Endophytic colonization and in planta nitrogen fixation by a Herbaspirillum sp. isolated from wild rice species.

Nitrogen-fixing bacteria were isolated from the stems of wild and cultivated rice on a modified Rennie medium. Based on 16S ribosomal DNA (rDNA) sequences, the diazotrophic isolates were phylogenetically close to four genera: Herbaspirillum, Ideonella, Enterobacter, and Azospirillum. Phenotypic properties and signature sequences of 16S rDNA indicated that three isolates (B65, B501, and B512) belong to the Herbaspirillum genus. To examine whether Herbaspirillum sp. strain B501 isolated from wild rice, Oryza officinalis, endophytically colonizes rice plants, the gfp gene encoding green fluorescent protein (GFP) was introduced into the bacteria. Observations by fluorescence stereomicroscopy showed that the GFP-tagged bacteria colonized shoots and seeds of aseptically grown seedlings of the original wild rice after inoculation of the seeds. Conversely, for cultivated rice Oryza sativa, no GFP fluorescence was observed for shoots and only weak signals were observed for seeds. Observations by fluorescence and electron microscopy revealed that Herbaspirillum sp. strain B501 colonized mainly intercellular spaces in the leaves of wild rice. Colony counts of surface-sterilized rice seedlings inoculated with the GFP-tagged bacteria indicated significantly more bacterial populations inside the original wild rice than in cultivated rice varieties. Moreover, after bacterial inoculation, in planta nitrogen fixation in young seedlings of wild rice, O. officinalis, was detected by the acetylene reduction and (15)N(2) gas incorporation assays. Therefore, we conclude that Herbaspirillum sp. strain B501 is a diazotrophic endophyte compatible with wild rice, particularly O. officinalis.     

Infection and Colonization of Sugar Cane and Other Graminaceous Plants by Endophytic Diazotrophs

Agriculturally important grasses such as sugar cane (Saccharum sp.), rice (Oryza sativa), wheat (Triticum aestivum) sorghum (Sorghum bicolor), maize (Zea mays), Panicum maximum, Brachiaria spp., and Pennisetum purpureum contain numerous diazotrophic bacteria, such as, Acetobacter diazotrophicus, Herbaspirillum spp., Azospirillum spp. These bacteria do not usually cause disease symptoms in the plants with which they are associated and the more numerous of them, for example, Herbaspirillum spp. and A. diazotrophicus, are obligate or facultative endo-phytes that do not survive well (or at all) in native soil; these are thought to be spread from plant generation to plant generation via seeds, vegetative propagation, dead plant material, and possibly by insect sap feeders. By contrast, Azospirillum spp. are not wholly endophytic but are root-associated, soil-dwelling bacteria that are also often found within plants, probably entering host plants via seeds or via wounds/cracks at lateral root junctions. Endophytic diazotrophs have been isolated from a number of grasses in which significant biological N₂ fixation (BNF) has been demonstrated, particularly Brazilian sugar cane varieties, but also in rice, maize, and sorghum. However, although the endophytic diazotrophs are held to be the causative agents of the observed BNF, direct evidence for this is lacking. Therefore, in this review we examine probable sites of bacterial multiplication and/or BNF within endophyte-containing grasses and discuss these in terms of potential benefits (or not) to both host plants and bacteria. In particular, we examine how potentially large numbers of bacteria, especially Herbaspirillum spp., A. diazotrophicus, and Azospirillum spp., can exist extracellularly within non-specialized (for symbiotic purposes) regions such as xylem vessels and intercellular spaces. The processes of infection and colonization of various grasses (particularly sugar cane) by diazotrophic endophytes are also described, and these are compared with those of important (nondiazotrophic) endophytic sugar cane pathogens such as Clavibacter xyli subsp. xyli and Xanthomonas albilineans.     

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.     

