Factors affecting the attachment of rhizospheric bacteria to bean and soybean roots

Plant growth-promoting rhizobacteria: a potential bio-asset for restoration of degraded soil and crop productivity with sustainable emerging techniques

The rapid expansion of degraded soil puts pressure on agricultural crop yield while also increasing the likelihood of food scarcity in the near future at the global level. The degraded soil does not suit plants growth owing to the alteration in biogeochemical cycles of nutrients, soil microbial diversity, soil organic matter, and increasing concentration of heavy metals and organic chemicals. Therefore, it is imperative that a solution should be found for such emerging issues in order to establish a sustainable future. In this context, the importance of plant growth-promoting rhizobacteria (PGPR) for their ability to reduce plant stress has been recognized. A direct and indirect mechanism in plant growth promotion is facilitated by PGPR via phytostimulation, biofertilizers, and biocontrol activities. However, plant stress mediated by deteriorated soil at the field level is not entirely addressed by the implementation of PGPR at the field level. Thus, emerging methods such as CRISPR and nanotechnological approaches along with PGPR could manage degraded soil effectively. In the pursuit of the critical gaps in this respect, the present review discusses the recent advancement in PGPR action when used along with nanomaterials and CRISPR, impacting plant growth under degraded soil, thereby opening a new horizon for researchers in this field to mitigate the challenges of degraded soil.

Purification of archetypal soybean root suberin mostly comprising alka(e)noic acids using an ionic liquid catalyst

Soybean (Glycine max) is an increasingly relevant crop due to its economic importance and also a model plant for the study of root symbiotic associations with nodule forming rhizobia. Plant polyesters mediate plant-microbe interactions with both pathogenic and beneficial microbes; suberin has been hypothesized to play a key role during the early steps of rhizobia attachment to the root. The downside is that suberin chemistry in soybean root is still scarcely studied. This study addresses this outstanding question by reporting a straightforward workflow for a speedy purification of suberin from soybean root and for its subsequent detailed chemical analysis. To purify suberin, cholinium hexanoate (an ionic liquid) was used as the catalyst. The ensuing suberin is highly esterified as observed by a precise Nuclear Magnetic Resonance quantification of each ester type, discriminating between primary and acylglycerol esters. Moreover, the composing hydrolysable monomers detected through GC-MS revealed that hexadecanoic acid is the most abundant monomer, similar to that reported before by others. Overall, this study highlights the adequacy of the ionic liquid catalyst for the isolation of suberin from soybean roots, where the polymer natural abundance is low, and builds new knowledge on the specificities of its chemistry; essential to better understand the biological roles of suberin in roots.

Bradyrhizobium diazoefficiens USDA 110 displays plasticity in the attachment phenotype when grown in different soybean root exudate compounds

Introduction

Bradyrhizobium diazoefficiens, a symbiotic nitrogen fixer for soybean, forms nodules after developing a symbiotic association with the root. For this association, bacteria need to move toward and attach to the root. These steps are mediated by the surface and phenotypic cell properties of bacteria and secreted root exudate compounds. Immense work has been carried out on nodule formation and nitrogen fixation, but little is known about the phenotype of these microorganisms under the influence of different root exudate chemical compounds (RECCs) or how this phenotype impacts the root attachment ability.

Methods

To address this knowledge gap, we studied the impact of 12 different RECCs, one commonly used carbon source, and soil-extracted solubilized organic matter (SESOM) on attachment and attachment-related properties of B. diazoefficiens USDA110. We measured motility-related properties (swimming, swarming, chemotaxis, and flagellar expression), attachment-related properties (surface hydrophobicity, biofilm formation, and attachment to cellulose and soybean roots), and surface polysaccharide properties (colony morphology, exopolysaccharide quantification, lectin binding profile, and lipopolysaccharide profiling).

Results and discussion

We found that USDA 110 displays a high degree of surface phenotypic plasticity when grown on the various individual RECCs. Some of the RECCs played specific roles in modulating the motility and root attachment processes. Serine increased cell surface hydrophobicity and root and cellulose attachment, with no EPS formed. Gluconate and lactate increased EPS production and biofilm formation, while decreasing hydrophobicity and root attachment, and raffinose and gentisate promoted motility and chemotaxis. The results also indicated that the biofilm formation trait on hydrophilic surfaces (polystyrene) cannot be related to the attachment ability of Bradyrhizobium to the soybean root. Among the tested phenotypic properties, bacterial cell surface hydrophobicity was the one with a significant impact on root attachment ability. We conclude that USDA 110 displays surface plasticity properties and attachment phenotype determined by individual RECCs from the soybean. Conclusions made based on its behavior in standard carbon sources, such as arabinose or mannitol, do not hold for its behavior in soil.

Molecular characterization of endophytic and ectophytic plant growth promoting bacteria isolated from tomato plants (Solanum lycopersicum L.) grown in different soil types

Background

Successful rhizosphere colonization by plant growth promoting rhizobacteria (PGPR) is of crucial importance to perform the desired plant growth promoting activities. Since rhizocompetence is a dynamic process influenced by surrounding environmental conditions. In the present study, we hypothesized that bacterial isolates obtained from different tomato plant microhabitats (balk soil, rhizosphere, endorhiza, phyllosphere, and endoshoot) grown in different soils (sand, clay, and peat moss) will show different rhizocompetence abilities.

Results

To evaluate this hypothesis, bacterial isolates were obtained from different plant microhabitats and screened for their phosphate solubilizing and nitrogen fixing activates. BOX-PCR fingerprint profiles showed high genotypic diversity among the tested isolates and that same genotypes were shared between different soils and/or plant microhabitats. 16S rRNA gene sequences of 25 PGP isolates, representing different plant spheres and soil types, were affiliated to eight genera: Enterobacter, Paraburkholderia, Klebsiella, Bacillus, Paenibacillus, Stenotrophomonas, Pseudomonas, and Kosakonia. The rhizocompetence of each isolate was evaluated in the rhizosphere of tomato plants grown on a mixture of the three soils. Different genotypes of the same bacterial species displayed different rhizocompetence potentials. However, isolates obtained from the above-ground parts of the plant showed high rhizocompetence. In addition, biological control-related genes, ituD and srfC, were detected in the obtained spore forming bacterial isolates.

Conclusion

This study evaluates, for the first time, the relationship between plant microhabitat and the rhizocompetence ability in tomato rhizosphere. The results indicated that soil type and plant sphere can influence both the genotypic diversity and rhizocompetence ability of the same bacterial species. Bacterial isolates obtained in this study are promising to be used as an environmentally friendly substitution of chemical fertilizers.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-022-00361-0.

