Rhizoplane Soil Research Articles

Microbial community assembly and functional profiles along the soil-root continuum of salt-tolerant Suaeda glauca and Suaeda salsa.

Developing and planting salt-tolerant plants has become a promising way to utilize saline-alkali land resources and ensure food security. Root-associated microbes of salt-tolerant plants have been shown to promote plant growth and alleviate high salt stress, yet very little is known about the salt resistance mechanisms of core microbes in different niches. This study characterized the microbial community structures, assembly processes, and functional profiles in four root-related compartments of two salt-tolerant plants by amplicon and shotgun metagenomic sequencing. The results showed that both plants significantly altered the microbial community structure of saline soils, with greater microbial alpha diversity in the rhizosphere or rhizoplane compared with bulk soils. Stochastic process dominated the microbial assembly processes, and the impact was stronger in Suaeda salsa than in S. glauca, indicating that S. salsa may have stronger resistance abilities to changing soil properties. Keystone species, such as Pseudomonas in the endosphere of S. glauca and Sphingomonas in the endosphere of S. salsa, which may play key roles in helping plants alleviate salt stress, were identified by using microbial co-occurrence network analysis. Furthermore, the microbiomes in the rhizoplane soils had more abundant genes involved in promoting growth of plants and defending against salt stress than those in bulk soils, especially in salt-tolerant S. salsa. Moreover, microbes in the rhizoplane of S. salsa exhibited higher functional diversities, with notable enrichment of genes involved in carbon fixation, dissimilar nitrate reduction to ammonium, and sulfite oxidation. These findings revealed differences and similarities in the microbial community assembly, functional profiles and keystone species closely related to salt alleviation of the two salt-tolerant plants. Overall, our study provides new insights into the ecological functions and varied strategies of rhizosphere microbes in different plants under salt stress and highlights the potential use of keystone microbes for enhancing salt resistance of plants.

Soil bacterial communities of paddy are dependent on root compartment niches but independent of growth stages from Mollisols of Northeast China.

Deep insights into adhering soil of root zones (rhizosphere and rhizoplane) microbial community could provide a better understanding of the plant-microbe relationship. To better understand the dynamics of these microbial assemblies over the plant life cycle in rhizodeposition along rice roots. Here, we investigated bacterial distribution in bulk, rhizosphere, and rhizoplane soils at tillering, heading, and mature stage, from rice (Oryza sativa) fields of the Northeast China. Our results revealed that soil bacterial α-diversity and community composition were significantly affected by root compartment niches but not by temporal change. Compared to rhizoplane soils in the same period, bulk in the heading and rhizosphere in the mature had the largest increase in Shannon's index, with 11.02 and 14.49% increases, respectively. Proteobacteria, Chloroflexi, Bacteroidetes, and Acidobacteria are predominant across all soil samples, bulk soil had more phyla increased across the growing season than that of root related-compartments. Deterministic mechanisms had a stronger impact on the bacterial community in the compartments connected to the roots, with the relative importance of the bulk soil, rhizoplane and rhizosphere at 83, 100, and 56%, respectively. Because of ecological niche drivers, the bacterial networks in bulk soils exhibit more complex networks than rhizosphere and rhizoplane soils, reflected by more nodes, edges, and connections. More module hub and connector were observed in bulk (6) and rhizoplane (5) networks than in rhizosphere (2). We also detected shifts from bulk to rhizoplane soils in some functional guilds of bacteria, which changed from sulfur and nitrogen utilization to more carbon and iron cycling processes. Taken together, our results suggest distinct bacterial network structure and distribution patterns among rhizosphere, rhizoplane, and bulk soils, which could possibly result in potential functional differentiation. And the potential functional differentiation may be influenced by plant root secretions, which still needs to be further explored.