Interaction of endophytic diazotrophic bacteria and Fusarium oxysporum f. sp. cubense on plantlets of banana ‘Maça’

The objective of this work was to evaluate the effect of endophytic diazotrophic bacteria Herbaspirillum and Burkholderia on the Fusarium oxysporum f. sp. cubense (Foc) establishment and on the plantlets growth of the ‘Maçã’ banana (Musa spp., group ABB). Two assays were carried out in a greenhouse at the National Center of Tropical Agroindustry in Fortaleza city, Ceará State (Brazil), using randomized block designs in the factorial arrangements 4?×?2 and 8?×?2. On the first trial plantlets were inoculated with the Burkholderia sp. AB202; Herbaspirillum sp., BA227, both of strains and controls bacteria, with and without Foc and cultivated in pots filled with an autoclaved mixture of washed sand and vermiculite (ratio 3:2 v v^?1), during 4 months. On a second assay, plants were subjected to following conditions: absence and presence of strains AB202, AB213 (Burkholderia spp.), BA227, BA234 (Herbaspirillum-like), AB202 plus BA227, AB213 plus BA234, the mixture of the four bacterial strain, absence and presence of the Foc; and cultivated in pots filled with autoclaved Haplic Arenosol, during 2 months. The plant association with diazotrophs and the Foc was confirmed, and factors interacted significantly on the most probable number of bacteria and the colony forming units of the pathogen on roots and plant rhizomes. The potential of the endophytes on the inhibition of Foc propagate units and on the plant growth promotion was demonstrated. The higher biomass was observed four and 2 months after plant inoculation with AB202 and BA234. Results showed that these endophytes may be used as potential biofertilizer and biocontrol agents.     

Photosynthetic Bradyrhizobia Are Natural Endophytes of the African Wild Rice Oryza breviligulata

We investigated the presence of endophytic rhizobia within the roots of the wetland wild rice Oryza breviligulata, which is the ancestor of the African cultivated rice Oryza glaberrima. This primitive rice species grows in the same wetland sites as Aeschynomene sensitiva, an aquatic stem-nodulated legume associated with photosynthetic strains of Bradyrhizobium. Twenty endophytic and aquatic isolates were obtained at three different sites in West Africa (Senegal and Guinea) from nodal roots of O. breviligulata and surrounding water by using A. sensitiva as a trap legume. Most endophytic and aquatic isolates were photosynthetic and belonged to the same phylogenetic Bradyrhizobium/Blastobacter subgroup as the typical photosynthetic Bradyrhizobium strains previously isolated from Aeschynomene stem nodules. Nitrogen-fixing activity, measured by acetylene reduction, was detected in rice plants inoculated with endophytic isolates. A 20% increase in the shoot growth and grain yield of O. breviligulata grown in a greenhouse was also observed upon inoculation with one endophytic strain and one Aeschynomene photosynthetic strain. The photosynthetic Bradyrhizobium sp. strain ORS278 extensively colonized the root surface, followed by intercellular, and rarely intracellular, bacterial invasion of the rice roots, which was determined with a lacZ-tagged mutant of ORS278. The discovery that photosynthetic Bradyrhizobium strains, which are usually known to induce nitrogen-fixing nodules on stems of the legume Aeschynomene, are also natural true endophytes of the primitive rice O. breviligulata could significantly enhance cultivated rice production.     

Preferential occurrence of diazotrophic endophytes, Azoarcus spp., in wild rice species and land races of Oryza sativa in comparison with modern races

Several diazotrophic species of Azoarcus spp. occur as endophytes in the pioneer plant Kallar grass. The purpose of this study was to screen Asian wild rice and cultivated Oryza sativa varieties for natural association with these endophytes. Populations of culturable diazotrophs in surface-sterilized roots were characterized by 16S rDNA sequence analysis, and Azoarcus species were identified by genomic fingerprints. A. indigens and Azoarcus sp. group C were detected only rarely, whereas Azoarcus sp. group D occurred frequently in samples of flooded plants: in 75% of wild rice, 80% of land races of O. sativa from Nepal and 33% of modern cultivars from Nepal and Italy. The putatively endophytic populations of diazotrophs differed with the rice genotype. The diversity of cultured diazotrophs was significantly lower in wild rice species than in modern cultivars. In Oryza officinalis (from Nepal) and O. minuta (from the Philippines), Azoarcus sp. group D were the predominant diazotrophic putative endophytes in roots. In contrast, their number was significantly lower in modern cultivars of O. sativa, whereas numbers and diversity of other diazotrophs, such as Azospirillum spp., Klebsiella sp., Sphingomonas paucimobilis, Burkholderia sp. and Azorhizobium caulinodans, were increased. In land races of O. sativa, the diazotrophic diversity was equally high; however, Azoarcus sp. was found in high apparent numbers. Similar differences in populations were also observed in a culture-independent approach comparing a wild rice (O. officinalis) and a modern-type O. sativa plant: in clone libraries of root-associated nitrogenase (nifH) gene fragments, the diazotrophic diversity was lower in the wild rice species. New lineages of nifH genes were detected, e.g. one deeply branching cluster within the anf (iron) nitrogenases. Our studies demonstrate that the natural host range of Azoarcus spp. extends to rice, wild rice species and old varieties being preferred over modern cultivars.     

