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Background: Growing evidence suggests that soil microbes can improve plant fitness under drought. However, in potato, the world's most important non-cereal crop, the role of the rhizosphere microbiome under drought has been poorly studied. Using a cultivation independent metabarcoding approach, we examined the rhizosphere microbiome of two potato cultivars with different drought tolerance as a function of water regime (continuous versus reduced watering) and manipulation of soil microbial diversity (i.e., natural (NSM), vs. disturbed (DSM) soil microbiome). Results: Water regime and soil pre-treatment showed a significant interaction with bacterial community composition of the sensitive (HERBST) but not the resistant cultivar (MONI). Overall, MONI had a moderate response to the treatments and its rhizosphere selected Rhizobiales under reduced watering in NSM soil, whereas Bradyrhizobium, Ammoniphilus, Symbiobacterium and unclassified Hydrogenedensaceae in DSM soil. In contrast, HERBST response to the treatments was more pronounced. Notably, in NSM soil treated with reduced watering, the root endophytic fungus Falciphora and many Actinobacteriota members (Streptomyces, Glycomyces, Marmoricola, Aeromicrobium, Mycobacterium and others) were largely represented. However, DSM soil treatment resulted in no fungal taxa and fewer enrichment of these Actinobacteriota under reduced watering. Moreover, the number of bacterial core amplicon sequence variants (core ASVs) was more consistent in MONI regardless of soil pre-treatment and water regimes as opposed to HERBST, in which a marked reduction of core ASVs was observed in DSM soil. Conclusions: Besides the influence of soil conditions, our results indicate a strong cultivar-dependent relationship between the rhizosphere microbiome of potato cultivars and their capacity to respond to perturbations such as reduced soil moisture. Our study highlights the importance of integrating soil conditions and plant genetic variability as key factors in future breeding programs aiming to develop drought resistance in a major food crop like potato. Elucidating the molecular mechanisms how plants recruit microbes from soil which help to mitigate plant stress and to identify key microbial taxa, which harbour the respective traits might therefore be an important topic for future research. [ABSTRACT FROM AUTHOR]

Human activities, industrialization and civilization have deteriorated the environment which eventually has led to alarming effects on plants and animals by heightened amounts of chemical pollutants and heavy metals in the environment, which create abiotic stress. Environmental conditions like drought, salinity, diminished macro-and micro-nutrients also contribute in abiotic stress, resulting in decrement of survival and growth of plants. Presence of pathogenic and competitive microorganisms, as well as pests lead to biotic stress and a plant alone can not defend itself. Thankfully, nature has rendered plant's rhizosphere with plant growth promoting rhizobacteria which maintain an allelopathic relationship with host plant to defend the plant and let it flourish in abiotic as well as biotic stress situations. This review discusses the mechanisms behind increase in plant growth via various direct and indirect traits expressed by associated microorganisms in the rhizosphere, along with their current scenario and promising future for sustainable agriculture. It also gives details of ten such bacterial species, viz. Acetobacter, Agrobacterium, Alcaligenes, Arthrobacter, Azospirillum, Azotobacter, Bacillus, Burkholderia, Enterobacter and Frankia, whose association with the host plants is famed for enhancing plant's growth and survival. [ABSTRACT FROM AUTHOR]

Quantifying carbon in root exudates and exploring their influencing factors are essential to understand soil organic carbon dynamics in cropland. A pot experiment was carried out to explore quantitative relations between root exuded carbon and root traits in wheat and rapeseed. The result showed that rapeseed had a similar pattern in root carbon exudation intensity (EI) as the wheat, but its EI per plant was obviously higher than that in wheat. Rapeseed plants had higher EI per root biomass than wheat plants in the early growth period but lower in the late growth period. EI per root biomass in both crops had significant exponential relationships with the specific root length (RL), surface area (RSA), volume (RV), root C/N ratio and root soluble sugar content. However, EI per plant of both crops had a markedly quadratic relationship with RL, RSA, RV and root biomass. During the whole growth period, the rapeseed had cumulative root carbon exudation of 14.09 g/plant, which was almost twice of that in the wheat plant. Root traits had close relations to root carbon exudation in both crops. Quantitative regression models between them could be referred to estimate root C exudation in rapeseed and wheat farmland. [ABSTRACT FROM AUTHOR]

