Immunoblotting detection of lectins in gluten and white rice flour

The gluten lectin was isolated by affinity chromatography, separated by sodium dodecyl sulphate-gel electrophoresis together with purified wheat germ agglutinin (WGA) and electrotransferred to nitrocellulose filters. The binding pattern of anti-WGA to the blotted filters confirmed the presence of WGA in gluten. A lectin from rice bran and white rice flour, respectively, was isolated by affinity chromatography. Both lectins reacted with anti-WA in immunoblotting. As patients with coeliac disease are known to tolerate rice flour, the finding of a WGA-like lectin questioned the suggestion that WGA in gluten is involved in the pathogenesis of coeliac disease. A second lectin was also isolated from rice flour which reacted only with antibodies against soybean lectin on immunoblots. This may indicate a contamination of soybean proteins in rice flour.     

Purification and saccharide-binding characteristics of a rice lectin

A lectin was purified from rice flour by aqueous extraction followed by precipitation by ammonium sulfate and affinity chromatography on p-aminobenzyl 2-acetamido-2-deoxy-1-thio-beta-D-glucoside-succinyl-aminohexylaminyl -Sepharose 4B. The molecular weight of the lectin is approximately 36,000, as determined by sedimentation-equilibrium analysis. It is a tetramer consisting of two different subunits (Mr = 12,000 +/- 1,000 and 9,000 +/- 1,000). Amino acid analysis indicated that the lectin contains very high proportions of half-cystine, glycine, and glutamic acid. All of the half-cystines are present as -S-S- bridges. The lectin agglutinates human A, B, AB, and O erythrocytes, rabbit erythrocytes, human leukocytes, and is mitogenic to human lymphocytes. The hemagglutinating activity of rice lectin is inhibited by 2-acetamido-2-deoxy-D-glucose, methyl 2-acetamido-2-deoxy-beta-D-glucoside, chitobiose, and chitotriose. N-Acetylneuraminic acid was a noninhibitor, but N-acetylneuramin-(2----3)-lactose showed weak inhibition. The agglutinating activity was also inhibited by various sialoglycoproteins. The immobilized rice-lectin bound glycophorin, alpha 1-acid glycoprotein, and fetuin. Asialoglycophorin, asialofetuin, ovomucoid, and human chorionic gonadotropin were bound only partially to the column.     

Lectin from rice

N-Acetyl-D-glucosamine-binding lectin was isolated and purified from rice by ammonium sulphate fractionation and affinity chromatography using N-acetyl-D-glucosamine linked Sepharose 6B column. It gave a single hand on Polyacrylamide disc gel. It was identified as a glycoprotein. The purified lectin dissociated into two components on Sephadex G-100 column chromatography,-a higher molecular weight fraction not containing any carbohydrate and a lower molecular weight glycoprotein fraction. The apparent molecular weights of these fractions were 85,000 and 14,500. The lectin agglutinated erythrocytes of human A,B,O groups and of several other mammals and its activity was inhibited only by N-acetyl-D-glucosamine. The glycopeptide isolated by pronase digestion of the lectin was homogeneous and did not possess agglutinating activity. It contained about 10% carbohydrate of which xylose, arabinose and glucose were the major components.     

Interaction of rice (Oryza sativa) lectin with N-acetylglucosaminides. Fluorescence studies.

The interaction of lectin isolated from rice (Oryza sativa) embryos with N-acetylglucosaminides was studied by equilibrium dialysis and fluorescence. Equilibrium dialysis with 4-methylumbelliferyl-(GlcNac)2 showed that rice lectin (Mr 38000) contains four equivalent saccharide-binding sites. Addition of the N-acetylglucosaminides GlcNac, (GlcNac)2 and (GlcNac)3 enhanced the intrinsic fluorescence of rice lectin and this was accompanied by a 10nm blue-shift of its maximum fluorescence with (GlcNac)2 and (GlcNac)3. These changes in intensity allowed determination of the association constants, which increased with the number of saccharide units: at 20 degrees C, Ka = (1.3 +/- 0.1) X 10(3), (5.1 +/- 0.4) X 10(4) and (2.6 +/- 0.1) X 10(5) M-1 for GlcNac, (GlcNac)2 and (GlcNac)3 respectively. The binding enthalpy, delta H0, for the three glucosaminides were very low and ranged from -12.1 to -20.6 kJ X mol-1. The results are compared with those obtained with wheat-germ agglutinin, another GlcNac-specific gramineaous lectin.     

