Diversity and activity of biosurfactant-producing Pseudomonas in the rhizosphere of black pepper in Vietnam

Aims: Phytophthora capsici is a major pathogen of black pepper and zoospores play an important role in the infection process. Fluorescent pseudomonads that produce biosurfactants with zoosporicidal activities were isolated from the black pepper rhizosphere in Vietnam, and their genotypic diversity and potential to control Phy. capsici root rot was determined. Methods: Biosurfactant‐producing pseudomonads were genotypically and biochemically characterized by BOX‐polymerase chain reaction (PCR), 16S‐rDNA sequencing, reverse‐phase‐high‐performance liquid chromatography and liquid chromatography‐masss spectrometry analyses. Results: Biosurfactant‐producing fluorescent pseudomonads make up c. 1.3% of the culturable Pseudomonas population in the rhizosphere of black pepper. Although BOX‐PCR revealed substantial genotypic diversity, the isolates were shown to produce the same biosurfactants and were all identified as Pseudomonas putida. When applied to black pepper stem cuttings, several of the biosurfactant‐producing strains provided significant disease control. In absence of the disease, several of the bacterial strains promoted shoot and root growth of black pepper stem cuttings. Conclusions: Biosurfactant‐producing pseudomonads indigenous to the rhizosphere of black pepper plants are genotypically diverse and provide a novel resource for the control of Phy. capsici root rot and growth promotion of black pepper stem cuttings. Significance and Impact of the Study: The results of this study provide a strong basis for further development of supplementary strategies with antagonistic bacteria to control foot and root rot of black pepper and to promote plant growth.     

Selecting Bacterial Strains for Use in the Biocontrol of Diseases Caused by <Emphasis Type="Italic">Phytophthora capsici</Emphasis> and <Emphasis Type="Italic">Alternaria alternata</Emphasis> in Sweet Pepper Plants

More than 500 isolates of bacteria were obtained from the aerial part and rhizosphere of sweet pepper (Capsicum annuum L.) plants harvested from different places in the Region of Murcia (Spain). The isolates were purified and assayed in vitro against Phytophthora capsici and Alternaria alternata. Sixty isolates (12 %) produced an inhibition zone against at least one of the pathogens, while ten had a strongly inhibitory effect on both pathogens assayed. Microscopic observation of interactions zone showed cell vacuolisation, hyphae lysis and spilling of cytoplasm content of the pathogens in the culture media. These ten isolates were then chosen for biocontrol of Phytophthora root rot and Alternaria leaf spots of pepper plants in vivo. Four of them denominated HS93, LS234, LS523 and LS674 reduced P. capsici root rot by 80, 51, 49 and 54 %, respectively, and A. alternata leaf spots by 54, 74, 62 and 53 %. HS93 belongs to the genus Bacillus and probably the species subtilis, while LS234, LS523 and LS674 belong to the genus Bacillus and probably the species licheniformis. Dry mass of plants treated with these bacteria was significantly higher than that of non-treated and inoculated plants.     

Tomato Growth in Soil Amended with Sugar Mill By-Products Compost

Sugar mill by-products compost may be a good soil amendment to promote tomato (Lycopersicon esculentum L.) growth. In addition, the compost may further promote plant growth by inoculation with N₂-fixing bacteria. Compost from sugar-mill waste was prepared with and without the N₂-fixing bacteria, Azotobacter vinelandii, Beijerinckia derxii and Azospirillum sp. and incubated for 50 days. Each compost type was added to 10 kg of soil in pots at rates of 0, 15, and 45 g with and without fertilizer N at rates of 0, 0.75, and 1.54 g. A blanket application of P and K was applied to all pots. Shoot and root dry weights and N content of the whole plant was measured at 55 days. Dry weight of tomato shoots was increased by 40% by addition of fertilizer N and root weight was increased by 66%. Without fertilizer N the high rate of inoculated compost increased shoot growth 180% and uninoculated compost increased shoot growth 112%. For most treatments with and without fertilizer N, inoculated compost enhanced shoot growth and nitrogen content more than uninoculated compost. Root weights were nearly doubled by addition of either compost in comparison to the 0 N treatment. At the low rate of compost addition without fertilizer N, root weight was the same for uninoculated and inoculated compost but at the high rate of compost addition root weight was 32% higher for inoculated compost. The N₂-fixing bacteria colonized roots when inoculated compost was used. Sugar mill by-products compost proved to be an effective soil amendment for promoting the growth of tomato plants.     