Microorganisms residing on root surfaces play a central role in plant development and performance and may promote growth in agricultural settings. Studies have started to uncover the environmental parameters and host interactions governing their assembly. However, soil microbial communities are extremely diverse and heterogeneous, showing strong variations over short spatial scales. Here, we quantify the relative effect of meter-scale variation in soil bacterial community composition among adjacent field microsites, to better understand how microbial communities vary by host plant genotype as well as soil microsite heterogeneity. We used bacterial 16S rDNA amplicon sequencing to compare rhizosphere communities from four Brassica rapa cultivars grown in three contiguous field plots (blocks) and evaluated the relative contribution of resident soil communities and host genotypes in determining rhizosphere community structure. We characterize concomitant meter-scale variation in bacterial community structure among soils and rhizospheres and show that this block-scale variability surpasses the influence of host genotype in shaping rhizosphere communities. We identified biomarker amplicon sequence variants (ASVs) associated with bulk soil and rhizosphere habitats, each block, and three of four cultivars. Numbers and percent abundances of block-specific biomarkers in rhizosphere communities far surpassed those from bulk soils. These results highlight the importance of fine-scale variation in the pool of colonizing microorganisms during rhizosphere assembly and demonstrate that microsite variation may constitute a confounding effect while testing biotic and abiotic factors governing rhizosphere community structure. [ABSTRACT FROM AUTHOR]

There is a great interest in mitigating soil salinity that limits plant growth and productivity. In this study, eighty-nine strains were isolated from the rhizosphere and endosphere of two halophyte species (Suaeda mollis and Salsola tetrandra) collected from three chotts in Algeria. They were screened for diverse plant growth-promoting traits, antifungal activity and tolerance to different physico-chemical conditions (pH, PEG, and NaCl) to evaluate their efficiency in mitigating salt stress and enhancing the growth of Arabidopsis thaliana and durum wheat under NaCl–stress conditions. Three bacterial strains BR5, OR15, and RB13 were finally selected and identified as Bacillus atropheus. The Bacterial strains (separately and combined) were then used for inoculating Arabidopsis thaliana and durum wheat during the seed germination stage under NaCl stress conditions. Results indicated that inoculation of both plant spp. with the bacterial strains separately or combined considerably improved the growth parameters. Three soils with different salinity levels (S1 = 0.48, S2 = 3.81, and S3 = 2.80 mS/cm) were used to investigate the effects of selected strains (BR5, OR15, and RB13; separately and combined) on several growth parameters of wheat plants. The inoculation (notably the multi-strain consortium) proved a better approach to increase the chlorophyll and carotenoid contents as compared to control plants. However, proline content, lipid peroxidation, and activities of antioxidant enzymes decreased after inoculation with the plant growth-promoting rhizobacteria (PGPR) that can attenuate the adverse effects of salt stress by reducing the reactive oxygen species (ROS) production. These results indicated that under saline soil conditions, halotolerant PGPR strains are promising candidates as biofertilizers under salt stress conditions. [ABSTRACT FROM AUTHOR]

In forest ecosystems, fine root respiration directly contributes to belowground carbon (C) cycling. Exudation from fine roots indirectly affects C cycling via enhanced microbial decomposition of soil organic matter. Although these root-derived C fluxes are essential components of belowground C cycling, how nitrogen (N) addition affects these fluxes and their correlations remains unclear. In this study, fine root exudation, respiration and chemical/morphological traits were measured in a dominant canopy species, Quercus crispula Blume, found in a cool temperate forest, the Tomakomai Experimental Forest, Hokkaido University, which has undergone 5-year N addition. Soil-dissolved organic carbon (DOC) was also measured in both bulk and rhizosphere soils to evaluate the impact of fine root exudation on soil C cycling. Compared with a control plot with no N treatment, fine roots in the N addition plot exhibited larger diameters and higher N concentrations, but lower specific root lengths and areas. On a root-weight basis, respiration was not different between plots, but exudation was slightly higher under N addition. On a root-area basis, exudation was significantly higher in the N addition plot. Additionally, differences in DOC between rhizosphere and bulk soils were two times higher in the N addition plot than the control plot. Although fine root respiration was positively correlated with exudation in both the control and N addition plots, the ratio of exudation C to respiration C decreased after 5-year N addition. Nitrogen addition also affected absolute C allocation to fine root exudation and changed the C allocation strategy between exudation and respiration fluxes. These findings will help enhance predictions of belowground C allocation and C cycling under N-rich conditions in the future. [ABSTRACT FROM AUTHOR]

