Mineral Phosphate Solubilizing Activity Research Articles

Endophytic bacteria Bacillus safensis and Pseudomonas hibiscicola and their ability to increase rice seedling growth

Endophytic bacteria Bacillus safensis RS95 and Pseudomonas hibiscicola RS121 were evaluated for their ability to promote the growth of rice seedlings and produce indole-acetic acid (IAA) and siderophores and to solubilize phosphates. ‘Guri’ rice seeds were immersed in bacterial endophyte cell suspensions (separated and two-strain mixed), as well as in Escherichia coli DH5α, phosphate-buffered saline (PBS) and water treatments (negative controls). Seeds were sown on agar-water in Petri plates placed vertically at an angle of 65°. The ability of plant growth-promoting endophytic bacteria (PGPEB) to produce IAA and siderophores was determined by Salkowski colorimetric and chrome azurol S (CAS) assays, respectively. Mineral phosphate solubilization activity was calculated by inoculating the endophytes onto medium containing insoluble phosphate. PGPEB showed a positive effect on the growth of rice seedlings, causing a mean growth of shoots and primary-roots of 60 and 67%, respectively. Bacterial strains also showed positive traits for IAA and siderophore production, as well as phosphate-solubilization activity.

Crc Regulates Succinate-Mediated Repression of Mineral Phosphate Solubilization in Acinetobacter sp. SK2 by Modulating Membrane Glucose Dehydrogenase.

The plant growth-promoting Acinetobacter sp. SK2 isolated from Vigna radiata rhizosphere was characterized for mineral phosphate solubilization (MPS). To understand the contribution of the membrane glucose dehydrogenase (mGDH) and soluble glucose dehydrogenase (sGDH) in glucose oxidation and MPS, insertional inactivation of the corresponding genes was carried out. The disruption of mGDH encoding gene gdhA resulted in complete loss of mGDH activity, which confirmed its role in periplasmic glucose oxidation and gluconate-mediated MPS phenotype. The inactivation of sGDH encoding gene gdhB resulted in loss of sGDH activity, which did not alter the MPS or mGDH activity. Thus, it was also concluded that the sGDH was dispensable in gluconate-mediated MPS. Supplementation of succinate in glucose-containing medium suppressed the activity of mGDH (and sGDH) and therefore repressed the MPS phenotype. The catabolite repression control protein (Crc) of Pseudomonas was implicated in Acinetobacter sp. for a similar function in the presence of preferred and non-preferred carbon sources. To understand the regulatory linkage between Crc and genes for glucose oxidation, crc mutants were generated. The inactivation of crc resulted in increased activity of the mGDH in glucose + succinate-grown cells, indicating derepression. An increase in phosphate solubilization up to 44% in glucose + succinate-grown crc– compared with glucose-grown cells was recorded, which was significantly repressed in the wild-type strain under similar conditions. It is therefore proposed that in Acinetobacter sp. SK2, Crc is involved in the succinate-provoked repression of the MPS phenotype. The gene expression data indicated that Hfq may also have a regulating role in preferential utilization of carbon source by perhaps modulating Crc–Hfq functionality. V. radiata plants inoculated with the wild type improved both root and shoot length by 1.3 to 1.4-fold. However, crc– increased the root and shoot length by 1.6-fold, compared with the uninoculated controls. In mimicking the soil condition (in the presence of multiple carbon sources, e.g., succinate along with glucose), the crc– strain of Acinetobacter sp. SK2 performed better in supporting the growth of V. radiata in pot experiments.

Solubilization of inorganic phosphate by Pseudomonas strains isolated from rice rhizosphere

The use of phosphate solubilizing bacteria to mobilize large reserves of insoluble P in soils instead of phosphate fertilizers is an ecologically safe and economically reasonable alternative to assure a sustainable agriculture. In this work, the mineral phosphate solubilization activity was evaluated for 4 Pseudomonas strains isolated from rice rhizosphere. For qualitative tests, different media were used; NBRIP, YED and Pikovskaya (PVK) amended with 0.5% Ca3(PO4)2 and modified PVK medium containing CaHPO4, Ca5HO13P3, FePO4 and AlPO4 as P source. All bacteria were able to dissolve calcium phosphates, while none was surrounded by transparent halos when FePO4 and AlPO4 were the P source. Quantitative test was assessed in PVK substituting glucose by sucrose and galactose. The highest levels of soluble P were recorded in the presence of glucose, while it didn’t exceed 43.64 μg / ml and 19.19 µg / ml in the presence of galactose and sucrose, respectively. Therefore, glucose has been chosen as a carbon source to assay the effect of carbon concentration on P solubilization ability. The level of dissolved P was enhanced with increasing amount of glucose (1%, 2% and 3% (w/v)). In all cases, the P biosolubilisation was accompanied by a significant pH decrease of the liquid media. A significant inverse correlation was found between P concentrations and pH reduction. Thus, acidification of the media seems to be the main mechanism for calcium phosphates solubilization in this study, and these test strains could serve as efficient biofertilizer candidates for improving crop production in low P soils.

