Isolation of monoclonal antibodies reacting with the core component of lipopolysaccharide from Rhizobium leguminosarum strain 3841 and mutant derivatives

Application of Recombinant Human scFv Antibody as a Powerful Tool to Monitor Nitrogen Fixing Biofertilizer in Rice and Legume

Bradyrhizobium is an endophytic bacterium under investigation as an efficient biofertilizer for sustainable legume-rice rotational cropping system. Monitoring and bio-imaging of this nitrogen fixing bacterium is essential for the study of plant-microbe evolution, soil microbiome, as well as quality control in organic farming. While phage display antibody technology has been widely used to generate recombinant antibody for myriad medical purposes, so far, this technology has been minimally applied in the agricultural sector. In this study, single-chain variable fragments (scFv) against two Bradyrhizobium strains SUTN9-2 (yiN92-1e10) and DOA9 (yiDOA9-162) were isolated from a human phage display antibody library. Specific binding of scFv was demonstrated by ELISA and confocal-immunofluorescence imaging techniques. Bradyrhizobium localization in both endophytic and bacteroid forms could be observed inside rice tissue and plant nodule, respectively. Moreover, successful application of the recombinant antibody for the evaluation of nodule occupancy was also demonstrated in comparison with standard GUS-staining method. The results of this study showed for the first time the potential use of human phage display scFv antibody for imaging and monitoring of Bradyrhizobium biofertilizer and thus could be further applied for point-of-detection of bacterial inoculum in the legume-rice rotational crop system.

IMPORTANCE Human scFv antibody generated from phage display technology was successfully used for the generation of specific recombinant antibodies: yiN92-1e10 and yiDOA9-162 for the detection of Bradyrhizobium strains SUTN9-2 and DOA9, respectively. These two recombinant scFv antibodies could be used for precise detection of the rhizobia both in symbiosis with legume and endophyte in rice tissue by ELISA and immunofluorescent staining, during legume-rice rotational cropping system in the field. This methodology can be further employed for the study of other plant-microbe interactions and monitoring of biofertilizer in diverse sustainable cropping systems as well as in precision agriculture.

Generation of a rabbit single-chain fragment variable (scFv) antibody for specific detection of Bradyrhizobium sp. DOA9 in both free-living and bacteroid forms

A simple and reliable method for the detection of specific nitrogen-fixing bacteria in both free-living and bacteroid forms is essential for the development and application of biofertilizer. Traditionally, a polyclonal antibody generated from an immunized rabbit was used for detection. However, the disadvantages of using a polyclonal antibody include limited supply and cross-reactivity to related bacterial strains. This is the first report on the application of phage display technology for the generation of a rabbit recombinant monoclonal antibody for specific detection and monitoring of nitrogen-fixing bacteria in both free-living form and in plant nodules. Bradyrhizobium sp. DOA9, a broad host range soil bacteria, originally isolated from the root nodules of Aeschynomene americana in Thailand was used as a model in this study. A recombinant single-chain fragment variable (scFv) antibody library was constructed from the spleen of a rabbit immunized with DOA9. After three rounds of biopanning, one specific phage-displayed scFv antibody, designated bDOA9rb8, was identified. Specific binding of this antibody was confirmed by phage enzyme-linked immunosorbent assay (phage ELISA). The phage antibody could bind specifically to DOA9 in both free-living cells (pure culture) and bacteroids inside plant nodules. In addition to phage ELISA, specific and robust immunofluorescence staining of both free-living and bacteroid forms could also be observed by confocal-immunofluorescence imaging, without cross-reactivity with other tested bradyrhizobial strains. Moreover, specific binding of free scFv to DOA9 was also demonstrated by ELISA. This recombinant antibody can also be used for the study of the molecular mechanism of plant-microbe interactions in the future.