Herbaspirillum-plant interactions: microscopical, histological and molecular aspects

Diazotrophic species in the genus Herbaspirillum (e.g. H. frisingense, H. rubrisubalbicans and H. seropedicae) associate with several economically important crops in the family Poaceae, such as maize (Zea mays), Miscanthus, rice (Oryza sativa), sorghum (Sorghum bicolor) and sugarcane (Saccharum sp.), and can increase their growth and productivity by a number of mechanisms, including nitrogen fixation. Hence, the improvement and use of these plant growth-promoting bacteria could provide economic and environmental benefits. We review the colonization processes of host plants by Herbaspirillum spp., including histological aspects and molecular mechanisms involved in these interactions, which may be epiphytic, endophytic, and even occasionally pathogenic. Herbaspirillum can recognize plant signals that modulate the expression of colonization traits and plant growth-promoting factors. Although a large proportion of herbaspirilla remain rhizospheric and epiphytic, plant-associated species in this genus are noted for their ability to colonize the plant internal tissues. Endophytic colonization by herbaspirilla begins with the attachment of the bacteria to root surfaces, followed by colonization at the emergence points of lateral roots and penetration through discontinuities of the epidermis; this appears to involve bacterial envelope structures, such as lipopolysaccharide (LPS), exopolysaccharide (EPS), adhesins and the type three secretion system (T3SS), but not necessarily the involvement of cell wall-degrading enzymes. Intercellular spaces are then rapidly occupied, proceeding to colonization of xylem and the aerial parts of the host plants. The response of the host plant includes both the recognition of the bacteria as non-pathogenic and the induction of systemic resistance to pathogens. Phytohormone signaling cascades are also activated, regulating the plant development. This complex molecular communication between some Herbaspirillum spp. and plant hosts can result in plant growth-promotion.     

Selection of phosphate-solubilizing diazotrophic Herbaspirillum and Burkholderia strains and their effect on rice crop yield and nutrient uptake

Background and Aims

Plant growth-promoting bacteria, mainly diazotrophs and phosphate solubilizers, can reduce the use of chemical fertilizers for rice crops. Here, diazotrophic bacteria isolated from rice were screened for their ability to solubilize inorganic P (P_i) in vitro and in association with rice plants cultivated in pots.

Methods

Forty-nine isolates were tested for the ability to solubilize P_i on NBRIP and GL agar plate media and seven selected strains were further evaluated in NBRIP liquid medium. Three of these strains were inoculated in rice plants grown in soil pots containing ¹⁵N-labeled fertilizer and two sources of P: tricalcium phosphate (TCP) or simple superphosphate (SSP). The dry matter, yield, N, P, and the ¹⁵N content accumulated in plant tissues were measured at 135 days after planting.

Results

Seven strains belonging to the genera Herbaspirillum and Burkholderia formed a halo of solubilized P_i on agar plates. The Burkholderia strains showed peak soluble P (around 200 mg P L^?1) on the fifth day when grown in NBRIP liquid medium for 14 days. Inoculation of Herbaspirillum strains (H18, ZA15) and a Burkholderia vietaminensis strain (AR114) increased rice grain yield from 33 to 47 % with TCP and 18 to 44 % with TSS, respectively. The bacterial inoculation led to enhanced N-use efficiency of the ¹⁵N-labeled fertilizer.

Conclusion

These results suggest that the selection and use of P-solubilizing diazotrophic bacteria are a good strategy to promote P solubilization and/or N use efficiency in rice plants.     

