植物促生菌接种对中国芥蓝根际固有微生物群落结构的影响

Plant growth-promoting rhizobacteria (PGPR) have been widely recognized as an important agent,especially as a biofertilizer,in agricultural systems.The objectives of this study were to select efective PGPR for Chinese kale (Brassica oleracea var.alboglabra) cultivation and to investigate the efect of their inoculation on indigenous microbial community structure.The Bacillus sp.SUT1 and Pseudomonas sp.SUT19 were selected for determining the efficiency in promoting Chinese kale growth in both pot and field experiments.In the field experiment,PGPR amended with compost gave the highest yields among all treatments.The Chinese kale growth promotion may be directly afected by PGPR inoculation.The changes of microbial community structure in the rhizosphere of Chinese kale following PGPR inoculation were examined by denaturing gradient gel electrophoresis (DGGE) and principal coordinate analysis.The DGGE fingerprints of 16S rDNA amplified from total community DNA in the rhizosphere confirmed that our isolates were established in the rhizosphere throughout this study.The microbial community structures were slightly diferent among all the treatments,and the major changes depended on stages of plant growth.DNA sequencing of excised DGGE bands showed that the dominant species in microbial community structure in the rhizosphere were not mainly interfered by PGPR,but strongly influenced by plant development.The microbial diversity as revealed by diversity indices was not diferent between the PGPR-inoculated and uninoculated treatments.In addition,the rhizosphere soil had more influence on eubacterial diversity,whereas it did not afect archaebacterial and fungal diversities.     

稻秸杆覆盖对小麦根围拮抗细菌组成和小麦纹枯病发生的影响

To assess the effect of rice straw mulching on changes of antagonistic bacteria and the incidence of wheat sharp eyespot, a multi-year field study was performed to compare unmulched plots and the plots mulched with rice straw for two or three years. Bacterial and fungal populations were evaluated in the cultures prepared from the wheat rhizosphere and bulk soils. Rice straw mulching increased the number of pseudomonas colony forming units in wheat rhizosphere and bulk soils. The proportion of total bacteria that were fluorescent pseudomonads was higher in mulched than in unmulched soil. Bacterial isolates antagonistic to Rhizoctonia cerealis were identified using an inhibition zone test. A series of these isolates were typed by partial sequencing of the 16S rRNA gene. Pseudomonads had higher antagonistic activity against R. cerealis than other species, and more than 80% of rhizosphere fluorescent pseudomonads were antagonistic to R. cerealis. The disease indices were lower in the mulched plots than in the unmulched control. These results suggest that rice straw mulching in a rice-wheat rotation increases the number of fluorescent pseudomonads. Additionally, these fluorescent pseudomonads may contribute to the control of wheat sharp eyespot.     

Soil organic carbon and phosphorus availability regulate abundance of culturable phosphate-solubilizing bacteria in paddy fields

Low availability of phosphorus(P) is a major constraint for optimal crop production, as P is mostly present in its insoluble form in soil. Therefore,phosphate-solubilizing bacteria(PSB) from paddy field soils of the Indo-Gangetic Plain, India were isolated, and their abundance was attempted to be correlated with the physicochemical characteristics of the soils. Ninety-four PSB were isolated on Pikovskaya's agar medium, and quantitative phosphate solubilization was evaluated using NBRIP medium. The isolates solubilized P up to a concentration of 1 006 μg mL~(-1) from tricalcium phosphate with the secretion of organic acids. These isolates were identified by 16 S rRNA gene sequence comparison, and they belonged to Gammaproteobacteria(56 isolates),Firmicutes(28 isolates), Actinobacteria(8 isolates), and Alphaproteobacteria(2 isolates). Phylogenetic analysis confirmed the identification by clustering the isolates in the clade of the respective reference organisms. The correlation analysis between PSB abundance and physicochemical characteristics revealed that the PSB population increased with increasing levels of soil organic carbon, insoluble P, K~+, and Mg~(2+). The promising PSB explored in this study can be further evaluated for their biofertilizer potential in the field and for their use as potent bio-inoculants.     