SARCOSYL-PAGE: a new method for the detection of MIRCERA- and EPO-doping in blood

The detection of doping with MIRCERA (the brand name for Continuous Erythropoietin Receptor Activator, or CERA) is hampered by the limited excretion of the rather large molecule (approximately 60 kDa) in urine. Blood (serum, plasma) in combination with sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) appears to be the ideal matrix for detecting all forms of doping with erythropoiesis-stimulating agents (ESAs) because the apparent molecular masses of ESAs are different from the mass of human serum erythropoietin (shEPO). While SDS-PAGE has proven the most sensitive method for the detection of doping with Dynepo, the sensitivity of SDS-PAGE for MIRCERA is drastically decreased. By exchanging the SDS for SARCOSYL (SAR) in the sample and running buffers the sensitivity problem was solved. SARCOSYL, a methyl glycine-based anionic surfactant, is only binding to the protein-part of MIRCERA but not to its polyethylene glycol (PEG)-chain, while SDS binds to both parts. In consequence, the monoclonal anti-EPO antibody (clone AE7A5) no longer interacts with the fully SDS-solubilized MIRCERA molecules. Only those molecules that contain SDS bound to the protein-chain are detected. Due to the inability of SARCOSYL to solubilize PEG-molecules, MIRCERA can be detected on SARCOSYL-PAGE with the same sensitivity as non-PEGylated epoetins. In a typical SAR-PAGE experiment, 200 microL of serum are used, which allows the direct detection of MIRCERA, recombinant epoetins (such as NeoRecormon, Dynepo, NESP), and shEPO in a single experiment and with high (i.e. femtogram) sensitivity.

Diversity of rhizobial bacteria isolated from nodules of the Gypsophyte Ononis tridentata L. growing in Spanish soils

The aim of this work is to describe the diversity and phylogeny of rhizobial bacteria associated to nodules of Ononis tridentata L. in different geographical regions of Spain. Twenty-two bacterial isolates were characterized using several molecular techniques (16S amplified ribosomal deoxyribonucleic acid restriction analysis, fingerprinting, and sequencing) and phylogenies were inferred from their 16S and nodC gene sequences. Phylogenetically, the isolates grouped with the genera Rhizobium, Mesorhizobium, Phylobacterium, and Bosea. The nodC gene, essential for nodulation, was detected for the first time in isolates close to the genera Bosea and Phyllobacterium. The bacteria isolated showed a high diversity at the genus, species, and strain level regardless of the geographical origin of the host plant. This is the first report describing bacteria associated to nodules of O. tridentata. This shrub legume is highly prized for the revegetation of gypsum soils in semiarid Mediterranean areas. Our molecular description of bacteria associated to this legume improves the current understanding of the ecology of this plant species. Our findings have implications for formulating suitable bacterial inocula to recover gypsum ecosystems.

Cell surface interactions of Rhizobium bacteroids and other bacterial strains with symbiosomal and peribacteroid membrane components from pea nodules

Samples of Rhizobium bacteroids isolated from pea nodule symbiosomes reacted positively with a monoclonal antibody recognizing N-linked glycan epitopes on plant glycoproteins associated with the peribacteroid membrane and peribacteroid fluid. An antiserum recognizing the symbiosomal lectin-like glycoprotein PsNLEC-1 also reacted positively. Samples of isolated bacteroids also reacted with an antibody recognizing a glycolipid component of the peribacteroid membrane and plasma membrane. Bacterial cells derived from free-living cultures then were immobilized on nitrocellulose sheets and tested for their ability to associate with components of plant extracts derived from nodule fractionation. A positive antibody-staining reaction indicated that both PsNLEC-1 and membrane glycolipid had become associated with the bacterial surface. A range of rhizobial strains with mutants affecting cell surface polysaccharides all showed similar interactions with PsNLEC-1 and associated plant membranes, with the exception of strain B659 (a deep-rough lipopolysaccharide mutant of Rhizobium leguminosarum). However, the presence of a capsule of extracellular polysaccharide apparently prevented interactions between rhizobial cells and these plant components. The importance of a close association between peribacteroid membranes, PsNLEC-1, and the bacterial surface is discussed in the context of symbiosome development.

