Screening for PGPR to improve growth of Cistus ladanifer seedlings for reforestation of degraded mediterranean ecosystems

A screening for PGPRs was carried out in the rhizosphere of wild populations of Cistus ladanifer. Two hundred and seventy 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 solubilisation. Fifty-eight percent of the isolates showed at least one of the evaluated activities, with phosphate solubilisation and siderophore production being the most abundant traits. After PCR-RAPDs (Randomly amplified polymorphic DNA) analysis, 11 groups appeared with 85% similiarity, revealing the low diversity in the system. One strain of each group was tested in a biological assay, and those that enhanced Cistus growth were identified by 16S rDNA sequencing.Although seven of the 11 assayed strains were phosphate solubilisers and able to produce siderophores, only one was really effective in increasing all biometric parameters in Cistus ladanifer seedlings, the lack of effect of the other six probably being due to the rich substrate used. This suggests that other mechanisms apart from nutrient mobilisation might be involved in growth promotion by this strain. However, the low diversity together with the high redundancy detected by PCR-RAPDs and the predominance of strains able to mobilise nutrients in the rhizosphere of Cistus reveals that the plant selects for bacteria that can help to supply scarce nutrients. This type of plant growth promoting rhizobacteria (PGPR) strains should be succesful in reforestation practices.