Long-term plastic mulching decreases rhizoplane soil carbon sequestration by decreasing microbial anabolism

Ridge-furrow with plastic mulching (RFPM) is a widely used agricultural practice in rain-fed farmlands. However, the impact of microbial related metabolism on soil organic carbon (SOC) is not fully understood. Amino sugar analysis, high-throughput sequencing, and high-throughput qPCR approaches are combined to investigate this topic, based on a long-term experiment. Treatments include flat planting without mulching (FP), ridge-furrow without mulching (RF), and RFPM. RFPM significantly decreases rhizoplane SOC contents, while bulk SOC contents change insignificantly across treatments. In terms of microbial metabolic pathways, RFPM decreases indicators of the in vivo metabolic pathway, whereas those of the ex vivo pathway are increased. In terms of microbial community features, core taxa module #1 is dominated by Sphingomonadaceae. These are putative high yield (Y) strategists, according to the microbial life-history strategy framework. They are closely related to the in vivo pathway and are most predictive for SOC; their abundance is highest under FP and lowest under RFPM. Core taxa module #2 is dominated by Chitinophagaceae, putative resource acquisition (A) strategists, that are closely related to the ex vivo pathway. Their abundance in the rhizoplane is highest under RFPM and lowest under FP. The RFPM-induced decline in SOC occurs simultaneously with the abundance of A-strategists with in vivo pathway but not the Y-strategists with ex vivo pathway. Overall, the result of this study shows a trade-off. In RFPM practice, the ex vivo microbial pathway is enhanced along with the abundance of A-strategists. This is not the case for the in vivo pathway and associated abundance of Y-strategists, which are closely associated with SOC. Our findings underlined the impact of rhizoplane microbial metabolic pathways on SOC status is key to agricultural practices in drylands such as RFPM, and advanced our understanding of how microbes affect the carbon cycling in dryland farming.

Spatiotemporal dynamics and functional characteristics of the composition of the main fungal taxa in the root microhabitat of Calanthe sieboldii (Orchidaceae)

BackgroundEndophytic fungi play a critical ecological role in the growth and development of orchids, but little is known about the spatial and temporal dynamics of fungal diversity or the ecological functions of fungi during orchid growth and reproduction. Calanthe sieboldii Decne. is listed in the Chinese National Key Protected Wild Plants as a class I protected wild plant. To understand the community characteristics of root and soil fungi of the orchid during its reproductive seasons, we investigated the community composition, spatial and temporal dynamics, and functional characteristics of the orchid microhabitat fungi by using diversity and ecological functional analyses.ResultsWe discovered that there were three, seven, and four dominant fungal families in the orchid's roots, rhizoplane soil, and rhizosphere soil, respectively. Tulasnellaceae, Aspergillaceae, and Tricholomataceae were the dominant fungi in this endangered orchid's microhabitats. The closer the fungal community was to the orchid, the more stable and the less likely the community composition to change significantly over time. The fungal communities of this orchid's roots and rhizoplane soil varied seasonally, while those of the rhizosphere soil varied interannually. Saprophytic fungi were the most abundant in the orchid's fungal community, and the closer the distance to the orchid, the more symbiotic fungi were present.ConclusionsThe fungi in different parts of the root microhabitat of C. sieboldii showed different spatiotemporal dynamic patterns. The fungal community near the orchid roots was relatively stable and displayed seasonal variation, while the community further away from the roots showed greater variation. In addition, compared with the soil fungi, the dominant endophytic fungi were more stable, and these may be key fungi influencing orchid growth and development. Our study on the spatiotemporal dynamics and functions of fungi provides a basis for the comprehensive understanding and utilization of orchid endophytic fungi.

Rhizosphere effect on phosphorus availability to soybean in Vertisols

ABSTRACT Vertisols of India are calcareous and have a heavy texture with high clay content but low to medium content of Olsen estimated phosphorus (P). The on-farm and on-station trials revealed that Vertisols with various levels of P2O5 responded negligibly to fertilizer application on soybean crops. In Vertisols, the inorganic P is primarily associated with Ca (Ca-P), which gradually solubilizes under acidic conditions surrounding rhizosphere. Hence, in order to correlate both the findings, the current study investigated the rhizosphere/rhizoplane pH. The study used agar plate technique to observe that the pH of the rhizosphere and rhizoplane soils, which was observed to be acidic although the soil pH was 7.6. The correlation between P uptake and soil-available P measured using the four test methods (Bray 2, Olsen, Truog, and Mehlich 3) revealed that Bray 2 had high correlation with the amount of P absorbed by soybean. Bray 2 method extracts approximately 10 times more P from Vertisols than does the Olsen extractant. This could be a plausible reason for insignificant difference in crop post fertilizer application due to sufficient P fertility of the Vertisols. Hence, Bray 2 method was established as a suitable method for the extraction of P from Vertisols, in which the pH of the extraction solution was the most acidic, among the selected four extraction methods and had high correlation with the amount of P absorbed by soybeans. Hence, this method is recommended for P estimation for Vertisols present in Central India. Essentially, this study provided information that an accurate measurement of P availability is required to explain the non-responsive nature of soybean to P fertilizers in Vertisols. Additionally, the current study emphasized the need for optimum recommendations of P from the view point of rhizosphere; it may facilitate the reduction of the application rates of P to the crops, without reducing the crop yields.

