Monocot chimeric jacalins: a novel subfamily of plant lectins

Lignin Biosynthesis and Its Diversified Roles in Disease Resistance

Lignin is complex, three-dimensional biopolymer existing in plant cell wall. Lignin biosynthesis is increasingly highlighted because it is closely related to the wide applications in agriculture and industry productions, including in pulping process, forage digestibility, bio-fuel, and carbon sequestration. The functions of lignin in planta have also attracted more attentions recently, particularly in plant defense response against different pathogens. In this brief review, the progress in lignin biosynthesis is discussed, and the lignin's roles in disease resistance are thoroughly elucidated. This issue will help in developing broad-spectrum resistant crops in agriculture.

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.

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.

Transcriptome Changes Reveal the Molecular Mechanisms of Humic Acid-Induced Salt Stress Tolerance in Arabidopsis

Humic acid (HA) is a principal component of humic substances, which make up the complex organic matter that broadly exists in soil environments. HA promotes plant development as well as stress tolerance, however the precise molecular mechanism for these is little known. Here we conducted transcriptome analysis to elucidate the molecular mechanisms by which HA enhances salt stress tolerance. Gene Ontology Enrichment Analysis pointed to the involvement of diverse abiotic stress-related genes encoding HEAT-SHOCK PROTEINs and redox proteins, which were up-regulated by HA regardless of salt stress. Genes related to biotic stress and secondary metabolic process were mainly down-regulated by HA. In addition, HA up-regulated genes encoding transcription factors (TFs) involved in plant development as well as abiotic stress tolerance, and down-regulated TF genes involved in secondary metabolic processes. Our transcriptome information provided here provides molecular evidences and improves our understanding of how HA confers tolerance to salinity stress in plants.

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.

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.

An Update on Jacalin-Like Lectins and Their Role in Plant Defense

Plant lectins are proteins that reversibly bind carbohydrates and are assumed to play an important role in plant development and resistance. Through the binding of carbohydrate ligands, lectins are involved in the perception of environmental signals and their translation into phenotypical responses. These processes require down-stream signaling cascades, often mediated by interacting proteins. Fusing the respective genes of two interacting proteins can be a way to increase the efficiency of this process. Most recently, proteins containing jacalin-related lectin (JRL) domains became a subject of plant resistance responses research. A meta-data analysis of fusion proteins containing JRL domains across different kingdoms revealed diverse partner domains ranging from kinases to toxins. Among them, proteins containing a JRL domain and a dirigent domain occur exclusively within monocotyledonous plants and show an unexpected high range of family member expansion compared to other JRL-fusion proteins. Rice, wheat, and barley plants overexpressing OsJAC1, a member of this family, are resistant against important fungal pathogens. We discuss the possibility that JRL domains also function as a decoy in fusion proteins and help to alert plants of the presence of attacking pathogens.

Identification, classification and transcriptional profiles of dirigent domain-containing proteins in sugarcane

Dirigent (DIR) proteins, encoded by DIR genes, are referred to as "dirigent" because they direct the outcome of the coupling of the monolignol coniferyl alcohol into (+) or (-) pinoresinol, the first intermediates in the enantiocomplementary pathways for lignan biosynthesis. DIR domain-containing or DIR-like proteins are, thus, termed for not having a clear characterization. A transcriptome- and genome-wide survey of DIR domain-containing proteins in sugarcane was carried out, in addition to phylogenetic, physicochemical and transcriptional analyses. A total of 120 non-redundant sequences containing the DIR domain were identified and classified into 64 groups according to phylogenetic and sequence alignment analyses. In silico analysis of transcript abundance showed that these sequences are expressed at low levels in leaves and genes in the same phylogenetic clade have similar expression patterns. Expression analysis of ShDIR1-like transcripts in the culm internodes of sugarcane demonstrates their abundance in mature internodes, their induction by nitrogen fertilization and their predominant expression in cells that have a lignified secondary cell wall, such as vascular bundles of young internodes and parenchymal cells of the pith of mature internodes. Due to the lack of information about the functional role of DIR in plants, a possible relationship is discussed between the ShDIR1-like transcriptional profile and cell wall development in parenchyma cells of sugarcane culm, which typically accumulates large amounts of sucrose. The number of genes encoding the DIR domain-containing proteins in sugarcane is intriguing and is an indication per se that these proteins may have an important metabolic role and thus deserve to be better studied.

Dirigent proteins in plants: modulating cell wall metabolism during abiotic and biotic stress exposure

Dirigent (DIR) proteins were found to mediate regio- and stereoselectivity of bimolecular phenoxy radical coupling during lignan biosynthesis. Here we summarize the current knowledge of the importance of DIR proteins in lignan and lignin biosynthesis and highlight their possible importance in plant development. We focus on the still rather enigmatic Arabidopsis DIR gene family, discussing the few members with known functional importance. We comment on recent discoveries describing the detailed structure of two DIR proteins with implications in the mechanism of DIR-mediated catalysis. Further, we summarize the ample evidence for stress-induced dirigent gene expression, suggesting the role of DIRs in adaptive responses. In the second part of our work, we present a preliminary bioinformatics-based characterization of the AtDIR family. The phylogenetic analysis of AtDIRs complemented by comparison with DIR proteins of mostly known function from other species allowed us to suggest possible roles for several members of this family and identify interesting AtDIR targets for further study. Finally, based on the available metadata and our in silico analysis of AtDIR promoters, we hypothesize about the existence of specific transcriptional controls for individual AtDIR genes and implicate them in various stress responses, hormonal regulations, and developmental processes.

Comparative Study of Lectin Domains in Model Species: New Insights into Evolutionary Dynamics

Lectins are present throughout the plant kingdom and are reported to be involved in diverse biological processes. In this study, we provide a comparative analysis of the lectin families from model species in a phylogenetic framework. The analysis focuses on the different plant lectin domains identified in five representative core angiosperm genomes (Arabidopsisthaliana, Glycine max, Cucumis sativus, Oryza sativa ssp. japonica and Oryza sativa ssp. indica). The genomes were screened for genes encoding lectin domains using a combination of Basic Local Alignment Search Tool (BLAST), hidden Markov models, and InterProScan analysis. Additionally, phylogenetic relationships were investigated by constructing maximum likelihood phylogenetic trees. The results demonstrate that the majority of the lectin families are present in each of the species under study. Domain organization analysis showed that most identified proteins are multi-domain proteins, owing to the modular rearrangement of protein domains during evolution. Most of these multi-domain proteins are widespread, while others display a lineage-specific distribution. Furthermore, the phylogenetic analyses reveal that some lectin families evolved to be similar to the phylogeny of the plant species, while others share a closer evolutionary history based on the corresponding protein domain architecture. Our results yield insights into the evolutionary relationships and functional divergence of plant lectins.