Phosphate Solubilizing Microorganisms in the rhizosphere of Pinus pinaster and in the mycosphere of associated Lactarius deliciosus

J. Barriuso Maicas¹, J.A. Lucas García¹, T. Pereyra de la Iglesia¹, A. Daza², B. Ramos¹ and F.J.Gutierrez Mañero¹

¹Universidad San Pablo CEU. Facultad CC. Experimentales y de la Salud. P.O. Box 67, Boadilla del Monte, 28668, Madrid. SPAIN. bramsol@ceu.es

²CIFA Las Torres-Tomejil. C/ Sevilla-Alcalá del Río Km.12,200. Apto. Oficial C.P.41200 Alcalá del Río. Sevilla. SPAIN.

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Plant hormone production, nutrient mobilization and plant protection are the generally accepted mechanisms for plant growth promotion. The rhizosphere of plants seems to be the most suitable place to search for these beneficial microorganisms since it's a widely accepted fact that plants select those bacteria that may be more benefitial for their health.

A screening for PGPRs was carried out in the rhizosphere of Pinus pinaster and in the mycosphere of associated Lactarius deliciosus. A total of two hundred and four strains, approximately 100 strains from each plant species were isolated, purified, grouped by morphological criteria and tested for ACC degradation, auxin and siderofore production and phosphate solubilization. Thirty two percent of strains showed at least one of these activities in vitro, and 47.7 % of these were phosphate solubilizers. Most of them were gram positive bacilli, esporulated if isolated from the rhizosphere, and non esporulated if isolated from the mycosphere.

All isolates that showed at least one of these activities were analyzed by PCR-RAPDs with six random primers. From this analysis and considering data from phosphate solubilizing gram postive bacilli only, 5 groups appeared at 85 % similiarity: 3 for espore forming bacilli, and 2 for non-espore forming bacilli. Results from PCR-RAPDs of phosphate solubilizing strains indicate the low diversity existing in the selective environments under the influence of plants or fungi. It's striking that some strains isolated from different areas and plant species showed 100 % similarity indexes.