The objective of the present study was to isolate and characterize most efficient phosphate solubilizing bacteria (PSB) from rice rhizosphere. The study was carried out during the Kharif season'2018 at Department of Soil Science and Agricultural Chemistry, Bihar Agricultural University, Sabour, Bhagalpur, Bihar. The availability of phosphorous to plants for uptake and utilization is limited in soil due to fixation in the form of Fe-P, Al-P and Ca-P. The use of phosphate solubilizing bacteria can prove to be helpful measure to supply phosphorous to the crops to increase the productivity. In the present investigation, a total of 10 isolates were obtained from rice rhizosphere soil samples. All ten isolated isolates were shown phosphorus solubilization. Out of ten isolates BAU3 was found to be most potent phosphate solubilizers showing clear halo zone around its colony. The isolate BAU3 showed 20.00 mm phosphate solubilizing halo zone around its colony. The solubilization index (SI) of the isolate BAU3 was also calculated at the end of the incubation period and observed phosphate solubilization index (SI) of 3.22. The isolate BAU3 showed maximum insoluble phosphate solubilization of 450.24 µg ml-1 and isolates BAU3 was selected for subsequent studies. The bacterial isolates BAU3 was gram negative, non-spore forming rods shaped. On the basis of the 16SrDNA sequencing, isolate BAU3 was identified as Enterobacter cloacae strain BAU3 (Genebank Accession No. MK033472). The isolated strain of bacterial has potential to solubilize insoluble phosphorus and it can be utilized for preparation of microbial inoculants or biofertilizers. [ABSTRACT FROM AUTHOR]

Aims: There is considerable interest in breeding for crop genotypes that harness rhizosphere microbial communities and processes in support of plant productivity. However, the extent to which past breeding efforts have altered plant rhizosphere traits and plant-microbe collaborations is unknown.Methods: We evaluated twelve best-selling and widely adapted maize hybrids released between 1936 and 2011 for changes in rhizosphere bacterial community composition (BCC) and plant access to endogenous soil nitrogen. Plants were grown in replicated monocultures fertilized with 0, 85 or 170 kg N ha−1 and measured for yield, nitrogen uptake and source. Rhizosphere BCC and function was assessed through potential extracellular enzyme assays and 16S rRNA gene amplicon sequencing.Results: Grain yield and nitrogen uptake from soil pools increased with year of hybrid release at all fertilization levels. Rhizosphere BCC and enzyme activity also varied among hybrids. However, releases from the 1960s and 70s were most distinct, while early and late releases shared similar BCC and enzyme activity.Conclusions: These results indicate that breeding has increased maize ability to acquire nitrogen from soil reserves, but has not resulted in a directional shift in rhizosphere bacterial community assembly. The variation observed among hybrids reveals the potential for future breeding efforts to influence rhizosphere traits. [ABSTRACT FROM AUTHOR]

Rhizobacteria associated with crops constitute an important source of potentially beneficial microorganisms with plant growth promoting activity or antagonistic effects against phytopathogens. In this study, we evaluated the plant growth promoting activity of 11 bacterial isolates that were obtained from the rhizosphere of healthy avocado trees and from that of avocado trees having survived root rot infestations. Seven bacterial isolates, belonging to the genera Bacillus, Pseudomonas and Arthrobacter, promoted in vitro growth of Arabidopsis thaliana. These isolates were then tested for antagonistic activity against Phytophthora cinnamomi, in direct dual culture assays. Two of those rhizobacterial isolates, obtained from symptomatic-declining trees, displayed antagonistic activity. Isolate A8a, which is closely related to Bacillus acidiceler, was also able to inhibit P. cinnamomi growth in vitro by 76% through the production of volatile compounds. Solid phase microextraction (SPME) and analysis by gas chromatography coupled with mass spectrometry (GC-MS) allowed to tentatively identify the main volatiles emitted by isolate A8a as 2,3,5-trimethylpyrazine, 6,10-dimethyl-5,9-undecadien-2-one and 3-amino-1,3-oxazolidin-2-one. These volatile compounds have been reported to show antifungal activity when produced by other bacterial isolates. These results confirm the significance of rhizobacteria and suggest that these bacteria could be used for biocontrol of soil borne oomycetes through their volatiles emissions. [ABSTRACT FROM AUTHOR]