Seed-Encapsulation of Desiccation-Tolerant Microorganisms for the Protection of Maize from Drought: Phenotyping Effects of a New Dry Bioformulation

Droughts and high temperatures deeply affect crop production. The use of desiccation-tolerant (or xerotolerant) microorganisms able to protect plants from droughts represents a promising alternative. These xerotolerant microorganisms have previously been used to modulate plant responses and improve their tolerance to drought. In addition, these microorganisms could be stored and used in dry formats, which would improve their viability and resilience at a much lower cost than current market alternatives. In the present study we analyze the possibility of using strains of xerotolerant Actinobacteria in encapsulated format on seeds. Under this formulation, we carried out greenhouse with farming soil with maize plants. Under greenhouse conditions, the plants showed greater resistance to drought, as well as increased growth and production yield, but not as well in field trials. This alternative could represent a useful tool to improve water efficiency in crops for drought-affected areas or affected by water scarcity.

Pseudomonas mediated nutritional and growth promotional activities for sustainable food security

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Fluorescent and non-fluorescent species of Pseudomonas are important for plant growth promotion, phytopathogenic control and plant disease management.

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Pseudomonas belong to Pseudomonadaceae family (10 groups on the basis of rRNA-DNA hybridization) classified into 6-subgroups of rRNA gene homology and RFLP.

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Pseudomonas species produce antagonistic mechanism such as ISR and compounds like cell wall degradation enzymes, and antibiotics to maintain a mutualistic relationship with the associated plant.

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Pseudomonas sp. synthesize auxins having properties similar to phytohormones like IAA, which act as signaling molecules for regulating plant growth.

Numerous microbial communities show synergistic and antagonistic interactions among themselves, resulting in benefit and harm to either or both the associated members. The association holds accountability for nutrients recycling and energy drift, resulting in the availability of macronutrients unavailable and insoluble forms of rhizospheric nutrients, crucial for vital processes in plants, e.g., act as co-factors of various phyto-enzyme and redox mediators. Plant growth promoting rhizobacteria are known to enhance plant growth by increasing these macronutrients availability during their plant root colonization. In comparison to any other genera, Pseudomonas is the most favored bioinoculant due to its significant properties in both plant growth and phytopathogen control during its synergistic association with the host plant. These properties include siderophore production, phosphate solubilization, nitrogen fixation, phenazines, antibiotics, and induced systemic resistance carried out by various Pseudomonas species like Pseudomonas fluorescens, Pseudomonas putida, and Pseudomonas syringae. The association of Pseudomonas with crop plants procures several secretory and electron-based feedback mechanisms in order to regulate the plant growth and phytopathogen control activities through the secretion of several phytohormones (auxins, gibberellins, Indole-3-acetic acid), secondary metabolites (flavonoids) and enzymes (aminocyclopropane-1-carboxylate, phenylalanine ammonia-lyase). Ecologically significant applications of Pseudomonas in biocontrol and bioaugmentation are crucial for maintaining food security.

Sclerotium oryzae biocontrol in flooded rice fields with floating microcarrier technology: The effect of chitosan molecular weight

BACKGROUND

Biocontrol strategies are of significant concern for their application in crops. Various green practices have been designed, but almost all of them had delivery constraints. In particular, to design biocontrol strategies against Sclerotium oryzae in flooded rice fields, the active agent should be retained on the plant leaves by spreading application, nevertheless the direct application onto the water produces the biocontrol agent dilution. An effective delivery model was needed. This work aimed to evaluate the effects of chitosan molecular weight on the formation of positively charged Pseudomonas fluorescens-chitosan complex as a floating microcarrier against Sclerotium oryzae. To this end, three different sizes of chitosan [molecular weights (MWs) 20 000, 250 000, and 1 250 000 g mol^-1 ] at different pH values (4, 6, and 7) were tested. The electrostatic interaction was analyzed through ζ-potential measurement. An adjustment of the experimental values was carried out for making predictions. The bacteria antifungal activity into the carrier with different chitosan MWs was analyzed.

RESULTS

Our results suggest that it is possible to form a bacteria-chitosan complex with a net positive charge under condition that improve bacteria incorporation to the microcarrier technology without harming bacteria viability and antifungal activity. Thus, high chitosan MW (1 250 000 g mol^-1 ) at pH 6 is preferable for microcarrier technology.

CONCLUSION

Our findings provide relevant information about bacteria-chitosan interaction and may be useful in biocontrol programs that involved these two components as well as situations in which bacteria adsorption to an anionic carrier or anionic surface is desirable.

Competition, Nodule Occupancy, and Persistence of Inoculant Strains: Key Factors in the Rhizobium-Legume Symbioses

Biological nitrogen fixation by Rhizobium-legume symbioses represents an environmentally friendly and inexpensive alternative to the use of chemical nitrogen fertilizers in legume crops. Rhizobial inoculants, applied frequently as biofertilizers, play an important role in sustainable agriculture. However, inoculants often fail to compete for nodule occupancy against native rhizobia with inferior nitrogen-fixing abilities, resulting in low yields. Strains with excellent performance under controlled conditions are typically selected as inoculants, but the rates of nodule occupancy compared to native strains are rarely investigated. Lack of persistence in the field after agricultural cycles, usually due to the transfer of symbiotic genes from the inoculant strain to naturalized populations, also limits the suitability of commercial inoculants. When rhizobial inoculants are based on native strains with a high nitrogen fixation ability, they often have superior performance in the field due to their genetic adaptations to the local environment. Therefore, knowledge from laboratory studies assessing competition and understanding how diverse strains of rhizobia behave, together with assays done under field conditions, may allow us to exploit the effectiveness of native populations selected as elite strains and to breed specific host cultivar-rhizobial strain combinations. Here, we review current knowledge at the molecular level on competition for nodulation and the advances in molecular tools for assessing competitiveness. We then describe ongoing approaches for inoculant development based on native strains and emphasize future perspectives and applications using a multidisciplinary approach to ensure optimal performance of both symbiotic partners.

Structure and Development of the Legume-Rhizobial Symbiotic Interface in Infection Threads

The intracellular infection thread initiated in a root hair cell is a unique structure associated with Rhizobium-legume symbiosis. It is characterized by inverted tip growth of the plant cell wall, resulting in a tunnel that allows invasion of host cells by bacteria during the formation of the nitrogen-fixing root nodule. Regulation of the plant-microbial interface is essential for infection thread growth. This involves targeted deposition of the cell wall and extracellular matrix and tight control of cell wall remodeling. This review describes the potential role of different actors such as transcription factors, receptors, and enzymes in the rearrangement of the plant-microbial interface and control of polar infection thread growth. It also focuses on the composition of the main polymers of the infection thread wall and matrix and the participation of reactive oxygen species (ROS) in the development of the infection thread. Mutant analysis has helped to gain insight into the development of host defense reactions. The available data raise many new questions about the structure, function, and development of infection threads.