Estimation of the pH of soybean rhizoplane, rhizosphere and bulk soil and its effect on availability and uptake of phosphorus in calcareous Vertisols

Vertisols are spread over central and western parts in Madhya Pradesh in India.As the Vertisolsare calcareous and/or alkaline in nature, mobility of P from soil to root surface is carried by diffusion process, and this diffusion rate is quite low i.e. 0.13mm day-1 (Jungk 1991). One of the major limitation is thatmany rhizosphere chemical interactions that can be involved in the changes ofP ion concentration in the soil solution and in the replenishment of the depleted soil solution (P buffering capacity)do not taken into account (Darrah, 1993).This prompted us to re-evaluate the P-fertility of Vertisols. In the study an attempt has been made to evaluate the most suitable method for P availability in calcareous Vertisols for crops considering the pH of rhizosphere. By agar plate technique, the pH of rhizoplane and rhizoplane soil was found acidic even though soil pH was7.6. The major portion of inorganic P in Vertisols is associated with Ca (Ca-P), which can be soluble more under acid condition than pH 8.5 of Olsen’s condition. The pH of bulk soil, that is unplanted soil which is treated in same way of applied nutrient and water as the planted pots, is 7.9. Soybean crop decreased the pH of rhizosphere and rhizoplane by 7.5and 6.0 respectively. Following the various crops the pH of rhizosphere decreased. Among various crops tested the lowest pH (5.8) of the rhizosphere and rhizoplane -attached soil was noticed in care of Chickpea. In case of pea, maize, sorghum and wheat the pH of rhizosphere and rhizoplane were 7.4 and 6.1, 7.6 and 6.4, 7.5 and 6.4, 7.5 and 6.3, respectively. Decreased pH due to rhizosphere can dissolve the phosphorus from the Calcium and increase the availability of P in Calcareous/ Alkaline soil.

Nitrogen transformation and microbial community structure varied in apple rhizosphere and rhizoplane soils under biochar amendment

Biochar amendment is commonly applied to ameliorate soil environment and alleviate nitrous oxide (N2O) emissions, while our understanding of biochar effects on microbial community and nitrogen transformation traits of orchard root region soil is limited. We observed the composition and diversity of bacterial, fungal, and archaeal communities in apple root region (non-rhizosphere, rhizosphere, and rhizoplane) soils amended with apple wood biochar using the Illumina sequencing methods. We used quantitative PCR to quantify the abundance of nitrogen-transforming functional marker genes involved in microbial nitrogen fixation (nifH), nitrification (archaeal amoA and bacterial amoA), and denitrification (nirK, nirS, and nosZ). Biochar amendment altered bacterial, fungal, and archaeal community compositions in apple root region soil. When compared to control, biochar additions dramatically increased the α-diversity of bacteria in the rhizoplane soil, whereas amendment significantly decreased the α-diversity of fungi and archaea in the rhizosphere soil. Moreover, the driving factors varied in non-rhizosphere, rhizosphere, and rhizoplane soils. The abundance of nitrogen-fixing microorganisms and microbial N2O reduction were enhanced by biochar amendment in rhizosphere and rhizoplane soils, which was associated with an observed reduction in N2O concentrations by 13.7 and 35.1% in the 15- and 20-cm soil layer. However, wood biochar amendment had insignificant effect on nifH and amoA gene abundances in non-rhizosphere soil. Network analysis revealed that nitrogen-transforming functional genes had an intensive potential link to multiple microbial phyla. The soil microbial community was altered in biochar-amended soil by changing soil properties, thereby regulating nitrogen transformation processes and N2O concentrations in apple root region soil.

Culture-based Methods for Studying the Bacterial Root Microbiome of Wheat.