外源铬对小白菜(Brassica chinensis L.)根际土壤微生物生物量和磷脂脂肪酸的剖面分布特征的影响

The effects of root activity on microbial response to cadmium （Cd） loading in the rhizosphere are not well understood. A pot experiment in greenhouse was conducted to investigate the effects of low Cd loading and root activity on microbial biomass and community structure in the rhizosphere of pakchoi （Brassica chinensis L.） on silty clay loam and silt loamy soil. Cd was added into soil as Cd（NO3）2 to reach concentrations ranging from 0.00 to 7.00 mg kg-1. The microbial biomass carbon （MBC） and community structure were affected by Cd concentration, root activity, and soil type. Lower Cd loading rates （〈 1.00 mg kg-1） stimulated the growth of pakchoi and microorganisms, but higher Cd concentrations inhibited the growth of microorganisms. The content of phospholipid fatty acids （PLFAs） was sensitive to increased Cd levels. MBC was linearly correlated with the total PLFAs. The content of general PLFAs in the fungi was positively correlated with the available Cd in the soil, whereas those in the bacteria and actinomycetes were negatively correlated with the available Cd in the soil. These results indicated that fungi were more resistant to Cd stress than bacteria or actinomycetes, and the latter was the most sensitive to Cd stress. Microbial biomass was more abundant in the rhizosphere than in the bulk soil. Root activity enhanced the growth of microorganisms and stabilized the microbial community structure in the rhizosphere. PLFA analysis was proven to be sensitive in detecting changes in the soil microbial community in response to Cd stress and root activity.     

水稻根际和周围土壤中微生物功能基因丰度与碳、氮状况及其通量之间的关系

Rapid nitrogen（N） transformations and losses occur in the rice rhizosphere through root uptake and microbial activities. However,the relationships between rice roots and rhizosphere microbes for N utilization are still unclear. We analyzed different N forms（NH＋4,NO-3, and dissolved organic N）, microbial biomass N and C, dissolved organic C, CH4 and N2O emissions, and abundance of microbial functional genes in both rhizosphere and bulk soils after 37-d rice growth in a greenhouse pot experiment. Results showed that the dissolved organic C was significantly higher in the rhizosphere soil than in the non-rhizosphere bulk soil, but microbial biomass C showed no significant difference. The concentrations of NH＋4, dissolved organic N, and microbial biomass N in the rhizosphere soil were significantly lower than those of the bulk soil, whereas NO-3in the rhizosphere soil was comparable to that in the bulk soil. The CH4 and N2O fluxes from the rhizosphere soil were much higher than those from the bulk soil. Real-time polymerase chain reaction analysis showed that the abundance of seven selected genes, bacterial and archaeal 16 S rRNA genes, amoA genes of ammonia-oxidizing archaea and ammonia-oxidizing bacteria, nosZ gene, mcrA gene, and pmoA gene, was lower in the rhizosphere soil than in the bulk soil, which is contrary to the results of previous studies. The lower concentration of N in the rhizosphere soil indicated that the competition for N in the rhizosphere soil was very strong, thus having a negative effect on the numbers of microbes. We concluded that when N was limiting, the growth of rhizosphere microorganisms depended on their competitive abilities with rice roots for N.     

引种植物江南桤木改变了土著芦苇根际细菌群落的结构和多样性

Alnus trabeculosa, a rhizobia-nodulating tree, was introduced into the Chongxi tidal wetland in the Yangtze River estuary of China to increase the biodiversity of plants and restore tidal wetland functions. However, the effect of the introduced plant on soil bacterial communities and restoration outcomes remains unknown. In this study, the rhizosphere bacterial community structure and diversity were compared between Phragmites australis monospecific community and A. trabeculosa-P. australis mixed communities,aiming to assess whether A. trabeculosa influenced the rhizosphere bacterial communities of P. australis and to investigate whether different taxonomic groups within a soil community may respond similarly to the presence of an introduced exotic plant. Among the14 phylogenetic phyla detected, Proteobacteria and Acidobacteria were the dominant bacterial taxa in the rhizosphere. Phylogenetic analysis of the predominant Proteobacteria showed that the clones from the rhizosphere soils of A. trabeculosa and P. australis in A. trabeculosa-P. australis mixed communities were more diverse than those in the rhizosphere soil of P. australis in P. australis monospecific community. The rhizosphere community in the wetland potentially included active microbial community related to carbon, nitrogen, and sulfur cycling in the Yangtze River estuary. The rhizosphere soil of P. australis in A. trabeculosa-P. australis mixed communities exhibited the highest Shannon diversity index(H′) and Simpson diversity index(1/D)(H = 4.52, 1/D =253). Correspondence analyses revealed that the bacterial community structures were altered after A. trabeculosa was introduced.     