Bacterial biomechanics-From individual behaviors to biofilm and the gut flora

Bacteria inhabit a variety of locations and play important roles in the environment and health. Our understanding of bacterial biomechanics has improved markedly in the last decade and has revealed that biomechanics play a significant role in microbial biology. The obtained knowledge has enabled investigation of complex phenomena, such as biofilm formation and the dynamics of the gut flora. A bottom-up strategy, i.e., from the cellular to the macroscale, facilitates understanding of macroscopic bacterial phenomena. In this Review, we first cover the biomechanics of individual bacteria in the bulk liquid and on surfaces as the base of complex phenomena. The collective behaviors of bacteria in simple environments are next introduced. We then introduce recent advances in biofilm biomechanics, in which adhesion force and the flow environment play crucial roles. We also review transport phenomena in the intestine and the dynamics of the gut flora, focusing on that in zebrafish. Finally, we provide an overview of the future prospects for the field.

Immobilization of Bradyrhizobium and Azospirillum in alginate matrix for long time of storage maintains cell viability and interaction with peanut

Immobilizarion of PGPR for agricultural applications aims to provide temporary physical protection from stressful environmental conditions and the gradual release of cells for successful root colonization, release the cells gradually. In this work, we immobilized Bradyrhizobium sp. SEMIA6144 or Azospirillum brasilense Az39 cells in 2% alginate beads prepared by ionic gelation process, and then stored up to 12 months at 4 °C. Alginate matrix showed interaction with the immobilized bacteria (FTIR), allowed a constant release of cells, and improved their viability and capability to interact with Arachis hypogaea. Cell number into beads reached 10⁷ CFU.bead^-1; however, viability decreased from 4 months of storage for Az39, while it was maintained up to 12 months for SEMIA6144, showing a low metabolic activity measured by the MTT assay. Adhesion of SEMIA6144 and Az39 from new beads to peanut root was 11.5% and 16%, respectively, higher than non-immobilized bacteria. Peanut inoculation with 12 months storage SEMIA6144 beads significantly increased root length and biomass at 30 days of growth, and under restrictive water condition (RWC), nodulation and total plant N content increased compared with liquid inoculation. Our results demonstrate that immobilization of SEMIA6144 and Az39 in alginate matrix is a potential alternative to enhance peanut growth even under RWC. KEY POINTS: • Alginate encapsulation enhances viability of SEMIA6144 or Az39 under storage at 4 °C for 1 year. • Alginate beads 2% ensure the gradual release of the microorganisms. • Cells from beads stored for long periods present chemotaxis and adhesion to peanut root. • Peanut inoculation with 1-year-old SEMIA6144 beads improves nodulation and growth in RWC.

Framework for Quantification of the Dynamics of Root Colonization by Pseudomonas fluorescens Isolate SBW25

Colonization of the root surface, or rhizoplane, is one of the first steps for soil-borne bacteria to become established in the plant microbiome. However, the relative contributions of processes, such as bacterial attachment and proliferation is not well characterized, and this limits our ability to comprehend the complex dynamics of microbial communities in the rhizosphere. The work presented here addresses this knowledge gap. A model system was developed to acquire quantitative data on the colonization process of lettuce (Lactuca sativa L. cultivar. All Year Round) roots by Pseudomonas fluorescens isolate SBW25. A theoretical framework is proposed to calculate attachment rate and quantify the relative contribution of bacterial attachment to colonization. This allows the assessment of attachment rates on the root surface beyond the short time period during which it can be quantified experimentally. All techniques proposed are generic and similar analyses could be applied to study various combinations of plants and bacteria, or to assess competition between species. In the future this could allow for selection of microbial traits that improve early colonization and maintenance of targeted isolates in cropping systems, with potential applications for the development of biological fertilizers.

Diversity patterns of soil microbial communities in the Sophora flavescens rhizosphere in response to continuous monocropping

Background

Continuous monocropping can affect the physicochemical and biological characteristics of cultivated soil. Sophora flavescens is a valuable herbal medicine and sensitive to continuous monocropping. Currently, diversity patterns of soil microbial communities in soil continuous monocropping with S. flavescens have not been extensively elucidated.

Results

In this study, comparative 16S rDNA and internal transcribed spacer (ITS) MiSeq sequencing analyses were used to examine the taxonomic community structure and microbial diversity in nonrhizosphere soil (CK) and rhizosphere soils (SCC, TCC, and FCC) sampled from fields that had undergone two, three, and five years of continuous monocropping, respectively. Among the microbial communities, a decreased abundance of Acidobacteria and increased abundances of Proteobacteria and Bacteroidetes were found with the increase in monocropping years of S. flavescens. As the continuous monocropping time increased, the diversity of the bacterial community decreased, but that of fungi increased. Redundancy analysis also showed that among the properties of the rhizosphere soil, the available phosphorus, organic matter, total nitrogen, and sucrase had the greatest impacts on the diversity of the rhizosphere microbial community. Moreover, a biomarker for S. flavescens soil was also identified using the most differentially abundant bacteria and fungi in soil samples.

Conclusions

Our study indicates that long-term monocropping exerted great impacts on microbial community distributions and soil physicochemical properties. The relationship between microbial community and physicochemical properties of rhizosphere soil would help clarify the side effects of continuous S. flavescens monocropping. Our study may aid in uncovering the theoretical basis underlying obstacles to continuous monocropping and provide better guidance for crop production.

Physical Properties of Carbon Nanomaterials and Nanoceria Affect Pathways Important to the Nodulation Competitiveness of the Symbiotic N2 -Fixing Bacterium Bradyrhizobium diazoefficiens

The pathogenicity and antimicrobial properties of engineered nanomaterials (ENMs) are relatively well studied. However, less is known regarding the interactions of ENMs and agriculturally beneficial microorganisms that affect food security. Nanoceria (CeO2 nanoparticles (NPs)), multiwall carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), and carbon black (CB) have been previously shown to inhibit symbiotic N2 fixation in soybeans, but direct rhizobial susceptibility is uncertain. Here, Bradyrhizobium diazoefficiens associated with symbiotic N2 fixation in soybeans is assessed, evaluating the role of soybean root exudates (RE) on ENM-bacterial interactions and the effects of CeO2 NPs, MWCNTs, GNPs, and CB on bacterial growth and gene expression. Although bacterial growth is inhibited by 50 mg L-1 CeO2 NPs, MWCNTs, and CB, all ENMs at 0.1 and 10 mg L-1 cause a global transcriptomic response that is mitigated by RE. ENMs may interfere with plant-bacterial signaling, as evidenced by suppressed upregulation of genes induced by RE, and downregulation of genes encoding transport RNA, which facilitates nodulation signaling. MWCNTs and CeO2 NPs inhibit the expression of genes conferring B. diazoefficiens nodulation competitiveness. Surprisingly, the transcriptomic effects on B. diazoefficiens are similar for these two ENMs, indicating that physical, not chemical, ENM properties explain the observed effects.