Beneficial plant-microbe interactions are important and desirable for sustainable intensification of agriculture. Here, we describe methods to isolate microbes from the roots of field-grown wheat plants. This includes the rhizosphere and rhizoplane soil, as well as the root endosphere. We also describe a method to test for endosphere competence of putative endophytes.

Shifts in the bacterial community along with root-associated compartments of maize as affected by goethite

Root-associated compartments, including rhizosphere soil, rhizoplane soil, and the endosphere, are found to harbor distinguished bacterial populations and community composition, but how microbiome in these rhizo-compartments are affected by edaphic variables remains largely unknown. Goethite is a prevalent crystalline iron (hydr)oxide mineral of the soil matrix and strongly interact with microbial communities. The objective of our study was to determine how goethite (α-FeOOH) amendment assemble bacterial communities in the rhizo-compartments of Maize (Zea mays. L). Using sequencing of microbial 16S ribosomal RNA gene amplicons, we revealed that goethite amendment into soil enriched Actinobacteria and depleted Proteobacteria in all rhizo-compartments. Also, goethite enlarged the differences in the alpha diversity (Chao) between rhizo-compartments, with much lower mean diversity in the endosphere and rhizoplane compared with rhizosphere soil, indicating a higher selection of the microbiome assemblage. This was supported by beta Nearest Taxon Index (βNTI > + 2), indicating that changes in environmental conditions progressively increase the strength of selection. It suggests that variable selection (a deterministic process) was the dominant process influencing the microbial assembly in soil amended with goethite. According to the distance-based linear modeling (distLM), the assemblage of bacterial communities in the rhizosphere compartments was regulated by specific edaphic variables, with the major contributors being goethite (62%), total C (52%), soil pH (50%), and FeOM (25%). Stabilization of rhizosphere C in the presence of goethite would be the selective step for its accessibility and consequent microbial community. For instance, the keystone microorganisms, e.g., Pseudomonas, had more negative links within the goethite added co-occurrence network, indicating its mutual exclusions and outcompete other microbes in C/nutrients limited conditions. Thus, goethite narrows the composition of rhizosphere mainly due to “gate selection” effects on rhizodeposits, which limited microbial penetrance into inner-compartments, consequently assemble the rhizosphere bacterial community via deterministic process.

Root growth, function and rhizosphere microbiome analyses show local rather than systemic effects in apple plant response to replant disease soil.

Apple replant disease (ARD) is the phenomenon of soil decline occurring after repeated planting of apple trees at the same site. This study aimed to elucidate whether ARD is systemic, i.e. whether the contact of parts of the root system with ARD soil causes the whole plant to show poor shoot and root growth. A split-root experiment was conducted with seedlings of ‘M26’, offering the same plant for its root system the choice between the substrates ARD soil (+ARD), γ-sterilized ARD soil (-ARD) or soil from a grass parcel (Control) with the following combinations: +ARD/+ARD, -ARD/-ARD; +ARD/-ARD; +ARD/Control. Root growth was analysed throughout the 34-day growing period. Samples from bulk, rhizosphere and rhizoplane soil were collected separately for each compartment, and analysed by fingerprints of 16S rRNA gene or ITS fragments amplified from total community (TC) DNA. The response of the plant to +ARD was not systemic as root growth in -ARD compartment was always superior to root growth in +ARD soil. Crosswise 15N-labelling of the N-fertilizer applied to the split-root compartments showed that nitrate-N uptake efficiency was higher for roots in -ARD soil compared to those in +ARD. Bacterial and fungal community composition in the rhizoplane and rhizosphere of the same plants differed significantly between the compartments containing +ARD/-ARD or +ARD/Control. The strongest differences between the bacterial fingerprints were observed in the rhizoplane and rhizosphere. Bacterial genera with increased abundance in response to ARD were mainly Streptomyces but also Sphingobium, Novosphingobium, Rhizobium, Lysobacter and Variovorax. The strongest differences between the fungal fingerprints were observed in bulk soil. Our data showed that the response of the apple plant to ARD soil is local and not systemic.

