Drought stress and plant ecotype drive microbiome recruitment in switchgrass rhizosheath

The rhizosheath, a layer of soil grains that adheres firmly to roots, is beneficial for plant growth and adaptation to drought environments. Switchgrass is a perennial C4 grass which can form contact rhizosheath under drought conditions. In this study, we characterized the microbiomes of four different rhizocompartments of two switchgrass ecotypes (Alamo and Kanlow) grown under drought or well-watered conditions via 16S ribosomal RNA amplicon sequencing. These four rhizocompartments, the bulk soil, rhizosheath soil, rhizoplane, and root endosphere, harbored both distinct and overlapping microbial communities. The root compartments (rhizoplane and root endosphere) displayed low-complexity communities dominated by Proteobacteria and Firmicutes. Compared to bulk soil, Cyanobacteria and Bacteroidetes were selectively enriched, while Proteobacteria and Firmicutes were selectively depleted, in rhizosheath soil. Taxa from Proteobacteria or Firmicutes were specifically selected in Alamo or Kanlow rhizosheath soil. Following drought stress, Citrobacter and Acinetobacter were further enriched in rhizosheath soil, suggesting that rhizosheath microbiome assembly is driven by drought stress. Additionally, the ecotype-specific recruitment of rhizosheath microbiome reveals their differences in drought stress responses. Collectively, these results shed light on rhizosheath microbiome recruitment in switchgrass and lay the foundation for the improvement of drought tolerance in switchgrass by regulating the rhizosheath microbiome.     

Rock phosphate solubilization by psychrotolerant Pseudomonas spp. and their effect on lentil growth and nutrient uptake under polyhouse conditions

Psychrotolerant Pseudomonas isolates (RT5RP2 and RT6RP) isolated from the rhizoplane of wild grass at 3,100 and 3,800 m above mean sea level, respectively, from Rudraprayag district of Uttarakhand (India), were found to solubilize Udaipur rock phosphate (URP). Both isolates grew at temperatures ranging from 4 to 30 °C. Kinetics of phosphate solubilization by the bacterial strains showed a nonlinear regression of the rate of P solubilization, which fitted best in the power model, and showed a declining trend across three different temperatures. Under pot culture conditions, bacterization of lentil seeds (cv. VL Masoor 507) with the psychrotolerant Pseudomonas strains when combined with URP as a sole source of phosphorus resulting in significant enhancement in P uptake of the plants, compared to the application of rock phosphate alone.     

Influence of Two Plant Species (Flax and Tomato) on the Distribution of Nitrogen Dissimilative Abilities within Fluorescent Pseudomonas spp

The distribution of nitrogen-dissimilative abilities among 317 isolates of fluorescent pseudomonads was studied. These strains were isolated from an uncultivated soil and from the rhizosphere, rhizoplane, and root tissue of two plant species (flax and tomato) cultivated on this same soil. The isolates were distributed into two species, Pseudomonas fluorescens (45.1%) and Pseudomonas putida (40.4%), plus an intermediate type (14.5%). P. fluorescens was the species with the greatest proportion of isolates in the root compartments and the greatest proportion of dissimilatory and denitrifying strains. According to their ability to dissimilate nitrogen, the isolates have been distributed into nondissimilatory and dissimilatory strains, nitrate reducers and true denitrifiers with or without N(inf2)O reductase. The proportion of dissimilatory isolates was significantly enhanced in the compartments affected by flax and tomato roots (55% in uncultivated soil and 90 and 82% in the root tissue of flax and tomato, respectively). Among these strains, the proportion of denitrifiers gradually and significantly increased in the root vicinity of tomato (44, 68, 75, and 94% in uncultivated soil, rhizosphere, rhizoplane, and root tissue, respectively) and was higher in the flax rhizoplane (66%) than in the uncultivated soil. A higher proportion of N(inf2)O reducers was also found in the root compartments. This result was particularly clear for tomato. It is hypothesized that denitrification could be a selective advantage for the denitrifiers in the root environment and that this process could contribute to modify the specific composition of the bacterial communities in the rhizosphere.     

Diversity of root-associated bacteria associated with field-grown canola (Brassica napus L.) and wheat (Triticum aestivum L.)

