Bacterial Biosynthesis of 1-Aminocyclopropane-1-Carboxylate (ACC) Deaminase and Indole-3-Acetic Acid (IAA) as Endophytic Preferential Selection Traits by Rice Plant Seedlings

In this study, bacteria were isolated from the rhizosphere and inside the roots and nodules of berseem clover plants grown in the field in Iran. Two hundred isolates were obtained from the rhizosphere (120 isolates), interior roots (57 isolates), and nodules (23 isolates) of clover plants grown in rotation with rice plants. Production of chitinase, pectinase, cellulase, siderophore, salicylic acid, hydrogen cyanide, indole acetic acid (IAA), 1-aminocyclopropane-1-carboxylate (ACC) deaminase, solubilization of phosphate, antifungal activity against various rice plant pathogen fungi, N₂ fixation, and colonization assay on rice seedlings by these strains was evaluated and compared (endophytic isolates vs. rhizosphere bacteria). The results showed both the number and the ability of plant growth-promoting (PGP) traits were different between endophytic and rhizosphere isolates. A higher percentage of endophytic isolates were positive for production of IAA, ACC deaminase, and siderophore than rhizosphere isolates. Therefore, it is suggested that clover plant may shape its own associated microbial community and act as filters for endophyte communities, and rhizosphere isolates with different (PGP) traits. We also studied the PGP effect of the most promising endophytic and rhizosphere isolates on rice seedlings. A significant relationship among IAA and ACC deaminase production, the size of root colonization, and plant growth (root elongation) in comparison with siderophore production and phosphate solubilization for the isolates was observed. The best bacterial isolates (one endophytic isolate and one rhizosphere isolate), based on their ability to promote rice growth and colonize rice roots, were identified. Based on 16S rDNA sequence analysis, the endophytic isolate CEN7 and the rhizosphere isolate CEN8 were closely related to Pseudomonas putida and Pseudomonas fluorescens, respectively. It seems that PGP trait production (such as IAA, ACC deaminase) may be required for endophytic and rhizosphere competence as compared to other PGP traits in rice seedlings under constant flooded conditions. The study also shows that the presence of diverse rhizobacteria with effective growth-promoting traits associated with clover plants may be used for sustainable crop management under field conditions.     

Endophytic colonization of Typha australis by a plant growth-promoting bacterium Klebsiella oxytoca strain GR-3

AIMS: To isolate and characterize endophytic diazotrophic bacteria from a semi-aquatic grass (Typha australis) which grows luxuriantly with no addition of any nitrogen source. METHODS AND RESULTS: Ten endophytic diazotrophic bacteria from surface-sterilized roots and culm of T. australis were isolated and screened for plant growth-promoting activities employing standard methods. Based on the rate of nitrogenase activity, indole acetic acid (IAA) production and phosphate (P) solubilization, one root isolate namely GR-3 was found to be the most efficient one. This isolate was identified as Klebsiella oxytoca on the basis of 16S rDNA sequence analysis. Amplification of nifH by polymerase chain reaction (PCR) and detection of dinitrogenase reductase by western blot confirmed the diazotrophic nature of GR-3. It was tagged with gusA fused to a constitutive promoter and the resulting transconjugant was inoculated onto endophyte-free rice variety Malviya dhan-36 seedlings to express cross-infection ability which resulted in a significant increase in root/shoot length and chlorophyll a content. CONCLUSIONS: Roots and culm of T. australis harbour several endophytic diazotrophic bacteria. One root isolate, identified as K. oxytoca GR-3, seems to be an efficient plant growth-promoting bacterium. SIGNIFICANCE AND IMPACT OF THE STUDY: Plant growth-promoting properties of GR-3 suggest that this promising isolate merits further investigations for potential application in agriculture.     