In the last decade cheaper and faster sequencing methods have resulted in an enormous increase in genomic data. High throughput genotyping, genotyping by sequencing and genomic breeding are becoming a standard in plant breeding. As a result, the collection of phenotypic data is increasingly becoming a limiting factor in plant breeding. Genetic studies on root traits are being hampered by the complexity of these traits and the inaccessibility of the rhizosphere. With an increasing interest in phenotyping, breeders and scientists try to overcome these limitations, resulting in impressive developments in automated phenotyping platforms. Recently, many such platforms have been thoroughly described, yet their efficiency to increase genetic gain often remains undiscussed. This efficiency depends on the heritability of the phenotyped traits as well as the correlation of these traits with agronomically relevant breeding targets. This review provides an overview of the latest developments in root phenotyping and describes the environmental and genetic factors influencing root phenotype and heritability. It also intends to give direction to future phenotyping and breeding strategies for optimizing root system functioning. A quantitative framework to determine the efficiency of phenotyping platforms for genetic gain is described. By increasing heritability, managing effects caused by interactions between genotype and environment and by quantifying the genetic relation between traits phenotyped in platforms and ultimate breeding targets, phenotyping platforms can be utilized to their maximum potential. [ABSTRACT FROM AUTHOR]

Background and aims: A fast and reliable phenotyping system forms a major bottleneck in root exudation research. Our goal was to develop such a system to quantify genetic and environmental effects on root exudation. Another aim was characterizing non-sterile phenotyping. Methods: We developed a system in which plants can be grown with non-sterile shoot environments and sterile rhizospheres. These sterile systems were compared with non-sterile controls. Results: In non-sterile rhizospheres exogenous carbon disappeared quickly with a half-life of 2 to 3 h and root exudate concentrations remained below detection limit. In sterile rhizospheres exogenous carbon levels were relatively stable or depleted slower than in non-sterile rhizospheres and organic acid build-up occurred. Tomato ( Solanum lycopersicum) could be grown with sterile roots for several months. Conclusions: The differences in carbon depletion in sterile and non-sterile rhizospheres was most likely due to the absence of microbial catabolism in sterile rhizospheres. These results prove that using a sterile phenotyping system is essential to study the quantity and composition of root exudates. The sterile system described in this paper eliminates the obscuring effect caused by microbes on exudation levels. It offers a stable, reliable and easy phenotyping method and can be used to investigate genetic and environmental effects on exudation. [ABSTRACT FROM AUTHOR]

Aims: Pulse labeling of crops using 13C is often employed to trace photosynthesized carbon (C) within crop-soil systems. However, few studies have compared the C distribution for different labeling periods. The overall aim of this study was to determine the length of the monitoring interval required after 13C-pulse labeling to quantify photosynthate C allocation into plant, soil and rhizosphere respiration pools for the entire growing season of maize ( Zea mays L.). Methods: Pot grown maize was pulse-labeled with 13CO 2 (98 at. %) at the beginning of emergence, elongation, heading and grainfilling growth stages. The routing of 13C into shoot and root biomass, soil CO 2 flux and soil organic carbon (SOC) pools was monitored for 27 days after 13C-pulse labeling at the beginning of each growth stage. Results: The majority of the 13C was recovered after 27 d in the maize shoots, i.e., 57 %, 53 %, 70 % and 80 %, at the emergence, elongation, heading, and grainfilling stages, respectively. More 13C was recovered in the root biomass at elongation (27 %) compared to the least at the grainfilling stage (3 %). The amount recovered in the soil was the smallest pool of 13C at emergence (2.3 %), elongation (3.8 %), heading and grainfilling (less than 2 %). The amount of 13C recovered in rhizosphere respiration, i.e. 13CO 2, was greatest at emergence (26 %), and similar at the elongation, heading and grainfilling stages (~16 %). Conclusions: At least 24 days is required to effectively monitor the recovery of 13C after pulse labeling with 13CO 2 for maize in plant and soil pools. The recovery of 13C differed between growth stages and corresponded to the changing metabolic requirements of the plant, which indicated labeling for the entire growth season would more accurately quantify the C budget in plant-soil system. [ABSTRACT FROM AUTHOR]

