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Root exudates as determinant of rhizospheric microbial diversity
... As abovementioned, the root exudate quality of each cultivar determines higher or lower rhizospheric effect of the inoculants. Singh andMukerji (2006) andJaboby et al. (2017) suggested a difference between primary or secondary root metabolites la RFSJN como los microorganismos BFN17 e IRMF94, podrían utilizarce como una estrategia amigable con el ambiente para el cultivo de lechuga. Como un resultado atractivo de la presente investigación, se puede señalar que la calidad nutricional, en relación al contenido de P, de la variedad BSS es mayor con respecto a la variedad GL, debido al mayor %P foliar mostrado; adicionalmente, se ha reportado mayor cantidad de carbohidratos, P, Ca, Fe y vitaminas para las lechugas de hoja suelta (tipo BSS) en relación a las de cabeza (tipo GL) (Vallejo y Estrada, 2004); por esta razón se debe considerar que las variedades de un cultivo, en este caso la lechuga, no posseen el mismo contenido nutricional, y por lo tanto sus necesidades nutricionales también variarán. ...
... As abovementioned, the root exudate quality of each cultivar determines higher or lower rhizospheric effect of the inoculants. Singh andMukerji (2006) andJaboby et al. (2017) suggested a difference between primary or secondary root metabolites la RFSJN como los microorganismos BFN17 e IRMF94, podrían utilizarce como una estrategia amigable con el ambiente para el cultivo de lechuga. Como un resultado atractivo de la presente investigación, se puede señalar que la calidad nutricional, en relación al contenido de P, de la variedad BSS es mayor con respecto a la variedad GL, debido al mayor %P foliar mostrado; adicionalmente, se ha reportado mayor cantidad de carbohidratos, P, Ca, Fe y vitaminas para las lechugas de hoja suelta (tipo BSS) en relación a las de cabeza (tipo GL) (Vallejo y Estrada, 2004); por esta razón se debe considerar que las variedades de un cultivo, en este caso la lechuga, no posseen el mismo contenido nutricional, y por lo tanto sus necesidades nutricionales también variarán. ...
... Como ya se ha dicho, la calidad de los exudados radicales de cada variedad va a incidir en un mayor o menor efecto rizosférico de los inoculantes. Singh y Mukerji (2006) y Jacoby et al. (2017) señalan que podría haber una diferencia entre los metabolitos radicales primarios o secundarios region of Venezuela must be based on the utilization of natural resources of the region such as the existing rock phosphate mines of San Joaquin de Navay, which is located at southeastern Tachira State. ...
The use of plant growth promoting microorganisms allows greater availability of nutrients for the development of plants under sustainable agriculture management. In this work, it was evaluated under field conditions. The biofertilizer potential use of two microorganisms (diazotrophic Enterobacter cloacae BFN17 strain and the phosphate solubilizing fungus Penicillium rugulosum IRMF94 strain), on the development of two lettuce cultivars, the Great Lakes (GL) and the Black Seeded Simpson (BSS), using a biological substrate mixture composed of San Joaquín de Navay rock phosphate (RFSJN) (10%), sand (85%) and an organic substrate composed of sugar - rice 50:50 (5%), the whole with 20% humidity. Four treatments were applied: an absolute control (without both, chemical fertilizers and biosubstrate), a commercial control (with chemical fertilizers but biosubstrate), a N chemical fertilization + biosubstrate, and a chemical fertilization without N + biosubstrate. The number of repetitions were five for each treatment. The N source was urea, and the biological treatments were the inoculants: BFN17, IRMF94, and the consortium BFN17+IRMF94. We evaluated number of leaves, fresh and dry weight, and %P foliar. Data were analyzed by ANOVA and LSD test (p<0.05) using the Statgraphics program 5.1. The implementation of the biological substrate mixture of RFSJN inoculated with both microorganisms in the form of consortium and with the N source in the GL cultivar, showed the best results for all variables, having statistically (p<0.05) similar results to the commercial treatment. It was found for the GL cultivar, 5% and 16.6% increases in the number of leaves and the %P foliar, respectively.
Key words: Inoculants, sustainability, microbial consortium, biofertilizer potential, N and P dynamics, agronomic effectiveness.
... However, because of the lower exudation rates of total C as corresponding to the biomass reduction, which was also reported in Aulakh et al. (2001), the proportion of LMWOAs to the total C was greater with the γ-irradiation, particularly in the P-applied compartment. This change in the proportion could be relevant to a selective response of the rice plant to the reduced microbiological activities to exudate smaller amounts of carbohydrates or sugars that are the dominant nutrient sources for microorganisms (Singh and Mukerji, 2006), and retain the exudation rates of LMWOAs. In addition, we observed a unique preferential citric acid exudation from the roots of the same rice plant toward the P-rich area of the split-box. ...
... The experimental soil was collected from the subsoil layer(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50) cm depth) at the JIRCAS Tropical Agricultural Research Front, Okinawa, Japan (26.5 o N, 135.7 o E). The physicochemical properties of the experimental soil were summarized by Oo et al. (2021). ...
Purpose
The response of rice plants to P deficiency together with rhizosphere microorganisms is yet to be explored in flooded soil cultures. This study aimed to identify how alterations in bacterial diversity are associated with underground metabolic and chemical reactions in the rhizosphere of rice.
Methods
Two rice varieties were grown in a split-root box under flooded conditions and filled with either P-applied or non-P-applied soils with or without γ-irradiation. At 41–42 days after transplanting, plant biomass, P uptake, exudation rates of total carbon and low-molecular-weight organic acids (LMWOAs) from the roots, and bacterial diversity, P fractions, and phosphatase activity in the rhizosphere were determined.
Results
γ-irradiation significantly decreased plant P uptake and biomass of both varieties and reduced the relative abundance of Alphaproteobacteria and microbiome beta diversity in the rhizosphere. Concurrently, the use of insoluble P in soils was evidently reduced by irradiation. These microbiological and chemical changes in the γ-irradiated rhizosphere occurred in both P-applied and non-P-applied soils. On the other hand, the proportion of LMWOAs and exudation of citric acid, high in P-solubilization capacity were increased in the γ-irradiated and P-applied rhizosphere soils at the expense of other carbons. No differences were detected in phosphatase activity in any treatments.
Conclusion
Dysbiosis in the rhizosphere microbiome negatively affected rice growth and the use of insoluble P pools in flooded soil cultures. With a reduction in microbiome diversity, rice plants may have a complementary strategy to increase the exudation proportion of citric acid toward the P-rich soil area.
