Screening for PGPRs in the rhizosphere of Cistus ladanifer

T. Pereyra de la Iglesia¹, A. Probanza¹, J.A. Lucas García¹, M. Mejías², F.J.Gutierrez Mañero¹, B. Ramos¹

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

²Fac. Farmacia. Universidad de Sevilla. C/ Profesor García González s/n 41012 Sevilla. SPAIN

__________________

The role of rhizosphere bacteria in plant health and nutrition is a widely accepted fact. Since the first description of Plant Growth Promoting Rhizobacteria (PGPRs) the benefits attributed to bacteria have been defined more precisely. Plant hormone production, nutrient mobilization and plant protection, either biocontrol or systemic induction are the most commonly accepted mechanisms. Researchers have looked for these beneficial microorganisms in the rhizosphere of diverse plant species, knowing that the plant selects those bacteria that may be more benefitial for its health .

A screening for PGPRs was carried out in the rhizosphere of Cistus ladanifer. One hundred and forty four bacteria were isolated, purified, grouped by morphological criteria and tested for ACC degradation, auxin and siderofore production and phosphate solubilization. 79 of the isolates showed at least one of the activities tested, and 30 were phosphate solubilizers. After PCR-RAPDs analysis, 11 groups appeared considering 85 % similiarity. One strain of each group was tested in biological assay, and identified by 16sDNA sequencing.

Although 7 of the 11 assayed strains were phosphate solubilizers, only one was really effective in increasing all biometric parameters in Cistus ladanifer. This effect is probably due to the rich substrate used. There is increasing evidence showing that genes related to nutrient acquisition are inducible by rhizosphere conditions. Hence, under low nutrient conditions, as it was in the original soil, phosphate solubilization genes would have been induced. This suggests that other mechanisms different to nutrient mobilization is involved on growth promotion by this strain.