Sulfate-reducing bacteria in rice field soil and on rice roots.

Rice plants that were grown in flooded rice soil microcosms were examined for their ability to exhibit sulfate reducing activity. Washed excised rice roots showed sulfate reduction potential when incubated in anaerobic medium indicating the presence of sulfate-reducing bacteria. Rice plants, that were incubated in a double-chamber (phylloshpere and rhizosphere separated), showed potential sulfate reduction rates in the anoxic rhizosphere compartment. These rates decreased when oxygen was allowed to penetrate through the aerenchyma system of the plants into the anoxic root compartment, indicating that sulfate reducers on the roots were partially inhibited by oxygen or that sulfate was regenerated by oxidation of reduced S-compounds. The potential activity of sulfate reducers on rice roots was consistent with MPN enumerations showing that H2-utilizing sulfate-reducing bacteria were present in high numbers on the rhizoplane (4.1 x 10(7) g-1 root fresh weight) and in the adjacent rhizosperic soil (2.5 x 10(7) g-1 soil dry weight). Acetate-oxidizing sulfate reducers, on the other hand, showed highest numbers in the unplanted bulk soil (1.9 x 10(6) g-1 soil dry weight). Two sulfate reducing bacteria were isolated from the highest dilutions of the MPN series and were characterized physiologically and phylogenetically. Strain F1-7b which was isolated from the rhizoplane with H2 as electron donor was related to subgroup II of the family Desulfovibrionaceae. Strain EZ-2C2, isolated from the rhizoplane on acetate, grouped together with Desulforhabdus sp. and Syntrophobacter wolinii. Other strains of sulfate-reducing bacteria originated from bulk soil of rice soil microcosms and were isolated using different electron donors. From these isolates, strains R-AcA1, R-IbutA1, R-PimA1 and R-AcetonA170 were Gram-positive bacteria which were affiliated with the genus Desulfotomaculum. The other isolates were members of subgroup II of the Desulfovibrionaceae (R-SucA1 and R-LacA1), were related to Desulforhabdus sp. (strain BKA11), Desulfobulbus (R-PropA1), or culstered between Desulfobotulus sapovorans and Desulfosarcina variabilis (R-ButA1 and R-CaprA1).     

棉花根际亲和性高效促生细菌的分离筛选

为了从棉花根际土壤筛选能与棉花凝集素具有亲和作用的高效促生细菌,以选择性培养基从棉花根部初步筛选具有固氮能力、解磷能力及解钾能力的促生细菌,再以异硫氰酸磺(FITC)标记的棉花凝集素为复筛工具,从棉花根际促生细菌中筛选能与棉花凝集素结合的亲和性菌株,分别挑选2株固氮菌、2株解磷细菌和2株解钾细菌作为微生物肥料接种到棉花根部进行盆栽试验.观察其在根部定殖情况.结果是在选择性平板上有20%～30%的菌株具有凝集素染色阳性.盆栽试验显示,接种的6株亲和性菌株能在棉花根部成功定殖,根际细菌数量约是灭活对照的`0倍.通过初步鉴定,固氮菌株N1111为固氮菌属(Azotobacter),N2121属于德克斯氏菌属(Derxia);解磷菌株P2126属于黄单胞菌属(Xanthomonas),P1108菌株为假单胞菌属(Pseudomonas);解钾菌株K2204和K2116属于芽孢杆菌属(Bacillus).     

Structure and activity of bacterial community inhabiting rice roots and the rhizosphere

Root-derived carbon provides a major source for microbial production and emission of CH4 from rice field soils. Therefore, we characterized the structure and activity of the bacterial community inhabiting rice roots and the rhizosphere. In the first experiment, DNA retrieved from rice roots was analysed for bacterial 16S rRNA genes using cloning, sequencing and in situ hybridization. In the second experiment, rice plants were pulse-labelled with 13CO2 (99% of atom 13C) for 7 days, and the bacterial RNA was isolated from rhizosphere soil and subjected to density gradient centrifugation. RNA samples from density fractions were analysed by terminal restriction fragment length polymorphism fingerprinting, cloning and sequencing. The experiments showed that the dominant bacteria inhabiting rice roots and the rhizosphere particularly belonged to the Alphaproteobacteria, Betaproteobacteria and Firmicutes. The RNA stable isotope probing revealed that the bacteria actively assimilating C derived from the pulse-labelled rice plants were Azospirillum spp. (Alphaproteobacteria) and members of Burkholderiaceae (Betaproteobacteria). Both anaerobic (e.g. Clostridia) and aerobic (e.g. Comamonas) degraders were present at high abundance, indicating that root environments and degradation processes were highly heterogeneous. The relative importance of iron and sulfate reducers suggested that cycling of iron and sulfur is active in the rhizosphere.     