Biocontrol of Meloidogyne incognita inciting disease in tomato by using a mixed compost inoculated with Paenibacillus ehimensis RS820

Meloidogyne spp. causes root-knot disease in tomato plants. Biological control of the disease may present economically feasible, agronomically durable and environmentally safe alternative of nematicides. A chitinolytic bacterial strain, Paenibacillus ehimensis RS820, previously isolated from the soil in Korea, produced lytic enzymes in higher amounts and inhibited the growth of phytopathogenic root-knot nematodes. Moreover, the juveniles and eggs of root-knot nematodes induced secretion of lytic enzymes by RS820 including chitinases, gelatinases and collagenases. Furthermore, mixed compost containing increased amounts of chitin and inoculated with RS820 was prepared in the present study. Use of the mixed compost not only reduced the disease caused by root-knot nematodes but also improved the plant growth. The extent of inoculation of the mixed compost with RS820 significantly influenced its ability to control the root-knot disease in tomato. The mixed compost also significantly altered the activity and density of the rhizosphere bacteria. Chitinase and gelatinase producing soil bacteria, as well as their enzyme activities, were significantly influenced by the mixed compost. The mixed compost proposed in the present study may represent a viable alternative to nematicides against the root-knot nematodes in tomato.     

Survival ofXanthomonas campestris pv.vesictoria in pepper seeds and roots in symptomless and dry leaves in non-host plants and in the soil

Summary A population ofXanthomonas campestris pv.vesicatoria developed as endophytes in the leaves and rhizosphere of apparently symptomless plants grown under mist but not under dry conditions. The pathogen survired for long periods on, and could be isolated from, the surface of infested dried seeds, inoculated sandy loam soil, dried leaves, and the rhizosphere of pepper and of other non-host plants. In addition, small numbers of the pathogen survived for 18 months in a field previously cropped with pepper diseased with bacterial scab. Healthy nursery or mature plants developed symptoms while growing in soil containing infested leaves, either buried or placed on the soil surface.     

Greenhouse and field evaluations of potassium phosphonate: the control of Phytophthora foot rot of black pepper in Vietnam

Phytophthora foot rot of black pepper caused by Phytophthora capsici is a major disease of black pepper throughout production areas in Vietnam. The disease causes collar, foot and tap root rots and eventual death of the infected vine. Potassium phosphonate was evaluated for the control of this disease in greenhouse and field trials. In greenhouse trials three-month-old vines treated with phosphonate by soil drenching (10–20 g a.i./l) and then inoculated with P. capsici mycelium (2% v/v soil) had significantly less foot rot compared to vines grown in non-treated soil. In field trials mature vines were treated with phosphonate at 50–100 g a.i/pole soil drenching or 10 g a.i./l by root infusion. After 10 days root, stem and leaf specimens were removed for bioassay by inoculation with 5 ml of P. capsici zoospores suspension (10⁶–10⁸ spores/ml). Soil drenching with phosphonate inhibited the colonisation of pathogen on excised leaf, stem and root tissues, significantly more than phosphonate root infusion. Our study provides further evidence supporting the efficacy of potassium phosphonate in the management of black pepper foot rot caused by P. capsici. The excised leaf and stem bioassay used in this study is a rapid and useful technique for testing the efficacy of systemic fungicides in controlling this disease.     