Identification of an ATP-binding cassette transporter for export of the O-antigen across the inner membrane in Rhizobium etli based on the genetic, functional, and structural analysis of an lps mutant deficient in O-antigen

For O-antigen lipopolysaccharide (LPS) synthesis in bacteria, transmembrane migration of undecaprenyl pyrophosphate-bound O-antigen oligosaccharide subunits or polysaccharide occurs before ligation to the core region of the LPS molecule. In this study, we identified by mutagenesis an ATP-binding cassette transporter in Rhizobium etli CE3 that is likely responsible for the translocation of the O-antigen across the inner plasma membrane. Mutant FAJ1200 LPS lacks largely the O-antigen, as shown by SDS-polyacrylamide gel electrophoresis and confirmed by immunoblot analysis. Furthermore, LPS isolated from FAJ1200 is totally devoid of any O-chain glycosyl residues and contains only those glycosyl residues that can be expected for the inner core region. The membrane component and the cytoplasmic ATP-binding component of the ATP-binding cassette transporter are encoded by wzm and wzt, respectively. The Tn5 transposon in mutant FAJ1200 is inserted in the wzm gene. This mutation resulted in an Inf- phenotype in bean plants.

Lipid A and O-chain modifications cause Rhizobium lipopolysaccharides to become hydrophobic during bacteroid development

Modifications to the lipopolysaccharide (LPS) structure caused by three different growth conditions were investigated in the pea-nodulating strain Rhizobium leguminosarum 3841. The LPSs extracted by hot phenol-water from cultured cells fractionated into hydrophilic water and/or hydrophobic phenol phases. Most of the LPSs from cells grown under standard conditions extracted into the water phase, but a greater proportion of LPSs were extracted into the phenol phase from cells grown under acidic or reduced-oxygen conditions, or when isolated from root nodules as bacteroids. Compared with the water-extracted LPSs, the phenol-extracted LPSs contained greater degrees of glycosyl methylation and O-acetylation, increased levels of xylose, glucose and mannose and increased amounts of long-chain fatty acids attached to the lipid A moiety. The water- and phenol-phase LPSs also differed in their reactivity with monoclonal antibodies and in their polyacrylamide gel electrophoretic banding patterns. Phenol-extracted LPSs from rhizobia grown under reduced-oxygen conditions closely resembled the bulk of LPSs isolated from pea nodule bacteria (i.e. mainly bacteroids) in their chemical properties, reactivities with monoclonal antibodies and extraction behaviour. This finding suggests that, during symbiotic bacteroid development, reduced oxygen tension induces structural modifications in LPSs that cause a switch from predominantly hydrophilic to predominantly hydrophobic molecular forms. Increased hydrophobicity of LPSs was also positively correlated with an increase in the surface hydrophobicity of whole cells, as shown by the high degree of adhesion to hydrocarbons of bacterial cells isolated from nodules or from cultures grown under low-oxygen conditions. The implications of these LPS modifications are discussed for rhizobial survival and function in different soil and in planta habitats.

Extracellular glycanases of Rhizobium leguminosarum are activated on the cell surface by an exopolysaccharide-related component

Rhizobium leguminosarum secretes two extracellular glycanases, PlyA and PlyB, that can degrade exopolysaccharide (EPS) and carboxymethyl cellulose (CMC), which is used as a model substrate of plant cell wall cellulose polymers. When grown on agar medium, CMC degradation occurred only directly below colonies of R. leguminosarum, suggesting that the enzymes remain attached to the bacteria. Unexpectedly, when a PlyA-PlyB-secreting colony was grown in close proximity to mutants unable to produce or secrete PlyA and PlyB, CMC degradation occurred below that part of the mutant colonies closest to the wild type. There was no CMC degradation in the region between the colonies. By growing PlyB-secreting colonies on a lawn of CMC-nondegrading mutants, we could observe a halo of CMC degradation around the colony. Using various mutant strains, we demonstrate that PlyB diffuses beyond the edge of the colony but does not degrade CMC unless it is in contact with the appropriate colony surface. PlyA appears to remain attached to the cells since no such diffusion of PlyA activity was observed. EPS defective mutants could secrete both PlyA and PlyB, but these enzymes were inactive unless they came into contact with an EPS(+) strain, indicating that EPS is required for activation of PlyA and PlyB. However, we were unable to activate CMC degradation with a crude EPS fraction, indicating that activation of CMC degradation may require an intermediate in EPS biosynthesis. Transfer of PlyB to Agrobacterium tumefaciens enabled it to degrade CMC, but this was only observed if it was grown on a lawn of R. leguminosarum. This indicates that the surface of A. tumefaciens is inappropriate to activate CMC degradation by PlyB. Analysis of CMC degradation by other rhizobia suggests that activation of secreted glycanases by surface components may occur in other species.