Solubilization of insoluble inorganic phosphates by a novel temperature-, pH-, and salt-tolerant yeast, <Emphasis Type="Italic">Rhodotorula</Emphasis> sp<Emphasis Type="Italic">.</Emphasis> PS4, isolated from seabuckthorn rhizosphere,growing in cold desert of Ladakh,India

The study was aimed to develop biofertilizer solubilizing inorganic phosphates for region experiencing temperature, pH and salt stressed conditions. A yeast strain PS4, which was temperature-, pH- and salt-tolerant and capable of solubilizing insoluble inorganic phosphate was isolated from rhizosphere of seabuckthorn (Hippophae rhamnoides L.), growing in the Indian Trans-Himalaya. Based on morphological, biochemical, whole cell FAME analysis and molecular characterization, strain PS4 was identified as Rhodotorula sp. The soluble phosphate production under optimal conditions at pH 7 and 30°C was 278.3 mg l⁻¹. Strain PS4 showed ability to solubilize insoluble phosphate under different stress conditions viz. 5–40°C temperature, 1–5% salt concentration and 3–11 pH range. Soluble phosphate production from Ca₃(PO₄)₂ under combined stress conditions at extreme values of temperature, pH and salt concentration showed 81.6–83.2% reduction as compare to optimal conditions after 5 days incubation. The strain solubilize Ca₃(PO₄)₂ to a great extent than FePO₄ and AlPO₄. The solubilization of insoluble phosphate was associated with drop in pH of the culture media. Inoculation of tomato seedling with the strain increased fruit yield, roots and shoot length. Rhodotorula sp. PS4 with phosphate-solubilizing ability under stress conditions appeared to be attractive for exploring their plant growth-promoting activity towards the development of microbial inoculants in stressed region.     

Plant growth promotion traits of phosphobacteria isolated from Puna,Argentina

The ability of soil microorganisms to solubilize phosphate is an important trait of plant growth-promoting bacteria leading to increased yields and smaller use of fertilizers. This study presents the isolation and characterization of phosphobacteria from Puna, northwestern Argentina and the ability to produce phosphate solubilization, alkaline phosphatase, siderophores, and indole acetic acid. The P-solubilizing activity was coincidental with a decrease in pH values of the tricalcium phosphate medium for all strains after 72 h of incubation. All the isolates showed the capacity to produce siderophores and indoles. Identification by 16S rDNA sequencing and phylogenetic analysis revealed that these strains belong to the genera Pantoea, Serratia, Enterobacter, and Pseudomonas. These isolates appear attractive for exploring their plant growth-promoting activity and potential field application.     

Growth Enhancement of Chickpea in Saline Soils Using Plant Growth-Promoting Rhizobacteria

Bacterial isolates with the ability to tolerate salinity and plant growth-promoting features were isolated from the saline areas of Gujarat, India, that is, Bhavnagar and Khambat. A total of 176 strains of rhizobacteria were isolated out of which 62 bacterial strains were able to tolerate 1 M NaCl. These were then further studied for their potential plant growth-promoting rhizobacteria characteristics like phosphate solubilization, siderophore production, and IAA production. Twenty-eight isolates of the 62 strains showed good tricalcium phosphate solubilization in solid medium in the range of 9–22 mm and 15 isolates showed good phosphate solubilization in liquid medium in the range of 9–45 μg/ml. Siderophore production was checked in all 15 isolates, and 13 were screened out that produced the hydroxamate type of siderophore in the range of 11–50 mM. Among the 13 isolates, 10 were able to produce indole acetic acid in the range of 10–26 μg/ml after 72 h of incubation. Pot trials were carried out on chickpea under 300 mM NaCl stress using the best five isolates. Plants inoculated with MSC1 or MSC4 isolates showed an increase in the parameters that evaluate plant growth when compared to uninoculated controls. Strains MSC1 and MSC4 were identified as Pseudomonas putida and Pseudomonas pseudoalcaligens, respectively, according to sequence analysis of the 16S rRNA gene.     