Changes in the soil labile carbon fractions and soil biochemical properties to elevated carbon dioxide (CO) and temperature reflect the changes in the functional capacity of soil ecosystems. The belowground root system and root-derived carbon products are the key factors for the rhizospheric carbon dynamics under elevated CO condition. However, the relationship between interactive effects of elevated CO and temperature on belowground soil carbon accrual is not very clear. To address this issue, a field experiment was laid out to study the changes of carbon allocation in tropical rice soil (Aeric Endoaquept) under elevated CO and elevated CO + elevated temperature conditions in open top chambers (OTCs). There were significant increase of root biomass by 39 and 44 % under elevated CO and elevated CO + temperature compared to ambient condition, respectively. A significant increase (55 %) of total organic carbon in the root exudates under elevated CO + temperature was noticed. Carbon dioxide enrichment associated with elevated temperature significantly increased soil labile carbon, microbial biomass carbon, and activities of carbon-transforming enzyme like β-glucosidase. Highly significant correlations were noticed among the different soil enzymes and soil labile carbon fractions. [ABSTRACT FROM AUTHOR]

In the present work, we studied the effectiveness of the predominant halophytes of Soliman sabkha (Tecticornia indica and Suaeda fruticosa) to promote soil biological activities and ecosystem productivity. Soil Arylsulphatese ARY, β-glucosidase β-GLU, phosphatase PHO, invertase INV, urease URE, and dehydogenase DES activities in Extra- and Intra-tuft halophytes and plant productivity were assessed. Results revealed a high increase of microbial community and ARY, β-GLU, PHO, INV, URE and DES activities (+298%, +400%, +800%, +350%, +320%, +25% and +759%, respectively) in Intra-tuft rhizosphere as compared to Extra-tuft one, which is likely due to the significant decrease of salinity in the rhizosphere of Tecticornia indica and Suaeda fruticosa. Both perennial plants exhibited high productivities (7.4 t dry weight ha−1 and 2.2 t dry weight ha−1, respectively) and Na+-hyperaccumulating capacities (0.75 t Na+ ha−1 and 0.22 t Na+ ha−1, respectively), reducing salt constraint and favouring soil fertility. This constitutes a promising alternative to enhance productivity in such a salt-affected biotope by offering suitable microhabitat for annual glycophytes. [ABSTRACT FROM PUBLISHER]

Two plant growth promoting bacteria designated as KiSII and RNF 267 isolated from the rhizosphere of coconut palms were identified as Serratia marcescens and Enterobacter sp. based on their phenotypic features, BIOLOG studies and 16S rRNA gene sequence analysis. Both bacteria exhibited phosphate solubilization, ammonification, and production of indole acetic acid, β-1, 3 glucanase activities and 1-aminocyclopropane-1-carboxylate-deaminase activity. They could also tolerate a range of pH conditions, low temperature and salinity (NaCl). In addition, S. marcescens KiSII exhibited N- fixation potential, chitinase activity, siderophore production and antibiotics production. Seed bacterization with these bacteria increased the growth parameters of test plants such as paddy and cowpea over uninoculated control in green house assay. In coconut seedlings, significant increase in growth and nutrient uptake accompanied with higher populations of plant beneficial microorganisms in their rhizospheres were recorded on inoculation with both the PGPRs. The present study clearly revealed that PGPRs can aid in production of healthy and vigorous seedlings of coconut palm which are hardy perennial crops. They offer a scope to be developed into novel PGPR based bioinoculants for production of elite seedlings that can benefit the coconut farming community and the coconut based ecology. [ABSTRACT FROM AUTHOR]