Rhizobium Mutant Deficient in Mineral Phosphate Solubilization Activity Shows Reduced Nodulation and Plant Growth in Green Gram

The aim of the present study was to understand the mineral phosphate solubilization (MPS) in Rhizobium sp. MR-54 (MPS+) by using forward genetic approach and its effect on nodulation as well as nitrogen accumulation on green gram. Tn5 mutants (MPS−) were generated from the MPS+ strain by Tn5 mutagenesis. Screening 1,603 colonies of MR-54::Tn5 resulted in identification of those mutants which failed to show any solubilization zone up to 72 h after incubation on Tri-calcium phosphate medium (TCP). pH drops and Pi release were highly correlated up to 15 days after incubation in TCP broth. Gluconic acid was detected in the culture supernatants of MPS+ isolates through thin layer chromatography indicating the presence of direct oxidation pathway in MR-54 strain. Detailed characterization of mutants was carried out for related properties like colony morphology, Pi release and pH change in TCP broth, organic acid production, surface characteristics and plant growth promoting ability etc. Mutants deficient in MPS showed reduced nodulation and nitrogen accumulation in green gram under green house conditions in leonard jar apparatus over the wild strain indicating the MPS by Rhizobium being involved in nodulation, nitrogen accumulation and plant growth.

Mineral phosphate solubilization by Streptomyces sp. CTM396 involves the excretion of gluconic acid and is stimulated by humic acids.

The actinomycetes isolates (128) which were taken from agricultural soil samples and collected near a rock phosphate processing unit were screened for mineral phosphate-solubilizing (MPS) ability. A significant MPS activity was observed for 30 isolates on various phosphate sources when grown in the National Botanical Research Institute's phosphate broth. CTM396 and CTM397 strains which showed the highest MPS abilities were identified by 16S rDNA sequencing as members of the genus Streptomyces. Their MPS activity was proved to be concomitant with a drop in pH due to the secretion of gluconic acid (GA). This was correlated with the simultaneous detection by PCR of genes gdh [encoding the glucose dehydrogenase (GDH) responsible for GA production from glucose] and pqq (involved in biosynthesis of the pyrroloquinoline quinone cofactor of GDH), as well as the highlighting of GHD enzyme activity, for the first time in a Streptomyces sp. strain producing GA. Furthermore, the 0.05% of humic acids proved to have a stimulatory effect on the growth and the ability of CTM396 to solubilize Gafsa rock phosphate. According to this study, it is possible to use humic acids and Gafsa rock phosphate in association with spores of ad hoc Streptomyces strains as natural and efficient amendments to improve plant growth with no need of costly and pollutant transformation of Gafsa rock phosphate.

Investigation on Phosphate Solubilization Potential of Agricultural Soil Bacteria as Affected by Different Phosphorus Sources, Temperature, Salt, and pH

Ten phosphate-solubilizing bacterial strains belonging to genera Pseudomonas, Burkholderia, Enterobacter, Serratia, Klebsiella, and Aeromonas were tested for mineral phosphate solubilization activity in Pikovskaya's broth using different phosphate sources at four temperatures (15, 25, 35, and 45 οC). Dicalcium and tricalcium phosphate were solubilized more effectively (≥1000 mg L−1) than ferric and rock phosphate (≥100 mg L−1) and 35 °C was found to be the optimum temperature. Although Klebsiella and Aeromonas spp. are well known for their dinitrogen (N2)–fixing ability, to the best of our knowledge, this is the first report of inorganic phosphate solubilization by Klebsiella terrigena and Aeromonas vaga. Interestingly, A. vaga BAM-77 is the most efficient strain at solubilizing inorganic phosphorus (P) even in the presence of 8% sodium chloride (NaCl) at pH 10. These findings indicate that all four strains are efficient P solubilizers under variable conditions of temperature, pH, and P source and thus can be recommended for P fertilization in different soils.