Subcellular site of lectin synthesis in developing rice embryos

Embryos of developing rice (Oryza sativa L. cv. Koshihikari) caryopses which actively synthesize lectin were labelled with [³⁵S]cysteine for different times and newly synthesized rice lectin was isolated by affinity chromatography. Gel filtration of embryo extracts on Sepharose-4B indicated that a large portion of the labelled lectin was associated with the particulate fraction. Experiments with detergent indicated that this lectin was sequestered within organelles. When extracts of pulse-labelled embryos were fractionated on isopycnic sucrose gradients, this detergent-released lectin banded in the same density-region as the endoplasmic reticulum (ER) marker enzyme NADH-cytochrome c reductase. Both radioactivity in rice lectin and the enzyme activity shifted towards a higher density in the presence of 2 mM Mg acetate, indicating that the labelled lectin was associated with the rough ER. The ER-bound lectin could be chased from this organelle when tissue was incubated in unlabelled cysteine following a 1 h pulse of labelled cysteine. Radioactivity chased out of the ER with a half-life of ˜4 h and accumulated in the soluble fraction. In the ER the lectin was present as a polypeptide with mol. wt. 23 000, while in the soluble fraction it occurred as polypeptides with mol. wt. 18 000, 10 000 and 8000. The rice lectin in the ER is capable of binding carbohydrates since it binds readily to the affinity gels. It is associated into dimers with an approximate mol. wt. of 46 000. The results show that newly synthesized rice lectin is transiently sequestered within the ER before further transport and processing take place.     

化感水稻根际微生物类群及酶活性变化

以化感水稻PI312777（PI）和非化感水稻Lemont（LE）为材料,分别测定不同水稻叶龄期（3～7叶期）根际微生物区系变化及根际土壤酶活性.结果表明,化感水稻明显影响土壤根际微生物类群及相关酶活性.化感水稻PI根际细菌、放线菌、固氮菌的数量高于非化感水稻LE,增幅分别在11.2%～28.3%、40%～78.6%和111.5%～173.9%之间,而真菌数量低于非化感水稻LE,最高仅为其值的25.5％,说明化感水稻PI对绝大多数细菌、放线菌、固氮菌生长有促进作用,对一些真菌生长有抑制作用.进一步分析表明,化感水稻PI对氨化细菌、亚硝酸细菌、硝酸细菌、好气性固氮菌、好气性纤维素分解菌、硫化细菌的生长具有促进作用,其中以氨化细菌、好气性固氮菌的更为明显,最低增幅分别为53.7％和57.6％;而对反硫化细菌、反硝化细菌生长有抑制作用,其值最高分别为非化感水稻的54.2％和50.6％.此外,化感水稻PI根系分泌物对脲酶、磷酸酶、蔗糖酶的活性具有促进作用,而对过氧化氢酶则呈抑制作用.     

Chemical characterization of root exudates from rice (Oryza sativa) and their effects on the chemotactic response of endophytic bacteria

Root exudates represent an important source of nutrients for microorganisms in the rhizosphere and seem to participate in early colonization inducing chemotactic responses of rhizospheric bacteria. We characterized the root exudates collected from rice plantlets cultured under hydroponic conditions and assessed their effects on the chemotaxis of two strains of endophytic bacteria, Corynebacterium flavescens and Bacillus pumilus, collected from the rice rhizosphere. We compared these chemotactic effects on endophytic bacteria with those on two strains of plant-growth-promoting bacteria, Azospirillum brasilense (isolated from the corn rhizosphere) and Bacillus sp. (from the rice rhizosphere). The root exudates were collected at different time intervals. The highest concentration and diversity of amino acids and carbohydrates were found during the first 2 weeks after seeding. Histidine, proline, valine, alanine, and glycine were the main amino acid residues identified during the 4 weeks of culture. The main carbohydrates identified were glucose, arabinose, mannose, galactose, and glucuronic acid. The chemotactic responses of the analyzed endophytic bacteria to root exudates were 3.9 to 5.1 times higher than those of A. brasilense and 2.2 to 2.8 times higher than Bacillus sp. Our results indicate that rice exudates may induce a higher chemotactic response for endophytic bacteria than for other bacterial strains present in the rice rhizosphere.     

