BIOLOGICAL ACTIVITY OF MICROBIAL RHIZOSPHERE OF SOME MEDICINAL PLANTS

Abstract

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 Matricaria chamomilla and Majorana hortensis. A total of 112 bacterial isolates were successfully isolated from the rhizosphere of the tested medicinal plants. Cultural and morphological characteristics of these isolates showed that they belong to bacilli, azotobacters, fluorescent pseudomonads and actinomycetes. Isolates were screened, in vitro, according to their capacities to produce plant growth promoting subastances i.e IAA, phosphate and potassium solublization, chitinase activities and hydrogen cyanide production ability. Results indicated that 36 isolates showed IAA production, 25 produced HCN, 57 had chitinase activities, 39 were dissolver’s phosphate and 105 were potassium solubilizers. Eleven selected isolates showed highest plant growth promoting activities and further subjected to estimate their siderophores and phenols production 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. 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. So, these strains were used to verify their effects on growth and chemical composition of Moringa oleifera and Mentha spicata as a promoting and biocontrol agents, respectively. Results indicated that mixed culture of three strains (Bacillus thuringiensis C110, Pseudomonas fluorescens Th98 and Pseudomonas poae Th75) gave the highest growth parameters of Moringa oleifera as compared to control treatments. In this respect, shoot and root dry weights were increased significantly from 5.75, 9.42 (g/plant) in uninoculated treatment (control) to 111.7, 54.83 (g/plant), the superiority of NPK uptake and the highest concentration of total phenols were recorded with the mixed culture treatment. This treatment gave high values of microelements, pigments and indole content of leaves as well as total carbohydrates in shoot and root. On the other hand, in case of Mentha spicata infested with Rhizoctonia solani, the mixed culture gave the highest percentages of plant survival being 100% and superior enhancers of total plant dry weight and microelements content. Plants grown with mixed bacterial culture contained the highest amounts of chlorophyll a, chlorophyll b and B- carotenes compared to uninoculated plants.

Key Words: Medicinal plants, PGPR, Antagonistic effect, IAA, GA3, Trans-zeatin riboside, 16SrRNA gene sequence, Moringa oleifera, Mentha spicata.