Biocontrol Ability of <Emphasis Type="Italic">Lysobacter antibioticus</Emphasis> HS124 Against <Emphasis Type="Italic">Phytophthora</Emphasis> Blight Is Mediated by the Production of 4-Hydroxyphenylacetic Acid and Several Lytic Enzymes

Several rhizobacteria play a vital role in plant protection, plant growth promotion and the improvement of soil health. In this study, we have isolated a strain of Lysobacter antibioticus HS124 from rhizosphere and demonstrate its antifungal activity against various pathogens including Phytophthora capsici, a destructive pathogen of pepper plants. L. antibioticus HS124 produced lytic enzymes such as chitinase, β-1,3-glucanase, lipase, protease, and an antibiotic compound. This antibiotic compound was purified by diaion HP-20, silica gel, sephadex LH-20 column chromatography and high performance liquid chromatography. The purified compound was identified as 4-hydroxyphenylacetic acid by gas chromatography-electron ionization (GC-EI) and gas chromatography-chemical ionization (GC-CI) mass spectrometry. This antibiotic exhibited destructive activity toward P. capsici hyphae. In vivo experiments utilizing green house grown pepper plants demonstrated the protective effect of L. antibioticus HS124 against P. capsici. The growth of pepper plants treated with L. antibioticus culture was enhanced, resulting in greater protection from fungal disease. Optimum growth and protection was found when cultures were grown in presence of Fe(III). Additionally, the activities of pathogenesis-related proteins such as chitinase and β-1,3-glucanase decreased in roots, but increased in leaves with time after treatment compared to controls. Our results demonstrate L. antibioticus HS124 as a promising candidate for biocontrol of P. capsici in pepper plants.     

Effects of the Biocontrol Agent Aspergillus flavipes on the Soil Microflora and Soil Enzymes in the Rooting Zone of Pepper Plants Infected with Phytophthora capsici

This study examined the effect of ASD strain (Aspergillus flavipes), isolated from continuous cropping soil for pepper and named by the sampling position, on soil microflora and soil enzymes in rooting zone soil of healthy and diseased (Phytophthora capsici) pepper plants. Results showed that the ASD strain could significantly reduce the number of bacteria and actinomycetes, with a significant increase in fungi in the rhizosphere soil of both healthy and diseased plants. With increasing colonization time of the ASD strain, the number of bacteria and actinomycetes decreased initially and then increased gradually, while the number of fungi was first increased significantly and later decreased slowly. The soil enzyme activities of urease, acid phosphatase, invertase and dehydrogenase were significantly increased by the ASD strain, while the activity of catalase was not significantly increased. As time from inoculation with the ASD strain increased, the activities of various enzymes were higher than controls. Maximum enzyme activities were found on the tenth day after ADS inoculation. The response of soil enzyme activities affected by the ASD strain was as follows: urease > dehydrogenase > invertase > acid phosphatase > catalase. These results suggest that the biocontrol of ASD strain could improve the micro ecology of rhizosphere soil.     

Competing Factors of Compost Concentration and Proximity to Root Affect the Distribution of Streptomycetes

Streptomycetes are important members of soil microbial communities and are particularly active in the degradation of recalcitrant macromolecules and have been implicated in biological control of plant disease. Using a streptomycetes-specific polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (PCR-DGGE) methodology coupled with band excision and sequence analysis, we examined the effect of grape marc compost amendment to soil on cucumber plant–associated streptomycetes community composition. We observed that both compost amendment and proximity to the root surface influenced the streptomycetes community composition. A strong root selection for a soil-derived Streptomycete, most closely related to Streptomyces thermotolerans, S. iakyrus, and S. thermocarboxydus, was independent of compost amendment rate. However, while the impact of compost amendment was mitigated with increasing proximity to the root, high levels of compost amendment resulted in the detection of compost-derived species on the root surface. Conversely, in rhizosphere and non-rhizosphere soils, the community composition of streptomycetes was affected strongly even by modest compost amendment. The application of a streptomycetes-specific PCR primer set combined with DGGE analysis provided a rapid means of examining the distribution and ecology of streptomycetes in soils and plant-associated environments.     