Characterization of mineral phosphate solubilization traits from a barley rhizosphere soil functional metagenome

Mineral phosphate solubilization (MPS) microorganisms are important for their provision of orthophosphate anions for plant growth promotion activity in soil. In this study, we applied a functional metagenomic approach to identify this trait directly from the microbiome in barley rhizosphere soil that had not received P fertilizer over a 15-year period. A fosmid system was used to clone the metagenome of which 18,000 clones (∼666 Mb of DNA) was screened for MPS. Functional assays and High Performance Liquid Chromatography analysis recognized gluconic acid production and MPS activity in the range 24.8–77.1 mmol/L and 27.6–38.16 μg/mL, respectively, when screened in an Escherichia coli host (at frequency of one MPS-positive clone hit per 114 Mb DNA tested). The MPS clones (with average insert size of ∼37 kb) were analysed by 454 Roche sequencing and annotated. A number of genes/operons with homology to Phosphorous (P) uptake, regulatory and solubilization mechanisms were identified, linking the MPS function to the uncultivated microbiome present in barley rhizosphere soil.

Coexpression of the Pyrroloquinoline Quinone and Glucose Dehydrogenase Genes from Serratia marcescens CTM 50650 Conferred High Mineral Phosphate-Solubilizing Ability to Escherichia coli

The genes gdh and pqqABCDE encoding glucose dehydrogenase and its pyrroloquinoline quinone cofactor were cloned from the mineral phosphate-solubilizing (MPS) bacterium Serratia marcescens CTM 50650. We investigated, for the first time, the impact of their coexpression in Escherichia coli on MPS ability. The production of recombinant PQQGDH conferred high MPS activity to the engineered E. coli. In fact, the amounts of soluble phosphorus (P) produced from tricalcium phosphate, hydroxyapatite, and Gafsa rock phosphate (GRP) were 574, 426, and 217 mg/L, respectively. In an attempt to increase the soluble P concentration, the E. coli strain coexpressing the gdh and pqqABCDE genes was immobilized in agar, calcium alginate, and k-carrageenan and was then further applied in a repeated batch (six batches) fermentation process to solubilize GRP. Compared to other encapsulated systems, alginate cell beads were noted to yield the highest concentration of soluble P, which attained 300 mg/L/batch. MPS efficiency was maximal in the presence of 5 and 40 g/L of GRP and glucose, respectively.

Assessment of mineral phosphate‐solubilizing properties and molecular characterization of zinc‐tolerant bacteria

Plant growth-promoting bacteria with the ability to tolerate heavy metals have importance both in sustainable agriculture and phytoremediation. The present study reports on the isolation and characterization of mineral phosphate-solubilizing (MPS) bacteria associated with the Achyranthes aspera L. plant (prickly chaff, flower plant). Out of 35 bacterial isolates, 6 isolates, namely RS7, RP23, EPR1, RS5, RP11 and RP19, with high MPS activity were selected and subjected to the assessment of MPS activity under various stress conditions, viz. ZnSO(4) (0.30-1.5 M), NaCl and temperature. MPS activity by the selected isolates was observed at concentrations of as high as >1.2 M ZnSO(4). Significant improvement in plant growth was observed on bacterization of seeds (pearl millet) with all of the six selected isolates. Plant growth was measured in terms of root length, shoot length, fresh weight and % increase in root biomass. The molecular diversity among the phosphate-solubilizing bacteria was studied employing enterobacterial repetitive intergenic sequence-PCR (ERIC-PCR). Representative strains from each ERIC type were identified, on the basis of a partial sequence of the 16S rRNA gene, as members of the genera Pseudomonas, Citrobacter, Acinetobacter, Serratia, and Enterobacter. Among all the isolates, RP19 was the best in terms of phosphate-solubizing activity and its response to various stresses. The ability of RP19 and other isolates to exhibit MPS activity at high ZnSO(4) concentrations suggests their potential as efficient biofertilizer for growing plants in metal (ZnSO(4))-contaminated soil.