Diversity of centromeric repeats in two closely related wild rice species, Oryza officinalis and Oryza rhizomatis

Oryza officinalis (CC, 2n=24) and Oryza rhizomatis (CC, 2n=24) belong to the Oryza genus, which contains more than 20 identified wild rice species. Although much has been known about the molecular composition and organization of centromeres in Oryza sativa, relatively little is known of its wild relatives. In the present study, we isolated and characterized a 126-bp centromeric satellite (CentO-C) from three bacterial artificial chromosomes of O. officinalis. In addition to CentO-C, low abundance of CentO satellites is also present in O. officinalis. In order to determine the chromosomal locations and distributions of CentO-C (126-bp), CentO (155 bp) and TrsC (366 bp) satellite within O. officinalis, fluorescence in situ hybridization examination was done on pachytene or metaphase I chromosomes. We found that only ten centromeres (excluding centromere 7 and 2) contain CentO-C arrays in O. officinalis, while centromere 7 comprises CentO satellites, and centromere 2 is devoid of any detectable satellites. For TrsC satellites, it was detected at multiple subtelomeric regions in O. officinalis, however, in O. rhizomatis, TrsC sequences were detected both in the four centromeric regions (CEN 3, 4, 10, 11) and the multiple subtelomeric regions. Therefore, these data reveal the evolutionary diversification pattern of centromere DNA within/or between close related species, and could provide an insight into the dynamic evolutionary processes of rice centromere.     

The roots of the halophyte Salicornia brachiata are a source of new halotolerant diazotrophic bacteria with plant growth-promoting potential

Soil salinity is the major cause limiting plant productivity worldwide. Nitrogen-fixing bacteria were enriched and characterised from roots of Salicornia brachiata, an extreme halophyte which has substantial economic value as a bioresource of diverse and valuable products. Nitrogen-free semisolid NFb medium with malate as carbon source and up to 4% NaCl were used for enrichment and isolation of diazotrophic bacteria. The isolates were tested for plant growth-promoting traits and 16S rRNA, nifH and acdS genes were analysed. For selected strains, plant growth-promoting activities were tested in axenically grown Salicornia seedlings at different NaCl concentrations (0–0.5M). New halotolerant diazotrophic bacteria were isolated from roots of S. brachiata. The isolates were identified as Brachybacterium saurashtrense sp. nov., Zhihengliuella sp., Brevibacterium casei, Haererehalobacter sp., Halomonas sp., Vibrio sp., Cronobacter sakazakii, Pseudomonas spp., Rhizobium radiobacter, and Mesorhizobium sp. Nitrogen fixation as well as plant growth-promoting traits such as indole acetic acid (IAA) production, phosphate solubilisation, and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity were demonstrated. For Brachybacterium saurashtrense and Pseudomonas sp., significant plant growth-promoting activities were observed in Salicornia in salt stress conditions. Salicornia brachiata is a useful source of new halotolerant diazotrophic bacteria with plant growth-promoting potential.     

厚朴野生种与栽培种花不同部位香气成分分析

以厚朴野生种和栽培种苞片刚裂开的花苞为试材,采用固相微萃取和气相色谱—质谱(GC/MS)分析技术对其进行香气成分和相对含量的测定,比较分析了花朵不同部位香气成分的差异。结果表明：厚朴野生种共有39种香气成分,雌雄蕊中26种,花瓣中22种,栽培种中75种香气成分,雌雄蕊中49种,花瓣中54种。萜烯类是两种厚朴花苞中最重要的香气化合物,莰烯、罗勒烯异构体混合物、石竹烯、芳樟醇是野生种和栽培种中共有的相对含量较高的香气成分。厚朴野生种和栽培种间以及同种厚朴雌雄蕊与花瓣之间的香气成分的种类和相对含量差异显著。     

Bacterial endophytes from rice cut grass (<Emphasis Type="Italic">Leersia oryzoides</Emphasis> L.) increase growth,promote root gravitropic response,stimulate root hair formation,and protect rice seedlings from disease

Background and Aims

Leersia oryzoides, a wild relative of rice (Oryza sativa), may carry potential seed-borne bacterial endophytes which could be used to enhance growth of rice. We hypothesized that seed-associated bacteria from L. oryzoides would be compatible with rice and promote seedling growth, development, and survival.

Methods

We isolated bacteria from seed of L. oryzoides and checked compatibility with rice as well as Bermuda grass seeds for seedling growth promotion. Internal colonisation of bacteria into root cells was observed by ROS staining and microscopic observation. Growth promoting bacteria were evaluated for IAA production, phosphate solubilization and antifungal activities.

Results

Overall, ten bacteria were found to be growth promoting in rice seedlings with effects including restoration of root gravitropic response, increased root and shoot growth, and stimulation of root hair formation. All bacteria were identified by 16S rDNA sequencing. Six bacteria were found to become intracellular in root parenchyma and root hairs in rice and in Bermuda grass seedlings. Six bacteria were able to produce IAA in LB broth with highest (47.06 ± 1.99 μg ml^?1) by LTE3 (Pantoea hericii). Nine isolates solubilized phosphate and inhibited at least one soil borne fungal pathogen.