Little is known about the composition and diversity of the bacterial community associated with plant roots. The purpose of this study was to investigate the diversity of bacteria associated with the roots of canola plants grown at three field locations in Saskatchewan, Canada. Over 300 rhizoplane and 220 endophytic bacteria were randomly selected from agar-solidified trypticase soy broth, and identified using fatty acid methyl ester (FAME) profiles. Based on FAME profiles, 18 bacterial genera were identified with a similarity index >0.3, but 73% of the identified isolates belonged to four genera: Bacillus (29%), Flavobacterium (12%), Micrococcus (20%) and Rathayibacter (12%). The endophytic community had a lower Shannon-Weaver diversity index (1.35) compared to the rhizoplane (2.15), and a higher proportion of Bacillus, Flavobacterium, Micrococcus and Rathayibacter genera compared to rhizoplane populations. Genera identified in the endophytic isolates were also found in the rhizoplane isolates. Furthermore, principal component analysis indicated three clusters of bacteria regardless of their site of origin, i.e., rhizoplane or endophytic. In addition, the rhizoplane communities of canola and wheat grown at the same site differed significantly. These results indicate that diverse groups of bacteria are associated with field-grown plants and that endophytes are a subset of the rhizoplane community.     

Relative contribution of seed phosphorus reserves and exogenous phosphorus uptake to maize (Zea mays L.) nutrition during early growth stages

Adequate phosphorus (P) nutrition during early stages is critical for maize growth. Our objective was to evaluate the relative contribution of seed P reserves and exogenous P to maize nutrition during early growth stages. Seedlings were grown with labeled nutrient solution (³²P). Seedlings were harvested periodically over the course of the three-week study. Initially, 87% and 77% of the total C and N in seeds were located in the endosperm, whereas 86% of seed P was located in the scutellum as phytate. Up to the 7th day after sowing, 96% of phytate was hydrolyzed. Hydrolyzed forms of P were temporarily stored in the seed before being translocated to growing organs, suggesting that the hydrolysis of phytate was not a limiting step for P supply to seedlings. Significant P uptake by roots was observed from the 5th day after sowing on. Both sources of P supplied roots and leaves, with a slightly higher proportion of P from seed reserves going to leaves rather than to roots. Of total seed P, 60% and 92% was exported towards newly growing seedlings till 7th and 17th days after sowing and ceased to be a significant source of P for growth thereafter. We conclude that although both P supply processes overlap in time, seed P was the main P source during early growth stages.     

Genetic and phenotypic microdiversity of Ochrobactrum spp

The diversity of Ochrobactrum anthropi, Ochrobactrum intermedium, Ochrobactrum tritici and Ochrobactrum grignonense in agricultural soil and on the wheat rhizoplane was investigated. O. anthropi was isolated both from soil and from the rhizoplane, O. intermedium and grignonense only from bulk soil, and O. tritici only from the wheat rhizoplane. On the genetic level, the immunotrapped isolates and a number of strains from culture collection mainly of clinical origin were compared with rep-PCR profiling using BOX primers, and a subset of these isolates and strains using REP primers. The isolates clustered according to their species affiliation. There was no correlation between rep clusters of O. anthropi isolates and habitat (place of isolation). The genetic diversity of Ochrobactrum at the species level as well as microdiversity of O. anthropi (number of BOX groups) was higher in soil than on the rhizoplane. Similarity values from genetic rep-PCR profiles correlated positively with DNA-DNA reassociation percentages. Isolates with >80.7% similarity in BOX profile and >86.4% in rep profile clustered within the same species. Similarity analysis of rep-PCR profiles is hence an alternative to DNA-DNA hybridization as a genomic criterion for species delineation within the genus Ochrobactrum. We used the substrate utilization system BIOLOG-GN to compare the immunotrapped isolates on the phenetic level. For the isolates from bulk soil, substrate utilization versatility (number of utilized substrates) and substrate utilization capacity (mean conversion rate of substrates) were slightly but significantly higher than for the isolates from the rhizoplane. This trend was also seen using API 20E and 20NE systems. Plate counts of total bacteria and the number of immunotrapped Ochrobactrum isolates per gram dry weight were higher for the rhizoplane than for the soil samples. The results of genetic and phenotypic analyses indicated a 'rhizosphere effect'; the diversity and metabolic capacity of Ochrobactrum isolates were higher in bulk soil, and the population density was higher on the wheat rhizoplane.     