Soft rot disease alters soil characteristics and root-associated, culturable microbial community of Amorphophallus konjac

To elucidate how soft rot disease affects soil characteristics and root-associated, culturable microorganisms in Amorphophallus konjac stands, the responses of soil around roots of A. konjac with soft rot disease were investigated in stands with and without soft rot. Changes in the root-associated culturable microbial community and diversity were investigated by dilution plating. Soil characteristics were compared between stands using standard techniques. A. konjac with soft rot had higher concentrations of available soil P and K, NH4–N, organic matter and water content and lower pH compared with plants without. The community composition of root-associated culturable microorganisms differed between stands with and without soft rot. The microbial community associated with soft rot in A. konjac was characterized by four types of abundant microorganisms (Fusarium solani, Fusarium oxysporum, Pseudomonas chlororaphis subsp. aureofaciens and Stenotrophomonas pavanii) and three types of less-abundant microorganisms (Rhizobium radiobacter, Bacillus thuringiensis and Streptomyces cellulosae), and a small number of Bacillus and Streptomyces species in the rhizosphere and rhizoplane soils. Particular microbial combinations were diametrically opposed between plants with and without soft rot. The richness and diversity of root-associated culturable microorganisms were higher in the stand without soft rot than in the stand with soft rot. A. konjac soft rot led to obvious differences in the diversity and community composition of root-associated culturable microorganisms and in soil characteristics.

Brachiaria Grasses (Brachiaria spp.) harbor a diverse bacterial community with multiple attributes beneficial to plant growth and development.

Endophytic and plant‐associated bacteria were isolated from plants and rhizoplane soil of naturally grown Brachiaria grasses at International Livestock Research Institute in Nairobi, Kenya. Eighty‐four bacterial strains were isolated from leaf tissues, root tissues, and rhizoplane soil on nutrient agar and 869 media. All bacterial strains were identified to the lowest possible taxonomic unit using 16S rDNA primers and were characterized for the production of Indole‐3‐acetic acid, hydrogen cyanide, and ACC deaminase; phosphate solubilization; siderophore production; antifungal properties; and plant biomass production. The 16S rDNA‐based identification grouped these 84 bacterial strains into 3 phyla, 5 classes, 8 orders, 12 families, 16 genera, and 50 unique taxa. The four most frequently isolated genera were Pseudomonas (23), Pantoea (17), Acinetobacter (9), and Enterobacter (8). The functional characterization of these strains revealed that 41 of 84 strains had a minimum of three plant beneficial properties. Inoculation of maize seedlings with Acinetobacter spp., Microbacterium spp., Pectobacterium spp., Pseudomonas spp., and Enterobacter spp. showed positive effects on seedling biomass production. The ability of Brachiaria grasses to host genetically diverse bacteria, many of them with multiple plant growth‐promoting attributes, might have contributed to high biomass production and adaptation of Brachiaria grasses to drought and low fertility soils.

EFFECTS OF FARMING SYSTEM ON ROOT-ZONE FUNGAL POPULATIONS IN WHEAT

Effects of farming system on the composition of the root-zone microbiota and on the occurrence of root and stem-base diseases in winter wheat cv. Rywalka were analysed. The wheat was grown in organic, integrated and conventional systems of production and in monoculture over three years. Using classical methods, saprotrophic and pathogenic microorganisms, mainly fungi, were isolated from the root zone and identified on the basis of morphology and, where possible, designated as potential pathogens or antagonists. In roots, rhizoplane, and rhizosphere and non-rhizosphere soil the total frequencies of dominants (with frequency >5%) was 80.7- 98.7% and subdominants (with frequency 1–5%) was 0.5-14.5% and were similar in all production systems. Potential major soil-borne pathogens included Fusarium + Haematonectria. Potential pathogen antagonists included Chaetomium, Clonostachys, Gliomastix, Sarocladium and Trichoderma species. The low ratio of fungal pathogens : antagonists (Fusarium + Haematonectria : Clonostachys + Trichoderma spp.) in soil associated with lowest incidence of root and stem-base diseases, lower occurrence of Fusarium and Rhizoctonia in root and stem-base region and highest yield of grain in a conventional system for production of wheat cv. Rywalka confirms the cultivar’s suitability for high-input conventional farming. Benefits of microbiological approaches in the study of suitability of cultivar to a particular farming system is demonstrated.