一株连香树根际促生细菌LWK2的分离鉴定及其全基因组序列分析

【背景】植物根际促生细菌是一类位于植物根际并能对植物生长产生促进作用的有益菌,在微生物肥料领域具有重要的应用价值。【目的】对濒危植物连香树根际的植物根际促生细菌进行分离筛选和连香树接种效应评价,挑选对连香树生长促进作用最为显著的菌种进行促生特性分析、菌种鉴定及全基因组序列测定与促生相关基因分析。【方法】利用相应筛选培养基对连香树根际土壤中解有机磷、溶无机磷和解钾细菌进行分离筛选,通过根际接种验证各菌株对连香树实生苗的促生能力。从中选取促生作用最为显著的细菌,进行解钾能力、产吲哚乙酸(indole-3-acetic acid,IAA)和1-氨基环丙烷-1-羧酸(1-aminocyclopropane-1-carboxylate,ACC)脱氨酶能力测定。利用菌体形态观察、16S rRNA基因序列分析及全基因组序列的平均核苷酸一致性比对进行菌种鉴定。最后利用基因组功能注释和比较基因组学分析对该菌株中的植物促生及重金属抗性相关基因进行解析。【结果】从连香树根际土壤中共筛选得到3株解有机磷细菌、2株溶无机磷细菌和2株解钾细菌,其中解钾细菌LWK2对连香树实生苗的生长促进作用最为显著。该菌株能够产...     

Isolation and characterization of plant growth-promoting rhizobacteria from wheat roots by wheat germ agglutinin labeled with fluorescein isothiocyanate

Thirty-two isolates were obtained from wheat rhizosphere by wheat germ agglutinin (WGA) labeled with fluorescein isothiocyanate (FITC). Most isolates were able to produce indole acetic acid (65.6%) and siderophores (59.3%), as well as exhibited phosphate solubilization (96.8%). Fourteen isolates displayed three plant growth-promoting traits. Among these strains, two phosphate-dissolving ones, WS29 and WS31, were evaluated for their beneficial effects on the early growth of wheat (Triticum aestivum Wan33). Strain WS29 and WS31 significantly promoted the development of lateral roots by 34.9% and 27.6%, as well as increased the root dry weight by 25.0% and 25.6%, respectively, compared to those of the control. Based on 16S rRNA gene sequence comparisons and phylogenetic positions, both isolates were determined to belong to the genus Bacillus. The proportion of isolates showing the properties of plant growth-promoting rhizobacteria (PGPR) was higher than in previous reports. The efficiency of the isolation of PGPR strains was also greatly increased by WGA labeled with FITC. The present study indicated that WGA could be used as an effective tool for isolating PGPR strains with high affinity to host plants from wheat roots. The proposed approach could facilitate research on biofertilizers or biocontrol agents.     

Fungus fruit body lytic enzyme from a myxomycete,Badhamia utricularis

Chitinase,β-1,3-glucanase, cellulase, xylanase and protease activity were detected in a crude enzyme preparation obtained from a slime mold (Badhamia utricularis) which was grown on autoclaved mycelia ofPholiota nameko in a petri dish. The optimal pH of the enzyme preparation for lytic activity against fruit bodies ofLentinus edodes was 4.0, and those ofβ-1,3-glucanase and cellulase were the same. On the other hand, chitinase and protease showed optimal activity at pH 5.0 and 8.0, respectively. The lytic activity was stable below 40°C but completely inactivated at 70°C, and was most stable at pH 5.0. The studies of the optimal pH, thermal stability, and pH stability, and isoelectric focusing analysis of the enzyme preparation suggest that chitinase,β-1,3-glucanase and cellulase activities may be responsible for lysis of fruit bodies of some mushrooms. The crude enzyme preparation from the slime mold lysed fruit bodies of several mushrooms more efficiently than did commercial lytic enzymes preparations (Driselase and Usukizyme).     

Effect of Abiotic Stress on Phosphate Solubilization by Biocontrol Fungus <Emphasis Type="Italic">Trichoderma</Emphasis> sp.

