-localization of the lectin and the dirigent domain occurred. Phylogenetic analyses revealed orthologs of OsJAC1 exclusively within the Poaceae plant family. Dicots, by contrast, only contain proteins with either JRL or dirigent domain(s). Altogether, our results identify OsJAC1 as a representative of a novel type of resistance protein derived from a plant lineage-specific gene fusion event for better function in local pathogen defense.

Lectin domains at the frontiers of plant defense

Plants are under constant attack from pathogens and herbivorous insects. To protect and defend themselves, plants evolved a multi-layered surveillance system, known as the innate immune system. Plants sense their encounters upon perception of conserved microbial structures and damage-associated patterns using cell-surface and intracellular immune receptors. Plant lectins and proteins with one or more lectin domains represent a major part of these receptors. The whole group of plant lectins comprises an elaborate collection of proteins capable of recognizing and interacting with specific carbohydrate structures, either originating from the invading organisms or from damaged plant cell wall structures. Due to the vast diversity in protein structures, carbohydrate recognition domains and glycan binding specificities, plant lectins constitute a very diverse protein superfamily. In the last decade, new types of nucleocytoplasmic plant lectins have been identified and characterized, in particular lectins expressed inside the nucleus and the cytoplasm of plant cells often as part of a specific plant response upon exposure to different stress factors or changing environmental conditions. In this review, we provide an overview on plant lectin motifs used in the constant battle against pathogens and predators during plant defenses.

Effects of an extracted lectin from Citrullus colocynthis L. (Cucurbitaceae) on survival, digestion and energy reserves of Ectomyelois ceratoniae Zeller (Lepidoptera: Pyralidae)

Lectins are the heterogeneous proteins in plants that serve as storage proteins via defensive mechanisms against herbivores. In the current study, a lectin was extracted and purified from seeds of Citrullus colocynthis by Sepharose 4B-Galactose and DEAE-cellulose fast flow chromatographies. Different concentrations of the lectin were added to artificial diet of Ectomyelois ceratoniae larvae finding out its effect on some biological parameters, digestive physiology and amount of storage macromolecules. It was found that CCA (C. colocynthis Agglutinin) increased life span from 23.44 days in control to 28.59 days in the treated individuals. Survival of larvae on control and CCA diets were 93.3 and 66.6%, respectively. Different concentrations of CCA significantly affected α-amylase and general proteolytic activities except for TAG-lipase activity. Activities of all specific proteases decreased when larvae were fed on different concentrations of CCA except for aminopeptidase. Meanwhile, amount of storage macromolecules in the larvae fed on different concentrations of CCA statistically decreased vs. control. These results demonstrated that CCA could intervene in physiology of E. ceratoniae and survival of larvae. Therefore, it can be taken into consideration in IPM of the pest through plant breeding programs.

Association of jacalin-related lectins with wheat responses to stresses revealed by transcriptional profiling

Jacalin-related lectins (JRLs) are carbohydrate-binding proteins widely present in plants and have one or more jacalin domains in common. However, JRLs' structural types and functions are still poorly understood. In the present study, a total of 67 wheat (Triticum aestivum) JRL genes were identified through an exhausted search of EST database coupling with genome walking using published 454 sequence reads of Chinese Spring. A comparison of the translated wheat JRL proteins with those from other plants showed plant JRLs generally had low sequence similarity within and between species but exhibited conserved modular domain structures. More JRL genes encoded multiple jacalin domains in Arabidopsis thaliana, whereas more genes encoded chimeric JRLs in cereal plants. Dirigent domain-containing JRL genes were Poaceae-specific and accounted for nearly half of the identified wheat JRL genes. The dirigent domains were evolutionarily significantly correlated with the covalently linked jacalin domains. A phylogenetic analysis showed JRL proteins have experienced a substantial diversification after speciation. Moreover, new structural features conserved across the taxa were identified. Digital expression analysis and RT-PCR assays showed the expression of wheat JRL genes was largely tissue specific, typically low, and mostly inducible by biotic and abiotic stresses and stress hormones. These results suggest plant JRLs are critical for plant adaptation to stressful environments.