Background: As roots penetrate soil, specialized cells called 'border cells' separate from root caps and contribute a large proportion of exudates forming the rhizosphere. Their function has been unclear. Recent findings suggest that border cells act in a manner similar to that of white blood cells functioning in defense. Histone-linked extracellular DNA (exDNA) and proteins operate as 'neutrophil extracellular traps' to attract and immobilize animal pathogens. DNase treatment reverses trapping and impairs defense, and mutation of pathogen DNase results in loss of virulence. Scope: Histones are among a group of proteins secreted from living border cells. This observation led to the discovery that exDNA also functions in defense of root caps. Experiments revealed that exDNA is synthesized and exported into the surrounding mucilage which attracts, traps and immobilizes pathogens in a host-microbe specific manner. When this plant exDNA is degraded, the normal resistance of the root cap to infection is abolished. Conclusions: Research to define how exDNA may operate in plant immunity is needed. In the meantime, the specificity and stability of exDNA and its association with distinct microbial species may provide an important new tool to monitor when, where, and how soil microbial populations become established as rhizosphere communities. [ABSTRACT FROM AUTHOR]

Here we describe the use of Pisum sativum L. as a model system to measure how short-term treatment of root tips with soluble metabolites can influence root growth and release of root exudates. The results revealed that even a 3-minute exposure of root tips to metabolites normally released from roots into the rhizosphere (e.g. rhamnose, ferulic acid, salicylic acid) can significantly influence root growth without affecting production of border cells and associated exudates. Conversely, products including caffeine, saccharide lactone, and pisatin alter production of border cells, without affecting root growth. Understanding how root-derived and exogenous metabolites can selectively impact root function may yield benefits in crop production, especially in greenhouse agriculture systems where growing roots can be exposed to a significant accumulation of plant exudates. [ABSTRACT FROM AUTHOR]

This review evaluates the importance of root exudates in determining rhizosphere bacterial community structure. We present evidence that indicates that: (1) the direct influence of root exudates on rhizosphere bacterial communities is limited to small spatiotemporal windows related to root apices; (2) upon rapid assimilation by microorganisms, root exudates are modified, independent of plant influences, before rerelease into the rhizosphere by the microorganisms themselves – thus, at short distances from root apices, rhizosphere carbon pools are unlikely to be dominated by root exudates; and (3) many of the major compounds found in root exudates are ubiquitous in the rhizosphere as they are found in other pools of rhizodeposits and in microbial exudates. Following this argument, we suggest that the importance of root exudates in structuring rhizosphere bacterial communities needs to be considered in the context of the wider contribution of other rhizosphere carbon pools. Finally, we discuss the implications of rhizosphere bacterial distribution trends for the development of effective strategies to manage beneficial plant–microorganism interactions. [ABSTRACT FROM AUTHOR]

The classical concept of arsenic transfer into plants through arsenate uptake via phosphate transporters, reduction to arsenite, complexation and compartmentation within vacuoles is challenged by recent identification of bidirectional transporters for arsenite and their potential role in plant As status regulation. Soil-based studies with chemical analysis of soil solution require root mat formation amplifying root effects on their surroundings and additionally denying investigations along individual roots differing in age and function. We tried to overcome these shortcomings by using bioreporter bacteria to visualise the spatial distribution of inorganic arsenic along roots and to characterize inorganic arsenic gradients in the rhizosphere concurrent with root age and branching. Therefore we developed an agar-based carrier element ensuring intimate contact between bioreporters and root-soil system and enabling fast and easy reporter output analysis. We show that inorganic arsenic distribution is related to root development with the highest bioreporter signal induction around lateral roots, which are known to show the highest expression of transporters responsible for bidirectional arsenite flux. Since there is so far no evidence for an arsenate efflux mechanism this is a strong indicator that we observed rather arsenite than arsenate efflux. No signal was detected along the distal region of young adventitious roots, i.e. the region of extension growth and root hair formation. The novel bioreporter assay may thus complement conventional measurements by providing information on the spatial distribution of inorganic arsenic on mm to cm-scale. [ABSTRACT FROM AUTHOR]

Root-secreted chemicals mediate multi-partite interactions in the rhizosphere, where plant roots continually respond to and alter their immediate environment. Increasing evidence suggests that root exudates initiate and modulate dialogue between roots and soil microbes. For example, root exudates serve as signals that initiate symbiosis with rhizobia and mycorrhizal fungi. In addition, root exudates maintain and support a highly specific diversity of microbes in the rhizosphere of a given particular plant species, thus suggesting a close evolutionary link. In this review, we focus mainly on compiling the information available on the regulation and mechanisms of root exudation processes, and provide some ideas related to the evolutionary role of root exudates in shaping soil microbial communities. [ABSTRACT FROM AUTHOR]