岑溪药用野生稻高效内生固氮菌分离及促生特性

【目的】以广西岑溪市野生稻保护区的药用野生稻为材料,分离纯化内生细菌,筛选固氮酶活性较高和对作物促生效果较好的菌株。【方法】利用乙炔还原法检测固氮酶活性,采用SDS-PAGE全细胞蛋白电泳和IS-PCR指纹图谱技术对分离到的固氮菌进行聚类。利用16S rRNA基因和nifH基因确定其系统发育地位。采用钼锑抗比色法、Salkowski比色法和CAS检测法分别测定菌株溶磷性、生长素的分泌能力和产铁载体能力。通过平板和盆栽试验检测其对水稻的促生作用。【结果】共分离得到35株内生固氮菌,分为6个类群。其中CX24固氮酶活性最高,经鉴定属于Klebsiella variicola,其固氮酶活性为298.64 μmol/(L·h),为参比模式菌株DSM15968的9倍。另外该菌株还具有较高的溶磷性、分泌生长素和产铁载体能力,能够有效地促进水稻的萌发和生长。【结论】菌株CX24属于Klebsiella variicola,是一株高效内生固氮菌,具有很好的生产应用前景。     

Colonization behavior of bacterium Burkholderia cepacia inside the Oryza sativa roots visualized using green fluorescent protein reporter

Colonization behavior of endophytic bacteria Burkholderia cepacia strains RRE-3 and RRE-5 was studied in the seedlings of rice variety NDR97 using confocal laser scanning microscopy under controlled laboratory and greenhouse conditions. For studying colonization pattern, bacterial strains were tagged with pHRGFPGUS plasmid. The role of bacterial strains (both gfp/gus-tagged and untagged) in growth promotion was also studied. After coming into contact with the host root system the bacteria showed an irregular spreading. Dense colonization was observed on the primary and secondary roots and also on the junction of emergence of the lateral roots. Results showed that the colonization pattern of Burkholderia cepacia strains was similar to that of other endophytic bacteria isolated from non-legumes. Burkholderia cepacia got entry inside the root at the sites of emergence of lateral roots, without formation of infection threads as in the case of symbiotic rhizobacteria. Observations suggested that the endophytic bacterial strains RRE-3 and RRE-5 entered inside the rice roots in a progressive manner. Bacteria were found to line up along the intercellular spaces of adjoining epidermal cells adjacent to the lateral root junction, indicating endophytic colonization pattern of Burkholderia cepacia strains. Experiments with the rice seedlings inoculated with RRE-3 and RRE-5 strains revealed that both strains enhanced plant growth considerably when observed under laboratory and greenhouse conditions and produced significantly higher plant biomass. No considerable difference was observed between the gfp/gus-tagged and non-gfp/gus-tagged strains in the plant growth experiments both in the laboratory and greenhouse conditions.     

Intercellular colonization and growth promoting effects of Methylobacterium sp. with plant-growth regulators on rice (Oryza sativa L. Cv CO-43)

The Methylobacterium sp. strain NPFM-SB3, isolated from Sesbania rostrata stem nodules possessed nitrogenase activity and nodA genes. Pure culture of NPFM-SB3 strain produced indole-3-acetic acid, cytokinins and on inoculation to rice plants resulted in numerous lateral roots. Inoculation of synthetic auxins 2,4-dichlorophenoxy acetic acid, naphthalene acetic acid or flavonoids naringenin and dihydroxy-4-methoxyisoflavone individually or to bacterial inoculated rice seedlings improved the plant growth and lateral root formation under hydroponic condition. The formation of nodule-like structure and nitrogenase activity which is purely auxin dependent was observed in 2,4-dichlorophenoxy acetic acid treatments to Methylobacterium sp. NPFM-SB3 inoculated rice plants. The rhizobia entered through fissures formed due to lateral root emergence and spread intercellularly in the nodular structures concluded that the effect of 2,4-dichlorophenoxy acetic acid treatment for rice seedlings grown under gnotobiotic conditions is to create a niche in which these bacteria can grow.     