The volatile-producing Flavobacterium johnsoniae strain GSE09 shows biocontrol activity against Phytophthora capsici in pepper

Aims: Previously, we selected a bacterial strain (GSE09) antagonistic to Phytophthora capsici on pepper, which produced a volatile compound (2,4‐di‐tert‐butylphenol), inhibiting the pathogen. In this study, we identified strain GSE09 and characterized some of the biological traits of this strain in relation to its antagonistic properties against P. capsici. In addition, we examined bacterial colonization on the root surface or in rhizosphere soil and the effect of various concentrations of the volatile compound and strain GSE09 on pathogen development and radicle infection as well as radicle growth. Methods and Results: Strain GSE09 was identified as Flavobacterium johnsoniae, which forms biofilms and produces indolic compounds and biosurfactant but not hydrogen cyanide (HCN) with little or low levels of antifungal activity and swimming and swarming activities. Fl. johnsoniae GSE09 effectively colonized on pepper root, rhizosphere, and bulk (pot) soil, which reduced the pathogen colonization in the roots and disease severity in the plants. Various concentrations of 2,4‐di‐tert‐butylphenol or strain GSE09 inhibited pathogen development (mycelial growth, sporulation, and zoospore germination) in I‐plate (a plastic plate containing a center partition). In addition, germinated seeds treated with the compound (1–100 μg ml^?1) or the strain (10²–10¹⁰ cells ml^?1) significantly reduced radicle infection by P. capsici without radicle growth inhibition. Conclusions: These results indicate that colonization of pepper root and rhizosphere by the Fl. johnsoniae strain GSE09, which can form biofilms and produce indolic compounds, biosurfactant, and 2,4‐di‐tert‐butylphenol, might provide effective biocontrol activity against P. capsici. Significance and Impact of the Study: To our knowledge, this is the first study demonstrating that the Fl. johnsoniae strain GSE09, as a potential biocontrol agent, can effectively protect pepper plants against P. capsici infection by colonizing the roots.     

Manganese reduction in the rhizosphere of mycorrhizal and nonmycorrhizal maize

The influence of rhizosphere microorganisms and vesicular-arbuscular (VA) mycorrhiza on manganese (Mn) uptake in maize (Zea mays L. cv. Tau) plants was studied in pot experiments under controlled environmental conditions. The plants were grown for 7 weeks in sterilized calcareous soil in pots having separate compartments for growth of roots and of VA mycorrhizal fungal hyphae. The soil was left either uninoculated (control) or prior to planting was inoculated with rhizosphere microorganisms only (MO-VA) or with rhizosphere microorganisms together with a VA mycorrhizal fungus [Glomus mosseae (Nicol and Gerd.) Gerdemann and Trappe] (MO+VA). Mycorrhiza treatment did not affect shoot dry weight, but root dry weight was slightly inhibited in the MO+VA and MO-VA treatments compared with the uninoculated control. Concentrations of Mn in shoots decreased in the order MO-VA > MO+VA > control. In the rhizosphere soil, the total microbial population was higher in mycorrhizal (MO+VA) than nonmycorrhizal (MO-VA) treatments, but the proportion of Mn-reducing microbial populations was fivefold higher in the nonmycorrhizal treatment, suggesting substantial qualitative changes in rhizosphere microbial populations upon root infection with the mycorrhizal fungi. The most important microbial group taking part in the reduction of Mn was fluorescent Pseudomonas. Mycorrhizal treatment decreased not only the number of Mn reducers but also the release of Mn-solubilizing root exudates, which were collected by percolation from maize plants cultivated in plastic tubes filled with gravel quartz sand. Compared with mycorrhizal plants, the root exudates of nonmycorrhizal plants had two fold higher capacity for reduction of Mn. Therefore, changes in both rhizosphere microbial population and root exudation are probably responsible for the lower acquisition of Mn in mycorrhizal plants.     

Influence of organic soil amendments on Verticillium dahliae and on the microbial composition of the strawberry rhizosphere

A technique was developed for assessing the saprophytic activity of Verticillium dahliae, using a strawberry root extract medium. The germination of conidia and microsclerotia, and mycelial growth in soil, was inhibited by the addition of chitin, laminarin, wheat straw and oven-dried green clover as soil amendments. A significant decrease in the number of viable propagules of the pathogen counted from soil, and in disease severity, was obtained with chitin and laminarin. More bacteria and actinomycetes were recorded from the rhizosphere of plants grown in chitin-amended soil than from those in natural soil.     