This study was undertaken to explore the role of Trichoderma sp. in phosphate (P) solubilization and antagonism against fungal phytopathogens. All fungal isolates (SE₆, KT₆, KT₂₈, and BRT₁₁) and a standard culture of T. harzianum (Th-std) were able to antagonize two fungal phytopathogens (Sclerotium rolfsii and Rhizoctonia solani) of chickpea (Cicer arietinum L.) wilt complex. Transmission electron microscopic studies (TEM) further confirmed ultra-cytological changes in the sclerotia of S. rolfsii parasitized by Trichoderma sp. All fungal cultures exhibited production of NH₃ and siderophore, but only BRT₁₁, SE₆, and Th-std could produce HCN. Among all the cultures tested, isolate KT₆ was found to be most effective for solubilization of ferric phosphate releasing 398.4 μg ml⁻¹ phosphate while isolates BRT₁₁ and SE₆ showed more potential for tricalcium phosphate (TCP) solubilization releasing 449.05 and 412.64 μg ml⁻¹ phosphate, respectively, in their culture filtrates. Part of this study focused on the influence of abiotic stress conditions such as pH, temperature, and heavy metal (cadmium) on phosphate (TCP) solubilizing efficiency. Two selected cultures KT₆ and T. harzianum retained their P solubilizing potential at varying concentrations of cadmium (0–1000 μg ml⁻¹). Isolate KT₆ and standard culture of T. harzianum released 278.4 and 287.6 μg ml⁻¹ phosphate, respectively, at 1000 μg ml⁻¹cadmium. Maximum solubilization of TCP was obtained at alkaline pH and at 28°C temperature. Isolate BRT₁₁ was found most alkalo-tolerant releasing 448.0 μg ml⁻¹ phosphate at pH 9.     

Xanthomonas campestris, a novel stress tolerant, phosphate-solubilizing bacterial strain from saline–alkali soils

A total of 198 bacterial strains were isolated from various niches of saline–alkali soils, out of which 85 strains were able to solubilize phosphate on plates at 30 °C. The strain RMLU-26, identified as Xanthomonas campestris, was the most efficient with its ability to solubilize P, subjected to N-methyl-N′-nitro-N-nitrosoguanidine (NTG) for development of mutants. The P solubilizing ability of X. campestris is reported for the first time. The wild type and mutant strains of X. campestris revealed a differential response to various stress factors (high pH, temperature, and salt concentration). The mutant strain revealed maximum P solubilization (67.1%) at 30 °C and pH 8.0 while the wild type strain showed maximum solubilization (41.9%) at 35 °C and pH 7.0. Percent P₂O₅ solubilization by both strains revealed a steep decline in tricalcium phosphate solubilization with an increase in NaCl concentration from 0.5 to 10% along with a concomitant drop in pH of the medium from 8.0 to 4.5 in wild type and 4.0 in mutant strain. However, a 1.5- to 2-fold increase in ‘P’ solubilization was observed in the mutant strain when compared to the wild type strain in the presence of NaCl. The overall improved tolerance of the strains to alkalinity and salinity could be due to accumulation and/or secretion of specific solute (xanthan).     

Phosphate solubilization by <Emphasis Type="Italic">Rhizobium</Emphasis> strains

Forty-six Rhizobium isolates from legume root and stem nodules were examined for their phosphate-solubilizing ability on Pikovskaya’s agar medium. Rhizobium isolates from root nodules of Cassia absus, Vigna trilobata and three strains from Sesbania sesban showed zone of tricalcium phosphate (TCP) solubilization. The isolate from C. absus showed maximum solubilization (620 μg/ml) after 12 d of incubation, while the Rhizobium sp. strain 26 (from S. sesban) showed the least amount (150 μg/ml) of phosphate solubilization. Among the carbon sources tested for their ability to solubilize TCP, maximum solubilization (620 μg/ml) was observed in glucose by Rhizobium isolate from C. absus. Phosphate solubilization increased with increase in glucose concentration steeply up to 2% and slowly above this concentration in four isolates. Among the nitrogen sources tested, maximum solubilization (620 μg/ml) was observed in ammonium sulphate by Rhizobium isolate from C. absus.     