... Some of these are well studied and ubiquitous genera, Pseudomonas and Bacillus (Berg et al. 2011). However, some degree of specificity in microhabitats is observed owing to the varying factors of the microenvironments such as plant species, soil type, pedoclimate, pesticides, and various other biotic and abiotic factors (Berg et al. 2005;Berg and Smalla 2009;Fürnkranz et al. 2012;Köberl et al. 2013a;Singh and Mukerji 2006). ...
... In the case of medicinal plants, however, the microbiome possesses a higher degree of specificity owing to their host plant's distinctive exudates and divergent secondary metabolites (Qi et al. 2012;Singh and Mukerji 2006). Actinobacteria are a phylum of Gram-positive bacteria, known for their antibiotic production potential, for example, the genus Streptomyces has yielded many novel antibiotics over the years (Goodfellow and Fiedler 2010;Raaijmakers and Mazzola 2012). ...
Medicinal plants hold a very important place in therapeutics. Plant growth is
affected by a number of abiotic and biotic factors. Among these factors, microorganisms
associated with these plants play an important role in the plant
health and thus indirectly influence humans as well. Many of these microbes
are known to be involved in the production of compounds that are not only useful
for the host plant, but also have commercial importance. For in-depth analysis
of these plant-associated microbiomes, metagenomic approaches provide
the necessary platform of robust, high-throughput techniques. This chapter
discusses the microbial communities associated with different medicinal
plants, and how metagenomics can be helpful in studying their diversity and
versatility.
... In the case of sugars, the tripartite ATP-independent periplasmic transporters (TRAP transporters) were found in robust colonizers of the genera Variovorax, Agrobacterium, and Ensifer. These transporters are responsible for the uptake of organic acids such as succinate, malate, and fumarate, all of which are found in root exudates of maize and sugarcane (Singh and Mukerji, 2006). It is likely that these transporters help robust colonizers to efficiently acquire C compounds provided by the plant, which would benefit competition for resources and ultimately increase microbe fitness for a robust colonization lifestyle. ...
... Because these transporters are linked to the uptake of a broad range of sugars, from monosaccharides to organic acids, it is likely that the diversity of transporters might be related to the nutritional complexity of soil environment and root exudates. Given that plant exudates vary among plant species and plant nutritional status ( Bais et al., 2006;Singh and Mukerji, 2006;Turner et al., 2013), by maintaining a broad repertoire of transporters the bacteria might efficiently capture resources from different types of hosts and plant organs and successfully compete during plant colonization. ...
Despite the availability of data on the functional and phylogenetic diversity of plant-associated microbiota, the molecular mechanisms governing the successful establishment of plant bacterial communities remain mostly elusive. To investigate bacterial traits associated with successful colonization of plants, we sequenced the genome of 26 bacteria of a synthetic microbial community (SynCom), 12 of which displayed robust and 14 displayed non-robust colonization lifestyles when inoculated in maize plants. We examined the colonization profile of individual bacteria in inoculated plants and inspected their genomes for traits correlated to the colonization lifestyle. Comparative genomic analysis between robust and non-robust bacteria revealed that commonly investigated plant growth-promoting features such as auxin production, nitrogen (N) fixation, phosphate acquisition, and ACC deaminase are not deterministic for robust colonization. Functions related to carbon (C) and N acquisition, including transporters of carbohydrates and amino acids, and kinases involved in signaling mechanisms associated with C and N uptake, were enriched in robust colonizers. While enrichment of carbohydrate transporters was linked to a wide range of metabolites, amino acid transporters were primarily related to the uptake of branched-chain amino acids. Our findings identify diversification of nutrient uptake phenotypes in bacteria as determinants for successful bacterial colonization of plants.
... One possible reason is that brackish water contains some trace elements that can stimulate crop growth [57]. If the initial soil salinity is relatively low, generally, the first 1-3 years of brackish water irrigation may be more beneficial to crop growth than freshwater irrigation, and crops will have more developed root systems compared with FWI and SWI treatments [7,58,59]. Root exudates are the key factors that regulate the vitality and function of rhizosphere micro-ecosystems. Under stress, many plants can actively or passively release various chemical substances from their roots into the environment, and the number and type of root exudates will increase. ...
Irrigation with saline water is a possible solution to alleviate freshwater shortages. The long-term use of saline water for irrigation requires consideration of the influence of salt on the environmental conditions of the soil. The objective of this field study was to determine the effects of three continuous years of saline water irrigation on physiochemical properties and microbial communities in drip-irrigated cotton fields. The three total dissolved solid (TDS) levels of irrigation water treatments were (i) 1 g L⁻¹ (fresh water, FWI), (ii) 3 g L⁻¹ (brackish water, BWI), and (iii) 7 g L⁻¹ (salt water, SWI). After three years, the electrical conductivity (EC), sodium adsorption ratio (SAR), and contents of K⁺, Na⁺, Mg²⁺, Cl⁻, and SO4²⁻ in the SWI treatment were significantly higher than those in the FWI and BWI treatments, but there were no significant differences in EC and K⁺ between the FWI and BWI treatments. BWI treatment significantly increased microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), urease, and sucrase contents. The diversity and abundance of bacteria and fungi were not affected by saline water irrigation, but the microbial community structure was altered. Saline water irrigation resulted in an elevation in the bacterial abundance of the phylum Chloroflexi and a decline in Proteobacteria and Actinobacteria. For fungi, the abundance of the phylum Ascomycota in the BWI treatment was greater than that in the FWI and SWI treatments. Linear discriminant analysis effect size (NMDS) results indicated clear variation in the microbiota profiles between the FWI, BWI, and SWI treatments for bacteria. Regarding the fungal microbiota profiles, the BWI and SWI treatments had similar microbiota profiles but were different from the FWI treatment. The number of bacterial biomarkers gradually increased with increasing total dissolved solids of irrigation water, while the number of fungal biomarkers gradually decreased. Additionally, cotton yield was significantly and positively correlated with the observed species of fungi, while it was significantly and negatively correlated with EC. Redundancy analysis (RDA) showed that bacterial community structure was regulated by SAR and fungal community structure was regulated by soil salinity and bulk density (BD). Future research will need to look into how the structure of the microbial community and the associated functional microorganisms are gradually changing with increased irrigation frequency under saline irrigation, as well as explore and screen for advantageous functional microorganisms.
