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

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.

Dirigent protein subfamily function and structure in terrestrial plant phenol metabolism

Aquatic plant transition to land, and subsequent terrestrial plant species diversification, was accompanied by the emergence and massive elaboration of plant phenol chemo-diversity. Concomitantly, dirigent protein (DP) and dirigent-like protein subfamilies, derived from large multigene families, emerged and became extensively diversified. DP biochemical functions as gateway entry points into new and diverse plant phenol skeletal types then markedly expanded. DPs have at least eight non-uniformly distributed subfamilies, with different DP subfamily members of known biochemical/physiological function now implicated as gateway entries to lignan, lignin, aromatic diterpenoid, pterocarpan and isoflavene pathways. While some other DP subfamily members have jacalin domains, both these and indeed the majority of DPs throughout the plant kingdom await discovery of their biochemical roles. Methods and approaches were developed to discover DP biochemical function as gateway entry points to distinct plant phenol skeletal types in land plants. Various DP 3D X-ray structural determinations enabled structure-based comparative sequence analysis and modeling to understand similarities and differences among the different DP subfamilies. We consider that the core DP β-barrel fold and associated characteristics are likely common to all DPs, with several residues conserved and nearly invariant. There is also considerable variation in residue composition and topography of the putative substrate binding pockets, as well as substantial differences in several loops, such as the β1-β2 loop. All DPs likely bind and stabilize quinone methide intermediates, while guiding distinctive regio- and/or stereo-chemical entry into Nature's chemo-diverse land plant phenol metabolic classes.

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.

Genome-wide screening of lectin putative genes from Sorghum bicolor L., distribution in QTLs and a probable implications of lectins in abiotic stress tolerance

BACKGROUND

Sorghum bicolor is one of the most important crops worldwide with the potential to provide resilience when other economic staples might fail against the continuous environmental changes. Many physiological, developmental and tolerance traits in plants are either controlled or influenced by lectins; carbohydrate binding proteins. Hence, we aimed at providing a comprehensive in silico account on sorghum's lectins and study their possible implication on various desired agronomical traits.

RESULTS

We have searched sorghum's genome from grain and sweet types for lectins putative genes that encode proteins with domains capable of differentially binding carbohydrate moieties and trigger various physiological responses. Of the 12 known plant lectin families, 8 were identified regarding their domain architectures, evolutionary relationships, physiochemical characteristics, and gene expansion mechanisms, and they were thoroughly addressed. Variations between grain and sweet sorghum lectin homologs in term of the presence/absence of certain other joint domains like dirigent and nucleotide-binding adaptor shared by APAF-1, R-proteins, and CED-4 (NB-ARC) indicate a possible neofunctionalization. Lectin sequences were found to be preferentially overrepresented in certain quantitative trait loci (QTLs) related to various traits under several subcategories such as cold, drought, salinity, panicle/grain composition, and leaf morphology. The co-localization and distribution of lectins among multiple QTLs provide insights into the pleiotropic effects that could be played by one lectin gene in numerous traits.

CONCLUSION

Our study offers a first-time inclusive details on sorghum lectins and their possible role in conferring tolerance against abiotic stresses and other economically important traits that can be informative for future functional analysis and breeding studies.

Growing Maize Root: Lectins Involved in Consecutive Stages of Cell Development

Proteins that carry specific carbohydrate-binding lectin domains have a great variety and are ubiquitous across the plant kingdom. In turn, the plant cell wall has a complex carbohydrate composition, which is subjected to constant changes in the course of plant development. In this regard, proteins with lectin domains are of great interest in the context of studying their contribution to the tuning and monitoring of the cell wall during its modifications in the course of plant organ development. We performed a genome-wide screening of lectin motifs in the Zea mays genome and analyzed the transcriptomic data from five zones of primary maize root with cells at different development stages. This allowed us to obtain 306 gene sequences encoding putative lectins and to relate their expressions to the stages of root cell development and peculiarities of cell wall metabolism. Among the lectins whose expression was high and differentially regulated in growing maize root were the members of the EUL, dirigent–jacalin, malectin, malectin-like, GNA and Nictaba families, many of which are predicted as cell wall proteins or lectin receptor-like kinases that have direct access to the cell wall. Thus, a set of molecular players was identified with high potential to play important roles in the early stages of root morphogenesis.

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.

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.