Inoculation of Paenibacillus illinoisensis alleviates root mortality, activates of lignification-related enzymes, and induction of the isozymes in pepper plants infected by Phytophthora capsici

To investigate the variations of the enzymes responsible for lignification, after inoculation with Phytophthora capsici and/or Paenibacillus illinoisensis KJA-424, in relation to biocontrol of Phytophthora blight in pepper, roots of two-month-old plants were inoculated with P. capsici inoculation (P), and co-inoculation of P. capsici and P. illinoisensis cell cultures (P + A). Root mortality of pepper plants induced by inoculation of P. capsici was completely recovered by co-inoculation with antagonistic KJA-424. At day 7, peroxidase (POD) activity increased by 36.7% in P-treated roots but by 7.1% only in P + A-treated, compared with control. Polyphenol oxidase (PPO) activity increased for 3 days and then drastically decreased in P-treated roots but maintained a constant level in control and P + A-treated. At day 7, PPO activity in P-treated leaves decreased but recovered to the level of control in the P + A-treated. Three major POD isozymes (45, 53, and 114 kDa) were shown in P-treated roots, while two major (53 and 114 kDa) in control and P + A-treated, suggesting that the 45 kDa of POD was actively induced in P-treated roots but not induced in P + A-treated roots. A PPO isozyme of 80 kDa was induced in P-treated roots but not induced by co-treated with KJA-424. In leaves, the POD isozyme of 45 kDa appears to be systemically induced in P-treated only. The PPO isozyme of 80 kDa in leaves was not induced by pathogen challenge but recovered by co-inoculated with P. illinoisensis. All these results suggest that the inoculation of an antagonist, P. illinoisensis alleviates root mortality, activates of lignification-related enzymes and induction of the isozymes in pepper plants infected by P. capsici.     

Pre-plant bacterisation: a strategy for delivery of beneficial endophytic bacteria and production of disease-free plantlets of black pepper (Pipernigrum L.)

Naturally occurring endophytic bacteria from black pepper vines were found to exhibit strong antagonistic activities against Phytophthora capsici and Radopholus similis. In order to deliver these bacterial strains, as well as to produce disease-free plantlets of black pepper, a pre-plant stem and root bacterisation was standardised. Stem bacterisation with endophytic Pseudomonas spp. was found to suppress P. capsici infection (over 90% reduction in lesion length) on cut shoots. Pre-plant root bacterisation with Pseudomonas aeruginosa, Pseudomonas putida and Bacillus megaterium yielded over 60% of plantlets free from P. capsici infection on roots. Curtobacterium luteum and B. megaterium recorded over 70% reduction of nematode population in soil with concomitant production of over 65% of nematode-free plantlets. Besides protecting the plants from the pathogens, the bacteria were also found to enhance the growth of rooted cuttings. The biocontrol potential of the above endophytic bacteria and their exploitation for disease management in the black pepper nursery are discussed.     

Changes in pathogenesis-related proteins in pepper plants with regard to biological control of phytophthora blight with <Emphasis Type="Italic">Paenibacillus illinoisensis</Emphasis>

To evaluate the biocontrol effectiveness of chitinase-producing bacterium, Paenibacillus illinoisensis strain KJA-424 against pathogenic strain of Phytophthora capsici in pepper plants, growth response and kinetics of pathogen related (PR) proteins were estimated after inoculation with P. capsici (P), and with a combination of P. capsici and strain KJA-424 cell culture (P+A). Fresh weight and chlorophyll content in shoots at P+A-treated plants significantly increased by 23.4 and 34.2%, respectively after 7days of inoculation, compared to P-treated plants. Root mortality in P+A-treated plants was significantly reduced compared to P-treated plants. Seven days after inoculation, the activities of -1,3-glucanase, cellulase and chitinase in P-treated roots had decreased by 54.8, 36.5 and 52.8%, respectively, compared to P+A-treated roots, while those in P-treated leaves increased by 22.8, 36.3 and 23.8%, respectively, compared to those in P+A-treated leaves. The activities of -1,3-glucanase, cellulase and chitinase in roots are negatively correlated with root mortality. All these results suggest that the inoculation of an antagonist, P. illinoisensis alleviates root mortality, reduction of PR proteins in roots, and activates of PR proteins in leaves infected by P. capsici.     