Jacalin domain in wheat jasmonate-regulated protein Ta-JA1 confers agglutinating activity and pathogen resistance

Ta-JA1 is a jacalin-like lectin from wheat (Triticum aestivum) plants. To date, its homologs are only observed in the Gramineae family. Our previous experiments have demonstrated that Ta-JA1 contains a modular structure consisting of an N-terminal dirigent domain and a C-terminal jacalin-related lectin domain (JRL) and this protein exhibits a mannose-specific lectin activity. The over-expression of Ta-JA1 gene provides transgenic plants a broad-spectrum resistance to diseases. Here, we report the differential activities of the dirigent and JRL domains of Ta-JA1. In vitro assay showed that the recombinant JRL domain could effectively agglutinate rabbit erythrocytes and pathogen bacteria Pseudomonas syringe pv tabaci. These hemagglutination activities could be inhibited by mannose but not by galactose. In contrast, the recombinant dirigent domain did not show agglutination activity. Corresponding to these differentiations of activities, similar to full-length of Ta-JA1, the over-expression of JRL domain in transgenic plants also increased resistance to the infection of P. syringe. Unlike JRL, the over-expression of dirigent domain in transgenic plants led to alteration of the seedling sensitivity to salts. In addition, a d(N)/d(S) ratio analysis of Ta-JA1 and its related proteins showed that this protein family functionally limited to a few crop plants, such as maize, rice and wheat.

Mechanisms of plant defense against insect herbivores

Plants respond to herbivory through various morphological, biochemicals, and molecular mechanisms to counter/offset the effects of herbivore attack. The biochemical mechanisms of defense against the herbivores are wide-ranging, highly dynamic, and are mediated both by direct and indirect defenses. The defensive compounds are either produced constitutively or in response to plant damage, and affect feeding, growth, and survival of herbivores. In addition, plants also release volatile organic compounds that attract the natural enemies of the herbivores. These strategies either act independently or in conjunction with each other. However, our understanding of these defensive mechanisms is still limited. Induced resistance could be exploited as an important tool for the pest management to minimize the amounts of insecticides used for pest control. Host plant resistance to insects, particularly, induced resistance, can also be manipulated with the use of chemical elicitors of secondary metabolites, which confer resistance to insects. By understanding the mechanisms of induced resistance, we can predict the herbivores that are likely to be affected by induced responses. The elicitors of induced responses can be sprayed on crop plants to build up the natural defense system against damage caused by herbivores. The induced responses can also be engineered genetically, so that the defensive compounds are constitutively produced in plants against are challenged by the herbivory. Induced resistance can be exploited for developing crop cultivars, which readily produce the inducible response upon mild infestation, and can act as one of components of integrated pest management for sustainable crop production.

Ipomoelin, a jacalin-related lectin with a compact tetrameric association and versatile carbohydrate binding properties regulated by its N terminus

Many proteins are induced in the plant defense response to biotic stress or mechanical wounding. One group is lectins. Ipomoelin (IPO) is one of the wound-inducible proteins of sweet potato (Ipomoea batatas cv. Tainung 57) and is a Jacalin-related lectin (JRL). In this study, we resolved the crystal structures of IPO in its apo form and in complex with carbohydrates such as methyl α-D-mannopyranoside (Me-Man), methyl α-D-glucopyranoside (Me-Glc), and methyl α-D-galactopyranoside (Me-Gal) in different space groups. The packing diagrams indicated that IPO might represent a compact tetrameric association in the JRL family. The protomer of IPO showed a canonical β-prism fold with 12 strands of β-sheets but with 2 additional short β-strands at the N terminus. A truncated IPO (ΔN10IPO) by removing the 2 short β-strands of the N terminus was used to reveal its role in a tetrameric association. Gel filtration chromatography confirmed IPO as a tetrameric form in solution. Isothermal titration calorimetry determined the binding constants (K(A)) of IPO and ΔN10IPO against various carbohydrates. IPO could bind to Me-Man, Me-Glc, and Me-Gal with similar binding constants. In contrast, ΔN10IPO showed high binding ability to Me-Man and Me-Glc but could not bind to Me-Gal. Our structural and functional analysis of IPO revealed that its compact tetrameric association and carbohydrate binding polyspecificity could be regulated by the 2 additional N-terminal β-strands. The versatile carbohydrate binding properties of IPO might play a role in plant defense.