VA菌根对土壤酸度的耐性

The ammonification,nitrifcation and denitrification in maize rhizosphere of alluvial sols were compared with those in the bulk soil after exposure to different kinds of heavy metals.The addition of cadmium at low levels (2 mg kg^-1 and 5 mg kg^-1）could stimulate the ammonification and nitrification in the soils,while inhibition influences were found at high levels of Cd addition (10 mg kg^-1 and 20mg kg^-1），The relationship between microbial activity and cadmium concentration varied with the kind of microorganisms.The nitrifying bacteria were more sensitive to cadmium pollution than the ammonifying bacteria.When Cd( Ⅱ），Cu（Ⅱ）and Cr（Ⅵ）wer compared at the same addition concentration of 20 mg kg^-1 soil,Cd(Ⅱ） was the most effective inhibitor of ammonification and denitrification. among the three investigated heavy metals,and Cr（Ⅵ）had the most strong inhibitory influence on the nitrifying bacteria.The microbial activities in rhizosphere were higher than those in the bulk soil for most of the treatments.Cr(Ⅵ)proved to be the most effective in enhacing the microbial activities in rhizosphere,and this could be caused by the poitive reduction of Cr(Ⅵ）to Cr(Ⅲ）in rhizosphere,and the relatively sufficient existence of organic matter which intensified the adsorption of the metal,It seemed that the rhizosphere had some mitigation effect on heavy metal toxicity.     

中国太湖流域水稻土中的反硝化细菌

This study attempted to determine the characteristics of the communities, the ecological factors, and the denitrifying enzyme activity for denitrifying bacteria found in the paddy soils of the Taihu Lake Basin, China. Samples of the six main soil types of the basin were taken from paddy fields with different fertilities. The total numbers of bacteria and denitrifying bacteria in the high fertility soils were much more than those in low fertility soils, and the number of denitrifying bacteria accounted for 49% to 80% of the total number of bacteria. The O2 content was an important ecological factor that affected denitrification. Of test the strains isolated from the paddy soils in the Taihu Lake Basin, some (e.g., Pseudomonas spp.) grew well under low oxygen partial pressure, while others (e.g., Bacillus spp.) had no strict predilection with O2 content. Another critical ecological factor was the nitrogen concentration. Three selected denitrifying bacteria grew better in aculture medium with 135 instead of 276mg L^-1 nitrogen. At the same time 67% of the test strains were able to reduce NO3^- to NO2^- and 56% had N2O reductase.     

Soil properties and habitats determine the response of bacterial communities to agricultural wastewater irrigation

Increasing temperatures and variability of precipitation events due to climate change will lead in the future to higher irrigation demands in agroecosystems.However,the use of secondary treated wasterwater(TWW)could have consequences for the receiving soil environment and its resident microbial communities.The objective of this study was to characterize the importance of soil properties and habitats to the response of soil bacteria and archaea to irrigation with TWW.Two agricultural soils with contrasting textures(loamy sand or silt loam)and,for each,three variants differing in soil organic carbon and nitrogen,as generated by long-term fertilization,were analyzed.For each of these six soils,prokaryotic communities from two habitats,i.e.,root-free bulk soil and the rhizosphere of developing cucumber plants in the greenhouse,were characterized.Communities were analyzed by the quantity and diversity of their polymerase chain reaction(PCR)-amplified 16S rRNA genes.To account for TWW-associated nutrient effects,potable water(PW)served as a control.Amplicon sequence analysis showed that prokaryotic communities mainly consisted of bacteria(99.8%).Upon irrigation,regardless of the water quality,prokaryotic diversity declined,p H increased,and no bacterial growth was detected in bulk soil.In contrast,the growth of cucumbers was stimulated by TWW,indicating that plants were the main beneficiaries.Moreover,strong responses were seen in the rhizosphere,suggesting an indirect effect of TWW by altered rhizodepositions.The main bacterial responders to TWW were Proteobacteria,Bacteroidetes,Actinobacteria,and Planctomycetes.Changes in bacterial communities due to TWW were less pronounced in all variants of the silt loam,indicating the importance of clay and soil organic carbon for buffering effects of TWW on soil bacterial communities.Hence,soil organic carbon and soil texture are important parameters that need to be considered when applying TWW in agriculture.     