... Plant roots continuously secrete large amounts of organic materials into the soil, forming rhizosphere deposits, which provide abundant nutrients and energy for microorganisms. The species and quantity of plant root exudates determine the species and quantity of rhizosphere microorganisms and affect their growth, reproduction and metabolism [52]. The dynamic changes in the rhizosphere microbial community in turn affect the material cycle and energy flow of the plant rhizosphere ecosystem, and thus affect the growth and development of plants and the changes in vegetation diversity [53]. ...
... However, because of the lower exudation rates of total C as corresponding to the biomass reduction, which was also reported in Aulakh et al. (2001), the proportion of LMWOAs to the total C was greater with the γ-irradiation, particularly in the P-applied compartment. This change in the proportion could be relevant to a selective response of the rice plant to the reduced microbiological activities to exudate smaller amounts of carbohydrates or sugars that are the dominant nutrient sources for microorganisms (Singh and Mukerji 2006) and retain the exudation rates of LMWOAs. In addition, we observed a unique preferential citric acid exudation from the roots of the same rice plant toward the P-rich area of the split-box. ...
Purpose
The response of rice (Oryza sativa L.) plants to phosphorus (P) deficiency and to alterations in the rhizosphere microbiome in flooded soils remains unexplored. We aimed to identify the association between alterations in bacterial diversity and underground metabolic and chemical reactions in the rhizosphere in P-deficient and γ-irradiated conditions.
Methods
Two rice varieties were grown in a split-root box under flooded conditions and filled with either P-applied or non-P-applied soils with or without γ-irradiation. At 41–42 days after transplanting, we determined plant biomass, P uptake, exudation rates of total carbon and low-molecular-weight organic acids (LMWOAs) from the roots, and bacterial diversity, P fractions, and phosphatase activity in the rhizosphere.
Results
γ-irradiation significantly decreased the P uptake and biomass of both rice varieties and reduced the relative abundance of Alphaproteobacteria and microbiome beta diversity in the rhizosphere. Concurrently, γ-irradiation reduced the uptake of insoluble P from soils. These microbiological and chemical changes in the γ-irradiated rhizosphere occurred irrespective of the P treatments of soils. On the contrary, the proportion of LMWOAs and citric acid exudation, which have a high P-solubilization capacity, increased in the γ-irradiated and P-applied rhizosphere soils at the expense of other carbons. No differences were detected in phosphatase activity in any treatments.
Conclusion
Dysbiosis of the rhizosphere microbiome negatively affected rice growth and the uptake of insoluble P in flooded soils. With a reduction in microbiome diversity, rice plants may have a complementary strategy to increase the proportion of citric acid exudation toward the P-rich soil area.
... VIII. Increasing microbial functioning and competition in the rhizosphere consequently forbidding the pathogens to acquire entrance to the roots [94]. ...
The 20thcentury witnessed an augmentation in agricultural production, mainly through the progress and use of pesticides, fertilizers containing nitrogen and phosphorus, and developments in plant breeding and genetic skills. In the naturally existing ecology, rhizospheric soils have innumerable biological living beings to favor the plant development, nutrient assimilation, stress tolerance, disease deter-rence, carbon seizing and others. These organisms include mycorrhizal fungi, bacteria, actinomycetes, etc. which solubilize nutrients and assist the plants in up taking by roots. Amongst them, arbuscular mycorrhizal (AM) fungi have key importance in natural ecosystem, but high rate of chemical fertilizer in agricultural fields is diminishing its importance. The majority of the terrestrial plants form association with Vesicular Arbuscular Mycorrhiza (VAM) or Arbuscular Mycorrhizal fungi (AMF). This symbiosis confers benefits directly to the host plant's growth and development through the acquisition of Phosphorus (P) and other mineral nutrients from the soil by the AMF. They may also enhance the protection of plants against pathogens and increases the plant diversity. This is achieved by the growth of AMF mycelium within the host root (intra radical) and out into the soil (extra radical) beyond. Proper management of Arbuscular Mycorrhizal fungi has the potential to improve the profitability and sustainability of agricultural systems. AM fungi are especially important for sustainable farming systems because AM fungi are efficient when nutrient availability is low and when nutrients are bound to organic matter and soil particles.
... Moreover, 20-60% of C fixed by plants through photosynthesis is transported to the roots, 40-70% of which is released into the rhizosphere as root exudates (Kuzyakov et al. 2001). Therefore, a large amount of C and N accumulates in the rhizosphere and provides microbes with available substrates, which affects microbial growth and the structure of the microbial community (Singh and Mukerji 2006), affecting the release and turnover of soil nutrients mediated by these microbes as a result (Fisk et al. 2015). Moreover, the increased C/N ratio of root exudates has been found to increase the abundance of soil microorganisms, with a higher C/N ratio promoting soil microorganisms to produce more extracellular enzymes to accelerate the decomposition of organic matter (Zhou et al. 2013). ...
AimsKin recognition has been used to explain plant interactions among siblings. However, the phenotypic-based measurements have drawn various results, and the mechanism of plant kin recognition remains unclear. Here, we tested plant kin recognition based on plant nitrogen (N) use efficiency induced by the rhizosphere effect, plant growth performance and fitness.Methods
Glycine max (L.) Merr. was planted pairwise either with siblings or with non-siblings (strangers). Several characteristics, including plant height, specific leaf area (SLA), plant biomass, fitness, chlorophyll content, and plant N use efficiency (NUE) were measured. The root exudation rates of carbon (C) and N, soil microbial biomass C (MBC) and N, and the abundance of functional genes, as well as soil archaeal, bacterial, and fungal abundance, were quantified.ResultsSignificant increases in plant height, seed and shoot biomass, chlorophyll content, and NUE were observed in G. max plants living with siblings compared to strangers, suggesting that plant kin recognition results in superior growth and fitness. The root exudation rate of C was significantly higher in individuals living with siblings than with strangers. The input of C from root exudates further induced the rhizosphere effect, enhancing the soil MBC and increasing the abundance of soil archaea and soil ammonia-oxidizing archaea (AOAs). Plants living with siblings increased NUE for enriched available N in soil, and increased aboveground growth and fitness compared to those living with strangers for G. max.Conclusions
The rhizosphere effect on soil microbes and N turnover triggered by the root exudates depend on the neighbor's identity and could be a potential underground feedback mechanism for plant kin recognition.
... VIII. Increasing microbial functioning and competition in the rhizosphere consequently forbidding the pathogens to acquire entrance to the roots [94]. ...
