The mechanisms of action of mineral phosphate solubilization (MPS) were studied in the wild-type Mps Penicillium rugulosum strain IR94-MF1 and in negative (Mps 3 ) and superpositive (Mps ) mutants derived from it. MPS activities were measured in liquid media using sucrose as C source, four N (arginine, nitrate, nitrate+ammonium and ammonium) and P sources (KH P PO R , hydroxyapatite, FePO R and AlPO R ). Ammonium significantly (P 6 0.01) decreased phosphate solubilization, and this activity was 1^66 times higher in the Mps mutant than in the wild-type depending on the P and N sources used. The Mps phenotype was strongly associated with the production of gluconic or citric acids. The results also suggest for the MPS 3 mutant the involvement of the H pump mechanism in the solubilization of small amounts of phosphates. z 1999 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.

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.

The abandoned ''Monte-Fresco'' rock phosphate mine in Ta´chira, Venezuela, was sampled to study the biodiversity of phosphate-solubilizing microorganisms (PSM). Rhizosphere and bulk soils were sampled from colonizer plant species growing at a mined site where pH and soluble P were higher than the values found at a near by unmined and shrubby soil. Counting and isolating of PSM choosing strains showing high solubilization halos in a solid minimal medium with hydroxyapatite as phosphate source were evaluated using ammonia or nitrate as nitrogen sources and dextrose, sucrose, and mannitol as carbohydrate sources. A larger number of PSM were found in the rhizospheric than in the bulk soil. Six fungal strains belonging to the genus Penicillium and with high hydroxyapatite dissolution capacities were isolated from bulk soil of colonizer plants. Five of these strains had similar phenotypes to Penicillium rugulosum IR-94MF1 but they solubilized hydroxyapatite at different degrees with both nitrogen sources. From 15 strains of Gram-negative bacteria isolated from the rhizosphere of colonizer plants, 5 were identified as diazotrophic free-living encapsulated Azotobacter species able to use ammonium and/or nitrate to dissolve hydroxyapatite with glucose, sucrose and/or mannitol. Different nitrogen and carbohydrate sources are parameters to be considered to further characterize the diversity of PSM.

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