Purification and characterization of a novel endo-β-1,4-glucanase from the thermoacidophilic <Emphasis Type="Italic">Aspergillus terreus</Emphasis>

An endo-β-1,4-glucanase from a thermoacidophilic fungus, Aspergillus terreus M11, was purified 18-fold with 14% yield and a specific activity of 67 U mg⁻¹ protein. The optimal pH was 2 and the cellulase was stable from pH 2 to 5. The cellulase had a temperature optimum of 60°C measured over 30 min and retained more than 60% of its activity after heating at 70°C for 1 h. The molecular mass of the cellulase was about 25 kDa. Its activity was inhibited by 77% by Hg²⁺ (2 mM) and by 59% by Cu²⁺ (2 mM).     

Bioprospecting in potato fields in the Central Andean Highlands: Screening of rhizobacteria for plant growth-promoting properties

The Central Andean Highlands are the center of origin of the potato plant (Solanum tuberosum). Ages of mutualism between potato plants and soil bacteria in this region support the hypothesis that Andean soils harbor interesting plant growth-promoting (PGP) bacteria. Therefore, the aim of this study was to isolate rhizobacteria from Andean ecosystems, and to identify those with PGP properties. A total of 585 bacterial isolates were obtained from eight potato fields in the Andes and they were screened for suppression of Phytophthora infestans and Rhizoctonia solani. Antagonistic mechanisms were determined and antagonistic isolates were further tested for phosphate solubilization, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, and production of NH₃- and indole-3-acetic acid (IAA). PGP was studied in healthy and R. solani diseased plantlets under growth room conditions. Performance was compared to the commercial strain B. subtilis FZB24^® WG. Isolates were dereplicated with matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS), and identified with 16S rRNA gene sequencing and multi locus sequence analysis (MLSA). A total of 10% of the isolates were effective antagonists, of which many were able to solubilize phosphate, and produce IAA, ACC deaminase, NH₃ and hydrogen cyanide (HCN). During growth room experiments, 23 antagonistic isolates were associated with plant growth-promotion and/or disease suppression. Ten isolates had a statistically significant impact on test parameters compared to the uninoculated control. Three isolates significantly promoted plant growth in healthy plantlets compared to the commercial strain, and seven isolates outperformed the commercial strain in in vitro R. solani diseased plantlets.     

Phosphate solubilization activity of rhizobia native to Iranian soils

Agricultural soils in Iran are predominantly calcareous with very low plant available phosphorus (P) content. In addition to their beneficial N₂-fixing activity with legumes, rhizobia can improve plant P nutrition by mobilizing inorganic and organic P. Isolates from different cross-inoculation groups of rhizobia, obtained from Iranian soils were tested for their ability to dissolve inorganic and organic phosphate. From a total of 446 rhizobial isolates tested for P solubilization by the formation of visible dissolution halos on agar plates, 198 (44%) and 341(76%) of the isolates, solubilized Ca₃(PO₄)₂ (TCP) and inositol hexaphosphate (IHP), respectively. In the liquid Sperber TCP medium, phosphate-solubilizing bacteria (Bacillus sp. and Pseudomonas fluorescens) used as positive controls released an average of 268.6 mg L⁻¹ of P after 360 h incubation. This amount was significantly (P < 0.05) higher than those observed with all rhizobia tested. The group of Rhizobium leguminosarum bv. viciae mobilized in liquid TCP Sperber medium significantly (P < 0.05) more P (197.1 mg L⁻¹ in 360 h) than other rhizobia tested,. This group also showed the highest dissolution halo on the TCP solid Sperber medium. The release of soluble P was significantly correlated with a drop in the pH of the culture filtrates indicating the importance of acid production in the mobilization process. None of the 70 bradyrhizobial isolates tested was able to solubilize TCP. These results indicate that many rhizobia isolated from soils in Iran are able to mobilize P from organic and inorganic sources and this beneficial effect should be tested with crops grown in Iran.     