The 20thcentury witnessed an augmentation in agricultural production, mainly through the progress and use of pesticides, fertilizers containing nitrogen and phosphorus, and developments in plant breeding and genetic skills. In the naturally existing ecology, rhizospheric soils have innumerable biological living beings to favor the plant development, nutrient assimilation, stress tolerance, disease deter-rence, carbon seizing and others. These organisms include mycorrhizal fungi, bacteria, actinomycetes, etc. which solubilize nutrients and assist the plants in up taking by roots. Amongst them, arbuscular mycorrhizal (AM) fungi have key importance in natural ecosystem, but high rate of chemical fertilizer in agricultural fields is diminishing its importance. The majority of the terrestrial plants form association with Vesicular Arbuscular Mycorrhiza (VAM) or Arbuscular Mycorrhizal fungi (AMF). This symbiosis confers benefits directly to the host plant's growth and development through the acquisition of Phosphorus (P) and other mineral nutrients from the soil by the AMF. They may also enhance the protection of plants against pathogens and increases the plant diversity. This is achieved by the growth of AMF mycelium within the host root (intra radical) and out into the soil (extra radical) beyond. Proper management of Arbuscular Mycorrhizal fungi has the potential to improve the profitability and sustainability of agricultural systems. AM fungi are especially important for sustainable farming systems because AM fungi are efficient when nutrient availability is low and when nutrients are bound to organic matter and soil particles.
... Firmicutes www.nature.com/scientificreports/ pH through root exudates 55 . In addition, studies have shown that plant growth was passively influenced by pH, regulating the amount and composition of root secretions 54,56 , which further influences nutrient availability, enzyme activity and microbial abundance 57,58 . ...
Ferula sinkiangensis ( F. sinkiangensis ) is a desert short-lived medicinal plant, and its number is rapidly decreasing. Rhizosphere microbial community plays an important role in plant growth and adaptability. However, F. sinkiangensis rhizosphere bacterial communities and the soil physicochemical factors that drive the bacterial community distribution are currently unclear. On this study, based on high-throughput sequencing, we explored the diversity, structure and composition of F. sinkiangensis rhizosphere bacterial communities at different slope positions and soil depths and their correlation with soil physicochemical properties. Our results revealed the heterogeneity and changed trend of F. sinkiangensis rhizosphere bacterial community diversity and abundance on slope position and soil depth and found Actinobacteria (25.5%), Acidobacteria (16.9%), Proteobacteria (16.6%), Gemmatimonadetes (11.5%) and Bacteroidetes (5.8%) were the dominant bacterial phyla in F. sinkiangensis rhizosphere soil. Among all soil physicochemical variables shown in this study, there was a strong positive correlation between phosphorus (AP) and the diversity of rhizosphere bacterial community in F. sinkiangensis . In addition, Soil physicochemical factors jointly explained 24.28% of variation in F. sinkiangensis rhizosphere bacterial community structure. Among them, pH largely explained the variation of F. sinkiangensis rhizosphere bacterial community structure (5.58%), followed by total salt (TS, 5.21%) and phosphorus (TP, 4.90%).
... In fact, the composition of soluble plant root exudates (mixtures of sugars, amino acids, sugar alcohols, organic acids, phenolics, vitamins, proteins, mucilage carbon IV oxide, aldehydes and various secondary metabolites) reportedly count for 10-40% of the total carbon fixed during photosynthesis (Bais et al., 2006). In addition, Singh and Mukerji (2006) and Neumann (2007) categorically described these organic substances (root exudates) as important drivers of microbial and fauna activity in soil due to their relatively high bioavailability, their role in controlling the bioavailability of nutrients (e.g., phosphorus) and phytotoxic elements (e.g., aluminium), together with the fact that they are added to soil on a regular/semi-continuous basis. One can therefore assume that the root exudates of T. diversifolia (TWL) as well as C. odorata (CWL) served as the carbon source or the precursors for the synthesis of soil humus, thereby enhancedthe availability and acquisition (uptake and transport) of nutrients by H. sabdariffa. ...
... (viii) Stimulating the microbial activity and competitions in the root zone (rhizosphere, rhizoplane) and thus preventing the pathogen to get access to the roots [43]. Roots colonized by VAM/AM fungi may also harbour more actinomycetes antagonistic to root pathogens [44]. ...
... (viii) Stimulating the microbial activity and competitions in the root zone (rhizosphere, rhizoplane) and thus preventing the pathogen to get access to the roots [43]. Roots colonized by VAM/AM fungi may also harbour more actinomycetes antagonistic to root pathogens [44]. ...
... That is why, the role of AM association in plant growth and their influence in biological control of soil or root-borne pathogens is of immense importance in the agricultural system. The AM fungi inhibit pathogen by mechanism of competition in root zones for its access, improved nutrition and/or collaboration of several mechanisms of resistance at the same time (Singh and Mukherji, 2006;Whipps, 2004). Likewise mycorrhiza, another symbiotic fungus with high taxonomic diversity popularly known for a long time as Trichoderma (teleomorph Hypocrea) is gaining importance which also establishes itself on groundnut plants ( Kokalis-Burelle et al., 1997;Hermosa et al., 2010;Jaklitsch et al., 2013). ...
Sclerotium rolfsii (Sacc.) is the causal agent of stem-rot of groundnut plants which is an important damaging soil-borne root pathogen worldwide. Arbuscular mycorrhizal (AM) fungi (Glomus fasciculatum) and Trichoderma asperelloides have shown potential bio-control agent properties against several soil-borne plant pathogens. Interactions between G. fasciculatum, T. asperelloides and soil-borne pathogen S. rolfsii were investigated in this present pot culture experiment. The inoculation of G. fasciculatum and T. asperelloides reduced the severity of disease but their combinations were most effective in reducing harmful effects of S. rolfsii. The arbuscule percentage of AM fungi was affected by presence of T. asperelloides but chlorophyll content got increased by AM fungi or T. asperelloides treatments during S. rolfsii attack. The defense related physiological, biochemical and anti-oxidant activities observed in roots of groundnut plant significantly increased by single inoculation of AM fungi or Trichoderma. But, the combined inoculations of AM fungi and Trichoderma species showed the highest defense related activities. Moreover, single application of either AM fungi or Trichoderma species showed potential for the biocontrol of soil-borne plant pathogen but their combined application attributed most substantial inhibition in development of pathogen.
... Each medicinal plant species have a specific rhizosphere microbiome dependent of the present soil community (Smalla et al., 2001). Besides plant species, the composition and diversity of microbial rhizosphere communities is shaped by soil type and pedoclimate, plant health and developmental stage, climate and season, pesticide treatments, grazers and animals, and several other biotic and abiotic factors (Singh and Mukerji, 2006;Berg and Smalla, 2009;Barnard et al., 2013). Some genera are ubiquitous and can be found distributed over the entire . ...