Antifungal properties exhibited by bacteria isolated from agriculturally cultivable soils and their antagonistic nature towards fungal phytopathogen suppression

Soil samples collected from rhizosphere soil, cultivable field soil, Rhizoplane soil of paddy plants and were used to isolate bacteria by culture methods of which thirty two bacterial cultures were identified with diverse colony characteristics on nutrient agar medium and biochemical characterization. Sixteen bacterial isolates were selected for screening fungal antagonism on PDA and were further tested against <italic>Fusarium</italic> and <italic>Rhizoctonia species.</italic> In this study, two <italic>Pseudomonas strains</italic> (RB15, RB30) and <italic>Bacillus cereus</italic> (RB13) were cyanogen producers. Both strains of <italic>Pseudomonas</italic> RB15 and RB30 and <italic>Bacillus cereus</italic> (RB13) were also siderophore producers. Only three isolates showed chitinolytic activity; <italic>Serratia marcesens (RB24), Bacillus cereus (RB13) and Enterobacter cancerogenus</italic> (RB17).

Effects of Biofertilizers on Mn and Zn Acquisition and Growth of Higher Plants: A Rhizobox Experiment

Plant disease resistance and susceptibility are greatly influenced by the availability of micronutrients, particularly manganese (Mn) and zinc (Zn). Take-all disease of wheat, caused by a strong Mn oxidizing fungus (Gaeumannomyces graminis var tritici, Ggt), results in a lack of availability of Mn to plants and increases disease severity in wheat. Three commercial Trichoderma harzianum (Vitalin T-50, BioHealth®-WSG, and BioHealth®-G) and one Bacillus subtilis (Vitalin SP-11) were investigated individually and in combination (Vitalin T-50 and Vitalin SP-11) for growth promotion and Mn/Zn uptake of take-all infected wheat in a rhizobox experiment under greenhouse conditions. Inoculation with Trichoderma and Bacillus biofertilizers did not increase the shoot dry weight and shoot to root ratio, whilst shoot length was significantly increased with Vitalin T-50 and Biohealth-G treatments in the final harvest. Biofertilizers inoculation that significantly (P < 0.05) enhanced root surface area and root dry weight were Vitalin T-50, BioHelath-G and combination of Vitalin (T-50 + SP-11). The bulk soil pH was not influenced by biofertilizer inoculation, whereas rhizosphere and rhizoplane soil pH were significantly reduced (0.3 – 0.4 pH scale) in Vitalin (T-50 + SP-11) and BioHealth-G treatments and to a lesser extent by Vitalin T-50 inoculation. Manganese uptake in shoots of wheat exhibited no significant differences among the biofertilizer treatments. On the contrary, Zn uptake was significantly higher in Vitalin T-50, Vitalin (T-50 + SP-11), BioHealth-G, and BioHealth-WSG (47, 64, 44, and 45%, respectively) inoculated plants. Therefore, Vitalin T-50 and Biohealth-G showed better performance in improving plant growth and Zn uptake.