The participation of the cell wall hydrolytic enzymes in the initial colonization of Azospirillum brasilense on wheat roots

The effect of cellulase and pectinase on bacterial colonization of wheat was studied by three different experiments. In the first experiment, the root colonization of 3 wheat cultivars (Ghods, Roshan and Omid) by two A. brasilense strains (Sp7 and Dol) was compared using pre-treated roots with cellulase and pectinase, and non-treated with these enzymes (control). Although the root colonization varied greatly among strain-plant combinations in controls, the pre-treatment of roots with polysaccharide degrading enzymes significantly increased the bacterial count in roots, regardless of the strain-plant combination. This might be an indication that cell wall may act as an important factor in plant-Azospirillum interaction. In the second experiment, the root cellulase activity of the same wheat cultivars treated with and without the two Azospirillum brasilense, strains (Sp7 and Dol) was compared. The pre-treatment of wheat roots with Azospirillum enhanced the cellulase activity of wheat root extracts. Thus, the cellulase activity might participate in the initial colonization of wheat roots by Azospirillum. The comparison of the cellulase activity of root extracts within inoculated and non-inoculated seedlings showed that the inoculation had enhanced the cellulase activity in root extracts, but this effect was directly dependent on the strain-plant combination. Strain Sp7 stimulated the highest cellulase activity in cv. Roshan, but strain Dol induced the highest enzyme activity in cv. Ghods. In the third experiment, several growth parameters of those 3 wheat cultivars treated with and without those two bacterial strains (Sp7 and Dol) were compared. The highest magnitude of growth responses caused by Sp7 strain was in the cv Roshan, but Dol strain stimulated the highest growth in cv Ghods. Therefore, effective colonization may contribute to more growth responses.     

Characterization of Novel Plant Growth Promoting Endophytic Bacterium <Emphasis Type="Italic">Achromobacter xylosoxidans</Emphasis> from Wheat Plant

Nine diazotrophic bacteria were isolated from surface-sterilized roots and culms of wheat variety Malviya-234, which is grown with very low or no inputs of nitrogen fertilizer. Out of the nine bacteria, four showed indole acetic acid (IAA) production, and five were positive for P solubilization. One isolate, WM234C-3, showed appreciable level of nitrogenase activity, IAA production, and P solubilization ability, and was further characterized with a view to exploiting its plant growth promoting activity. Based on 16S rDNA sequence analysis, this isolate was identified as Achromobacter xylosoxidans. Diazotrophic nature of this particular isolate was confirmed by Western blot analysis of dinitrogenase reductase and amplification of nifH. Analysis of the nifH sequence showed close homology with typical diazotrophic bacteria. Endophytic nature and cross-infection ability of WM234C-3 were tested by molecular tagging with gusA fused to a constitutive promoter followed by inoculation onto rice seedlings in axenic conditions. At 21 days after inoculation, the roots showed blue staining, the most intense color being at the emergence of lateral roots and root tips. Microscopic observation confirmed colonization of gus-tagged WM234C-3 in the intercellular spaces of cortical as well as vascular zones of roots. Inoculation of gus-tagged WM234C-3 to rice plants resulted in significant increase in root/shoot length, fresh weight, and chlorophyll a content. Plant growth promoting features coupled with cross-infection ability suggest that this endophytic bacterium may be exploited as agricultural agent for various crops after a thorough and critical pathogenicity test.     