The rhizosphere is a narrow region of soil that is directly and associated soil microorganisms. Research on rhizosphere microbes of various medicinal plants is essential for microbial ecology, applied microbiology and industrial biotechnology with regard to the sustainable utilization of culturing most rhizosphere microorganisms (around 99%) in the laboratory obviates the research progress. In recent years, there advances and omics to the study of rhizosphere microbial diversity and plant interactions terminal restriction fragment length polymorphism (Dry Rhizosphere of medicinal plants collec treated represents a common physiological stress for the microbial communities residing in surface of these medicinal plants. A dry and wetting by water induce lysis in a significant proportion of the microbial biomass and, microbial community composition. In this study Dry sand and water treated as common stress in the laboratory by exposing three different rhizosphere immersed in water to 10 day and 20 day period. medicinal plants were collected from Saint Katherine Mountain, Sinai, Egypt. Fungal every 10 day of incubation. Total DNA was extracted from sand samples and characterize revealed that water changed with dry sand due to water may induced shifts in bacterial community.
... Each medicinal plant species have a specific rhizosphere microbiome dependent of the present soil community ( Smalla et al., 2001). Besides plant species, the composition and diversity of microbial rhizosphere communities is shaped by soil type and pedoclimate, plant health and developmental stage, climate and season, pesticide treatments, grazers and animals, and several other biotic and abiotic factors ( Singh and Mukerji, 2006;Berg and Smalla, 2009;Barnard et al., 2013). Some genera are ubiquitous and can be found distributed over the entire Table 4. Similarity index of microbial community of Terminal restriction fragment length polymorphism (TRFLP) results for all control and microcosms' samples after 10 and 20 day 0.66 0.72 0.78 0.7 0.69 0.84 0.69 0.68 0.85 0.79 C20 0 0.68 0.66 0.81 0.8 0.79 0.81 0.78 0.75 0.9 0.84 CM10 0 0.56 0.74 0.74 0.73 0.79 0.77 0.76 0.88 0.82 CM20 0 0.74 0.63 0.64 0.65 0.76 0.73 0.9 0.81 CH10 0 0.49 0.44 0.54 0.72 0.65 0.8 0.76 CH20 0 0.25 0.57 0.61 0.49 0.86 0.74 CHM10 0 0.48 0.55 0.45 0.83 0.78 CHM20 0 0.77 0.7 0.79 0.74 HE10 0 0.43 0.81 0.77 HE20 0 0.85 0.8 HEM10 0 0.57 HEM20 0 Cophenetic Correlation Coefficient (CP) = 0.89 plant, such as the well-known plant-associated genera Bacillus and Pseudomonas ( Berg et al., 2011). ...
The rhizosphere is a narrow region of soil that is directly and associated soil microorganisms. Research on rhizosphere microbes of various medicinal plants is essential for microbial ecology, applied microbiology and industrial biotechnology with regard to the sustainable utilization of culturing most rhizosphere microorganisms (around 99%) in the laboratory obviates the research progress. In recent years, there advances and omics to the study of rhizosphere microbial diversity and plant interactions terminal restriction fragment length polymorphism (Dry Rhizosphere of medicinal plants collec treated represents a common physiological stress for the microbial communities residing in surface of these medicinal plants. A dry and wetting by water induce lysis in a significant proportion of the microbial biomass and, microbial community composition. In this study Dry sand and water treated as common stress in the laboratory by exposing three different rhizosphere immersed in water to 10 day and 20 day period. medicinal plants were collected from Saint Katherine Mountain, Sinai, Egypt. Fungal every 10 day of incubation. Total DNA was extracted from sand samples and characterize revealed that water changed with dry sand due to water may induced shifts in bacterial community.
... The microbial and faunal activities are particularly driven by root exudates present in soil. This is because of the fact that they are incorporated into the soil on a regular/ semi-continuous basis, their comparatively greater bioavailability than aged plant detritus and their role in the regulation of bioavailability of nutrients (e.g., phosphorus) and phytotoxic compounds (e.g., aluminium) (Singh and Mukerji 2006 ;Neumann 2007 ). ...
Soil is the incoherent matter on the earth’s surface having organic and mineral content. It is subjected to environmental changes and hence shows effects of climate change as well as organisms over a period of time. Hence, it is the high time to find ways to increase the crop productivity in soil as green revolution cannot withstand this need. An alternative to this problem is the use of soil microorganism to increase the fertility of soil. Soil enzymes originate from soil microbes and regulate the nutrient cycle. Potential soil isolates can be used to increase nutrients in soil. In addition, these isolates can help in reducing the increase of carbon dioxide by sequestering carbon in soil. It is known that CO2 is one of the major greenhouse gases that contributes to global warming and CO2 fluxes are controlled by soil biota. Thus, soil act as buffer compartment to sequester carbon in relation to climate change. The sequestered soil carbon may further be utilized in agriculture and forestry and as a powerful option for global change mitigation. With this background, the present chapter aims to provide an insight into the contribution of microbial communities to soil carbon sequestration and its benefits to sustainable agriculture.
... In fact, the composition of soluble plant root exudates (mixtures of sugars, amino acids, sugar alcohols, organic acids, phenolics, vitamins, proteins, mucilage carbon IV oxide, aldehydes and various secondary metabolites) reportedly count for 10-40% of the total carbon fixed during photosynthesis (Bais et al., 2006). In addition, Singh and Mukerji (2006) and Neumann (2007) categorically described these organic substances (root exudates) as important drivers of microbial and fauna activity in soil due to their relatively high bioavailability, their role in controlling the bioavailability of nutrients (e.g., phosphorus) and phytotoxic elements (e.g., aluminium), together with the fact that they are added to soil on a regular/semi-continuous basis. One can therefore assume that the root exudates of T. diversifolia (TWL) as well as C. odorata (CWL) served as the carbon source or the precursors for the synthesis of soil humus, thereby enhancedthe availability and acquisition (uptake and transport) of nutrients by H. sabdariffa. ...
... When considering the differences in chemical characteristics of the tested products, we hypothesize that all these factors could have also accounted for the changes observed among Archaea and Eubacteria communities in our trial. Nevertheless, we cannot exclude that the treatments affected the amount and composition of plant exudates as well (Nardi et al., 2002;Singh and Mukerji, 2006), as result of a different rate of N mineralization for the two organic fertilizers, because of their different composition. The resulting different availability of nitrate or ammonia N, which would be expected considering the different nitrogen composition of the mineral and organic fertilizers, would thus induce the release of different rates of hydrogen ions by the plant (Sas et al., 2002), modifying the rhizosphere pH and altering the conditions for bacterial growth (Huang et al., 2014). ...
