Evaluation of rhizobacteria of some medicinal plants for plant growth promotion and biological control

Densities of microbial count in the rhizosphere of eleven medicinal plants viz., Ocimum basilicum, Marrubium vulgare, Melissa officinals, Origanum syriacum, Quisqualis indica, Solidago virgaurea, Melilotus officinalis, Cymbopogon citratus, Matricaria chamomilla, Thymus vulgaris and Majorana hortensis were determined. The lowest populations were found in the rhizosphere of M. chamomilla and M. hortensis. A total of 112 bacterial cultures were successfully isolated in pure form from the rhizosphere of the tested medicinal plants. Cultural and morphological characteristics of these isolates showed that they belong to bacilli, azotobacter, fluorescent pseudomonads and actinomycetes. These isolates were screened, in vitro, according to their capacities to produce plant growth promoting substance i.e. IAA, phosphate, potassium solubilization, chitinase activities and hydrogen cyanide. Results indicated that 36 bacterial isolates showed IAA production, 25 HCN production, 57 chitinase activities, 39 phosphate and 105 potassium solubilizers. Eleven bacterial isolates were selected which showed highest plant growth promoting activities and further subjected to estimate siderophores and phenols produced in liquid culture along with antifungal activity against two phytopathogenic fungi. The most potent isolates were identified on the basis of 16S rRNA gene sequence as Bacillus thuringiensis C110, Pseudomonas fluorescens Th98 and Pseudomonas poae Th75. Their amounts of soluble phosphate in liquid culture were 2.2, 4.62 and 14.53 ppm in respective order. The indol-3-acetic acid (IAA), gibberellic acid (GA3) and trans-zeatin riboside (t-zr) produced by the identified strains were determined by HPLC analysis. These strains proved to be effective PGPR inoculants, as they possess a number of traits useful for plant growth.