Restrictive water condition modifies the root exudates composition during peanut-PGPR interaction and conditions early events, reversing the negative effects on plant growth

Water deficit is one of the most serious environmental factors that affect the productivity of crops in the world. Arachis hypogaea is a legume with a high nutritional value and 70% is cultivated in semi-arid regions. This research aimed to study the effect of water deficit on peanut root exudates composition, analyzing the importance of exudates on peanut-PGPR interaction under restrictive water condition. Peanut seedlings were subjected to six treatments: 0 and 15 mM PEG, in combination with non-inoculated, Bradyrhizobium sp. and Bradyrhizobium-Azospirillum brasilense inoculated treatments. We analyzed the 7-day peanut root exudate in response to a water restrictive condition and the presence of bacterial inocula. Molecular analysis was performed by HPLC, UPLC and GC. Bacteria motility, chemotaxis, bacterial adhesion to peanut roots and peanut growth parameters were analyzed. Restrictive water condition modified the pattern of molecules exuded by roots, increasing the exudation of Naringenin, oleic FA, citric and lactic acid, and stimulation the release of terpenes of known antioxidant and antimicrobial activity. The presence of microorganisms modified the composition of root exudates. Water deficit affected the first events of peanut-PGPR interaction and the root exudates favored bacterial mobility, the chemotaxis and attachment of bacteria to peanut roots. Changes in the profile of molecules exuded by roots allowed A. hypogaea-Bradyrhizobium and A.hypogaea-Bradyrhizobium-Azospirillum interaction thus reversing the negative effects of restrictive water condition on peanut growth. These findings have a future potential application to improve plant-PGPR interactions under water deficit by formulating inoculants containing key molecules exuded during stress.

Effect of water availability on changes in root amino acids and associated rhizosphere on root exudation of amino acids in Pisum sativum L

Root exudation is considered to regulate the abundance of the microbial community. It may vary both qualitatively and quantitatively in response to the environment in which the plant is growing. A part of exuded N derives from amino acids (AAs). This, in turn, may help plants to cope with abiotic stresses by favouring positive interactions with the rhizosphere environment, thus playing a potential role in maintaining healthy plants. In this respect, an under-investigated area is the effect of stress due to water deficit (WD). It is proposed that the AA profile in the rhizosphere may be altered by WD, reflecting a modulation of root AA exudation linked to a physiological response of the plant to water stress. To investigate this, Pisum sativum L. plants, grown in unsterilised Rhizobium leguminosarum-enriched soil, were stem-labelled with 15N-urea for 96 h, and then subjected/not subjected to 72 h of WD. The concentrations and abundance of 15N-labelling in individual AAs were determined in both roots and the associated rhizosphere at 24, 48 and 72 h after stress application. It was found that both AAs metabolism in the pea root and AAs exudation were strongly modified in WD conditions. After 24 h of WD, the concentrations of all measured AAs increased in the roots, accompanied by a dramatic stress-related increase in the 15N-labelling of some AAs. Furthermore, after 48-72 h of WD, the concentrations of Pro, Ala and Glu increased significantly within the rhizosphere, notably with a concomitant increase in 15N-enrichment in Pro, Ser, Asn, Asp, Thr and Ile. These results support the concept that, in response to WD, substantial amounts of recently assimilated N are rapidly translocated from the shoots to the roots, a portion of which is exuded as AAs. This leads to the rhizosphere being relatively augmented by specific AAs (notably HSer, Pro and Ala) in WD conditions, with a potential impact on soil water retention.

Fluorescent Pseudomonas -FAP2 and Bacillus licheniformis interact positively in biofilm mode enhancing plant growth and photosynthetic attributes

Compatible interaction between commonly used plant growth promoting rhizobacteria (PGPR) in biofilm mode in vitro and in the rhizosphere is expected to provide better understanding for the development of effective consortium. With the above hypothesis, the present study evaluated two characterized PGPR (Pseudomonas fluorescens FAP2 and Bacillus licheniformis B642) for their biofilm-related functions using standard protocols. The interaction between the FAP2 and B642 in planktonic mode was studied by plate spot/overlay method and competitive growth assessment. Biofilm development on a microtitre plate and a glass surface was studied by standard methods. Biofilm formation was characterized by SEM. Rhizosphere and rhizoplane colonization of wheat seedlings by both isolates individually and by co-inoculation was studied by determining CFU/g of soil/root samples. Biofilm development on the root surface was further analyzed by SEM. Both isolates demonstrated multiple plant growth promoting (PGP) traits (production of IAA, siderophore, and ammonia; phosphate solubilization) and biofilm-related functions such as production of EPS, alginate, cell surface hydrophobicity and swarming motility. Both strains formed strong biofilms on a glass cover slip in vitro. Interaction between the two strains under the planktonic mode revealed no antagonism in terms of growth inhibition and competitive growth kinetics. Similarly, FAP2 and B642 strains formed a mixed biofilm on a glass cover slip as well as on seedling roots. Wheat rhizosphere and rhizoplane were colonized by both isolates as evidenced from their viable counts in single and co-inoculation. The effect of single and co inoculation revealed the significant enhancement of vegetative growth and photosynthetic parameters such as chlorophyll content, transpiration rate (E), internal CO₂ concentration (Ci), stomatal conductance (g_s), and net photosynthetic rate (P_N) and leaf water potential (LWP) as compared to uninoculated control. Indigenous Pseudomonas fluorescens FAP2 strain and Bacillus licheniformis B642 are compatible PGPR in both planktonic and biofilm modes of growth and threfore could be developed effective consortium of PGPR. Further indepth investigation is required to understand molecular mechanism of the interaction in biofilm mode of growth under natural condition.

Rhizobia: from saprophytes to endosymbionts

Rhizobia are some of the best-studied plant microbiota. These oligotrophic Alphaproteobacteria or Betaproteobacteria form symbioses with their legume hosts. Rhizobia must exist in soil and compete with other members of the microbiota before infecting legumes and forming N₂-fixing bacteroids. These dramatic lifestyle and developmental changes are underpinned by large genomes and even more complex pan-genomes, which encompass the whole population and are subject to rapid genetic exchange. The ability to respond to plant signals and chemoattractants and to colonize nutrient-rich roots are crucial for the competitive success of these bacteria. The availability of a large body of genomic, physiological, biochemical and ecological studies makes rhizobia unique models for investigating community interactions and plant colonization.