This study evaluates the effect of two different kinds of liquid organic fertilizers and a mineral fertilizer on microbial populations (Eubacteria and Archaea) living in the tomato rhizosphere. The organic fertilizers comprised a stillage from bakery yeasts production, characterized by acidic pH and high organic nitrogen content, as well as a vermicompost extract with alkaline pH and low nitrogen content. The number of operational taxonomic units (OTUs) of Archaea species significantly increased after the addition of both the organic and mineral fertilizers, in comparison to the unfertilized control. Eubacteria OTUs increased only in plants treated with the stillage. T-RFLPs profiles identified different clusters of soil rhizospheric Archaea and Eubacteria in treatments of both organic and mineral fertilizers. However, multivariate analyses performed on indicators of genetic diversity, based on the T-RFLPs profiles, and Shannon and equitability indexes of the Archaea and Eubacteria communities, pointed out marked differences between the control (or the mineral fertilization) and the organic fertilizers. The latter treatments induced also a higher root growth and a modification of the root architecture in comparison to both control and mineral fertilizer. The results are discussed with reference to the effect of the organic products on rhizospheric soil characteristics and the relationships between the plant and the rhizospheric microorganisms.
... A number of studies show synergistic effect of plant growth promoting rhizobacteria and symbiotic fungi (Kohler et al., 2007). Root colonization (endophytic & exophytic) is a pre requisite to derive benefits from a PGPR (Chabot et al., 1996;Singh and Mukerji, 2006). ...
Effect of plant genotype on the root endophytic colonization ability of a plant growth promoting rhizobacteria (PGPR), Pseudomonas striata was undertaken in this study. Use of a lac-Z tagged P. striata strain showed that, it can exist as an endophyte and the plant genotype determines the performance of the inoculated PGPR. The cultivars of Zea mays L. (maize) and Vigna radiata L. (mung bean) tested showed differential affinity to the PGPR (P. striata) as reflected by a significant variation in the root endophytic colonization ability of P. striata. Coinoculation with a novel symbiotic fungus Piriformospora indica was found to stimulate endophytic colonization of P. striata in both maize and mungbean. The root exudates of maize and mungbean cultivars showed variations in the total sugar and amino acid contents. However, no consistent relationship was recorded between the concentrations of these metabolites and endophytic colonization of the added PGPR.
... Soluble plant root exudates account for 10-40% of the total carbon fixed by photosynthesis and are composed mainly of mixtures of sugars, amino acids, sugar alcohols, organic acids and secondary metabolites (Bais et al. 2006). Root exudates are particularly important drivers of microbial and faunal activity in soil due to a combination of their relatively high bioavailability compared with senescent plant detritus, their role in controlling the bioavailability of nutrients (e.g., phosphorus) and phytotoxic elements (e.g., aluminium), together with the fact that they are added to soil on a regular/semi-continuous basis (Singh and Mukerji 2006;Neumann 2007). Thus microbial numbers and activity in the immediate vicinity of growing roots (1-3 mm -rhizosphere) are commonly orders of magnitude greater than in non-rhizosphere soil. ...
Organic matter is mainly present in the top 20–30 cm of most soil profiles and is essentially an array of organic macromolecules
consisting principally of combinations of carbon, oxygen, hydrogen, nitrogen, phosphorus and sulphur. Soil organic matter
is commonly measured as the quantity of organic carbon. The global pool of organic carbon in soil to a depth of 1 m has been
estimated at 1,200–1,550 Pg (2 m: 2,370–2,450 Pg), and as such is significantly greater than either the biological-biota (560
Pg) or atmospheric (760 Pg) carbon pools (Baldock 2007). Almost all organic matter in soil is directly and indirectly derived from plants via photosynthesis. Thus atmospheric carbon
dioxide is transformed by reduction into simple and complex organic carbon compounds, which in combination with key nutrients
enable the plant to function and grow. Carbon dioxide is released directly from plants by respiration, but most of the fixed
carbon is retained and ultimately transferred to the soil ecosystem via a combination of spatially distinct pathways over
a variety of timescales. The most important pathways are the direct addition of senes-cent material as above-ground and below-ground
detritus, return of ingested plant matter in animal faeces, and exudation of soluble organic compounds from roots (Howarth
2007).
Orobanche cumama wallr. is the sunflower root parasitic weed with special life stage in which seed germination and parasitism take place in the soil. In practice, applying microbial agents and trapping crop rotation are utilized separately, or just one of them is selected to control O. cumana. The development of the sunflower industry is severely constrained on the farmland, where there is high density of O. cumana’s seed banks. In this study, two biological control methods were combined to solve the problem of O. cumana parasitism. The bioassay experiment showed that the high concentration fermentation filtrates of Streptomyces rochei D74 could effectively inhibit the germination and growth of the germ tube of O. cumana seeds. As the concentration was increased to 3.1 mg/mL, O. cumana was almost unable to sprout. A two-year pot experiment revealed that the use of D74 agents and sunflower–maize–sunflower rotation together promoted sunflower growth, as shown by the biomass accumulation, plant height, and denser root systems. The combined method resulted in a significant decrease in the number of O. cumana parasitism, compared to one method alone. Additionally, it affected the bacterial community composition of sunflower rhizosphere, mostly leading to an increase in Streptomyces and Brevibacterium and a decrease in Arthrobacter. This experiment, combined with multiple biological control, means significantly reducing the parasitism of O. cumana, which provides an effective foundation for practical application.
Rhizosphere bacteria affect the diversity of soil functions, playing important roles in the growth and expansion of Chinese fir. Understanding the driving factors of rhizosphere bacterial distribution is imperative when comparing bacterial diversity and composition under different Chinese fir provenances. We investigated the growth of Chinese fir belts and the effects of climate, geographic location, and soil nutrients. Using 16S rDNA next-generation sequencing analysis, the bacterial communities of 16 Chinese fir provenances were compared. The bacterial compositionsof Dechang, Junlian, Shangrao, Zhenxiong, Yangxin, Xinyang, Luotian, and Tianmushan provenances weredistinct from others. Generally, higher-latitude provenances showed more biomarkers (LDA = 2). Rhizosphere bacterial α-diversity was the highest in Hunan Youxian and lowest in Henan Xinyang (p < 0.05). From south to north, bacterial α-diversity initially increased and then decreased. From east to west in the middle belt, bacterial α-diversity followed a “W” trend, with the eastern middle belt having the highest values, especially near Hunan, Fujian, and Zhejiang provinces. Amongst environmental factors, soil nutrient content (Mg, P and K) and stoichiometric ratio (Ca/Mg, K/Ca and N/P), along with precipitationrate primarily controlled rhizosphere bacterial diversity. Soil pH had a significant impact on the relative abundance of rhizosphere soil bacteria. Our findings offer insight into the evolution of Chinese fir and provide a scientific basis for soil microbial community improvement of Chinese fir provenances.