Effect of soil type and plant species on the fluorescent pseudomonads nitrate dissimilating community

The distribution of nitrogen dissimilative abilities among 618 isolates of fluorescent pseudomonads was studied. These strains were isolated from two uncultivated soils (C and D; collected at Chateaurenard and Dijon, France, respectively) and from rhizosphere, rhizoplane and root tissue of two plant species (flax and tomato) cultivated on these two soils. According to their ability to dissimilate nitrogen, the isolates have been distributed into three metabolic types: non-dissimilators, NO₂ ^- accumulators and denitrifiers. While the three metabolic types were recovered in all the compartments of soil D experiments, only two (non-dissimilators and denitrifiers) were recovered in all the compartments of soil C experiments. Even under the contrasting conditions of the two soil types, both plants were able to select the nitrate dissimilating community among the total community of fluorescent Pseudomonas, but the mode of this selection seems to be dependent on both plant and soil type. The soil type appears to be unable to significantly modulate the strong selective effect of tomato. Indeed, similar dissimilator to non-dissimilator ratios were found in the root tissue of this plant species cultivated in both soils. In contrast, the different dissimilator to non-dissimilator ratios observed in flax roots between soils C and D suggest that the selective effect of flax was modulated by the soil type. Taxonomic identifications showed that the 618 isolates were distributed among three species (P. chlororaphis, P. fluorescens, P. putida) plus an intermediate type between P. fluorescens and P. putida. However, no clear relationship between the distribution of the metabolic types (functional diversity) and the distribution of bacterial species has been found. This revised version was published online in June 2006 with corrections to the Cover Date.     

Screening of bacteria of the genus Bacillus for the control of the plant-pathogenic fungus Macrophomina phaseolina

This study evaluated the efficiency of 19 Bacillus isolates, obtained from the rhizosphere and rhizoplane of wild and cultivated castor bean plants, to control the pathogenic fungus Macrophomina phaseolina. Using in vitro assays, we examined the antifungal effects of volatile and non-volatile compounds, the production of siderophores and the activity of chitinase in these isolates. In vivo experiments were conducted to determine the potential of the Bacillus isolates to colonise castor bean plant roots and to control the fungus. In general, results showed that isolates from wild castor bean, compared with isolates from cultivated castor bean, were more efficient producers of antifungal compounds, better colonisers of plant roots and more effective protectors of plant seedlings against infection with M. phaseolina. Altogether, isolate RP 5, originating from the rhizoplane of wild castor bean, was the most promising candidate for future evaluation as a biological control agent of M. phaseolina.     

Isolation and identification of phytate-degrading rhizobacteria with activity of improving growth of poplar and Masson pine

A number of soil microorganisms can convert insoluble forms of phosphorus (P) to an accessible form to increase plant yields. Phytate is such a large kind of insoluble organic phosphorus that plants cannot absorb directly in soil, so the objectives of this study were to isolate, screen phytate-degrading rhizobacteria (PDRB), and to select potential microbial inocula that could increase the P uptake by plants. In this study, a total of 24 soil samples were collected from natural habitats of eight poplar and pine planting areas from the eastern to southern China. 17 PDRB strains were preliminarily screened from the rhizosphere soil of poplars and pines by the visible decolorization in the phytate selective medium. The highest ratio of the total diameter (colony + halo zone) to the colony diameter of the isolates was JZ-GX1, 3.85. Afterward, 17 PDRB strains were further determined for their abilities to degrade sodium phytate based on the amount of liberated inorganic P in liquid phytate specific medium. The results showed that the phytase ability of the three highest PDRB strains: JZ-GX1, JZ-DZ1 and JZ-ZJ1 were up to 2.58, 2.36 and 2.24 U/mL, respectively, much better than most of the bacteria reported in previous studies. In the soil–plant experiment, compared to CK, the best three strains of PDRB all could significantly promote growth of poplar and Masson pine under container growing. The three efficient PDRB strains were identified as follow: JZ-GX1, Rahnella aquatilis, both JZ-DZ1 and JZ-ZJ1 being autofluorescent, Pseudomonas fluorescens, by 16S rDNA gene sequencing technology, Biolog Identification System and biological characterization. The present study suggests that the three screened PDRB strains would have great potential application as biological fertilizers in the future.     