Bacteriome from Pinus arizonica and P. durangensis: Diversity, Comparison of Assemblages, and Overlapping Degree with the Gut Bacterial Community of a Bark Beetle That Kills Pines

Symbioses between plants and microorganims have been fundamental in the evolution of both groups. The endophytic bacteria associated with conifers have been poorly studied in terms of diversity, ecology, and function. Coniferous trees of the genera Larix, Pseudotsugae, Picea and mainly Pinus, are hosts of many insects, including bark beetles and especially the Dendroctonus species. These insects colonize and kill these trees during their life cycle. Several bacteria detected in the gut and cuticle of these insects have been identified as endophytes in conifers. In this study, we characterized and compared the endophytic bacterial diversity in roots, phloem and bark of non-attacked saplings of Pinus arizonica and P. durangensis using 16S rRNA gene pyrosequencing. In addition, we evaluated the degree of taxonomic relatedness, and the association of metabolic function profiles of communities of endophytic bacteria and previously reported gut bacterial communities of D. rhizophagus; a specialized bark beetle that colonizes and kills saplings of these pine species. Our results showed that both pine species share a similar endophytic community. A total of seven bacterial phyla, 14 classes, 26 orders, 43 families, and 51 genera were identified. Enterobacteriaceae was the most abundant family across all samples, followed by Acetobacteraceae and Acidobacteriaceae, which agree with previous studies performed in other pines and conifers. Endophytic communities and that of the insect gut were significantly different, however, the taxonomic relatedness of certain bacterial genera of pines and insect assemblages suggested that some bacteria from pine tissues might be the same as those in the insect gut. Lastly, the metabolic profile using PICRUSt showed there to be a positive association between communities of both pines and insect gut. This study represents the baseline into the knowledge of the endophytic bacterial communities of two of the major hosts affected by D. rhizophagus.

Transcriptional analysis of genes involved in competitive nodulation in Bradyrhizobium diazoefficiens at the presence of soybean root exudates

Nodulation competition is a key factor that limits symbiotic nitrogen fixation between rhizobia and their host legumes. Soybean root exudates (SREs) are thought to act as signals that influence Bradyrhizobium ability to colonize roots and to survive in the rhizosphere, and thus they act as a key determinant of nodulation competitiveness. In order to find the competitiveness-related genes in B. diazoefficiens, the transcriptome of two SREs treated B. diazoefficiens with completely different nodulation abilities (B. diazoefficiens 4534 and B. diazoefficiens 4222) were sequenced and compared. In SREs treated strain 4534 (SREs-4534), 253 unigenes were up-regulated and 204 unigenes were down-regulated. In SREs treated strain 4534 (SREs-4222), the numbers of up- and down-regulated unigenes were 108 and 185, respectively. There were considerable differences between the SREs-4534 and SREs-4222 gene expression profiles. Some differentially expressed genes are associated with a two-component system (i.g., nodW, phyR-σEcfG), bacterial chemotaxis (i.g., cheA, unigene04832), ABC transport proteins (i.g., unigene02212), IAA (indole-3-acetic acid) metabolism (i.g., nthA, nthB), and metabolic fitness (i.g., put.), which may explain the higher nodulation competitiveness of B. diazoefficiens in the rhizosphere. Our results provide a comprehensive transcriptomic resource for SREs treated B. diazoefficiens and will facilitate further studies on competitiveness-related genes in B. diazoefficiens.

Microbial Community Dynamics and Response to Plant Growth-Promoting Microorganisms in the Rhizosphere of Four Common Food Crops Cultivated in Hydroponics

Plant growth promoting microorganisms (PGPMs) of the plant root zone microbiome have received limited attention in hydroponic cultivation systems. In the framework of a project aimed at the development of a biological life support system for manned missions in space, we investigated the effects of PGPMs on four common food crops (durum and bread wheat, potato and soybean) cultivated in recirculating hydroponic systems for a whole life cycle. Each crop was inoculated with a commercial PGPM mixture and the composition of the microbial communities associated with their root rhizosphere, rhizoplane/endosphere and with the recirculating nutrient solution was characterised through 16S- and ITS-targeted Illumina MiSeq sequencing. PGPM addition was shown to induce changes in the composition of these communities, though these changes varied both between crops and over time. Microbial communities of PGPM-treated plants were shown to be more stable over time. Though additional development is required, this study highlights the potential benefits that PGPMs may confer to plants grown in hydroponic systems, particularly when cultivated in extreme environments such as space.

Hypothetical protein Avin_16040 as the S-layer protein of Azotobacter vinelandii and its involvement in plant root surface attachment

A proteomic analysis of a soil-dwelling, plant growth-promoting Azotobacter vinelandii strain showed the presence of a protein encoded by the hypothetical Avin_16040 gene when the bacterial cells were attached to the Oryza sativa root surface. An Avin_16040 deletion mutant demonstrated reduced cellular adherence to the root surface, surface hydrophobicity, and biofilm formation compared to those of the wild type. By atomic force microscopy (AFM) analysis of the cell surface topography, the deletion mutant displayed a cell surface architectural pattern that was different from that of the wild type. Escherichia coli transformed with the wild-type Avin_16040 gene displayed on its cell surface organized motifs which looked like the S-layer monomers of A. vinelandii. The recombinant E. coli also demonstrated enhanced adhesion to the root surface.

Arachis hypogaea PGPR isolated from Argentine soil modifies its lipids components in response to temperature and salinity

The aim of this work was to clarify the mechanism related to plant growth promoting of a bacterial strain (L115) isolated from Arachis hypogaea rhizospheres and the effects of high growth temperature and salinity on phospholipids and fatty acids composition. L115 was isolated from peanut rhizospheres and identified according to the sequence analysis of the 16S rRNA gene. Phenotypic, metabolic and plant growth promoting rhizobacteria (PGPR) characteristics of L115 were tested. Inoculation test in plant growth chamber was performed. In addition, L115 was exposed to a 37 °C and 300 mM NaCl and phospholipids and fatty acid composition were evaluated. L115 strain was identified as Ochrobactrum intermedium and was able to increase the peanut shoot and root length as well as dry weight, indicating a PGPR role by being able to produce indole acetic acid and siderophores and present ACC deaminase activity. In addition, L115 showed tolerance to both high growth temperature and 300 mM NaCl. The most striking change was a decreased percentage of 18:1 fatty acid and an increase in 16:0 and 18:0 fatty acids, under high growth temperature or a combination of increased temperature and salinity. The most important change in phospholipid levels was an increase in phosphatidylcholine biosynthesis in all growth conditions. L115 can promote the growth of peanut and can tolerate high growth temperature and salinity modifying the fatty acid unsaturation degree and increasing phosphatidylcholine levels. This work is the first to report the importance of the genus Ochrobactrum as PGPR on peanut growth as well as on the metabolic behaviour against abiotic stresses that occur in soil. This knowledge will be useful for developing strategies to improve the growth of this bacterium under stress and to enhance its bioprocess for the production of inoculants.