Acibenzolar-S-methyl induced resistance to <Emphasis Type="Italic">Phytophthora capsici</Emphasis> in pepper leaves

The leaves of pepper (Capsicum anuum L.) were inoculated with Phytophthora capsici Leonian 3 d after treatment with acibenzolar-S-methylbenzo [1,2,3]thiadiazole-7-carbothioic acid-S-methyl ester (ASM) and resistance to Phytophthora blight disease was investigated. Results showed that P. capsici was significantly inhibited by ASM treatment by up to 45 % in planta. The pepper plants responded to ASM treatments by rapid and transient induction of L-phenylalanine ammonia-lyase (PAL), increase in total phenol content and activities of chitinase and β-1,3-glucanase. No significant increases in enzyme activities were observed in water-treated control plants compared with the ASM-treated plants. Therefore it may be suggested that ASM induces defense-related enzymes, PAL activity, PR proteins and phenol accumulation in ASM-treated plants and contribute to enhance resistance against P. capsici.     

Biocontrol of Late Blight Disease (Phytophthora capsici) of Pepper and the Plant Growth Promotion by Paenibacillus ehimensis KWN38

For field application of a bacterial strain used to control Phythophthora capsici, we will need a biologically and economically efficient carrier medium. The known antagonist Paenibacillus ehimensisKWN38 was grown in a grass medium where it showed high antifungal and lytic enzyme activities. To demonstrate the potential of P. ehimensisKWN38 for biocontrol of late blight disease in pepper, pot trials were conducted by treating the 1‐month‐old plants with water (W), a selected grass medium (G3), G plus P. ehimensisKWN38 inoculation (G3P) or synthetic fungicide (F). The shoot dry weight in G3P was higher than that in W and F treatments at 15 days after zoospore infection (DZI). The root dry weight in G3P was also higher than that in W. The root mortality of G3 and W increased over 58 and 80% at 15 DZI, and some plants in those treatments wilted due to the failure of root physiology. The plants in G3P and F survived well because of their better root health conditions. Soil cellulase activity of G3P was consistently higher than that of W and F at earlier observation times (0, 2 and 6 DZI). The root β‐1,3‐glucanase activity of G3P promptly increased to maximum shortly after zoospore infection and reached the maximum value of 51.12 unit g^?1 of fresh weight at 2 DZI. All these results indicate that inoculation of P. ehimensisKWN38 to the root zone of potted pepper plants increases plant growth, root and soil enzyme activities and alleviates the root death caused by infection with P. capsici zoospores.     