Mineral phosphate solubilization activity of gluconacetobacter diazotrophicus under P-limitation and plant root environment

The ability to solubilize insoluble inorganic pho- sphate compounds by Gluconacetobacter diazotrophicus was studied using different cul-ture approaches. Qualitative plate assays using tricalcium phosphate as the sole P-source showed that G. diazotrophicus produced solu-bilization only when aldoses were used as the C-source. Extracellular aldose oxidation via a pyrroloquinoline quinone-linked glucose dehy-drogenase (PQQ-GDH) is the main pathway for glucose metabolism in G. diazotrophicus. In batch cultures with 5 g l-1 of hydroxyapatite as the P-source and glucose as the C-source, more than 98% of insoluble P was solubilized. No solubilization was observed neither using glyc-erol nor culturing a PQQ-GDH mutant of G. di-azotrophicus. Solubilizaton was not affected by adding 100 mmol l-1 of MES buffer. Continuous cultures of G. diazotrophicus showed significant activities of PQQ-GDH either under C or P limi-tation. An intense acidification in the root envi-ronment of tomato and wheat seedlings inocu-lated with a G. diazotrophicus PAL5 was ob-served. Seedlings inoculated with a PQQ-GDH mutant strain of G. diazotrophicus showed no acidification. Our results suggest that G. di-azotrophicus is an excellent candidate to be used as biofertilizer because in addition to the already described plant growth-promoting abili-ties of this organism, it shows a significant mineral phosphate solubilization capacity.

Characterization of the mineral phosphate solubilizing activity of Serratia marcescens CTM 50650 isolated from the phosphate mine of Gafsa

The mineral phosphate solubilizing (MPS) ability of a Serratia marcescens strain, namely CTM 50650, isolated from the phosphate mine of Gafsa, was characterized on a chemically defined medium (NBRIP broth). Various insoluble inorganic phosphates, including rock phosphate (RP), calcium phosphate (CaHPO(4)), tri-calcium phosphate (Ca(3)(PO(4))(2)) and hydroxyapatite were tested as sole sources of phosphate for bacterial growth. Solubilization of these phosphates by S. marcescens CTM 50650 was very efficient. Indeed, under optimal conditions, the soluble phosphorus (P) concentration it produced reached 967, 500, 595 and 326 mg/l from CaHPO(4), Ca(3)(PO(4))(2), hydroxyapatite and RP, respectively. Study of the mechanisms involved in the MPS activity of CTM 50650, showed that phosphate solubilization was concomitant with significant drop in pH. HPLC-analysis of culture supernatants revealed the secretion of gluconic acid (GA) resulting from direct oxidation pathway of glucose when the CTM 50650 cells were grown on NBRIP containing glucose as unique carbon source. This was correlated with the simultaneous detection by PCR for the first time in a S. marcescens strain producing GA, of a gene encoding glucose dehydrogenase responsible for GA production, as well as the genes pqqA, B, C and E involved in biosynthesis of its PQQ cofactor. This study is expected to lead to the development of an environmental-friendly process for fertilizer production considering the capacity of S. marcescens CTM 50650 to achieve yields of P extraction up to 75% from the Gafsa RP.

EFFECT OF NITROGEN AND CARBON SOURCES ON THE INORGANIC PHOSPHATE SOLUBILIZATION BY DIFFERENT ASPERGILLUS NIGER STRAINS

The Mineral phosphate solubilization (MPS) was studied in ten Aspergillus niger strains. MPS activity was measured in solid (Pikovskaya's medium) as well as liquid media using different phosphate sources (tricalcium, dicalcium, ferric and aluminium phosphates, and bone meal), carbon sources (glucose, mannitol, fructose, sucrose, xylose, and sorbitol), and nitrogen sources (ammonium sulfate, ammonium chloride, ammonium nitrate, sodium nitrate, potassium nitrate, and urea). All the strains showed a zone of clearance of tricalcium phosphate in Pikovskaya's medium in plates and solubilized dicalcium and tricalcium phosphates in broth efficiently. Solubilization was lower in ferric and aluminium phosphates. The activity was very poor in bone meal medium. Among the carbon sources the fungi preferred mannitol for higher P solubilization. Nitrogen in the form of nitrate was very effective in solubilizing inorganic phosphates. Xylose and urea were the poorest sources of carbon and nitrogen for all the strains of Aspergillus. Phosphate release was associated with reduction in pH.