我国不同地区杨树根际磷细菌的分离与鉴定

A few soil microorganisms can convert insoluble forms of phosphorus(P) to an accessible form to increase plant yields.The objectives of this study were to isolate,screen and identify the phosphobacteria,i.e.,phosphate-solubilizing bacteria(PSB) and phosphate-mineralizing bacteria(PMB),in some poplar plantations and to select potential microbial inoculants that could increase the P uptake by plants.Seventy four PSB strains and 138 PMB strains were preliminarily screened from 797 colonies isolated from the rhizosphere soil of poplar by the visible decolorization in the National Botanical Research Institute’s phosphate growth medium(NBRIP) containing a pH indicator of bromophenol and by the clear/halo zone on yeast malt agar plate,respectively.Then,based on the solubilization of Ca 3(PO 4) 2 in liquid NBRIP medium and the mineralization of lecithin in liquid Menkina medium,15 PSB strains and 7 PMB strains were further determined for their abilities to solubilize Ca 3(PO 4) 2 and to mineralize lecithin.The results showed that the 15 PSB strains exhibited inorganic P-solubilizing abilities ranging between 376.62 and 669.56 mg L -1 and the 7 PMB strains exhibited organic P-mineralizing abilities varying from 0.32 to 1.56 mg L -1 three days after incubation.Three of PSB strains with the highest P-solubilizing ability,named JW-JS1,JW-JS2 and JW-SX2,were all preliminarily identified as Pseudomonas fluorescens,while two of PMB strains with the highest P-mineralizing ability,named JYZ-SD1 and JYT-JS2,were preliminarily identified as Bacillus cereus and Bacillus subtilis,respectively.All the efficient PSB and PMB strains should be further studied under both pot culture and field conditions to test their beneficial effects on poplar.     

Improvement of Brassica napus growth under cadmium stress by cadmium-resistant rhizobacteria

This study focuses on the characterization of four bacterial isolates from heavy metal-polluted rhizosphere in order to examine their plant growth promoting (PGP) activity. The PGP activity on the canola (Brassica napus) of the strains which showed cadmium resistance and multiple PGP traits was assessed in the presence and in the absence of Cd²⁺. The strains, Pseudomonas tolaasii ACC23, Pseudomonas fluorescens ACC9, Alcaligenes sp. ZN4 and Mycobacterium sp. ACC14 showed 1-aminocyclopropane-1-carboxylate deaminase (ACCD) activity. They also synthesized ACCD enzyme in vitro when 0.4 mM Cd²⁺ was added to the growth medium. The presence of the metal, however, reduced the ACCD activity in Alcaligenes sp. ZN4 and Mycobacterium sp. ACC14, while it did not affect the ACCD activity of P. tolaasii ACC23 and P. fluorescens ACC9. ACC9 and ACC23 produced indole acetic acid (IAA) and siderophores, while ACC14 produced only IAA. IAA and siderophores were produced more actively under Cd-stress.Root elongation assays conducted on B. napus under gnotobiotic conditions demonstrated increases (from 34% up to 97%) in root elongation of inoculated canola seedlings compared to the control plants. Subsequently, the effect of inoculation with these strains on growth and uptake of Cd²⁺ in roots and shoots of canola was studied in pot experiments using Cd-free and Cd-treated (15 μg Cd²⁺ g^?1 dw) soil. Inoculation with P. tolaasii ACC23, P. fluorescens ACC9 and Mycobacterium sp. ACC14 promoted the growth of plants at concentrations of 0 and 15 μg Cd²⁺ g^?1 soil. The maximum growth was observed in the plants inoculated with P. tolaasii ACC23. The strains did not influence the specific accumulation of cadmium in the root and shoot systems, but all increased the plant biomass and consequently the total cadmium accumulation.The present observations showed that the bacterial strains used in this study protect the plants against the inhibitory effects of cadmium, probably due to the production of IAA, siderophores and ACCD activity.     

Characterization of phosphate solubilizing fluorescent pseudomonads from the rhizosphere of Aloe vera (L.)