Conclusions

Seed bacteria of L. oryzoides are compatible with rice. Many of these bacteria become intracellular, induce root gravitropic response, increase root and shoot growth, and stimulate root hair formation in both rice and Bermuda grass seedlings. Presence of bacteria protects seedlings from soil pathogens during seedling establishment. This research suggests that bioprospecting microbes on near relatives of rice and other crop plants may be a viable strategy to obtain microbes to improve cultivation of crops.

     

Rice,beans and pulses at Vadnagar: An early historical site with a Buddhist monastery in Gujarat,western India

We present the plant macroremains from the site of Vadnagar, an important centre of Buddhist learning in western India spanned between first century BCE and fourth century CE. The cultural relics and AMS dates of the recovered carbonized seeds from the cultural horizon also confirm the archaeological context. The study indicate that the likely staples were cereals (Oryza sativa, Hordeum vulgare, Triticum aestivum), leguminous crops (Vigna unguiculata, Vigna radiata, Vigna mungo, and Macrotyloma uniflorum), and oil and fibre-yielding (Linum usitatissimum and Gossypium sp.) crops. In addition, there is an evidence that fruits such as Tamarindus indica and Ziziphus sp. may have been consumed by the settlers. Few weeds and wild taxa (Paspalum scrobiculatum, Vicia sp., and Ipomoea sp.), which turned up as an admixture with the above economically important seeds/grains have also been encountered. Abundance of Oryza sativa (rice) grains and spikelet bases indicate crop-processing at the site and played significant role in the subsistence economy. Moreover, the overall assemblage of summer and winter crops suggests the prevalence of multi-cropping system attributed to conducive environmental conditions during ~100 BCE–400 CE.     

Effects of Colonization of a Bacterial Endophyte,Azospirillum sp. B510, on Disease Resistance in Rice

Agriculturally important grasses contain numerous diazotrophic bacteria, the interactions of which are speculated to have some other benefits to the host plants. In this study, we analyzed the effects of a bacterial endophyte, Azospirillum sp. B510, on disease resistance in host rice plants. Rice plants (Oryza sativa cv. Nipponbare) were inoculated with B510 exhibited enhanced resistance against diseases caused by the virulent rice blast fungus Magnaporthe oryzae and by the virulent bacterial pathogen Xanthomonas oryzae. In the rice plants, neither salicylic acid (SA) accumulation nor expression of pathogenesis-related (PR) genes was induced by interaction with this bacterium, except for slight induction of PBZ1. These results indicate the possibility that strain B510 is able to induce disease resistance in rice by activating a novel type of resistance mechanism independent of SA-mediated defense signaling.     

Isolation and characterization of plant growth promoting endophytic diazotrophic bacteria from Korean rice cultivars

We have isolated 576 endophytic bacteria from the leaves, stems, and roots of 10 rice cultivars and identified 12 of them as diazotrophic bacteria using a specific primer set of nif gene. Through 16S rDNA sequence analysis, nifH genes were confirmed in the two species of Penibacillus, three species of Microbacterium, three Bacillus species, and four species of Klebsiella. Rice seeds treated with these plant growth-promoting bacteria (PGPB) showed improved plant growth, increased height and dry weight and antagonistic effects against fungal pathogens. In addition, auxin and siderophore producing ability, and phosphate solubilizing activity were studied for the possible mechanisms of plant growth promotion. Among 12 isolates tested, 10 strains have shown higher auxin producing activity, 6 isolates were confirmed as strains with high siderophore producing activity while 4 isolates turned out to have high phosphate-solubilizing activity. These results strongly suggest that the endophytic diazotrophic bacteria characterized in this study could be successfully used to promote plant growth and inducing fungal resistance in plants.     