EM复合菌对新疆色素辣椒生长及根际细菌群落的影响

【背景】Effective microorganisms （EM）复合菌在我国农业种植上的应用越来越广泛,但对色素辣椒的促生作用与根际细菌群落结构的影响未见报道。【目的】评估EM复合菌对新疆色素辣椒的促生长作用,并分析其对色素辣椒根际细菌群落组成的影响。【方法】通过随水灌溉方式将EM复合菌接种到色素辣椒根部,在收获期测定辣椒生长指标、土壤养分和酶活活性,明确EM复合菌对辣椒生长和土壤质量的影响;利用16S rRNA基因高通量测序技术测定EM复合菌对辣椒根系细菌群落组成和结构的影响。【结果】与对照组相比,EM复合菌的施用使辣椒株高、鲜重、单个果重和单株结果数分别提高23.89%、85.41%、42.31%和46.04%;土壤中碱解氮和速效磷含量分别提高5.83%和13.39%,土壤中脲酶、蔗糖酶和过氧化物酶的活性分别提高11.47%、9.42%和21.43%;施用EM复合菌显著改变辣椒根际微生物群落的α多样性和β多样性,提高有益菌群变形菌门（Proteobacteria）、酸杆菌门（Acidobacteria）、芽单胞菌门（Gemmatimonadetes）、放线菌门（Actinobacteria）和厚壁菌门（Firmicutes）的相对丰度,其中变形菌门黄单胞菌科（Xanthomonadaceae）的相对丰度增加119.32%;在属的水平上,施用EM复合菌显著增加了藤黄色杆菌属（Luteitalea）、藤黄单胞菌属（Luteimonas）、鞘脂单胞菌属（Sphingobacterium）和盐单胞菌属（Halomonas）的相对丰度,尤其是藤黄单胞菌属的丰度提高244.17%,同时显著降低黄杆菌属（Flavobacterium）的相对丰度。此外,与土壤理化指标呈正相关的微生物菌群相对丰度也显著升高。【结论】EM复合菌能够通过提高土壤营养成分与酶活活性,调控根系微生物群落结构,富集大量在盐碱地生存能力较强的有益菌群,进而起到促进色素辣椒生长的功效。     

Effects of a vesicular-arbuscular mycorrhizal fungus and other soil microorganisms on growth,mineral nutrient acquisition and root exudation of soil-grown maize plants

Maize (Zea mays L. cv. Alize) plants were grown in a calcareous soil in pots divided by 30-m nylon nets into three compartments, the central one for root growth and the outer ones for hyphal growth. Sterle soil was inoculated with either (1) rhizosphere microorganisms other than vesicular-arbuscular mycorrhizal (VAM) fungi, (2) rhizosphere microorganisms together with a VAM fungus [Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappel], or (3) with a gamma-irradiated inoculum as control. Plants were grown under controlled-climate conditions and harvested after 3 or 6 weeks. VAM plants had higher shootroot ratios than non-VAM plants. After 6 weeks, the concentrations of P, Zn and Cu in roots and shoots had significantly increased with VAM colonization, whereas Mn concentrations had significantly decreased. Root exudates were collected on agar sheets placed on the interface between root and hyphal compartments. Six-week-old VAM and non-VAM plants had similar root exudate compositions of 72–73% reducing sugars, 17–18% phenolics, 7% organic acids and 3% amino acids. In another experiment in which root exudates were collected on agar sheets with or without antibiotics, the amounts of amino acids and carbohydrates recovered were similar in VAM and non-VAM plants. However, threeto sixfold higher amounts of carbohydrates, amino acids and phenolics were recovered when antibiotics were added to the agar sheets. Thus, the high microbial activity in the rhizosphere and on the rhizoplane limits the exudates recovered from roots.     

Effect of Medicago sativa L. and compost on organic and inorganic pollutant removal from a mixed contaminated soil and risk assessment using ecotoxicological tests

Several Gentle Remediation Options (GRO), e.g., plant-based options (phytoremediation), singly and combined with soil amendments, can be simultaneously efficient for degrading organic pollutants and either stabilizing or extracting trace elements (TEs). Here, a 5-month greenhouse trial was performed to test the efficiency of Medicago sativa L., singly and combined with a compost addition (30% w/w), to treat soils contaminated by petroleum hydrocarbons (PHC), Co and Pb collected at an auto scrap yard. After 5 months, total soil Pb significantly decreased in the compost-amended soil planted with M. sativa, but not total soil Co. Compost incorporation into the soil promoted PHC degradation, M. sativa growth and survival, and shoot Pb concentrations [3.8 mg kg^?1 dry weight (DW)]. Residual risk assessment after the phytoremediation trial showed a positive effect of compost amendment on plant growth and earthworm development. The O₂ uptake by soil microorganisms was lower in the compost-amended soil, suggesting a decrease in microbial activity. This study underlined the benefits of the phytoremediation option based on M. sativa cultivation and compost amendment for remediating PHC- and Pb-contaminated soils.