Stabilization of dextransucrase from <Emphasis Type="Italic">Leuconostoc mesenteroides</Emphasis> NRRL B-640

Phosphate solubilizing yeast (PSY) were isolated from rhizosphere, non-rhizosphere and fruits from Bhavnagar district. The potential of 25 yeasts were analyzed on the basis of phosphate solubilizing zone to growth on solid medium denoted as solubilization index (SI) which ranged from 1.10 to 1.50. Among 25 yeast isolates, 6 yeast belonging to genus Saccharomyces (2), Hansenula, Klockera, Rhodotorula and Debaryomyces exhibited highest SI (1.33–1.50) were further examined for in vitro tricalcium phosphate (TCP) and low grade rock phosphate (RP) solubilization. TCP proved superior to RP with all the yeasts. Within low grade RPs tested, except isolate Y5, all isolates showed maximum solubilization with Hirapur RP (HRP) ranging from 7.24 to 19.30 mg% P₂O₅. Among six PSY screened, Debaryomyces hansenii showing maximal HRP solubilization was chosen for further physiological studies. Maximum HRP solubilization was expressed in following condition: pH optima 7.0, temperature optima 28°C and optimal period of incubation were 15 days. Acidic pH of the spent media was a constant feature in all the cases. No correlation could be established between final acidity produced by yeasts and the quantity of phosphate liberated.     

Plant growth-promoting traits of epiphytic and endophytic yeasts isolated from rice and sugar cane leaves in Thailand

A total of 1035 yeast isolates, obtained from rice and sugar cane leaves, were screened primarily for indole-3-acetic acid (IAA) production. Thirteen isolates were selected, due to their IAA production ranging from 1.2 to 29.3 mg g⁻¹ DCW. These isolates were investigated for their capabilities of calcium phosphate and ZnO₃ solubilisation, and also for production of NH₃, polyamine, and siderophore. Their 1-aminocyclopropane-1-carboxylate (ACC) deaminase, catalase and fungal cell wall-degrading enzyme activities were assessed. Their antagonism against rice fungal pathogens was also evaluated. Strain identification, based on molecular taxonomy, of the thirteen yeast isolates revealed that four yeast species – i.e. Hannaella sinensis (DMKU-RP45), Cryptococcus flavus (DMKU-RE12, DMKU-RE19, DMKU-RE67, and DMKU-RP128), Rhodosporidium paludigenum (DMKU-RP301) and Torulaspora globosa (DMKU-RP31) – were capable of high IAA production. Catalase activity was detected in all yeast strains tested. The yeast R. paludigenum DMKU-RP301 was the best IAA producer, yielding 29.3 mg g⁻¹ DCW, and showed the ability to produce NH₃ and siderophore. Different levels of IAA production (7.2–9.7 mg g⁻¹ DCW) were found in four strains of C. flavus DMKU-RE12, DMKU-RE19, and DMKU-RE67, which are rice leaf endophytes, and strain DMKU-RP128, which is a rice leaf epiphyte. NH₃ production and carboxymethyl cellulase (CMCase) activity was also detected in these four strains. Antagonism to fungal plant pathogens and production of antifungal volatile compounds were exhibited in T. globosa DMKU-RP31, as well as a moderate level of IAA production (4.9 mg g⁻¹ DCW). The overall results indicated that T. globosa DMKU-RP31 might be used in two ways: enhancing plant growth and acting as a biocontrol agent. In addition, four C. flavus were also found to be strains of interest for optimal IAA production.     

Isolation and performance evaluation of halotolerant phosphate solubilizing bacteria from the rhizospheric soils of historic Dagong Brine Well in China

The use of halotolerant phosphate solubilizing bacteria as inoculants to convert insoluble phosphorus of salt-affected soils to an accessible form is a promising strategy to improve the phosphorus ingestion of plants in salt-affected agriculture. A total of four aerobic isolates with biggest clear halos on the 10% NaCl NBRIP medium plate containing tricalcium phosphate were isolated from the rhizospheric soils of native plants growing on the wall of Dagong Ancinet Brine Well, located in Sichuan of China. And these four isolates were classified to the same strain, named QW10-11, and closely related to Bacillus megatherium var. phosphaticum DSM 3228 and B. megaterium ATCC 14581 according to their phenotype and 16S rRNA. However, the Molecular evolutionary evidences of 16S-23S rRNA ISR further suggested that QW10-11, DSM 3228 and ATCC 14581 have respectively fall into the different sub-divisions in intra specific phylogeny. Strain QW10-11 has significantly better ability of tricalcium phosphate solubilization than that of lecithin solubilization. When it grows under pH 4.8–8.0, 24–33°C and 5–10% NaCl, it can exhibit the higher values of solubilized tricalcium phosphate between 59.3 and 71.4 μg ml⁻¹. Furthermore, its tricalcium phosphate solubilizing activity was associated with the release of organic acids. Taken together, our results indicted that QW10-11 from the rhizospheric soils of halobiot of Dagong Ancinet Brine Well is attractive as efficient phosphate solubilizing candidates in the salt-affected agriculture.     