Identification of monocot chimeric jacalin family reveals functional diversity in wheat

MAIN CONCLUSION

46 monocot chimeric jacalins (MCJs) were mined from wheat genome. They were divided into three subfamilies with the activity of mannose-specific lectins and had effects on dehydration tolerance or disease resistance. Monocot chimeric jacalin (MCJ) is a newly identified subfamily of plant lectins that exclusively exists in Poaceae. The MCJs are modular proteins consisting of a dirigent domain and a jacalin-related lectin domain. Their unique evolution and various functions are not fully understood as only few members of MCJ have so for been investigated. From wheat, 46 MCJs were identified and phylogenetically classified into three subfamilies, in which subfamily I represented the early evolutionary cluster. MCJ genes are evenly distributed among three subgenomes of wheat, indicating that MCJ might be an ancient gene in Poaceae. qRT-PCR analysis showed that TaMCJ1 and TaMCJ2 were mainly expressed in leaves while TaMCJ3 in root tissues. All these TaMCJ genes are JA or ABA inducible. All three proteins exhibited agglutinating activity but different preference to mannose-binding. The overexpression of TaMCJ3 in tobacco increased dehydration tolerance, while TaMCJ1 enhanced wildfire disease resistance. The lignin biosynthetic genes were temporarily induced after pathogen inoculation in transgenic tobacco overexpressing TaMCJ, but the specific association with TaMCJ was not established. This evidence argued against the notion that the dirigent domain in TaMCJ is directly linked with lignin metabolism. Taken together, these results pave the way for a better understanding of the manifold functionality of MCJs and offer important insights to the evolutionary history of MCJ.

Prediction of Dhurrin Metabolism by Transcriptome and Metabolome Analyses in Sorghum

Sorghum (Sorghum bicolor (L.)) Moench is an important food for humans and feed for livestock. Sorghum contains dhurrin which can be degraded into toxic hydrogen cyanide. Here, we report the expression patterns of 14 candidate genes related to dhurrin ((S)-4-Hydroxymandelnitrile-β-D-glucopyranoside) metabolism and the effects of the gene expression on specific metabolite content in selected sorghum accessions. Dhurrin-related metabolism is vigorous in the early stages of development of sorghum. The dhurrin contents of most accessions tested were in the range of approximately 6-22 μg mg-1 fresh leaf tissue throughout growth. The p-hydroxybenzaldehyde (pHB) contents were high at seedling stages, but almost nonexistent at adult stages. The contents of p-hydroxyphenylacetic acid (pHPAAc) were relatively low throughout growth compared to those of dhurrin or pHB. Generally, the expression of the candidate genes was higher at seedling stage than at other stages and decreased gradually as plants grew. In addition, we identified significant SNPs, and six of them were potentially associated with non-synonymous changes in CAS1. Our results may provide the basis for choosing breeding materials to regulate cyanide contents in sorghum varieties to prevent HCN toxicity of livestock or to promote drought tolerance or pathogen resistance.

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.

Monocot chimeric jacalins: a novel subfamily of plant lectins

Monocot chimeric jacalins are a small group of lectins (currently with nine members), each typically consisting of a dirigent domain and a jacalin-related lectin domain. This unique module structure, along with their limited taxonomic distribution and short time window in molecular evolution, makes them a novel family of lectins. Recent studies have shown that these proteins play important roles in plant stress responses and development. Our knowledge of these proteins in functional domain and evolution has also made significant progress.

A sorghum xylanase inhibitor-like protein with highly potent antifungal, antitumor and HIV-1 reverse transcriptase inhibitory activities

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The antifungal protein purified in this study is a pH stable and thermostable xylanase inhibitor.

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Sorghum antifungal protein is inhibitory toward various fungal species.

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The sorghum antifungal protein exerts suppressive action on the proliferating hepatoma (HepG2) cells and breast cancer (MCF7) cells.

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Sorghum antifungal protein exerts a highly potent inhibitory activity against HIV-1 reverse transcriptase.

A 25-kDa protein, with an N-terminal amino acid sequence homologous to that of xylanase inhibitor and designated as xylanase inbibitor-like protein (XILP) was purified from sorghum seeds. The isolation protocol consisted of affinity chromatography, ion exchange chromatography, and gel filtration. XILP inhibited mycelial growth in various phytopathogenic fungi. The antifungal activity was thermostable and pH-stable. XILP inhibited proliferation of various cancer cell lines but did not do so in human embryonic liver (WRL 68) cells. There was no mitogenic activity toward mouse splenocytes. XILP reduced the activity of HIV-1 reverse transcriptase with an IC₅₀ of 11.1 μM, but lacked inhibitory activity toward HIV-1 integrase and SARS coronavirus proteinase. In conclusion, sorghum XILP is thermostable and pH stable and exhibits potent antifungal, antiproliferative, and HIV-1 reverse transcriptase inhibitory activities.

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.

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.

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.