Ascending Migration of Endophytic Rhizobia, from Roots to Leaves, inside Rice Plants and Assessment of Benefits to Rice Growth Physiology

Rhizobia, the root-nodule endosymbionts of leguminous plants, also form natural endophytic associations with roots of important cereal plants. Despite its widespread occurrence, much remains unknown about colonization of cereals by rhizobia. We examined the infection, dissemination, and colonization of healthy rice plant tissues by four species of gfp-tagged rhizobia and their influence on the growth physiology of rice. The results indicated a dynamic infection process beginning with surface colonization of the rhizoplane (especially at lateral root emergence), followed by endophytic colonization within roots, and then ascending endophytic migration into the stem base, leaf sheath, and leaves where they developed high populations. In situ CMEIAS image analysis indicated local endophytic population densities reaching as high as 9 × 10¹⁰ rhizobia per cm³ of infected host tissues, whereas plating experiments indicated rapid, transient or persistent growth depending on the rhizobial strain and rice tissue examined. Rice plants inoculated with certain test strains of gfp-tagged rhizobia produced significantly higher root and shoot biomass; increased their photosynthetic rate, stomatal conductance, transpiration velocity, water utilization efficiency, and flag leaf area (considered to possess the highest photosynthetic activity); and accumulated higher levels of indoleacetic acid and gibberellin growth-regulating phytohormones. Considered collectively, the results indicate that this endophytic plant-bacterium association is far more inclusive, invasive, and dynamic than previously thought, including dissemination in both below-ground and above-ground tissues and enhancement of growth physiology by several rhizobial species, therefore heightening its interest and potential value as a biofertilizer strategy for sustainable agriculture to produce the world's most important cereal crops.     

The xylem of rice (Oryza sativa) is colonized by Azorhizobium caulinodans

Following inoculation with Azorhizobium caulinodans ORS571 (pXLGD4), lateral root development of rice and colonization of lateral root cracks by bacteria were shown to be stimulated by the flavonoid naringenin. Rice seedlings growing aseptically in the presence of naringenin were inoculated with ORS571 (pXLGD4), carrying the lacZ reporter gene. By microscopic analysis of sections of inoculated rice roots, it has been demonstrated that the xylem of rice roots can be colonized by Azorhizobium caulinodans. We discuss whether this colonization of the xylem of rice roots by azorhizobia could provide a suitable niche for endophytic nitrogen fixation.     

Endophytic colonization of olive roots by the biocontrol strain Pseudomonas fluorescens PICF7

Confocal microscopy combined with three-dimensional olive root tissue sectioning was used to provide evidence of the endophytic behaviour of Pseudomonas fluorescens PICF7, an effective biocontrol strain against Verticillium wilt of olive. Two derivatives of the green fluorescent protein (GFP), the enhanced green and the red fluorescent proteins, have been used to visualize simultaneously two differently fluorescently tagged populations of P. fluorescens PICF7 within olive root tissues at the single cell level. The time-course of colonization events of olive roots cv. Arbequina by strain PICF7 and the localization of tagged bacteria within olive root tissues are described. First, bacteria rapidly colonized root surfaces and were predominantly found in the differentiation zone. Thereafter, microscopy observations showed that PICF7-tagged populations eventually disappeared from the root surface, and increasingly colonized inner root tissues. Localized and limited endophytic colonization by the introduced bacteria was observed over time. Fluorescent-tagged bacteria were always visualized in the intercellular spaces of the cortex region, and no colonization of the root xylem vessels was detected at any time. To the best of our knowledge, this is the first time this approach has been used to demonstrate endophytism of a biocontrol Pseudomonas spp. strain in a woody host such as olive using a nongnotobiotic system.     

Enhancement of polygalacturonase activity during auxin induced para nodulation and endorhizosphere colonization of Azospirillum in rice roots