Endogenous production of endo-β-1,4-glucanase by decapod crustaceans

The potential ability to produce cellulase enzymes endogenously was examined in decapods crustaceans including the herbivorous gecarcinid land crabs Gecarcoidea natalis and Discoplax hirtipes, the amphibious freshwater crab Austrothelphusa transversa, the terrestrial hermit crab, Coenobita variabilis the parastacid crayfish Euastacus, and the crayfish Cherax destructor. The midgut gland of both G. natalis and D. hirtipes contained substantial total cellulase activities and activities of the cellulase enzymes endo-β-1,4-glucanase and β-glucosidase. With the exception of total cellulase and β-glucosidase from D. hirtipes, the enzyme activities within the midgut gland were higher than those within the digestive juice. Hence, the enzyme activities appear to reside predominantly within midgut gland, providing indirect evidence for endogenous synthesis of cellulase enzymes by this tissue. A 900 bp cDNA fragment encoding a portion of the endo-β-1,4-glucanase amino acid sequence was amplified by RT-PCR using RNA isolated from the midgut gland of C. destructor, Euastacus, A. transversa and C. variabilis. This provided direct evidence for the endogenous production of endo-β-1,4-glucanase. The 900 bp fragment was also amplified from genomic DNA isolated from the skeletal muscle of G. natalis and D. hirtipes, clearly indicating that the gene encoding endo-β-1,4-glucanase is also present in these two species. As this group of evolutionary diverse crustacean species possesses and expresses the endo-β-1,4-glucanase gene it is likely that decapod crustaceans generally produce cellulases endogenously and are able to digest cellulose.     

Cellulase production and activity in a species ofCladosporium

ACladosporium species produced large amounts of cellulase enzyme components when grown in shake-culture with medium containing carboxymethylcellulose. There was significantly less activity when Avicel, filter paper or cotton were used as substrates. KNO₃ was better than NH₄Cl or urea for the production of cellulase. Tween 80 at 0.1% (w/v) increased the production of cellulase by 1.5 to 4.5-fold. All the cellulase components were optimally active in the assay at pH 5.0 and 60°C.     

Enhanced atrazine removal by hydrophyte–bacterium associations and in vitro screening of the isolates for their plant growth-promoting potential

Emergent hydrophytes Acorus calamus, Typha latifolia, and Phragmites karka and epiphytic root bacteria isolated from their rhizoplanes were exposed to atrazine (5 and 10 mg l^?1) individually and in plant-bacterium combination for 15 days hydroponically. It was observed that A. calamus-Pseudomonas sp. strain, the ACB combination, was best in decontamination, showing 91% and 87% removal of 5 and 10 mg l^?1 atrazine. Plant-bacterium association led to significant increase in atrazine decontamination as compared to decontamination by either plant or bacterium alone, indicating a synergistic action of the hydrophytes and isolates which led to enhanced atrazine removal. To the best of our knowledge this is the first report on the potential of plant–bacterium combinations for atrazine decontamination. The isolates showed augmented growth in the presence of plants and were able to alleviate atrazine stress in them. These isolates exhibited plant growth-promoting traits such as auxin, siderophore, Poly(3–hydroxybutyric acid)/succinogycan, ammonia, catalase production and solubilization of inorganic phosphate in vitro. The use of plant-bacterium mutualistic symbiosis for atrazine mitigation is a relatively simple, inexpensive, and clean technique and this phytoremediation-rhizoremediation combination is suggested to be tried on field to establish their potential for clean-up of contaminated sites.     