A jacalin-related lectin-like gene in wheat is a component of the plant defence system

Jacalin-related lectins (JRLs) are a subgroup of proteins with one or more jacalin-like lectin domains. Although JRLs are often associated with biotic or abiotic stimuli, their biological functions in plants, as well as their relationships to plant disease resistance, are poorly understood. A mannose-specific JRL (mJRL)-like gene (TaJRLL1) that is mainly expressed in stem and spike and encodes a protein with two jacalin-like lectin domains was identified in wheat. Pathogen infection and phytohormone treatments induced its expression; while application of the salicylic acid (SA) biosynthesis inhibitor paclobutrazol and the jasmonic acid (JA) biosynthesis inhibitor diethyldithiocarbamic acid, respectively, substantially inhibited its expression. Attenuating TaJRLL1 through virus-induced gene silencing increased susceptibility to the facultative fungal pathogen Fusarium graminearum and the biotrophic fungal pathogen Blumeria graminis. Arabidopsis thaliana transformed with TaJRLL1 displayed increased resistance to F. graminearum and Botrytis cinerea. JA and SA levels in transgenic Arabidopsis increased significantly. A loss or increase of disease resistance due to an alteration in TaJRLL1 function was correlated with attenuation or enhancement of the SA- and JA-dependent defence signalling pathways. These results suggest that TaJRLL1 could be a component of the SA- and JA-dependent defence signalling pathways.

Plant lectins as defense proteins against phytophagous insects

One of the most important direct defense responses in plants against the attack by phytophagous insects is the production of insecticidal peptides or proteins. One particular class of entomotoxic proteins present in many plant species is the group of carbohydrate-binding proteins or lectins. During the last decade a lot of progress was made in the study of a few lectins that are expressed in response to herbivory by phytophagous insects and the insecticidal properties of plant lectins in general. This review gives an overview of lectins with high potential for the use in pest control strategies based on their activity towards pest insects. In addition, potential target sites for lectins inside the insect and the mode of action are discussed. In addition, the effect of plant lectins on non-target organisms such as beneficial insects as well as on human/animal consumers is discussed. It can be concluded that some insecticidal lectins are useful tools that can contribute to the development of integrated pest management strategies with minimal effect(s) on non-target organisms.

Deletion of the N-terminal dirigent domain in maize beta-glucosidase aggregating factor and its homolog sorghum lectin dramatically alters the sugar-specificities of their lectin domains

Maize beta-glucosidase aggregating factor (BGAF) and its homolog Sorghum Lectin (SL) are modular proteins consisting of an N-terminal dirigent domain and a C-terminal jacalin-related lectin (JRL) domain. BGAF is a polyspecific lectin with a monosaccharide preference for galactose, whereas SL displays preference for GalNAc. Here, we report that deletion of the N-terminal dirigent domain in the above lectins dramatically changes their sugar-specificities. Deletions in the N-terminal region of the dirigent domain of BGAF abolished binding to galactose/lactose, but binding to mannose was unaffected. Glucose, which was a poor inhibitor of hemagglutinating activity of BGAF, displayed higher inhibitory effect on the hemagglutinating activity of deletion mutants. Deletion of the dirigent domain in SL abolished binding to GalNAc, but binding to mannose was not affected. Surprisingly, fructose, an extremely poor inhibitor (minimum inhibitory concentration (MIC) = 125 mM) of SL hemagglutinating activity, was found to be a very potent inhibitor (MIC = 1 mM) of hemagglutinating activity of its JRL domain. These results indicate that the dirigent domain in this class of modular lectins, at least in the case of maize BGAF and SL, influences sugar specificity.

Overexpression of a wheat jasmonate-regulated lectin increases pathogen resistance

Jasmonates are known to induce the transcriptional activation of plant defense genes, which leads to the production of jasmonate-regulated proteins (JRP). We previously cloned and characterized a novel jacalin-like lectin gene (Ta-JA1) from wheat (Triticum aestivum L.), which codes a modular JRP with disease response and jacalin-related lectin (JRL) domains and is present only in the Gramineae family. The function of this protein is still unclear. Phylogenetic analysis indicated that Ta-JA1 and related proteins from cereals grouped together, which diverged from JRL with an additional N-terminal disease response domain. The recombinant Ta-JA1 proteins agglutinated rabbit erythrocytes, and this hemagglutination activity was preferentially inhibited by mannose. The Ta-JA1 protein was able to inhibit E. coli cell growth. Overexpression of Ta-JA1 in transgenic tobacco plants increased their resistance to infection by tobacco bacterial, fungal and viral pathogens. Our results suggest that Ta-JA1 belongs to a mannose-specific lectin, which may confer a basal but broad-spectrum resistance to plant pathogens. Ta-JA1 and its homologues in maize, rice, sorghum and creeping bentgrass may represent a new type of monocot lectin with a modular structure and diversity of physiological functions in biotic and abiotic stress responses.