Structure and biological roles of Sinorhizobium fredii HH103 exopolysaccharide

Here we report that the structure of the Sinorhizobium fredii HH103 exopolysaccharide (EPS) is composed of glucose, galactose, glucuronic acid, pyruvic acid, in the ratios 5∶2∶2∶1 and is partially acetylated. A S. fredii HH103 exoA mutant (SVQ530), unable to produce EPS, not only forms nitrogen fixing nodules with soybean but also shows increased competitive capacity for nodule occupancy. Mutant SVQ530 is, however, less competitive to nodulate Vigna unguiculata. Biofilm formation was reduced in mutant SVQ530 but increased in an EPS overproducing mutant. Mutant SVQ530 was impaired in surface motility and showed higher osmosensitivity compared to its wild type strain in media containing 50 mM NaCl or 5% (w/v) sucrose. Neither S. fredii HH103 nor 41 other S. fredii strains were recognized by soybean lectin (SBL). S. fredii HH103 mutants affected in exopolysaccharides (EPS), lipopolysaccharides (LPS), cyclic glucans (CG) or capsular polysaccharides (KPS) were not significantly impaired in their soybean-root attachment capacity, suggesting that these surface polysaccharides might not be relevant in early attachment to soybean roots. These results also indicate that the molecular mechanisms involved in S. fredii attachment to soybean roots might be different to those operating in Bradyrhizobium japonicum.

Swimming and swarming motility properties of peanut-nodulating rhizobia

Motility allows populations of bacteria to rapidly reach and colonize new microniches or microhabitats. The motility of rhizobia (symbiotic nitrogen-fixing bacteria that nodulate legume roots) is an important factor determining their competitive success. We evaluated the effects of temperature, incubation time, and seed exudates on swimming and swarming motility of five strains of Bradyrhizobium sp. (peanut-nodulating rhizobia). Swimming motility was increased by exudate exposure for all strains except native Pc34. In contrast, swarming motility was increased by exudate exposure for native 15A but unchanged for the other four strains. All five strains displayed the ability to differentiate into swarm cells. Morphological examination by scanning electron microscopy showed that the length of the swarm cells was variable, but generally greater than that of vegetative cells. Our findings suggest the importance of differential motility properties of peanut-nodulating rhizobial strains during agricultural inoculation and early steps of symbiotic interaction with the host.

Cytokinin producing bacteria stimulate amino acid deposition by wheat roots

Phytohormone production is one mechanism by which rhizobacteria can stimulate plant growth, but it is not clear whether the bacteria gain from this mechanism. The hypothesis that microbial-derived cytokinin phytohormones stimulate root exudation of amino acids was tested. The rhizosphere of wheat plants was drenched with the synthetic cytokinin trans-zeatin or inoculated with Bacillus subtilis IB-22 (which produces zeatin type cytokinins) or B. subtilis IB-21 (which failed to accumulate cytokinins). Growing plants in a split root system allowed spatial separation of zeatin application or rhizobacterial inoculation to one compartment and analyses of amino acid release from roots (rhizodeposition) into the other compartment (without either microbial inoculation or treatment with exogenous hormone). Supplying B. subtilis IB-22 or zeatin to either the whole root system or half of the roots increased concentrations of amino acids in the soil solution although the magnitude of the increase was greater when whole roots were treated. There was some similarity in amino acid concentrations induced by either bacterial or zeatin treatment. Thus B. subtilis IB-22 increased amino acid rhizodeposition, likely due to its ability to produce cytokinins. Furthermore, B. subtilis strain IB-21, which failed to accumulate cytokinins in culture media, did not significantly affect amino acid concentrations in the wheat rhizosphere. The ability of rhizobacteria to produce cytokinins and thereby stimulate rhizodeposition may be important in enhancing rhizobacterial colonization of the rhizoplane.

Comparison of the rhizosphere bacterial communities of Zigongdongdou soybean and a high-methionine transgenic line of this cultivar

Previous studies have shown that methionine from root exudates affects the rhizosphere bacterial population involved in soil nitrogen fixation. A transgenic line of Zigongdongdou soybean cultivar (ZD91) that expresses Arabidopsis cystathionine γ-synthase resulting in an increased methionine production was examined for its influence to the rhizosphere bacterial population. Using 16S rRNA gene-based pyrosequencing analysis of the V4 region and DNA extracted from bacterial consortia collected from the rhizosphere of soybean plants grown in an agricultural field at the pod-setting stage, we characterized the populational structure of the bacterial community involved. In total, 87,267 sequences (approximately 10,908 per sample) were analyzed. We found that Acidobacteria, Proteobacteria, Bacteroidetes, Actinobacteria, Chloroflexi, Planctomycetes, Gemmatimonadetes, Firmicutes, and Verrucomicrobia constitute the dominant taxonomic groups in either the ZD91 transgenic line or parental cultivar ZD, and that there was no statistically significant difference in the rhizosphere bacterial community structure between the two cultivars.

Involvement of Trichoderma asperellum strain T6 in regulating iron acquisition in plants

Iron (Fe) deficiency is a major plant nutritional disorder in many parts of the world, particularly in areas with saline soils. Among the numerous root-associated microbes that are beneficial for plant nutrient uptake, Trichoderma spp. are the most effective rhizosphere fungi for enhancing plant growth and plant resistance to biotic and abiotic stresses. To investigate the potential mechanisms of action of Trichoderma on insoluble Fe in the soil, which is difficult for plants to absorb and utilize, a high siderophore-producing strain of Trichoderma T6, was isolated from the rhizosphere of cucumber plants. The strain was identified as T. asperellum based on the morphological features and molecular phylogenetic analyses. Applying strain T6 to sterile soil could increase soil levels of Fe(2+) and siderophores, as well as increase Fe(2+) and Fe(3+)-chelate reductase (FCR) activity in cucumber tissues. Purified siderophore eluent (PSE) increased plant growth, thus confirming its role in plant growth promotion. Moreover, extracellular Fe(3+) reducing activity and three kinds of organic acids were detected in the culture filtrate of strain T6. These results indicate that strain T6 influences plant Fe absorption in several ways. Siderophore-based Fe chelation is effective in providing Fe to plants, organic acids, and Fe(3+) reducing enzymes may participate in the solubilization and reduction of insoluble Fe(3+) to Fe(2+).

