Screening for putative PGPR to improve establishment of the symbiosis Lactarius deliciosus-Pinus sp.

A screening for plant growth promoting rhizobacteria (PGPR) was carried out in the mycorrhizosphere of wild populations of Pinus pinea and P. pinaster, and in the mycosphere of associated Lactarius deliciosus. A total of 720 bacteria were isolated, purified, and grouped by morphological criteria. Fifty percent of the isolates were selected and tested for aminocyclopropanecarboxylic acid (ACC) degradation, auxin and siderophore production, and phosphate solubilization. Thirty eight percent of the isolates showed at least one of the evaluated activities. Nutrient-related traits were associated with P. pinaster, whereas hormone production traits predominated in P. pinea. These activities were found mostly in Gram positive isolates. After PCR-RAPDs (random amplified polymorphic DNA) analysis, 10 groups appeared with 85% similiarity when considering all isolates, indicating the low diversity in the system. One strain of each group was identified by 16S rDNA sequencing. Our results suggest that P. pinaster selects for mycorrhizosphere bacteria that mobilize nutrients, whereas P. pinea selects for bacteria that have the capacity to increase root growth via production of plant growth regulators.