Isolation and purification of a membrane hyaluronate-binding protein from embryonic human brain]

A membrane hyaluronate-binding protein from cerebral cortex of human embryonic brain (22-24 weeks) was purified by affinity, ion exchange chromatographies and gel-filtration. While gel-filtration analysis the protein had Mm 250 kDa. Electrophoresis under reduction conditions in the presence of DS-Na revealed a major band with Mm 85 kDa and two minor binds with Mm 68 and 36 kDa. The isolated protein did not react with antibodies against known hyaluronate-binding and other proteins with similar mass. The results show that a new membrane hyaluronate-binding protein was isolated and purified from human embryonic brain.     

Expression and purification of the recombinant SALT lectin from rice (Oryza sativa L.)

The SALT protein is a 14.5 kDa mannose-binding lectin, originally described as preferentially expressed in rice plant roots in response to NaCl stress. Recombinant SALT lectin was produced in Escherichia coli from a cDNA clone encoding protein. After isopropyl-beta-d-thiogalactopyranoside induction, the expression level achieved was 23% of the soluble protein. The recombinant agglutinin was purified by a single-step process by dialyses against a high concentrated salt solution. After purification, hemagglutination assays of rabbit erythrocytes revealed that the recombinant SALT protein is a potent agglutinin (0.078 microg ml(-1) minimal concentration). The purified recombinant lectin was also used for comparative estimation of native protein amounts in protein extracts from rice plants by Western blot assay.     

A novel mannose-binding tuber lectin from Typhonium divaricatum (L.) Decne (family Araceae) with antiviral activity against HSV-II and anti-proliferative effect on human cancer cell lines

A novel mannose-binding tuber lectin with in vitro antiproliferative activity towards human cancer cell lines and antiviral activity against HSV-II was isolated from fresh tubers of a traditional Chinese medicinal herb, Typhonium divaricatum (L.) Decne by a combined procedure involving extraction, ammonium sulfate precipitation, ion exchange chromatography on DEAE-SEPHAROSE, CM-SEPHAROSE and gel-filtration on sephacryl S-200. The apparent molecular mass of the purified Typhonium divaricatum lectin (TDL) was 48 kDa. TDL exhibits hemagglutinating activity toward rabbit erythrocytes at 0.95 microg/ml, and its activity could be strongly inhibited by mannan, ovomucoid, asialofetuin and thyroglobulin. TDL showed antiproliferative activity towards some well established human cancer cell lines, e.g. Pro-01 (56.7 +/- 6.8), Bre-04 (41.5 +/- 4.8), and Lu-04 (11.4 +/- 0.3). The anti-HSV-II activity of TDL was elucidated by testing its HSV-II infection inhibitory activity in Vero cells with TC(50) and EC(50) of 5.176 mg/ml and 3.054 microg/ml respectively. The full-length cDNA sequence of TDL was 1145 bp and contained an 813-bp open reading frame (ORF) encoding a 271 amino acid precursor of 29-kDa. Homology analysis showed that TDL had high homology with many other mannose-binding lectins. Secondary and three-dimensional structures analyses showed that TDL is heterotetramer and similar with lectins from mannose-binding lectin superfamily, especially those from family Araceae.     

A novel lectin isolated from the hemolymph of the marine hair crab Erimacrus isenbeckii

A lectin that induces hemagglutination activity in mouse and rabbit erythrocytes has been purified from the hemolymph of the marine hair crab Erimacrus isenbeckii. The results of SDS-PAGE, gel-filtration, affinity and anion-exchange chromatography indicate that this lectin, designated EIL (E. isenbeckii lectin), was successfully purified as a single protein, and comprises a mixture of a major (90%) dimeric and a minor (10%) oligomeric protein with a molecular mass of 116 kDa, with covalent linking between two subunits of 62 and 54 kDa. The activity was maximal at pH 5.6-8.0 and at temperatures below 50 degrees C. The N-terminal amino acid sequences were determined, and these differed greatly from those of other reported lectins from invertebrates, vertebrates, or plants. EIL binds with high specificities to both the O-acetylsialic acid and mannose that are present in bacterial pathogens, which suggests that EIL can act as a defense protein against infection in this crab.     