Soil phosphorous (P) deficiency is a major constraint to plant production which is overcome by adding inorganic-phosphate as chemical fertilizers. Fluorescent pseudomonads are the diverse group of bacteria able to mobilize sparingly soluble phosphate form. Total three hundred seven fluorescent Pseudomonas isolates were obtained from the Aloe barbadensis (Miller) rhizosphere. These Pseudomonas strains were further evaluated in vitro for their ability to solubilize phosphate and to produce indole acetic acid (IAA), hydrogen cyanide (HCN), siderophore and 1-aminocyclopropane 1- carboxylate (ACC) deaminase. Fifty three (36.8%) isolates produced IAA and 52 (36.1%) isolates produced siderophores whereas 36 (25.0%) and 31 (21.5%) isolates produced HCN and ACC deaminase, respectively. A positive correlation existed between siderophore and ACC deaminase producers. Cluster analysis showed rhizosphere as the major factor influencing the ecological distribution and physiological characterization of phosphate solubilizing bacteria (PSB). Based on partial 16S rRNA gene sequencing PSB were identified as Pseudomonas putida, Pseudomonas sp. and Pseudomonas plecoglossicida with highest phosphate solubilization ability. In conclusion, these phosphate solubilizing fluorescent pseudomonads would help in understanding their role in phosphorus solubilization and identification of potent phosphorus solubilizers from the rhizosphere of commercially grown A. barbadensis.     

Effects of two new siderophore-producing rhizobacteria on growth and iron content of maize and canola plants

Siderophore-producing rhizobacteria beneficially affect plant growth by providing available iron to plants. In this study, bacteria were isolated from the rhizosphere of canola (Brassica napus L.) plants grown in the central fields in Iran, for the presence of siderophore-producing bacteria. A total of 45 distinct isolates were found to produce siderophore using qualitative chrome azurol sulfonate (CAS)-agar assay. Of them, ten isolates, based on the highest halo diameter/colony diameter ratios, were selected to quantify the rate of siderophore production using CAS-liquid assay. A variety of biochemical assays was used to determine the type(s) of siderophores produced by each of the ten isolates. The best isolates, based on production of the highest rates of either hydroxamates or carboxylates, were identified and used in further studies. Based on 16S ribosomal ribonucleic acid (rRNA) sequence analysis and a variety of phenotypic properties, the isolates were identified as Micrococcus yunnanensis YIM 65004 (T) and Stenotrophomonas chelatiphaga LPM-5 (T). We also studied the plant growth-promoting effect of the most promising isolates (YIM 65004 and LPM-5) on canola and maize plants under greenhouse conditions. The results of this study showed that M. yunnanensis and S. chelatiphaga increased gain weight and iron (Fe) content of roots and shoots significantly, in comparison with control, indicating beneficial effects of these rhizobacteria on plant growth and development. This study reports M. yunnanensis and S. chelatiphaga, as new records for Iran. The latter is reported for the first time from plant (canola) rhizosphere. Besides, the ability of both M. yunnanensis and S. chelatiphaga to produce siderophores is documented for the first time.     

Growth stimulation and fruit yield improvement of greenhouse tomato plants by inoculation with Pseudomonas putida or Trichoderma atroviride: Possible role of indole acetic acid (IAA)

Five bacteria (Pseudomonas fluorescens, P. fluorescens subgroup G strain 2, P. marginalis, P. putida subgroup B strain 1 and P. syringae strain 1) and three fungi (Penicillium brevicompactum, P. solitum strain 1 and Trichoderma atroviride) were evaluated to determine their promoting effect on the growth of mature healthy tomato plants grown under hydroponic conditions. P. putida and T. atroviride were shown to improve fruit yields in rockwool and in organic medium. The production or degradation of indole acetic acid (IAA) by the two microorganisms was investigated as possible mechanisms for plant growth stimulation. Both P. putida and T. atroviride were shown to produce IAA. The production of IAA by the two microorganisms was stimulated in vitro by the addition of l-tryptophan, tryptamine and tryptophol (200 μg ml⁻¹) in the culture medium. P. putida and T. atroviride also increased the fresh weight of both the shoot and the roots of tomato seedlings grown in the presence of increasing concentrations of l-tryptophan (up to 0.75 mM). Both microorganisms showed partial degradation of IAA in vitro when grown in a minimal medium with or without sucrose. In addition, the capacity of these microorganisms to reduce the deleterious effect of exogenous IAA was investigated using tomato seedlings. The results showed that the roots of tomato seedlings grown in the presence of increasing concentrations of IAA (0-10 μg ml⁻¹) were significantly longer when seeds were previously treated with P. putida or T. atroviride. The reduction in the detrimental effect of IAA on root elongation could be associated with a reduced ethylene production resulting from a decrease of its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) by microbial degradation of IAA in the rhizosphere and/or by ACC deaminase activity present in both microorganisms.     