Effect of different auxins, namely, 2,4-dichlorophenoxyacetic acid (2,4-D), naphthalene acetic acid (NAA) and indole acetic acid (IAA) and Azospirillum brasilense bioinoculation on the enhancement of polygalacturonase (PG) activity in rice roots during para nodulation and endorhizosphere colonization of Azospirillum was studied under in vitro condition. It was observed that Azospirillum bioinoculation could augment PG activity of rice roots to a lesser extent without any root morphogenesis whereas auxin application together with Azospirillum bioinoculation enhanced PG activity of rice roots to a higher level which resulted in better root morphogenesis (para nodule) and endorhizosphere colonisation of A. brasilense. Among the three auxins tested, 2,4-D, even at lower concentration (0.5 ppm) enhanced the rice root PG activity, root morphogenesis and endorhizosphere colonization of Azospirillum while it was 2.0 ppm with NAA and variable with IAA. It is concluded that there is a positive correlation existing among PG activity, degree of root morphogenesis and endorhizosphere colonization of Azospirillum brasilense in rice roots and the degree of correlation is determined by the chemical composition, concentration and mode of action of the auxin utilised.     

Upregulation of jasmonate-inducible defense proteins and differential colonization of roots of Oryza sativa cultivars with the endophyte Azoarcus sp

The endophyte Azoarcus sp. strain BH72 expresses nitrogenase (nif) genes inside rice roots. We applied a proteomic approach to dissect responses of rice roots toward bacterial colonization and jasmonic acid (JA) treatment. Two sister lineages of Oryza sativa were analyzed with cv. IR42 showing a less compatible interaction with the Azoarcus sp. resulting in slight root browning whereas cv. IR36 was successfully colonized as determined by nifHi::gusA activity. External addition of JA inhibited colonization of roots and caused browning in contrast to the addition of ethylene, applied as ethephon (up to 5 mM). Only two of the proteins induced in cv. IR36 by JA were also induced by the endophyte (SalT, two isoforms). In contrast, seven JA-induced proteins were also induced by bacteria in cv. IR42, indicating that IR42 showed a stronger defense response. Mass spectrometry analysis identified these proteins as pathogenesis-related (PR) proteins (Prb1, RSOsPR10) or proteins sharing domains with receptorlike kinases induced by pathogens. Proteins strongly induced in roots in both varieties by JA were identified as Bowman-Birk trypsin inhibittors, germinlike protein, putative endo-1,3-beta-D-glucosidase, glutathion-S-transferase, and 1-propane-1-carboxylate oxidase synthase, peroxidase precursor, PR10-a, and a RAN protein previously not found to be JA-induced. Data suggest that plant defense responses involving JA may contribute to restricting endophytic colonization in grasses. Remarkably, in a compatible interaction with endophytes, JA-inducible stress or defense responses are apparently not important.     

Ascending migration of endophytic rhizobia, from roots to leaves, inside rice plants and assessment of benefits to rice growth physiology

Rhizobia, the root-nodule endosymbionts of leguminous plants, also form natural endophytic associations with roots of important cereal plants. Despite its widespread occurrence, much remains unknown about colonization of cereals by rhizobia. We examined the infection, dissemination, and colonization of healthy rice plant tissues by four species of gfp-tagged rhizobia and their influence on the growth physiology of rice. The results indicated a dynamic infection process beginning with surface colonization of the rhizoplane (especially at lateral root emergence), followed by endophytic colonization within roots, and then ascending endophytic migration into the stem base, leaf sheath, and leaves where they developed high populations. In situ CMEIAS image analysis indicated local endophytic population densities reaching as high as 9 x 10(10) rhizobia per cm3 of infected host tissues, whereas plating experiments indicated rapid, transient or persistent growth depending on the rhizobial strain and rice tissue examined. Rice plants inoculated with certain test strains of gfp-tagged rhizobia produced significantly higher root and shoot biomass; increased their photosynthetic rate, stomatal conductance, transpiration velocity, water utilization efficiency, and flag leaf area (considered to possess the highest photosynthetic activity); and accumulated higher levels of indoleacetic acid and gibberellin growth-regulating phytohormones. Considered collectively, the results indicate that this endophytic plant-bacterium association is far more inclusive, invasive, and dynamic than previously thought, including dissemination in both below-ground and above-ground tissues and enhancement of growth physiology by several rhizobial species, therefore heightening its interest and potential value as a biofertilizer strategy for sustainable agriculture to produce the world's most important cereal crops.     