Comparative study of endophytic and endophytic diazotrophic bacterial communities across rice landraces grown in the highlands of northern Thailand

Communities of bacterial endophytes within the rice landraces cultivated in the highlands of northern Thailand were studied using fingerprinting data of 16S rRNA and nifH genes profiling by polymerase chain reaction–denaturing gradient gel electrophoresis. The bacterial communities’ richness, diversity index, evenness, and stability were varied depending on the plant tissues, stages of growth, and rice cultivars. These indices for the endophytic diazotrophic bacteria within the landrace rice Bue Wah Bo were significantly the lowest. The endophytic bacteria revealed greater diversity by cluster analysis with seven clusters compared to the endophytic diazotrophic bacteria (three clusters). Principal component analysis suggested that the endophytic bacteria showed that the community structures across the rice landraces had a higher stability than those of the endophytic diazotrophic bacteria. Uncultured bacteria were found dominantly in both bacterial communities, while higher generic varieties were observed in the endophytic diazotrophic bacterial community. These differences in bacterial communities might be influenced either by genetic variation in the rice landraces or the rice cultivation system, where the nitrogen input affects the endophytic diazotrophic bacterial community.     

An-1 Encodes a Basic Helix-Loop-Helix Protein That Regulates Awn Development,Grain Size,and Grain Number in Rice

Long awns are important for seed dispersal in wild rice (Oryza rufipogon), but are absent in cultivated rice (Oryza sativa). The genetic mechanism involved in loss-of-awn in cultivated rice remains unknown. We report here the molecular cloning of a major quantitative trait locus, An-1, which regulates long awn formation in O. rufipogon. An-1 encodes a basic helix-loop-helix protein, which regulates cell division. The nearly-isogenic line (NIL-An-1) carrying a wild allele An-1 in the genetic background of the awnless indica Guangluai4 produces long awns and longer grains, but significantly fewer grains per panicle compared with Guangluai4. Transgenic studies confirmed that An-1 positively regulates awn elongation, but negatively regulates grain number per panicle. Genetic variations in the An-1 locus were found to be associated with awn loss in cultivated rice. Population genetic analysis of wild and cultivated rice showed a significant reduction in nucleotide diversity of the An-1 locus in rice cultivars, suggesting that the An-1 locus was a major target for artificial selection. Thus, we propose that awn loss was favored and strongly selected by humans, as genetic variations at the An-1 locus that cause awn loss would increase grain numbers and subsequently improve grain yield in cultivated rice.     

Studies on Endophytic Colonization Ability of Two Upland Rice Endophytes, <Emphasis Type="Italic">Rhizobium</Emphasis> sp. and <Emphasis Type="Italic">Burkholderia</Emphasis> sp., Using Green Fluorescent Protein Reporter

Colonization ability of the two endophytic bacteria, isolated from surface sterilized roots of upland cultivated rice viz., Rhizobium sp. and Burkholderia sp., was compared after genetically tagging them with a constitutively expressing green fluorescent protein gene (gfp/gusA). Confocal laser scanning microscopy (CLSM) of gnotobiotically grown seedlings of Narendradhan 97, inoculated with gfp/gusA-tagged endophytes, revealed that both Rhizobium sp. and Burkholderia sp. colonized the intercellular spaces in the root cortex when inoculated separately. Colonization by gfp/gusA-tagged Rhizobium sp. was severely inhibited when co-inoculated with an equal number (10⁶ cfu ml⁻¹) of wild type Burkholderia sp. Burkholderia sp. was a more aggressive endophytic colonizer of rice than Rhizobium sp. The potential of using gfp/gusA reporter and CLSM as tools in evaluating competitive ability of colonization among endophytes is demonstrated in this study.     

Identification of Quantitative Trait Loci Controlling Drought Tolerance at Seedling Stage in Chinese Dongxiang Common Wild Rice (Oryza rufipogon Griff.)

Common wild rice (Oryza rufipogon Griff.) is the ancestor of cultivated rice (O. sativa L.), which has a greater genetic diversity and important traits that remain to be employed in cultivated rice. In this study, a set of introgression lines (BC₄F₅ and/or BC₄F₆) carrying various introgressed segments from common wild rice, collected from Dongxiang county, Jiangxi Province, China, in the background of an Indica (O. sativa L. ssp. indica) cultivar, Guichao 2, was used. A total of 12 drought-related quantitative trait loci (QTL) were identified by investigating drought tolerance of introgression lines under 30% PEG treatment at the young seedlings stage. Of these QTLs, the alleles of 4 QTLs on chromosome 2, 6 and 12 from Dongxiang common wild rice were responsible for increased drought tolerance of the introgression lines. In particular, a QTL qSDT12-2, near RM17 on chromosome 12, was consistently detected in different replications, and expressed stably under PEG stress throughout the study. It was also found that the QTLs located on different chromosomes might express at different stages.