Cadmium-tolerant plant growth-promoting bacteria associated with the roots of Indian mustard (Brassica juncea L. Czern.)

Eleven cadmium-tolerant bacterial strains were isolated from the root zone of Indian mustard (Brassica juncea L. Czern.) seedlings grown in Cd-supplemented soils as well as sewage sludge and mining waste highly contaminated with Cd. The bacteria also showed increased tolerance to other metals including Zn, Cu, Ni and Co. The isolated strains included Variovorax paradoxus, Rhodococcus sp. and Flavobacterium sp., and were capable of stimulating root elongation of B. juncea seedlings either in the presence or absence of toxic Cd concentrations. Some of the strains produced indoles or siderophores, but none possessed C₂H₂-reduction activity. All the strains, except Flavobacterium sp. strain 5P-3, contained the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which hydrolyses ACC (the immediate precursor of plant hormone ethylene) to NH₃ and α-ketobutyrate. V. paradoxus utilized ACC as a sole source of N or energy. A positive correlation between the in vitro ACC deaminase activity of the bacteria and their stimulating effect on root elongation suggested that utilization of ACC is an important bacterial trait determining root growth promotion. The isolated bacteria offer promise as inoculants to improve growth of the metal accumulating plant B. juncea in the presence of toxic Cd concentrations and for the development of plant-inoculant systems useful for phytoremediation of polluted soils.     

Growth,P uptake and rhizosphere properties of wheat and canola genotypes in an alkaline soil with low P availability

The aim of the present study was to assess the role of soil type on growth, P uptake and rhizosphere properties of wheat and canola genotypes in an alkaline soil with low P availability. Two wheat (Goldmark and Janz) and two canola genotypes (Drum and Outback) were grown in a calcareous soil (pH 8.5) at two P levels [no P addition (0P) or addition of 200 mg kg⁻¹ P as Ca₃(PO₄)₂ (200P)] and harvested at flowering or maturity. Shoot and root dry weight, root length and shoot P content were greater in the two canola genotypes than in wheat. There were no consistent differences in available P, microbial P and phosphatase activity in the rhizosphere of the different genotypes. Shoot P content was significantly positively correlated with root length, pH and phosphatase activity in the rhizosphere. The microbial community composition, assessed by fatty acid methylester analysis, of the canola genotypes differed strongly from that of the wheat genotypes. The weight percentage bacterial fatty acids, the bacteria/fungi (b/f) ratio and the diversity of fatty acids were greater in the rhizosphere of the canolas than in the rhizosphere of the wheat genotypes. In contrast to the earlier studies in an acidic soil, only small differences in growth and P uptake between the genotypes of one crop were detected in the alkaline soil used here. The results confirmed the importance of root length for P uptake in soils with low P availability and suggest that the rhizosphere microbial community composition may play a role in the better growth of the canola compared to the wheat genotypes.     

Förderung des Wachstums und der Nährstoffaufnahme bei kruziferen Ölund Zwischenfrüchten durch inokulierte Rhizosphärenmikroorganismen

Promotion of plant growth and nutrient uptake of cruciferous oil- and intercrops by inoculated rhizosphere microorganisms The vegetative growth of rape, oil radish and mustard was repeatedly stimulated by isolates of Pseudomonas fluorescens (PsIA12), P. putida (A1 A2), Stenotrophomonas maltophilia (PsIB2, Psl2), Agrobacterium rhizogenes (A1A4) and Rhizobium trifolii (R39) by improved root development and nutrient uptake (N. P. K. Mg) in pot and field experiments on loamy sand and sandy loam at different yield levels. The seed yield of rape could be increased only by 2 isolates (PsIA12, PsIB2). The growth stimulating bacteria produced phytohormones (auxins and partly cytokinins). No direct relations between plant growth stimulation and the potential microbial metabolic activities nitrogenase, nitrate reductase, P-mobilization, pectinase, cellulase could be found. Effective bacteria are in close association with the plants and can survive in the rhizosphere.     