Nitrogen dynamics in the intact grasses Poa trivialis and Panicum maximum receiving contrasting supplies of nitrogen

The C(3) grass Poa trivialis and the C(4) grass Panicum maximum were grown in sand culture and received a complete nutrient solution with nitrogen supplied as 1.5 mol m(-3) NH(4)NO(3). (15)N tracer techniques were used to quantify the relative use of root uptake and mobilization in supplying nitrogen to growing leaves in intact plants which either continued to receive nitrogen or which received the complete nutrient solution without nitrogen. The allocation of both (15)N-labelled nitrogen uptake and unlabelled mobilized nitrogen indicated that, under their conditions of growth, the sink strength of growing leaves was relatively greater in P. maximum than P. trivialis. The supply of nitrogen by mobilization to side tillers of P. trivialis was completely stopped as the external nitrogen supply was reduced, whilst in P. maximum some allocation of mobilized nitrogen to side tillers, roots and growing leaves was maintained. In both plant species receiving an uninterrupted supply of nitrogen the allocation pattern of mobilized nitrogen differed from that of nitrogen derived from root uptake. Differences exist in the degree to which P. trivialis and P. maximum utilized uptake and mobilization to supply nitrogen to the growing leaves. In P. trivialis roots were always a net sink of mobilized nitrogen, irrespective of the external nitrogen supply. In P. maximum, roots were a net sink of mobilized nitrogen when external nitrogen was withdrawn, but exhibited both source and sink behaviour when nitrogen supply was continued.     

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).     

Effect of a Nickel-Tolerant ACC Deaminase-Producing Pseudomonas Strain on Growth of Nontransformed and Transgenic Canola Plants

Four bacterial strains were isolated from soils at nickel-contaminated sites based on their ability to utilize 1-aminocyclopropane-1-carboxylate (ACC) as a sole source of nitrogen. The four isolates were all identified as Pseudomonas putida Biovar B, and subsequent testing revealed that they all exhibited traits previously associated with plant growth promotion (i.e., indoleacetic acid and siderophore production and ACC deaminase activity). These four strains were also tolerant of nickel concentrations of up to 13.2 mM in the culture medium. The strain, HS-2, selected for further characterization, was used in pot experiments to inoculate both nontransformed and transgenic canola plants (expressing a bacterial ACC deaminase gene in its roots). Plants inoculated with the HS-2 strain produced an increase in plant biomass as well as in nickel (Ni) uptake by shoots and roots. The results suggest that this strain is a potential candidate to be used as an inoculant in both phytoremediation protocols and in plant growth promotion.     

Comparison of aerobic heterotrophic taxa isolated from four root domains of mature sugar beet (Beta vulgaris)

Abstract: The distribution of bacteria in the rhizosphere, rhizoplane, interior root tissues (core) and lower root (all tissues) of mature sugar beet roots ( Beta vulgaris ) was compared. Of 556 isolates, 102 species from 40 genera were identified by fatty acid methyl ester gas-chromatographic (FAME-GC) analysis. The ten most common genera ( Bacillus , 14%; Arthrobacter , 12%; Pseudomonas , 11%; Aureobacterium , 9%; Micrococcus , 6%; Xanthomonas , 5%; Alcaligenes , 4%; Flavobacterium , 3%; Agrobacterium , 3%; Microbacterium , 3%) accounted for 70% of isolates, and were found in each of three root domains (rhizosphere, rhizoplane and interior root tissues) on the two principal sampling occasions. Gram-positive strains were more abundant in the rhizosphere than the rhizoplane. Compared to the rhizoplane, rhizosphere bacterial communities were represented by a less diverse, more hierarchical distribution of species where twice as many isolates formed late developing colonies on isolation plates. Between October and January, the bacteria isolated from root interior tissues acquired a distinct change in taxonomic pattern, with decreased diversity and increased hierarchy. A bacterial continuum of similar taxa was observed which extended from the rhizosphere to interior root tissues.     