Phosphate solubilization potential and modeling of stress tolerance of rhizobacteria from rice paddy soil in northern Iran

The purposes of this study were to evaluate the phosphate solubilization activity of bacteria isolated from the rhizosphere of rice paddy soil in northern Iran, and to study the effect of temperature, NaCl and pH on the growth of these isolates by modeling. Three of the most effective strains from a total of 300 isolates were identified and a phylogenetic analysis was carried out by 16S rDNA sequencing. The isolates were identified as Pantoea ananatis (M36), Rahnella aquatilis (M100) and Enterobacter sp. (M183). These isolates showed multiple plant growth-promoting attributes such as phosphate solubilization activity and indole-3-acetic acid (IAA) production. The M36, M100 and M183 isolates were able to solubilize 172, 263 and 254 µg ml^?1 of Ca₃(PO₄)₂ after 5 days of growth at 28 °C and pH 7.5, and to produce 8.0, 2.0 and 3.0 μg ml^?1 of IAA when supplemented with l-tryptophan (1 mg ml^?1) for 72 h, at 28 °C and pH 7.0, respectively. The solubilization of insoluble phosphate was associated with a drop in the pH of the culture medium and there was an inverse relationship between pH and solubilized P (r = ?0.98, P < 0.0952). There were no significant differences among isolates in terms of acidity tolerance based on their confidence limits as assessed by segmented model analysis and all isolates were able to grow at pH 4.3–11 (with optimum at 7.0–7.5). Based on a sigmoidal trend of a three-parameter logistic model, the salt concentration required for 50 % inhibition was 8.15, 6.30 and 8.23 % NaCl for M36, M100 and M183 isolates, respectively. Moreover, the minimum and maximum growth temperatures estimated by the segmented model were 5.0 and 42.75 °C for M36, 12.76 and 40.32 °C for M100, and 10.63 and 43.66 °C for M183. The three selected isolates could be deployed as inoculants to promote plant growth in an agricultural environment.     

Bacterial Biosynthesis of 1-Aminocyclopropane-1-Carboxylate (ACC) Deaminase and Indole-3-Acetic Acid (IAA) as Endophytic Preferential Selection Traits by Rice Plant Seedlings

In this study, bacteria were isolated from the rhizosphere and inside the roots and nodules of berseem clover plants grown in the field in Iran. Two hundred isolates were obtained from the rhizosphere (120 isolates), interior roots (57 isolates), and nodules (23 isolates) of clover plants grown in rotation with rice plants. Production of chitinase, pectinase, cellulase, siderophore, salicylic acid, hydrogen cyanide, indole acetic acid (IAA), 1-aminocyclopropane-1-carboxylate (ACC) deaminase, solubilization of phosphate, antifungal activity against various rice plant pathogen fungi, N₂ fixation, and colonization assay on rice seedlings by these strains was evaluated and compared (endophytic isolates vs. rhizosphere bacteria). The results showed both the number and the ability of plant growth-promoting (PGP) traits were different between endophytic and rhizosphere isolates. A higher percentage of endophytic isolates were positive for production of IAA, ACC deaminase, and siderophore than rhizosphere isolates. Therefore, it is suggested that clover plant may shape its own associated microbial community and act as filters for endophyte communities, and rhizosphere isolates with different (PGP) traits. We also studied the PGP effect of the most promising endophytic and rhizosphere isolates on rice seedlings. A significant relationship among IAA and ACC deaminase production, the size of root colonization, and plant growth (root elongation) in comparison with siderophore production and phosphate solubilization for the isolates was observed. The best bacterial isolates (one endophytic isolate and one rhizosphere isolate), based on their ability to promote rice growth and colonize rice roots, were identified. Based on 16S rDNA sequence analysis, the endophytic isolate CEN7 and the rhizosphere isolate CEN8 were closely related to Pseudomonas putida and Pseudomonas fluorescens, respectively. It seems that PGP trait production (such as IAA, ACC deaminase) may be required for endophytic and rhizosphere competence as compared to other PGP traits in rice seedlings under constant flooded conditions. The study also shows that the presence of diverse rhizobacteria with effective growth-promoting traits associated with clover plants may be used for sustainable crop management under field conditions.