Methane oxidation and the competition for oxygen in the rice rhizosphere

A mechanistic approach is presented to describe oxidation of the greenhouse gas methane in the rice rhizosphere of flooded paddies by obligate methanotrophic bacteria. In flooded rice paddies these methanotrophs compete for available O(2) with other types of bacteria. Soil incubation studies and most-probable-number (MPN) counts of oxygen consumers show that microbial oxygen consumption rates were dominated by heterotrophic and methanotrophic respiration. MPN counts of methanotrophs showed large spatial and temporal variability. The most abundant methanotrophs (a Methylocystis sp.) and heterotrophs (a Pseudomonas sp. and a Rhodococcus sp.) were isolated and characterized. Growth dynamics of these bacteria under carbon and oxygen limitations are presented. Theoretical calculations based on measured growth dynamics show that methanotrophs were only able to outcompete heterotrophs at low oxygen concentrations (frequently < 5 microM). The oxygen concentration at which methanotrophs won the competition from heterotrophs did not depend on methane concentration, but it was highly affected by organic carbon concentrations in the paddy soil. Methane oxidation was severely inhibited at high acetate concentrations. This is in accordance with competition experiments between Pseudomonas spp. and Methylocystis spp. carried out at different oxygen and carbon concentrations. Likely, methane oxidation mainly occurs at microaerophilic and low-acetate conditions and thus not directly at the root surface. Acetate and oxygen concentrations in the rice rhizosphere are in the critical range for methane oxidation, and a high variability in methane oxidation rates is thus expected.     

A lectin from the thigh muscle of Rana tigerina

Lectin activity has been detected in the thigh muscle extracts of Rana tigerina, which was found to agglutinate both trypsinized and untrypsinized rabbit erythrocytes. The lectin has been purified to homogeneity by MEPBS (0.01 M phosphate-buffered saline (pH 7.2) with 4 mM beta-mercaptoethanol) buffer extraction of the tissue and affinity chromatography on acid-treated Sepharose 6B. The molecular weight (Mr) of the purified lectin was determined by SDS-polyacrylamide gel electrophoresis and gel filtration on Sephadex G-75, which gave values of 15,500 +/- 1000 and 32,000 +/- 1000, respectively, suggesting that the lectin is a dimer. Amino acid composition data of the lectin has revealed that it contains a high proportion of glycine and alanine, and low amounts of sulphur-containing amino acids. Hapten-inhibition study of this lectin has shown that it is galactose-specific. Hemagglutination activity of the lectin can also be inhibited by beta-galactoside containing oligosaccharides.     

Isolation, molecular and biological properties of a lectin from rice embryo: relationship with wheat germ agglutinin properties

Rice lectin (Oryza sativa, var. Balilla 28) was purified from defatted embryos by aqueous acid extraction at pH 1.3 followed by ammonium sulfate precipitation between 2 and 4 M, affinity chromatography on agarose-p-aminophenyl-beta-D-N-acetylglucosamine, and gel filtration on AcA 54. The homogeneity of the lectin was checked by polyacrylamide gel electrophoresis, gel filtration, and immunodiffusion. The amino acid analysis revealed a high half-cystine content (9%) and a low aromatic and hydrophobic amino acid content. The lectin contained neither neutral carbohydrates nor amino sugars. The isoelectric point was estimated to be 8.1. The molecular weight of rice lectin was estimated to be 38,000. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions showed two polypeptides with Mr 19,000 and 15,000. The circular dichroism spectrum of rice lectin in far ultraviolet was characterized by a positive maximum at 228 nm and a negative band at 203 nm suggesting the presence of a beta-pleated sheet and the absence of alpha-helix. Rice lectin had no human blood group specificity and agglutinated rabbit erythrocytes more efficiently than erythrocytes from other animal species. Furthermore, agglutination was enhanced by trypsin treatment of erythrocytes. The erythroagglutinating activity was very high since the minimal concentration needed to agglutinate erythrocytes was 0.05 micrograms/ml. Although [methyl-3H]thymidine incorporation was stimulated in human lymphocytes, rice lectin could not be considered as a mitogenic lectin since it stimulated neither blast transformation nor lymphocyte proliferation. The saccharide specificity of rice lectin was related to N-acetylglucosamine and its oligomers: N,N',N"-triacetylchitotriose was the most powerful inhibitor. Furthermore, the N-acetylneuraminic acid was not a specific rice determinant. Finally, the double immunodiffusion method revealed a cross-reactivity between rice lectin and wheat germ agglutinin, indicating that these lectins were closely antigenically related. The analogies and differences between biological and immunological properties of rice lectin and wheat germ agglutinin are discussed and the possibility of their evolution from a common ancestor is put forward.     