Fungi Colonization of the Rhizoplane of Okra (Hibiscusesculentus) Plant

The frequency of occurrence of the colonizing fungal species in the rhizoplane soil of okra plant (Hibiscus esculentus) were studied at different stages of development. The most frequently isolated fungal species in this examined okra plant were; Schizosaccharomyces pombe, Aspergillus flavus and Aspergillus fumigatus, others were; Penicillium spinulosum, Mucos racemosus, Saccharomyces cerevisiae, Aspergillus nidulans, Penicillium digitatum, Aspergillus niger, Cladiosporium resinae, Monascus ruber and Alternaria tenius.Investigation was also carried out on how factors such as soil moisture content organic matter content and soil pH affect the relative abundance of the fungal with okra plant growth. I. Introduction Microorganisms found in the soil and its study can be equated with soil microbiology. Varieties of plant and animal found inhabiting in the soil are called soil organisms. Some of them are macro while some are micro, those classified as plants are called microflora, fungi is included (Brandy & Weil, 1999).The plant roots or tree roots are classified under soil macroorganism, the root system of plants is associated with inanimate environment composed of organic and inorganic substances also with metabolically active community of microorganisms. The fungi found around cultivated soil is quite different from uncultivated soil community purposely because growing plant creates unique habitat for microorganisms and are inturn affected by their population. Relationship that occurred amongmicroorganisms, soil and roots were observed by Foster and Marks, (1967) through transmission electron microscopy (TEM) of the soil-root interface.The rhizoplane is the root surface of the plant (Rovira, 1965) and different from the rhizosphere which is the zone (soil) surrounding the plant root (Rovira, 1965).The Rhizoplane which is the root surface of the growing plants in the soil provide unique habitat, the rhizosphere, which is particularly favourable for the development of the microorganisms in the soil. However, the majority of the microorganisms stimulated by rhizoplane are usually harmless saprophytes which live mainly on dead, decomposing root tissues.The root excretions such as volatile and dissolved exudates affect microbial growth in the root surrounding soil, also most microbial activities in the soil is associated with the roots or fresh decomposing organic materials, therefore, soil microorganisms including Fungi depend on available organic compounds for survival while activity of microorganisms in non- cultivated soil will be dormant.It is claimed that the young root presents a virgin niche available for soil microorganism colonization and as the root grows through soil, the root exudates promote the fungal spores germination as propagules and growth of fungal hyphae. The initial root microorganism colonizers disappeared as roots ages leaving stable root surface microflora and microbial species eventually become dominant members of the final stable root surface population which depends on plants species and the soil (Peterson, 1958).The great variability that occurred in the microbial cover of the roots was reported by Bowen and Theodoros (1973) that the surface of the three weeks old plant root contains less than 10% microorganisms and increased to 37% in 90 days old plan root.The low surface occupancy by microorganisms in the root surface (rhizoplane) occurred as result of relative few points of inoculation in the soil followed by little spread over the surface and also, soil parts of the root surface may not be conducive for microbial growth.Foster, (1962) observed that the relatively small number of microbial cover on the roots at the early age offers little protection against plant pathogens like Phytophthoracinnamomi and biological control of such organism occurred by attack on the soil propagule. The hypothesis that organic debris is major source of inoculum for the plant root is consistent with Gray, et al; (1967) observation, that in sand dune soil, organic matter particles provide less than 15% of the available solid surfaces.Microbial growth can be affected by volatile and dissolved exudates from the seeds and roots for relatively large distance for example, the germination of Chlymadospores of Fusarium is 10mm from the planted seeds as been reported by Stanghellini and Hancock, 1971. Plant root exudates can infect the susceptible plants and can as well inhibit spores germination of resistant roots varieties as been reported by Buxton, (1957).Some saprophytic fungal initiated by the rhizoplane inhibiting rhizosphere were known to produce antibiotics which might possibly antagonize other organisms including plant pathogens.Some environmental factors like temperature, moisture content, soil pH etc. can influence

Phytotoxicity assessment of γ-Fe2O3 nanoparticles on root elongation and growth of rice plant

This study was conducted to determine the phytotoxicity of 6 nm γ-Fe2O3 nanoparticles (IONPs) in terms of root elongation and the physiological performance of rice plants. Rice seeds (Oryza sativa L. var. Koshihikari) exposed to IONPs at 500, 1,000 and 2,000 mg/L, had a significantly higher root elongation than the control and its bulk counterparts (IOBKs), indicating that the effect can be nanospecific. In a 14-week greenhouse pot experiment, the CO2 assimilation rate in IOBK and IONP-treated pots (500 and 1,000 mg/pot) decreased over time, with the decline (maximum 42.5 %) being less pronounced for IONPs, indicating that the effect cannot be inferred from the toxicity of nanoscale size iron oxide. Excessive adsorption of IONPs onto soil colloids with subsequent low water extractable iron was responsible for the unremarkable phytotoxic nature of IONPs in the rice plants. Amendment of IONPs coated with 20 mmol citric acid (IONPs-Cit) significantly diminished the CO2 assimilation rate and the decrease was similar to its bulk counterpart (IOBKs-Cit). However, maximum shoot growth inhibition (37 %) was associated with the application of IOBKs-Cit. It was concluded that massive accumulations of Fe plaque on the root surfaces of IOBKs-Cit treatments due to a decline in the pH of rhizoplane soils compared to the IONPs-Cit treatments were responsible for the remarkable shoot growth reduction. This study provided evidence of the phytotoxicity of γ-Fe2O3 nanoparticles, demonstrating the lower toxicity of nanosized iron oxide compared to a microsized preparation under reductive conditions.