Phosphate solubilization by Penicillium spp. closely associated with wheat roots

In this study we found that Penicillium spp. exhibiting P-solubilizing activity are common both on and in the roots of wheat plants grown in southern Australian agricultural soils. From 2,500 segments of washed and surface-disinfested root pieces, 608 and 223 fungi were isolated on a selective medium, respectively. All isolates were screened for P solubilization on solid medium containing hydroxyapatite (HA); 47 isolates (5.7%) solubilized HA and were identified as isolates of Penicillium or its teleomorphs. These isolates were evaluated for solubilization of Idaho rock phosphate (RP) in liquid culture. Penicillium bilaiae strain RS7B-SD1 was the most effective, mobilizing 101.7 mg P l−1 after 7 days. Other effective isolates included Penicillium simplicissimum (58.8 mg P l−1), five strains of Penicillium griseofulvum (56.1–47.6 mg P l−1), Talaromyces flavus (48.6 mg P l−1) and two unidentified Penicillium spp. (50.7 and 50 mg P l−1). A newly isolated strain of Penicillium radicum (KC1-SD1) mobilized 43.3 mg P l−1. RP solubilization, biomass production and solution pH for P. bilaiae RS7B-SD1, P. radicum FRR4718 or Penicillium sp. 1 KC6-W2 was determined over time. P. bilaiae RS7B-SD1 solubilized the greatest amount of RP (112.7 mg P l−1) and had the highest RP-solubilizing activity per unit of biomass produced (up to 603.2 μg P l−1 mg biomass−1 at 7 days growth). This study has identified new isolates of Penicillium fungi with high mineral phosphate solubilizing activity. These fungi are being investigated for the ability to increase crop production on strong P-retaining soils in Australia.

Rock phosphate solubilization and colonization of maize rhizosphere by wild and genetically modified strains of Penicillium rugulosum.

Maize root colonization and phosphate solubilizing activity of the fungus Penicillium rugulosum were assessed in a greenhouse trial using soil-plant microcosms. The bacterial gene hph conferring resistance to hygromicin B was introduced by electroporation in the wild-type strain IR-94MF1 of P. rugulosum and one transformant, w-T3, was selected. Maize plants were grown for 5 weeks in a P-poor soil and fertilized with a Florida apatite mineral, with Navay, an apatite rock deposit from Venezuela, or with simple superphosphate. Inoculation treatments included strain IR-94MF1, transformant w-T3 and two IR-94MF1 UV-induced mutants with enhanced (Mps++) or reduced (Mps-) in vitro mineral phosphate solubilizing activity. In the absence of P fertilization, inoculation with any P. rugulosum isolate significantly reduced the size of the total and P-solubilizing bacterial community present in maize rhizosphere. The bacterial community significantly increased in maize inoculated with IR-94MF1 and w-T3 when P was added as apatites Navay or Florida. All P. rugulosum strains were able to stimulate the growth of maize plants as indicated by 3.6 to 28.6% increases in dry matter yields. In the presence of rock phosphate, P uptake by maize plants inoculated with the two mutants Mps++ and Mps- was not always in agreement with their P-solubilizing phenotypes. Strain IR-94MF1 and transformant w-T3 increased P assimilation by the plants fertilized with Navay rock phosphate by 26 and 38%, respectively. In this treatment, w-T3 showed its highest significant maize rhizosphere colonization. With the simple superphosphate treatment, w-T3 increased P uptake in plants by 8% over the uninoculated control and also decreased significantly the community size of total bacteria, total fungi, and P-solubilizing fungi in the rhizosphere.

Cloning of a Serratia marcescens DNA fragment that induces quinoprotein glucose dehydrogenase-mediated gluconic acid production in Escherichia coli in the presence of stationary phase Serratia marcescens

Serratia marcescens ER2 was isolated from an endorhizosphere sample based on its high level of mineral phosphate solubilizing (MPS) activity. This phenotype was correlated with expression of the direct oxidation pathway. An ER2 plasmid library constructed in Escherichia coli strain DH5α was screened for MPS activity. A recombinant clone DH5α (pKG3791) was capable of gluconic acid (GA) production and tricalcium phosphate solubilization but only in the presence of stationary phase ER2 cells. GA production in DH5α (pKG3791) was apparently the result of the quinoprotein glucose dehydrogenase activity because AG121 (a Tn5 knockout of gcd) carrying pKG3791 did not produce GA under the same conditions. GA production by DH5α (pKG3791) was not observed when ER2 was replaced by another PQQ-producing strain bacterium. These data add to a growing body of evidence that E. coli contains some type of PQQ biosynthesis pathway distinct from those previously characterized in Gram-negative bacteria and that these genes may be induced under appropriate conditions.