Ferula sinkiangensis ( F.sinkiangensis ) is a desert short-lived medicinal plant, and its number is rapidly decreasing. Rhizosphere microbial community plays an important role in plant growth and adaptability. However, F.sinkiangensis rhizosphere bacterial communities and the soil physicochemical factors that drive the bacterial community distribution are currently unclear. On this study, based on high-throughput sequencing, we explored the diversity, structure and composition of F.sinkiangensis rhizosphere bacterial communities at different slope positions and soil depths and their correlation with soil physicochemical properties. Our results revealed the heterogeneity and changed trend of F.sinkiangensis rhizosphere bacterial community diversity and abundance on slope position and soil depth and found Actinobacteria (25.5%), Acidobacteria (16.9%), Proteobacteria (16.6%), Gemmatimonadetes (11.5%) and Bacteroidetes (5.8%) were the dominant bacterial phyla in F.sinkiangensis rhizosphere soil. Among all soil physicochemical variables shown in this study, there was a strong positive correlation between phosphorus (AP) and the diversity of rhizosphere bacterial community in F.sinkiangensis . In addition, Soil physicochemical factors jointly explained 24.28% of variation in F.sinkiangensis rhizosphere bacterial community structure. Among them, pH largely explained the variation of F.sinkiangensis rhizosphere bacterial community structure (5.58%), followed by total salt (TS, 5.21%) and phosphorus (TP, 4.90%).
Ferula sinkiangensis is a desert short-lived medicinal plant, and its number is rapidly decreasing. Rhizosphere microbial community plays an important role in plant growth and adaptability. However, Ferula sinkiangensis rhizosphere bacterial communities and the soil physicochemical factors that drive the bacterial community distribution are currently unclear. On this study, based on high-throughput sequencing, we explored the diversity, structure and composition of Ferula sinkiangensis rhizosphere bacterial communities at different slope positions and soil depths and their correlation with soil physicochemical properties. Our results revealed the heterogeneity and variation trends of Ferula sinkiangensis rhizosphere bacterial community diversity and abundance on a fine spatial scale (Slope position and soil depth) and Found Actinobacteria (25.5%), Acidobacteria (16.9%), Proteobacteria (16.6%), Gemmatimonadetes (11.5%) and Bacteroidetes (5.8%) were the dominant bacterial phyla in Ferula sinkiangensi s rhizosphere soil. Among all soil physicochemical variables shown in this study, there was a strong positive correlation between phosphorus (AP) and the diversity of rhizosphere bacterial community in Ferula sinkiangensis . In addition, Soil physicochemical factors jointly explained 24.28% of variation in Ferula sinkiangensis rhizosphere bacterial community structure. Among them, pH largely explained the variation of Ferula sinkiangensis rhizosphere bacterial community structure (5.58%), followed by total salt (TS, 5.21%) and phosphorus (TP, 4.90%).
Citrus is vegetatively propagated by grafting for commercial production, and most rootstock cultivars of citrus have scarce root hairs, thus heavily relying on mutualistic symbiosis with arbuscular mycorrhizal fungi (AMF) for mineral nutrient uptake. However, the AMF community composition, and its differences under different citrus scion/rootstock genotypes, were largely unknown. In this study, we investigated the citrus root-associated AMF diversity and richness, and assessed the influence of citrus scion/rootstock genotypes on the AMF community composition in a controlled condition, in order to exclude interferences from environmental factors and agricultural practices. As a result, a total of 613,408 Glomeromycota tags were detected in the citrus roots, and 46 AMF species were annotated against the MAARJAM database. Of these, 39 species belonged to Glomus, indicating a dominant role of the Glomus AMF in the symbiosis with citrus. PCoA analysis indicated that the AMF community’s composition was significantly impacted by both citrus scion and rootstock genotypes, but total samples were clustered according to rootstock genotype rather than scion genotype. In addition, AMF α diversity was significantly affected merely by rootstock genotype. Thus, rootstock genotype might exert a greater impact on the AMF community than scion genotype. Taken together, this study provides a comprehensive insight into the AMF community in juvenile citrus plants, and reveals the important effects of citrus genotype on AMF community composition.
The persistent and undiscriminating application of chemicals as means to improve crop growth, development and yields for several years has become problematic to agricultural sustainability because of the adverse effects these chemicals have on the produce, consumers and beneficial microbes in the ecosystem. Therefore, for agricultural productivity to be sustained there are needs for better and suitable preferences which would be friendly to the ecosystem. The use of microbial metabolites has become an attractive and more feasible preference because they are versatile, degradable and ecofriendly, unlike chemicals. In order to achieve this aim, it is then imperative to explore microbes that are very close to the root of a plant, especially where they are more concentrated and have efficient activities called the rhizosphere. Extensive varieties of bacteria, archaea, fungi and other microbes are found inhabiting the rhizosphere with various interactions with the plant host. Therefore, this review explores various beneficial microbes such as bacteria, fungi and archaea and their roles in the environment in terms of acquisition of nutrients for plants for the purposes of plant growth and health. It also discusses the effect of root exudate on the rhizosphere microbiome and compares the three domains at molecular levels.
The soil-plant ecosystem harbors an immense microbial diversity that challenges investigative approaches to study traits underlying plant-microbe association. Studies solely based on culture-dependent techniques have overlooked most microbial diversity. Here we describe the concomitant use of culture-dependent and -independent techniques to target plant-beneficial microbial groups from the sugarcane microbiome. The community-based culture collection (CBC) approach was used to access microbes from roots and stalks. The CBC recovered 399 unique bacteria representing 15.9% of the rhizosphere core microbiome and 61.6–65.3% of the endophytic core microbiomes of stalks. By cross-referencing the CBC (culture-dependent) with the sugarcane microbiome profile (culture-independent), we designed a synthetic community comprised of naturally occurring highly abundant bacterial groups from roots and stalks, most of which has been poorly explored so far. We then used maize as a model to probe the abundance-based synthetic inoculant. We show that when inoculated in maize plants, members of the synthetic community efficiently colonize plant organs, displace the natural microbiota and dominate at 53.9% of the rhizosphere microbial abundance. As a result, inoculated plants increased biomass by 3.4-fold as compared to uninoculated plants. The results demonstrate that abundance-based synthetic inoculants can be successfully applied to recover beneficial plant microbes from plant microbiota.