In situ gradient distribution of polycyclic aromatic hydrocarbons (PAHs) in contaminated rhizosphere soil: a field study

Little information is available heretofore on the gradient distribution of persistent organic pollutants in rhizosphere on a field scale. In this field study, we seek to explore the in situ distribution gradient of polycyclic aromatic hydrocarbons (PAHs) in rhizosphere soil proximal to the roots. Clover (Trifolium pratense L.) and hyssop (Hyssopus officinalis L.) grew in situ in the contaminated field soil near a petrochemical plant and were harvested when about 30 cm tall with mature roots. Rhizosphere soils of the plants were sampled including the rhizoplane, strongly adhering soil, and loosely adhering soil. Eleven EPA-priority PAHs were detected in each layer of rhizosphere soils in proximity to the root surface. The PAH concentrations followed the descending order of bulk soil, loosely adhering soil, strongly adhering soil, and rhizoplane soil in proximity to the root surface of clover and hyssop. The rhizosphere effect (R, in percent) on PAH distribution clearly decreased with increasing distance from the root, and a more significant decrease was observed for hyssop compared to clover. R values were generally lower for three- and four-ringed PAHs in the rhizosphere, which were more significant in loosely and strongly adhering rhizosphere layers. Our field observations combined with previous potted studies demonstrated that PAH concentrations in rhizosphere soils increased with distance from the root. Results of this work provide new information on the fate of PAHs in rhizosphere.

Chemical Speciation of Heavy Metals in the Fractionated Rhizosphere Soils of Sunflower Cultivated in a Humic Andosol

Chemical speciation and bioaccumulation factor of iron (Fe), manganese (Mn), and zinc (Zn) were investigated in the fractionated rhizosphere soils and tissues of sunflower plants grown in a humic Andosol. The experiment was conducted for a period of 35 days in the greenhouse, and at harvest the soil system was differentiated into bulk, rhizosphere, and rhizoplane soils based on the collection of root-attaching soil aggregates. The chemical speciations of heavy metals in the soil samples were determined after extraction sequentially into fractions classified as exchangeable, carbonate bound, metal–organic complex bound, easily reducible metal oxide bound, hydrogen peroxide (H2O2)–extractable organically bound, amorphous mineral colloid bound, and crystalline Fe oxide bound. Iron and Zn were predominantly crystalline Fe oxide bound in the initial bulk soils whereas Mn was mainly organically bound. Heavy metals in the exchangeable form accumulated in the rhizosphere and rhizoplane soils, comprising <4% of the total content, suggesting their relatively low availability in humic Andosol. Concentrations of organically bound Fe and Mn in soils decreased with the proximity to roots, suggesting that organic fraction is the main source for plant uptake. Concentrations of Mn and Zn in the metal–organic complex also decreased, indicating a greater ability of sunflower to access Mn from more soil pools. Sunflower showed bioaccumulation factors for Zn, Fe, and Mn as large as 0.39, 0.05, and 0.04 respectively, defining the plant as a metal excluder species. This result suggests that access to multiple metal pools in soil is not necessarily a major factor that governs metal accumulation in the plant.

Screening for fluorescent pseudomonades, isolated from the rhizosphere of tomato, for antagonistic activity toward Clavibacter michiganensis subsp. michiganensis

Bacterial canker of tomato, caused by Clavibacter michiganensis subsp. michiganensis, continues to be a problem for tomato growers in the Souss-Massa Draa valley, South of Morocco. Assuming that biological control is an alternative for the management of this disease, a total of 303 fluorescent pseudomonads strains isolated from roots and rhizospheric soil of tomato plants were in vitro tested against C. michiganensis subsp. michiganensis. Fluorescent pseudomonads strains which showed the highest antagonistic properties were thereafter investigated for their ability to colonize tomato roots. Our results showed that fluorescent pseudomonads are more represented in rhizospheric soils. However, the most efficient fluorescent pseudomonads isolates were found in the rhizoplane soil and the endorhizosphere. Among 42 spontaneous antibiotic resistant mutants obtained by treatment of the wild-type isolates with five antibiotics (rifampicine, nalidixic acid, ampicilline and chloramphenicol), 28 completely colonized the roots of all tomatoes seedlings used in this investigation. The 42 wild type isolates were then used for in vivo screening with the cotyledon test. Using this test, eight isolates from 42 tested induced a significant decrease of disease incidence and disease symptoms. The eight efficient isolates were then tested for their effectiveness in the protection of tomato plants in pots under greenhouse conditions. Results obtained showed that all tested isolates applied as seed and root treatments reduced significantly (P ≤ 0.001) the incidence of bacterial canker.