Phosphate-solubilizing microorganisms in the rhizosphere of Pinus pinaster and in the mycosphere of associated Lactarius deliciosus

Abstract

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 beneficial 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 204 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. About 32% 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, five groups appeared at 85% similiarity: three for espore forming bacilli, and two 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.