Isolation and characterization of agar-degrading Paenibacillus spp. associated with the rhizosphere of spinach

Agar-degrading bacteria in spinach plant roots cultivated in five soils were screened, and four strains of Paenibacillus sp. were isolated from roots cultivated in three soils. The agar-degrading bacteria accounted for 1.3% to 2.5% of the total bacteria on the roots. In contrast, no agar-degrading colony was detected in any soil (non-rhizosphere soil samples) by the plate dilution method, and thus these agar-degrading bacteria may specifically inhabit plant roots. All isolates produced extracellular agarase, and could grow using agar in the culture medium as the sole carbon source. Zymogram analyses of agarase showed that all four isolates extracellularly secreted multiple agarases (75-160 kDa). In addition, the isolates degraded not only agar but also various plant polysaccharides, i.e., cellulose, pectin, starch, and xylan.     

Isolation and Characterization of Agar-degrading Paenibacillus spp. Associated with the Rhizosphere of Spinach

Agar-degrading bacteria in spinach plant roots cultivated in five soils were screened, and four strains of Paenibacillus sp. were isolated from roots cultivated in three soils. The agar-degrading bacteria accounted for 1.3% to 2.5% of the total bacteria on the roots. In contrast, no agar-degrading colony was detected in any soil (non-rhizosphere soil samples) by the plate dilution method, and thus these agar-degrading bacteria may specifically inhabit plant roots. All isolates produced extracellular agarase, and could grow using agar in the culture medium as the sole carbon source. Zymogram analyses of agarase showed that all four isolates extracellularly secreted multiple agarases (75-160 kDa). In addition, the isolates degraded not only agar but also various plant polysaccharides, i.e., cellulose, pectin, starch, and xylan.     

Role of dissimilatory fermentative iron-reducing bacteria in Fe uptake by common bean (Phaseolus vulgaris L.) plants grown in alkaline soil

Iron (Fe) is an essential element for plant growth and development. Some plant growth-promoting rhizobacteria can increase Fe uptake by plants through reduction of Fe(III) to Fe(II) at the root surface. The aim of this work was to identify novel bacterial strains with high Fe(III) reduction ability and to evaluate their role in plant Fe uptake. Four bacterial strains (UMCV1 to UMCV4) showing dissimilatory Fe-reducing activity were isolated from the rhizosphere of bean and maize plants and further identified by 16S rDNA amplification and sequence analysis. From these analyses, UMCV1 and UMCV2 isolates were identified as Bacillus megaterium and Arthrobacter spp., respectively, whereas UMCV3 and UMCV4 were identified as Stenotrophomonas maltophilia. All four isolates showed Fe reduction in a nonflooded soil and when associated with roots of bean plants grown in alkaline soil or in mineral medium. In addition, the bacterial isolates were able to stimulate plant growth in vitro and on a broad level, plants grown in inoculated soil were generally bigger and with higher Fe content than those grown in sterilized soil. These results indicate that bacterial species isolated from the rhizosphere of bean and maize plants contribute significantly to Fe uptake by plants likely through increased Fe(III) reduction in the rhizosphere.     

Screening for phosphate solubilizing bacteria inhabiting the rhizoplane of rice grown in acidic soil in Bangladesh

The objectives of the research were to isolate phosphate solubilizing bacteria (PSB) from the rhizoplane of rice (Oryza sativa L.) cv. BRRIdhan 29 cultivated in acidic soils of Tangail in Bangladesh and evaluate their performances in phosphate solubilization in both in vitro and in vivo conditions. A total of 10 bacterial strains were isolated and purified by repeated streak culture on nutrient agar medium. Upon screening, five isolates (OS01, OS03, OS07, OS08 and OS10) showed varying levels of phosphate solubilizing activity in agar plate and broth assays. Among them, the strain OS07 (B1) and two previously isolated PSB strains B2 and B3 were selected for evaluation for their performances in rice alone or in combination of TSP (triple super phosphate: P1) and rock phosphate (P2). Plant height and the number of tillers per plant were significantly increased by all PSB isolates when used in combination with TSP but PSB alone did not influence much on plant height and the number of tillers except B1. The levels of mineral nutrients content in rice plant tissues were generally increased by the application of the PSB in combination with TSP, while the performances of B1 isolate was superior in all aspects to B2 and B3 isolates.     