Inoculation of wheat with Azospirillum brasilense and Pseudomonas fluorescens: Impact on the production and culturable rhizosphere microflora

Scientific evidence recognizes that the operation of a terrestrial ecosystem depends on soil microbial activity. Some Azospirillum strains produce plant growth regulators, increase the development of roots, and fix atmospheric nitrogen (N₂). Some Pseudomonas strains are capable of producing cytokinins and solubilizing organic phosphorus. A sustainability analysis requires a detailed knowledge of the interrelationships between the microorganisms added to the system and those present in the soil. This study examines the effect of three commercial inoculants Azospirillum brasilense Az1 and Az2 as well as Pseudomonas fluorescens Pf on biomass production, grain yield and rhizosphere colonization of wheat, combined with two levels of N-addition. Plate counts of rhizosphere soil showed that the inoculation and N-addition did not affect the number of P. fluorescens, whereas it significantly affected the number of Azospirillum. N-addition and inoculation did not change the communities of actinomycetes and bacteria but they changed the number of fungi at the rhizosphere of wheat plants. Community-level physiological profiles of carbon-source utilization of rhizosphere soil microbial communities were changed after inoculation with Az1, Az2 and Pf depending on the phenological stage of the crop. Although no significant responses were observed, in average, PGPB inoculation increased aerial biomass by 12%, root biomass by 40% and grain yield by 16%. These increases represent important earnings for the farmer and they may help to obtain a greater sustainability of the agroecosystems.     

Study the Effects of Siderophore-Producing Bacteria on Zinc and Phosphorous Nutrition of Canola and Maize Plants

Plant growth-promoting rhizobacteria (PGPR) are soil bacteria that are able to colonize rhizosphere and to enhance plant growth by means of a wide variety of mechanisms. In the present study, Myristica yunnanensis and Stenotrophomonas chelatiphaga strains were recognized as new records in Iran flora. According to the results, these strains significantly affected plants’ zinc and phosphorous contents which could be due to the production of phytosiderophore. Siderophore-producing bacteria increased canola zinc (Zn) content as strategy-I plant, while in maize, it can be said that probably the effect of phytosiderophore produced by plant on increasing root and shoot Zn content was more than siderophore produced by bacteria. These isolates could be used as bio-input for improving the plant productivity as a substitute to chemical fertilizers and also to correct the nutrient deficiencies in canola and maize for sustainable agriculture.     

Seed priming with Pseudomonas putida isolated from rhizosphere triggers innate resistance against Fusarium wilt in tomato through pathogenesis-related protein activation and phenylpropanoid pathway

Pathogenesis-related (PR) proteins are one of the major and preliminary proteins accumulated as a defense against biotic stress. This defense response can be induced by using beneficial rhizobacteria, which has been studied in various host-pathogen interactions. In the present study, eleven Pseudomonas isolates were assessed for their potential to ferment sorbitol, reduce nitrate, and produce mycolytic enzymes, 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, phenazine antibiotics, and N-acyl homoserine lactones (AHLs). All isolates were tested against the host-specific pathogen Fusarium oxysporum MTCC1755 in tomato under greenhouse conditions, and shortlisted isolates were tested for their rhizosphere competence. In-vitro test results showed that the isolates were able to produce mycolytic enzymes, including protease, lipase, chitinase, cellulase, and amylase, and the antibiotic phenazine and were negative for pyoluteorin. All the isolates except two were positive for ACC deaminase production. Greenhouse results showed that the isolates M80, M96, and T109 significantly reduced symptoms of Fusarium wilt. Extended greenhouse tests under autoclaved and unautoclaved soil conditions showed that M80, M96, and T109 were excellent rhizosphere competitors and were identified as Pseudomonas putida. In brief, the defense-specific biochemical variations in the host could describe the improved defense against Fusarium wilt occurring in the primed plants. These three Pseudomonas strains could be used as potential biocontrol agents, along with their rhizosphere competence.