Characterization and screening of plant probiotic traits of bacteria isolated from rice seeds cultivated in Argentina

Many seeds carry endophytes, which ensure good chances of seedling colonization. In this work, we have studied the seed-borne bacterial flora of rice varieties cultivated in the northeast of Argentina. Surface-sterilized husked seeds of the rice cultivars CT6919, El Paso 144, CAMBA, and IRGA 417 contained an average of 5×10⁶ CFU/g of mesophilic and copiotrophic bacteria. Microbiological, physiological, and molecular characterization of a set of 39 fast-growing isolates from the CT6919 seeds revealed an important diversity of seed-borne mesophiles and potential plant probiotic activities, including diazotrophy and antagonism of fungal pathogens. In fact, the seed-borne bacterial flora protected the rice seedlings against Curvularia sp. infection. The root colonization pattern of 2 Pantoea isolates from the seeds was studied by fluorescence microscopy of the inoculated axenic rice seedlings. Both isolates strongly colonized the site of emergence of the lateral roots and lenticels, which may represent the entry sites for endophytic spreading. These findings suggest that rice plants allow grain colonization by bacterial species that may act as natural biofertilizers and bioprotectives early from seed germination.     

水稻内生优势成团泛菌GFP标记菌株的性质与标记丢失动力学

为研究内生细菌对宿主植物侵染定殖的机理和其共生生物学作用 ,对水稻内生优势成团泛菌 (Pantoeaagglomerans)YS19与绿色荧光蛋白 (GFP)标记的YS19B ::gfp菌株的生长动力学进行了比较研究 ,探讨了成团泛菌YS19B ::gfp的标记稳定性和荧光性质 .标记菌株与野生型菌株相比 ,最大比生长速率和最大生物量仅减小 12 4 %和 6 % ,代时延长 14 0 % .成团泛菌YS19B ::gfp在指数期连续传代培养 10 0代后 ,GFP标记的保持率为 89 1% ,建立了标记菌株在有标记丢失存在时的生长动力学模型 :dX+ dt =μ+ (1-p)X+ ,解析出细胞分裂时标记丢失的概率p =9 75 6× 10 -7,确定了方程的模型参数 .标记菌株的荧光光谱在激发波长为 4 0 0nm时 ,最大发射波长为 5 0 8nm ,与供体菌株完全相同 .在LB培养基上生长时 ,成团泛菌YS19B ::gfp的GFP产生时间在指数期末期到稳定期较快 ,并于培养至 2 0h时达到最高 ,同时单位菌体生物量的荧光强度也达到最大 .结果说明 ,在GFP标记后成团泛菌YS19B ::gfp的生长仅受到较小影响 ,不致对成团泛菌的生理活动造成大的改变 ,同时由于该菌对宿主的侵染能力比其它内生细菌要强得多 ,因而该菌对植物的侵染活性影响也较小 ,该菌仍然可以保持其内生优势地位 .该标记的稳定性比较高 ,荧光产生正常 ,很适     

攀枝花地区烤烟可培养内生固氮菌的多样性

【目的】认识烤烟(Flue-cured tobaccos)内生固氮菌多样性,挖掘内生固氮菌资源,丰富内生固氮菌基因库。【方法】运用纯培养法、重复因子扩增(BOX-PCR)分析技术、16S r RNA基因测序和系统发育分析对内生固氮菌多样性和系统发育进行研究,并测定分离菌株的固氮酶活性、溶磷溶钾特性、吲哚乙酸(IAA)含量等指标。【结果】通过Ashby培养基共分离得到62株固氮菌。基于BOX-PCR图谱选取16株代表菌株进行16S r RNA基因序列测定。16S r RNA基因序列系统发育分析显示,62株菌株分属于芽孢杆菌属(Bacillus)、泛菌属(Pantoea)、短小杆菌属(Curtobacterium)等3个属,其中芽孢杆菌属(Bacillus)为优势菌属。62株菌株中有20株菌株(占总分离菌株的32.3%)具有固氮酶活性,8株菌株(占总分离菌株的12.9%)能产IAA,有4株(占总分离菌株的6.5%)表现溶磷活性,有3株(占总分离菌株的4.8%)表现溶钾活性。【结论】攀枝花烤烟有较为丰富的内生固氮菌,具有潜在应用价值。     