Molecular characterization of a wheat protein induced by vernalisation

Using a PCR strategy we isolated from winter wheat (Triticum aestivum L. cv. Bolero) the ver2 gene coding for a modular protein constituted by an N-terminal domain called "dirigent", found in several defence-related genes, and a C-terminal domain related to jacalin-related lectin (JRL). ver2 transcript as well as native Ver2 levels increased during vernalisation and upon methyl jasmonate treatment of young seedlings. ver2 transcript levels were kept constant either in infected tissues or in wounded samples indicating that Ver2 is not directly involved in plant defence mechanisms. The Ver2 protein was expressed in bacteria as a recombinant GST-Ver2 fusion protein. The purified recombinant protein was further characterized using an affinity chromatography approach based on its interaction with mannose-agarose beads. GST-Ver2 tightly bound to the matrix. Molecular modelling of the jacalin domain and mannose docking confirmed that Ver2 possesses D: -mannose binding capacity.

Isolation of a low-sulfur tolerance gene from Eichhornia crassipes using a functional gene-mining approach

Genes enhancing nutrient utilization efficiency are needed for crop improvement. Here, we report the isolation of a gene conferring low-sulfur tolerance from water hyacinth (Eichhornia crassipes) using a functional gene-mining method. In doing this, an entry cDNA library was constructed from the roots of nutrient-starved water hyacinth using recombination cloning and subsequently shuttled into the plant transformation- and expression-ready vector. The plant transformation- and expression-ready library was transferred into Arabidopsis and a seed library of 50,000 independent transgenic lines was generated. Three transgenic lines with enhanced low-sulfur tolerance were isolated from the seed library. One of the transgenic lines, shl143-1, with improved tolerance to sulfate deficiency and an improved root system was further analyzed. It was found that a water hyacinth jacalin-related lectin gene (EcJRL-1) was overexpressed in shl143-1. Recapitulation analysis confirmed that the overexpression of the EcJRL-1 cDNA caused the phenotype. Therefore, this study demonstrates that a jacalin-related lectin is involved in root elongation under sulfur-deficient conditions.

Dendrobium findleyanum agglutinin: production, localization, anti-fungal activity and gene characterization

The recently reported Dendrobium findleyanum agglutinin (DFA) was identified and determined in different parts of D. findleyanum pseudobulbs by using Western blot analysis, LC-MS/MS, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and histochemical procedure. Western blot analysis of crude protein extract with horseradish peroxidase (HRP), a mannose-rich glycoprotein, showed only one band at 14.5 kDa, which had the same molecular mass as DFA. This band was a major band when the membrane was stained with Coomassie Brilliant Blue. The protein profiles from SDS-PAGE showed higher band intensity of the 14.5 kDa mannose-binding protein in nearly mature and mature stages, compared to very young and young stages of the orchid. In addition, the band intensity was to a great extent different between the swollen and the non-swollen internode of the pseudobulb. Using LC-MS/MS, the sequence tags of the 14.5-kDa protein bands from the node, swollen internode and non-swollen internode revealed that the protein was DFA. Histochemical procedure in the transverse section of the pseudobulbs demonstrated major HRP binding sites, which reflected the location of DFA, in periphery of parenchymal cells. The purified DFA showed anti-fungal activity against Alternaria alternata and Collectotrichum sp. Using reverse transcription polymerase chain reaction and DNA sequencing, the deduced amino acid sequence of the DFA precursor revealed 94% homology with a lectin precursor from D. officinale. N-terminal sequencing demonstrated the processing site between residues 24 and 25 of the DFA precursor.

Nucleocytoplasmic plant lectins

During the last decade it was unambiguously shown that plants synthesize minute amounts of carbohydrate-binding proteins upon exposure to stress situations like drought, high salt, hormone treatment, pathogen attack or insect herbivory. In contrast to the 'classical' plant lectins, which are typically found in storage vacuoles or in the extracellular compartment this new class of lectins is located in the cytoplasm and the nucleus. Based on these observations the concept was developed that lectin-mediated protein-carbohydrate interactions in the cytoplasm and the nucleus play an important role in the stress physiology of the plant cell. Hitherto, six families of nucleocytoplasmic lectins have been identified. This review gives an overview of our current knowledge on the occurrence of nucleocytoplasmic plant lectins. The carbohydrate-binding properties of these lectins and potential ligands in the nucleocytoplasmic compartment are discussed in view of the physiological role of the lectins in the plant cell.

Phosphorylation modification of wheat lectin VER2 is associated with vernalization-induced O-GlcNAc signaling and intracellular motility

BACKGROUND

O-linked beta-N-acetylglucosamine (O-GlcNAc) modification of proteins mediates stress response and cellular motility in animal cells. The plant lectin concanavalin A can increase nuclear O-GlcNAc levels and decrease cytoplasmic O-GlcNAc levels in T lymphocytes. However, the functions of O-GlcNAc signaling in plants, as well as the relation between plant lectins and O-GlcNAc in response to environmental stimuli are largely undefined.