The rhizosphere represents one of the most complex ecosystems on earth with almost every root on the planet expected to have a chemically, physically and biologically unique rhizosphere. Despite its intrinsic complexity, understanding the rhizosphere is vital if we are to solve some of the world’s most impending environmental crises such as sustainable food, fibre and energy production, preservation of water resources and biodiversity, and mitigation against climate change. One of the key challenges that faces rhizosphere ecologists is how to translate their fundamental research into practical real-world applications. In addition, they need to convince policy makers that consideration of the rhizosphere is vital in the formulation and implementation of any environmental policy relating to plant growth. This is highlighted by the recent biofuel and carbon debt debate whereby rhizosphere processes such as priming were largely ignored leading to destabilization of national policies. Recent advances in our understanding of the tangled web of rhizosphere interactions have been largely driven by technological innovations in analytical, bioinformatic and imaging tools, and this is likely to continue for the foreseeable future. However, there is also a critical need to incorporate this more reductionist information into mathematical models that are capable of incorporating the rhizosphere to allow simulation of plot- or landscape-level processes that are particularly relevant to policymakers. Consequently, as the multidisciplinary rhizosphere science community grows, there will be increasing need to both integrate scientific information and to subsequently convey this in an effective manner to stakeholders. If we can achieve this we will be in a good position to help prevent ongoing global environmental degeneration. These issues were addressed at the RHIZOSPHERE 2 International Conference which was held at Montpellier, France in August 2007. This special issue gathers some of the research presented during this major event. [ABSTRACT FROM AUTHOR]

The rhizosphere is a complex zone of multitrophic interactions comprising plant roots, associated bacteria, fungi, and micro-, meso-, and macro-fauna. Of considerable importance in this system is the production of antibiotics by root-associated or “rhizo” bacteria. This is a widespread phenomenon of which a much-studied exemplar is the production by pseudomonads of 2,4-diacetylphloroglucinol (DAPG), known to be effective in the suppression of soil-borne fungal pathogens. Rapid advances in understanding the molecular and biochemical bases of antibiotic (particularly DAPG) production have been made. However, our understanding of in situ antibiotic production currently lags behind this. There is therefore a need for a rapid soil-based screen with which to identify antibiotic producers under rhizosphere C-flow conditions. Here, a novel “rhizocosm,” comprising porous pipe and Rhizon sampler®, was superior to a simple, nonperfusing incubation-type microcosm with respect to supporting a rhizobacterial inoculum. Its use as a screening tool is illustrated by screening known DAPG-producing inocula in soil continuously supplied with simulated rhizosphere carbon flow. The DAPG was then extracted from soil using acetone and quantified by high-performance liquid chromatography (HPLC). Simple sugars (glucose and fructose) stimulated the greatest DAPG production, while glucose with an additional organic or amino acid, or with a signal molecule, resulted in strongly variable DAPG expression. [ABSTRACT FROM AUTHOR]

Plant roots support the growth and activities of a wide variety of microorganisms that may have a profound effect on the growth and/or health of plants. Among these microorganisms, a high diversity of bacteria have been identified and categorized as deleterious, beneficial, or neutral with respect to the plant. The beneficial bacteria, termed plant growth-promoting rhizobacteria (PGPR), are widely studied by microbiologists and agronomists because of their potential in plant production.Azospirillum, a genus of versatile PGPR, is able to enhance the plant growth and yield of a wide range of economically important crops in different soils and climatic regions. Plant beneficial effects ofAzospirillumhave mainly been attributed to the production of phytohormones, nitrate reduction, and nitrogen fixation, which have been subject of extensive research throughout the years. These elaborate studies madeAzospirillumone of the best-characterized genera of PGPR. However, the genetic and molecular determinants involved in the initial interaction betweenAzospirillumand plant roots are not yet fully understood. This review will mainly highlight the current knowledge onAzospirillumplant root interactions, in the context of preceding and ongoing research on the association between plants and plant growth-promoting rhizobacteria. [ABSTRACT FROM AUTHOR]