Concomitant colonization of nifH positive endophytic Burkholderia sp. in rice (Oryza sativa L.) promotes plant growth

Six diazotrophic bacteria were isolated from surface-sterilized roots of rice variety HUR-36, which is grown with very low or no inputs of nitrogen fertilizer. Out of six bacteria one isolate, RREM25, showed appreciable level of nitrogenase activity, IAA production, and Phosphate solubilization ability, and was further characterized with a view to exploiting its plant growth promoting activity. Based on 16S rRNA gene sequence analysis, this isolate was identified as Burkholderia cepacia. Diazotrophic nature of this particular isolate was confirmed by Western blot analysis of dinitrogenase reductase and amplification of nifH. Microscopic observation confirmed colonization of gfp/gusA-tagged RREM25 in the intercellular spaces of cortical as well as vascular zones of roots. Inoculation of RREM25 to rice plants resulted in significant increase in plant height, dry shoot and root weight, chlorophyll content, nitrogen content and nitrogenase activity. Plant growth promoting features suggest that this endophytic bacterium may be exploited in rice cultivation after a thorough and critical pathogenicity test.     

Characterization of a small cryptic plasmid from endophytic Pantoea agglomerans and its use in the construction of an expression vector

A circular cryptic plasmid named pPAGA (2,734 bp) was isolated from Pantoea agglomerans strain EGE6 (an endophytic bacterial isolate from eucalyptus). Sequence analysis revealed that the plasmid has a G+C content of 51% and contains four potential ORFs, 238(A), 250(B), 131(C), and 129(D) amino acids in length without homology to known proteins. The shuttle vector pLGM1 was constructed by combining the pPAGA plasmid with pGFPmut3.0 (which harbors a gene encoding green fluorescent protein, GFP), and the resulting construct was used to over-express GFP in E. coli and P. agglomerans cells. GFP production was used to monitor the colonization of strain EGE6gfp in various plant tissues by fluorescence microscopy. Analysis of EGE6gfp colonization showed that 14 days after inoculation, the strain occupied the inner tissue of Eucalyptus grandis roots, preferentially colonizing the xylem vessels of the host plants.     

Interactions of rhizobia with rice and wheat

Recently, evidence has been obtained that naturally occurring rhizobia, isolated from the nodules of non-legume Parasponia species and from some tropical legumes, are able to enter the roots of rice, wheat and maize at emerging lateral roots by crack entry. We have now investigated whether Azorhizobium caulinodans strain ORS571, which induces root and stem nodules on the tropical legume Sesbania rostrata as a result of crack entry invasion of emerging lateral roots, might also enter rice and wheat by a similar route. Following inoculation with ORS571 carrying a lacZ reporter gene, azorhizobia were observed microscopically within the cracks associated with emerging lateral roots of rice and wheat. A high proportion of inoculated rice and wheat plants had colonized lateral root cracks. The flavanone naringenin at 10 and 10 M stimulated significantly the colonization of lateral root cracks and also intercellular colonization of wheat roots. Naringenin does not appear to be acting as a carbon source and may act as a signal molecule for intercellular colonization of rice and wheat by ORS571 by a mechanism which is nod gene-independent, unlike nodule formation in Sesbania rostrata. The opportunity now arises to compare and to contrast the ability of Azorhizobium caulinodans with that of other rhizobia, such as Parasponia rhizobia, to intercellularly colonize the roots of non-legume crops.