Phosphorus Acquisition Strategies within Arbuscular Mycorrhizal Fungal Community of a Single Field Site

Diversity in phosphorus (P) acquisition strategies was assessed among eight isolates of arbuscular mycorrhizal fungi (AMF) belonging to three Glomus species, all obtained from the same field site. Maize (Zea mays L. cv. Corso) was used as a test plant. Compartmented cultivation containers coupled with ³³P radioisotope labeling of soil P were employed to estimate (1) the distance from the roots that AMF were able to acquire soil P from, (2) the rate of soil colonization, (3) the efficiency of uptake of soil P by AMF, (4) benefits provided to maize in terms of P acquisition and growth. Glomus mosseae and G. intraradices took up P 10 cm from roots, whereas G. claroideum only up to 6 cm from the roots. G. mosseae most rapidly colonized the available soil volume and transported significant amounts of P to maize from a distance, but provided no net P uptake benefit to the plants. On the other hand, both G. intraradices and three out of four G. claroideum isolates significantly improved net P uptake by maize. These effects seem to be related to variability between and to a limited extent also within AMF species, in mycelium development, efficiency of hyphal P uptake and effects on plant P acquisition via the root pathway. In spite of absence of maize growth responses to inoculation with any of the AMF isolates, this study indicates remarkable functional diversity in the underground component of the studied field site.     

Survival of genetically modified Pseudomonas fluorescens introduced into subtropical soil microcosms

Abstract A genetically modified strain of Pseudomonas fluorescens and its parent showed grossly similar decline rates following introduction into subtropical clay and sandy soils. In unplanted clay soit at pH 6.9 and 25°C, population densities declined progressively from about 10⁸ to 10³ colony forming units (cfu) g⁻¹ dry soil over 75 days, but in unplanted sandy soil the introduced populations could not be detected after 25 days. In clay soil at pH 8.7 or 4.7, or at environmental temperature, decay rates were enhanced as compared to those at pH 6.9 and 25°C. Counts of introduced strains in clay bulk soil and in rhizosphere and rhizoplane of maize suggested that the introduced bacteria competed well with the native bacteria, and colonized the roots at about 10⁶ cfu g⁻¹ dry root at 25°C, over 20 days. However, rhizoplane colonization was lower at environmental temperature. The decay rate of both strains was slower in planted than in unplanted sandy soil. The population densities in the rhizosphere and rhizoplane in the sandy soil were significantly lower than those in the clay soil. Both introduced strains colonized the maize roots in both soils, using seeds coated with bacteria in 1% carboxymethyl cellulose. Introduced cells were localized at different sites along the roots of plants developing in clay soil, with higher densities in the original (near the seeds) and root hair zones as compared to the intermediate zones. No significant difference was observed between the extent of root colonization of the genetically modified strain and its parent.     

Tomato genotype and Azospirillum inoculation modulate the changes in bacterial communities associated with roots and leaves

AIMS: To evaluate the effect of plant variety and Azospirillum brasilense inoculation on the microbial communities colonizing roots and leaves of tomato (Lycopersicon esculentum Mill.) plants. METHODS AND RESULTS: Seeds of cherry and fresh-market tomato were inoculated with A. brasilense BNM65. Sixty days after planting, plants were harvested and the microbial communities of the rhizoplane and phyllosphere were analysed by community-level physiological profiles (CLPP) using BIOLOG EcoPlates and denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA genes. Differences on the rhizoplane and phyllosphere bacterial communities between the two tomato types were detected by principal component analysis of the CLPP; DGGE fingerprints also showed differences at the phyllosphere level. Fresh-market tomato had a more complex phyllosphere bacterial community than cherry tomato, as determined by DGGE profiles. Physiological and genetic changes on phyllosphere and rhizoplane bacterial communities by Azospirillum seed inoculation were evident only on cherry tomato. CONCLUSIONS: Tomato genotype affects the response of native bacterial communities associated with the roots and leaves to A. brasilense seed inoculation. SIGNIFICANCE AND IMPACT OF THE STUDY: The successful implementation of Azospirillum inoculation requires not only the consideration of the interactions between A. brasilense strains and plant genotypes, but also the plant-associated microflora.