元阳梯田地方水稻品种根部内生菌及根际微生物的分离与鉴定

采用组织分离法和土壤稀释涂板法,对元阳哈尼梯田2个地方品种‘月亮谷’和‘红脚老粳’的根部内生细菌及根际土壤细菌进行了分离,研究元阳梯田传统水稻品种特殊的内生菌组成.结果表明: 试验共得到399个菌株.经形态特征及生理生化鉴定,月亮谷根部和其根际土壤分别分离到8和5个属,其中5个属是共有的;红脚老粳的根部和其根际土壤中分别分离到10和7个属,其中6个属是共有的.经分子生物学鉴定,月亮谷根部分离到11个种和5个属,根际土壤分离到8个种和4个属,其中5个种和4个属是共有的;红脚老粳根部分离到9个种和5个属,根际土壤分离到10个种和3个属,其中4个种和2个属是共有的.通过分子生物学鉴定,大部分菌株都可以鉴定到种,而通过形态及生理生化特性只能初步鉴定到属,但两种方法在属层次上的鉴定结果基本一致.元阳地方水稻根部内生细菌及根际土壤细菌具有一定的种属同源性与特异性.     

A new C-type animal lectin isolated from Bothrops pirajai is responsible for the snake venom major effects in the isolated kidney

We investigated the biochemical and biological effects of a new C-type galactoside specific lectin termed BPL that was isolated from the snake venom of Bothrops pirajai. This lectin was purified using size exclusion HPLC followed by an immobilized lactose affinity column. The purified BPL was homogeneous by reverse phase HPLC and SDS-PAGE. We evaluated the nephrotoxicity of the whole venom of B. pirajai and its lectin. The whole venom of B. pirajai (10 microg/mL) showed similar results as those observed for BPL (3, 10 and 30 microg/mL) evaluated by the perfused rat kidney method. They caused reductions in perfusion pressure (Control120 = 110.28 +/- 3.69; BP120 = 70.70 +/- 2.40*; BPL3(120) = 113.20 +/- 4.40; BPL10(120) = 67.80 +/- 3.00*; BPL30(120) = 64.90 +/- 3.50* mmHg; *: P < 0.05), renal vascular resistance, urinary flow, glomerular filtration rate (Control90 = 0.695 +/- 0.074; BP90 = 0.142 +/- 0.032*; BPL3(90) = 0.314 +/- 0.064; BPL10(90) = 0.250 +/- 0.038*; BPL30(90) = 0.088 +/- 0.021* mLg(-1) min(-1); *: P < 0.05) and sodium (Control120 = 81.28 +/- 0.26; BP120 = 55.71 +/- 5.72*; BPL3(120) = 80.94 +/- 0.93; BPL10(120) = 65.23 +/- 1.47*; BPL30(120) = 76.03 +/- 1.70* %; *: P < 0.05), potassium and chloride tubular transport. Neither whole venom nor purified BPL induced direct vasoactive effects in perfused arteriolar mesenteric bed, and BPL did not potentiate bradykinin contraction in the ileum. We postulate that both B. pirajai and BPL promoted the same renal effects probably caused by the release of inflammatory mediators.     

Arsenic biotransformation by Streptomyces sp. isolated from rice rhizosphere

Isolation and functional analysis of microbes mediating the methylation of arsenic (As) in paddy soils is important for understanding the origin of dimethylarsinic acid (DMA) in rice grains. Here, we isolated from the rice rhizosphere a unique bacterium responsible for As methylation. Strain GSRB54, which was isolated from the roots of rice plants grown in As‐contaminated paddy soil under anaerobic conditions, was classified into the genus Streptomyces by 16S ribosomal RNA sequencing. Sequence analysis of the arsenite S‐adenosylmethionine methyltransferase (arsM) gene revealed that GSRB54 arsM was phylogenetically different from known arsM genes in other bacteria. This strain produced DMA and monomethylarsonic acid when cultured in liquid medium containing arsenite [As(III)]. Heterologous expression of GSRB54 arsM in Escherichia coli promoted methylation of As(III) by converting it into DMA and trimethylarsine oxide. These results demonstrate that strain GSRB54 has a strong ability to methylate As. In addition, DMA was detected in the shoots of rice grown in liquid medium inoculated with GSRB54 and containing As(III). Since Streptomyces are generally aerobic bacteria, we speculate that strain GSRB54 inhabits the oxidative zone around roots of paddy rice and is associated with DMA accumulation in rice grains through As methylation in the rice rhizosphere.