Cloning of a Serratia marcescens DNA fragment that induces quinoprotein glucose dehydrogenase-mediated gluconic acid production in Escherichia coli in the presence of stationary phase Serratia marcescens

Serratia marcescens ER2 was isolated from an endorhizosphere sample based on its high level of mineral phosphate solubilizing (MPS) activity. This phenotype was correlated with expression of the direct oxidation pathway. An ER2 plasmid library constructed in Escherichia coli strain DH5alpha was screened for MPS activity. A recombinant clone DH5alpha (pKG3791) was capable of gluconic acid (GA) production and tricalcium phosphate solubilization but only in the presence of stationary phase ER2 cells. GA production in DH5alpha (pKG3791) was apparently the result of the quinoprotein glucose dehydrogenase activity because AG121 (a Tn5 knockout of gcd) carrying pKG3791 did not produce GA under the same conditions. GA production by DH5alpha (pKG3791) was not observed when ER2 was replaced by another PQQ-producing strain bacterium. These data add to a growing body of evidence that E. coli contains some type of PQQ biosynthesis pathway distinct from those previously characterized in Gram-negative bacteria and that these genes may be induced under appropriate conditions.

Evidence for mutualism between a plant growing in a phosphate-limited desert environment and a mineral phosphate solubilizing (MPS) rhizobacterium

Alkaline desert soils are high in insoluble calcium phosphates but deficient in soluble orthophosphate (Pi) essential for plant growth. In this extreme environment, one adaptive strategy could involve specific associations between plant roots and mineral phosphate solubilizing (MPS) bacteria. The most efficient MPS phenotype in Gram-negative bacteria results from extracellular oxidation of glucose to gluconic acid via the quinoprotein glucose dehydrogenase. A unique bacterial population isolated from the roots of Helianthus annus jaegeri growing at the edge of an alkaline dry lake in the Mojave Desert showed no MPS activity and no gluconic acid production. Addition of a concentrated solution containing material washed from the roots to these bacteria in culture resulted in production of high levels of gluconic acid. This effect was mimicked by addition of the essential glucose dehydrogenase redox cofactor 2,7,9-tricarboxyl-1H-pyrrolo[2,3]-quinoline-4,5-dione (PQQ) but the bioactive component was not PQQ. DNA hybridization data confirmed that this soil bacterium carried a gene with homology to the Escherichia coli quinoprotein glucose dehydrogenase. These data suggest that expression of the direct oxidation pathway in this bacterium may be regulated by signaling between the bacteria and the plant root. The resultant acidification of the rhizosphere may play a role in nutrient availability and/or other ecophysiological parameters essential for the survival of this desert plant.

Evidence for mutualism between a plant growing in a phosphate-limited desert environment and a mineral phosphate solubilizing (MPS) rhizobacterium

Alkaline desert soils are high in insoluble calcium phosphates but deficient in soluble orthophosphate (Pi) essential for plant growth. In this extreme environment, one adaptive strategy could involve specific associations between plant roots and mineral phosphate solubilizing (MPS) bacteria. The most efficient MPS phenotype in Gram-negative bacteria results from extracellular oxidation of glucose to gluconic acid via the quinoprotein glucose dehydrogenase. A unique bacterial population isolated from the roots of Helianthus annus jaegeri growing at the edge of an alkaline dry lake in the Mojave Desert showed no MPS activity and no gluconic acid production. Addition of a concentrated solution containing material washed from the roots to these bacteria in culture resulted in production of high levels of gluconic acid. This effect was mimicked by addition of the essential glucose dehydrogenase redox cofactor 2,7,9-tricarboxyl-1H-pyrrolo[2,3]-quinoline-4,5-dione (PQQ) but the bioactive component was not PQQ. DNA hybridization data confirmed that this soil bacterium carried a gene with homology to the Escherichia coli quinoprotein glucose dehydrogenase. These data suggest that expression of the direct oxidation pathway in this bacterium may be regulated by signaling between the bacteria and the plant root. The resultant acidification of the rhizosphere may play a role in nutrient availability and/or other ecophysiological parameters essential for the survival of this desert plant.

Characteristics of phosphate solubilization by an isolate of a tropical Penicillium rugulosum and two UV-induced mutants

After screening several fungal isolates obtained from Venezuelan soils, the isolate IR-94MF1 of Penicillium rugulosum was selected for its high mineral phosphate solubilizing activity (Mps+) with hydroxyapatite. Mutants with altered (Mps−) or amplified activity (Mps++) were obtained by UV irradiation of conidia. When glucose was used as the sole C source by the fungus, gluconic acid was associated with the hydroxyapatite activity. In solid media, in comparison to glucose and maltose, sucrose appeared to be the best C source for the solubilization of hydroxy-apatite and FePO4.