Responses to elevated c-di-GMP levels in mutualistic and pathogenic plant-interacting bacteria

Despite a recent burst of research, knowledge on c-di-GMP signaling pathways remains largely fragmentary and molecular mechanisms of regulation and even c-di-GMP targets are yet unknown for most bacteria. Besides genomics or bioinformatics, accompanying alternative approaches are necessary to reveal c-di-GMP regulation in bacteria with complex lifestyles. We have approached this study by artificially altering the c-di-GMP economy of diverse pathogenic and mutualistic plant-interacting bacteria and examining the effects on the interaction with their respective host plants. Phytopathogenic Pseudomonas and symbiotic Rhizobium strains with enhanced levels of intracellular c-di-GMP displayed common free-living responses: reduction of motility, increased production of extracellular polysaccharides and enhanced biofilm formation. Regarding the interaction with the host plants, P. savastanoi pv. savastanoi cells containing high c-di-GMP levels formed larger knots on olive plants which, however, displayed reduced necrosis. In contrast, development of disease symptoms in P. syringae-tomato or P. syringae-bean interactions did not seem significantly affected by high c-di-GMP. On the other hand, increasing c-di-GMP levels in symbiotic R. etli and R. leguminosarum strains favoured the early stages of the interaction since enhanced adhesion to plant roots, but decreased symbiotic efficiency as plant growth and nitrogen contents were reduced. Our results remark the importance of c-di-GMP economy for plant-interacting bacteria and show the usefulness of our approach to reveal particular stages during plant-bacteria associations which are sensitive to changes in c-di-GMP levels.

Water-limiting conditions alter the structure and biofilm-forming ability of bacterial multispecies communities in the alfalfa rhizosphere

Biofilms are microbial communities that adhere to biotic or abiotic surfaces and are enclosed in a protective matrix of extracellular compounds. An important advantage of the biofilm lifestyle for soil bacteria (rhizobacteria) is protection against water deprivation (desiccation or osmotic effect). The rhizosphere is a crucial microhabitat for ecological, interactive, and agricultural production processes. The composition and functions of bacterial biofilms in soil microniches are poorly understood. We studied multibacterial communities established as biofilm-like structures in the rhizosphere of Medicago sativa (alfalfa) exposed to 3 experimental conditions of water limitation. The whole biofilm-forming ability (WBFA) for rhizospheric communities exposed to desiccation was higher than that of communities exposed to saline or nonstressful conditions. A culture-dependent ribotyping analysis indicated that communities exposed to desiccation or saline conditions were more diverse than those under the nonstressful condition. 16S rRNA gene sequencing of selected strains showed that the rhizospheric communities consisted primarily of members of the Actinobacteria and α- and γ-Proteobacteria, regardless of the water-limiting condition. Our findings contribute to improved understanding of the effects of environmental stress factors on plant-bacteria interaction processes and have potential application to agricultural management practices.

High NaCl concentrations induce the nod genes of Rhizobium tropici CIAT899 in the absence of flavonoid inducers

The nodulation (nod) genes of Rhizobium tropici CIAT899 can be induced by very low concentrations (micromolar to nanomolar range) of several flavonoid molecules secreted by the roots of leguminous plants under a number of different conditions. Some of these conditions have been investigated and appear to have a great influence on the concentration and the number of different Nod factors, which can induce root nodule primordia and pseudonodules in several leguminous plant roots. In one such condition, we added up to 300 mM NaCl to the induction medium of R. tropici CIAT899 containing the nod gene inducer apigenin. At the higher concentrations of NaCl, larger amounts and more different Nod factors were produced than in the absence of extra NaCl. To our surprise, under control conditions (300 mM NaCl without apigenin), some Nod-factor-like spots were also observed on the thin-layer plates used to detect incorporation of radiolabeled glucosamine into newly synthesized Nod factors. This phenomenon was further investigated with thin-layer plates, fusions of nod genes to the lacZ gene, high-performance liquid chromatography, mass spectrometry, and the formation of pseudonodules on bean roots. Here, we report that, in the absence of flavonoid inducers, high concentrations of NaCl induced nod genes and the production of Nod factors.

Light regulates attachment, exopolysaccharide production, and nodulation in Rhizobium leguminosarum through a LOV-histidine kinase photoreceptor

Rhizobium leguminosarum is a soil bacterium that infects root hairs and induces the formation of nitrogen-fixing nodules on leguminous plants. Light, oxygen, and voltage (LOV)-domain proteins are blue-light receptors found in higher plants and many algae, fungi, and bacteria. The genome of R. leguminosarum bv. viciae 3841, a pea-nodulating endosymbiont, encodes a sensor histidine kinase containing a LOV domain at the N-terminal end (R-LOV-HK). R-LOV-HK has a typical LOV domain absorption spectrum with broad bands in the blue and UV-A regions and shows a truncated photocycle. Here we show that the R-LOV-HK protein regulates attachment to an abiotic surface and production of flagellar proteins and exopolysaccharide in response to light. Also, illumination of bacterial cultures before inoculation of pea roots increases the number of nodules per plant and the number of intranodular bacteroids. The effects of light on nodulation are dependent on a functional lov gene. The results presented in this work suggest that light, sensed by R-LOV-HK, is an important environmental factor that controls adaptive responses and the symbiotic efficiency of R. leguminosarum.

Cyclic-β-glucans of Rhizobium (Sinorhizobium) sp. strain NGR234 are required for hypo-osmotic adaptation, motility, and efficient symbiosis with host plants

Cyclic-β-glucans (CβG) consist of cyclic homo-polymers of glucose that are present in the periplasmic space of many Gram-negative bacteria. A number of studies have demonstrated their importance for bacterial infection of plant and animal cells. In this study, a mutant of Rhizobium (Sinorhizobium) sp. strain NGR234 (NGR234) was generated in the cyclic glucan synthase (ndvB)-encoding gene. The great majority of CβG produced by wild-type NGR234 are negatively charged and substituted. The ndvB mutation abolished CβG biosynthesis. We found that, in NGR234, a functional ndvB gene is essential for hypo-osmotic adaptation and swimming, attachment to the roots, and efficient infection of Vigna unguiculata and Leucaena leucocephala.

Technologies for beneficial microorganisms inocula used as biofertilizers

The increasing need for environmentaly friendly agricultural practices is driving the use of fertilizers based on beneficial microorganisms. The latter belong to a wide array of genera, classes, and phyla, ranging from bacteria to yeasts and fungi, which can support plant nutrition with different mechanisms. Moreover, studies on the interactions between plant, soil, and the different microorganisms are shedding light on their interrelationships thus providing new possible ways to exploit them for agricultural purposes. However, even though the inoculation of plants with these microorganisms is a well-known practice, the formulation of inocula with a reliable and consistent effect under field conditions is still a bottleneck for their wider use. The choice of the technology for inocula production and of the carrier for the formulation is key to their successful application. This paper focuses on how inoculation issues can be approached to improve the performance of beneficial microorganisms used as a tool for enhancing plant growth and yield.