The coordinated behaviors, at the population level, occur in bacteria due to communication between the cells, which is defined as quorum sensing (QS). These communications help the bacteria to respond in terms of extracellular factors which are involved in scavenging for nutrients; biosynthesis of exopolysaccharides, extracellular hydrolytic enzymes, siderophores, antibiotics, pigments, hypersensitive reaction; aiding motility; and providing scaffolding for biofilms to grow as well as those involved in conjugation and epiphytic fitness. Even in parasitic species, these signals help in regulation of bacterial growth, virulence, and damage caused to hosts. QS signal molecules are regulated by autoinduction and positive feedback mechanism at high cell densities. A detailed mechanism involved in QS is discussed in this chapter with special reference to rhizosphere microflora. The rhizosphere is a digestive system of the plants wherein several microorganisms influenced by root exudates in turn favor its growth. In the future, understanding QS related to plant growth-promoting bacteria and its interspecific bacterial communication may offer good opportunities to manipulate bacterial community structure in the rhizosphere for the benefit of agriculture.
Root exudates are chemical compounds that are released from living plant roots and provide significant energy, carbon, nitrogen and phosphorus sources for microbes inhabiting the rhizosphere. The exudates shape the microflora associated with the plant, as well as influences the plant health and productivity. Therefore, a better understanding of the trophic link that is established between the plant and the associated bacteria is necessary. In this study, a comprehensive survey on the utilization of grapevine and rootstock related organic acids were conducted on a vineyard soil isolate which is Pseudomonas mendocina strain S5.2. Phenotype microarray analysis has demonstrated that this strain can utilize several organic acids including lactic acid, succinic acid, malic acid, citric acid and fumaric acid as sole growth substrates. Complete genome analysis using single molecule real-time technology revealed that the genome consists of a 5,120,146 bp circular chromosome and a 252,328 bp megaplasmid. A series of genetic determinants associated with the carbon utilization signature of the strain were subsequently identified in the chromosome. Of note, the coexistence of genes encoding several iron–sulfur cluster independent isoenzymes in the genome indicated the importance of these enzymes in the events of iron deficiency. Synteny and comparative analysis have also unraveled the unique features of d-lactate dehydrogenase of strain S5.2 in the study. Collective information of this work has provided insights on the metabolic role of this strain in vineyard soil rhizosphere.
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The online version of this article (doi:10.1186/s13568-017-0437-7) contains supplementary material, which is available to authorized users.
This study was carried out to investigate the population densities, R/S ratios, and identification of heterotrophic bacteria on the rhizosphere soil of halophyte Suaeda japonica found on the western and southern mudflats of Korea. The population densities of aerobic and anaerobic heterotrophic bacteria on the rhizosphere soil of Suaeda japonica were in the range of 1.3\;?\;0.3\;{\times}\;10^6\;{\sim}\;6.3\;?\;3.3\;{\times}\;10^7\;and\;2.8\;?\;1.3\;{\times}\;10^4\;{\sim}\;1.8\;?\;0.7\;{\times}\;10^7\;cfu\;g^{-1}\;d.\;wt., respectively. In case of physiologically specific bacteria, population densities of amylolytic bacteria on the rhizosphere soil of Suaeda japonica were in the range of 4.4\;?\;0.6\;{\times}\;10^6\;{\sim}\;2.5\;?\;1.2\;{\times}\;10^7\;cfu\;g^{-1}\;d.\;wt., those of cellulolytic bacteria were from 8.5\;?\;6.0\;{\times}\;10^4\;{\sim}\;2.3\;?\;1.6\;{\times}\;10^6\;cfu\;g^{-1}\;d.\;wt., and those of proteolytic bacteria were from 3.8\;?\;1.8\;{\times}\;10^5\;{\sim}\;4.2\;?\;2.9\;{\times}\;10^6\;cfu\;g^{-1}\;d.\;wt., respectively. The R/S ratios were ranged from 2.33 to 2.39. Among eleven isolates from the roots of halophyte Suaeda japonica of Goheung bay by using 16S rDNA analysis, five clones were closely related to {\gamma}-Proteobacteria group and six clones were closely related to {\alpha}-Proteobacteria group. Among four isolates from Suncheon bay, two strains were related to {\gamma}-Proteobacteria group and another two were related to Actinobacteria and Bacilli group, respectively.
Allelochemals induced in mycorrhizal plants play an important role in disease resistance. Mycorrhizal associations are the
most important symbiosis systems in terrestrial ecosystems and offer many benefits to the host plant. Arbuscular mycorrhizal
associations can reduce damage caused by soil and root - borne pathogens.
Plant growth-promoting rhizobacteria (PGPR) naturally occur in the rhizospheres of pasture, but still little is understood regarding how soil agricultural practices affect them. Here, we examined the effects of long-term nitrogen (N) fertilisation on the occurrence of potential culturable PGPR in rhizosphere soils from pastures grown in Chilean Andisols. We also evaluated in vitro the effects of organic acids (citric, malic and oxalic acids), metals (Al and Mn), and N supply (urea and ammonium sulphate) on indole acetic acid (IAA) production and phosphorus (P) liberation by selected strains. Compared with non-N-fertilised pasture, N fertilisation significant increased (30%) the occurrence of culturable phosphobacteria, but decreased (7%) the occurrence of IAA-producing rhizobacteria. Most efficient IAA-producing phosphobacteria (IAAP) were identified as Bacillus, Enterobacter, Pseudomonas and Serratia. At low pH (4.8), the assays showed that the IAA production by Serratia sp. N0-10LB was increased (31-74%) by organic acids. On the other hand, the IAA production by Pseudomonas sp. N1-55PA was increased 2-5 fold by metals. In all strains, the growth and IAA production were significant decreased by 500 μM of Al, except Serratia sp. N0-10LB, suggesting its potential as PGPR for Chilean Andisols. When urea was added as main N source the bacterial growth and P utilization significant increased compared with ammonium sulphate. The influence of environmental factors that are typical of Chilean Andisols on rhizobacterial communities will provide better management practices to enhance their PGPR functions as well as a better selection biofertilisers to be used in Chilean Andisols.
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