METHODOLOGY/PRINCIPAL FINDINGS

We describe a jacalin-like lectin VER2 in wheat that shows N-acetylglucosamine and galactose specificity. Immunocytochemical localization showed VER2 expression induced predominantly at potential nuclear structures in shoot tips and young leaves and weakly in cytoplasm in response to vernalization. In contrast, under devernalization (continuous stimulation with a higher temperature after vernalization), VER2 signals appeared predominantly in cytoplasm. 2-D electrophoresis, together with western blot analysis, showed phosphorylation modification of VER2 under vernalization. Immunoblot assay with O-GlcNAc-specific antibody revealed that vernalization increased O-GlcNAc modification of proteins at the global level. An O-GlcNAc-modified protein co-immunoprecipitated with VER2 in vernalized wheat plants but not in devernalized materials. The dynamic of VER2 was observed in transgenic Arabidopsis overexpressing the VER2-GFP fusion protein. Overexpressed VER2 accelerated nuclear migration. Immunogold labeling and indirect immunofluoresence colocalization assay indicated that VER2-GFP was targeted to the secretory pathway.

CONCLUSIONS/SIGNIFICANCE

O-GlcNAc signaling is involved in the vernalization response in wheat, and phosphorylation is necessary for the lectin VER2 involving O-GlcNAc signaling during vernalization. Our findings open the way to studies of O-GlcNAc protein modification in response to environmental signals in plants.

Homolog of the maize beta-glucosidase aggregating factor from sorghum is a jacalin-related GalNAc-specific lectin but lacks protein aggregating activity

Recently, we identified the maize beta-glucosidase aggregating factor (BGAF) as a jacalin-related lectin (JRL) and showed that its lectin domain is responsible for beta-glucosidase aggregation. By searching for BGAF homologs in sorghum, we identified and obtained an EST clone and determined its complete sequence. The predicted protein had the same modular structure as maize BGAF, shared 67% sequence identity with it, and revealed the presence of two potential carbohydrate-binding sites (GG...ATYLQ, site I and GG...GVVLD, site II). Maize BGAF1 is the only lectin from a class of modular JRLs containing an N-terminal dirigent and a C-terminal JRL domain, whose sugar specificity and beta-glucosidase aggregating activity have been studied in detail. We purified to homogeneity a BGAF homolog designated as SL (Sorghum lectin) from sorghum and expressed its recombinant version in Escherichia coli. The native protein had a molecular mass of 32 kD and was monomeric. Both native and recombinant SL-agglutinated rabbit erythrocytes, and inhibition assays indicated that SL is a GalNAc-specific lectin. Exchanging the GG...GVVLD motif in SL with that of maize BGAF1 (GG...GIAVT) had no effect on GalNAc-binding, whereas binding to Man was abolished. Substitution of Thr(293) and Gln(296) in site I to corresponding residues (Val(294) and Asp(297)) of maize BGAF1 resulted in the loss of GalNAc-binding, indicating that site I is responsible for generating GalNAc specificity in SL. Gel-shift and pull-down assays after incubating SL with maize and sorghum beta-glucosidases showed no evidence of interaction nor were any SL-protein complexes detected in sorghum tissue extracts, suggesting that the sorghum homolog does not participate in protein-protein interactions.

RNAi knockdown of Oryza sativa root meander curling gene led to altered root development and coiling which were mediated by jasmonic acid signalling in rice

Jasmonic acid (JA) is a well-known defence hormone, but its biological function and mechanism in rice root development are less understood. Here, we describe a JA-induced putative receptor-like protein (OsRLK, AAL87185) functioning in root development in rice. RNA in situ hybridization revealed that the gene was expressed largely in roots, and a fusion protein showed its localization on the plasma membrane. The primary roots in RNAi transgenic rice plants meandered and curled more easily than wild-type (WT) roots under JA treatment. Thus, this gene was renamed Oryza sativa root meander curling (OsRMC). The transgenic primary roots were shorter, the number of adventitious roots increased and the number of lateral roots decreased as compared to the WT. As well, the second sheath was reduced in length. Growth of both primary roots and second sheaths was sensitive to JA treatment. No significant change of JA level appeared in the roots between the transgenic rice line and WT. Expression of RSOsPR10, involved in the JA signalling pathway, was induced in transgenic rice. Western blotting revealed OsRMC induced by JA. Our results suggest that OsRMC of the DUF26 subfamily involved in JA signal transduction mediates root development and negatively regulates root curling in rice.