Genotypic diversity among rhizospheric bacteria of three legumes assessed by cultivation-dependent and cultivation-independent techniques

The genotypic diversity of rhizospheric bacteria of 3 legumes including Vigna radiata, Arachis hypogaea and Acacia mangium was compared by using cultivation-dependent and cultivation-independent methods. For cultivation-dependent method, Random amplified polymorphic DNA (RAPD) profiles revealed that the bacterial genetic diversity of V. radiata and A. mangium rhizospheres was higher than that of A. hypogaea rhizosphere. For cultivation-independent method, Denaturing gradient gel electrophoresis (DGGE) profiles of PCR-amplified 16S rRNA genes revealed the difference in bacterial community and diversity of rhizospheres collected from 3 legumes. The ribotype richness which indicates species diversity, was highest in V. radiata rhizosphere, followed by A. hypogaea and A. mangium rhizospheres, respectively. Three kinds of media were used to cultivate different target groups of bacteria. The result indicates that the communities of cultivable bacteria in 3 rhizospheres recovered from nutrient agar (NA) medium were mostly different from each other, while Bradyrhizobium selective medium (BJSM) and nitrogen-free medium shaped the communities of cultivable bacteria. Nine isolates grown on BJSM were identified by 16S rRNA gene sequence analysis. These isolates were very closely related (with 96% to 99% identities) to either one of the three groups including Cupriavidus-Ralstonia group, Bacillus group and Bradyrhizobium-Bosea-Afipia group. The rhizospheres were also examined for their enzymatic patterns. Of 19 enzymes tested, 3 rhizospheres were distinguishable by the presence or the absence of leucine acrylamidase and acid phosphatase. The selected cultivable bacteria recovered from NA varied in their abilities to produce indole-acetic acid and ammnonia. The resistance to 10 antibiotics was indistinguishable among bacteria isolated from different rhizospheres.

Overexpression of hns in the plant growth-promoting bacterium Enterobacter cloacae UW5 increases root colonization

AIMS

Plant growth-promoting rhizobacteria (PGPR) introduced into soil often do not compete effectively with indigenous micro-organisms for plant colonization. The aim of this study was to identify novel genes that are important for root colonization by the PGPR Enterobacter cloacae UW5.

METHODS AND RESULTS

A library of transposon mutants of Ent. cloacae UW5 was screened for mutants with altered ability to colonize canola roots using a thermal asymmetric interlaced (TAIL)-PCR-based approach. A PCR fragment from one mutant was reproducibly amplified at greater levels from genomic DNA extracted from mutant pools recovered from seedling roots 6 days after seed inoculation compared to that from the cognate inoculum cultures. Competition assays confirmed that the purified mutant designated Ent. cloacae J28 outcompetes the wild-type strain on roots but not in liquid cultures. In Ent. cloacae J28, the transposon is inserted upstream of the hns gene. Quantitative RT-PCR showed that transposon insertion increased expression of hns on roots.

CONCLUSIONS

These results indicate that increased expression of hns in Ent. cloacae enhances competitive colonization of roots.

SIGNIFICANCE AND IMPACT OF THE STUDY

A better understanding of the genes involved in plant colonization will contribute to the development of PGPR that can compete more effectively in agricultural soils.

Different and new Nod factors produced by Rhizobium tropici CIAT899 following Na+ stress

The root nodule bacterium Rhizobium tropici strain CIAT899 is highly stress resistant. It grows under acid conditions, in large amounts of salt, and at high osmotic pressure. An earlier study reported a substantial qualitative and quantitative effect of acid stress on the biosynthesis of Nod factors. The aim of the present work was to investigate the effect of high salt (NaCl) concentrations, another common stress factor, on Nod factor production. For this purpose, thin-layer chromatography, HPLC and MS analyses were carried out. The expression of nodulation genes was also studied using a nodP:lacZ fusion. High concentrations of sodium enhanced nod gene expression and Nod factor biosynthesis. The effect is sodium specific because high potassium or chloride concentrations did not have this effect. Under salt stress conditions, 46 different Nod factors were identified in a CIAT899 culture, compared with 29 different Nod factors under control conditions. Only 15 Nod factor structures were common to both conditions. Under salt stress conditions, 14 different new Nod factor structures were identified that were not observed as being produced under neutral or acid conditions. The implications of our results are that stress has a great influence on Nod factor biosynthesis and that new, very interesting regulatory mechanisms, worth investigating, are involved in controlling Nod factor biosynthesis.

Biofilm formation by plant-associated bacteria

Plants support a diverse array of bacteria, including parasites, mutualists, and commensals on and around their roots, in the vasculature, and on aerial tissues. These microbes have a profound influence on plant health and productivity. Bacteria physically interact with surfaces to form complex multicellular and often multispecies assemblies, including biofilms and smaller aggregates. There is growing appreciation that the intensity, duration, and outcome of plant-microbe interactions are significantly influenced by the conformation of adherent microbial populations. Biofilms on different tissues have unique properties, reflecting the prevailing conditions at those sites. Attachment is required for biofilm formation, and bacteria interact with plant tissues through adhesins including polysaccharides and surface proteins, with initial contact often mediated by active motility. Recognition between lectins and their cognate carbohydrates is a common means of specificity. Biofilm development and the resulting intimate interactions with plants often require cell-cell communication between colonizing bacteria.

Attachment of bacteria to the roots of higher plants

Attachment of soil bacteria to plant cells is supposedly the very early step required in plant-microbe interactions. Attachment also is an initial step for the formation of microbial biofilms on plant roots. For the rhizobia-legume symbiosis, various mechanisms and diverse surface molecules of both partners have been proposed to mediate in this process. The first phase of attachment is a weak, reversible, and unspecific binding in which plant lectins, a Ca(+2)-binding bacterial protein (rhicadhesin), and bacterial surface polysaccharide appear to be involved. The second attachment step requires the synthesis of bacterial cellulose fibrils that cause a tight and irreversible binding of the bacteria to the roots. Cyclic glucans, capsular polysaccharide, and cellulose fibrils also appear to be involved in the attachment of Agrobacterium to plant cells. Attachment of Azospirillum brasilense to cereals roots also can be divided in two different steps. Bacterial surface proteins, capsular polysaccharide and flagella appear to govern the first binding step while extracellular polysaccharide is involved in the second step. Outer cell surface proteins and pili are implicated in the adherence of Pseudomonas species to plant roots.