Abstract
Plant growth promoting rhizobacteria (PGPR) consisting a wide range of beneficial soil bacteria inhabiting rhizosphere of plant. Fluorescent pseudomondas are the most important plant growth promoting rhizosphere bacteria in different crop plants. In this research, twenty-five isolates of fluorescent pseudomonads isolated from forty soil samples of wheat rhizosphere and forty four isolates of Flavobacterium selected from the bacterial strains collection were employed. The ability of these isolates for production of chitinase, Salicylic acid, siderophore and hydrogen cyanide and antifungal activity against Rhizoctonia solani were determined. Results revealed that all the fluorescent pseudomonads isolates were able to produce siderophore on Chrome Azurol S (CAS) agar plates and siderophore is an important secondary metabolite in plant growth promotion and antifungal activity. In CAS agar method, the range of siderophore production was 0.34-1.21 halo diameter per colony diameter. None of these isolates were capable for chitinase production. Salicylic acid (SA) production by the isolates ranging from 0-10.91 g.ml-1 (average 2.48 g.ml-1). Bacterial ability for production of hydrogen cyanide had a great variation and based on this ability they were qualitatively classified in to high, relatively high, moderate and no production. Productions of studied metabolites were not detected in Flavobacterium isolates. Three isolates (PA24, PA1 and PA18) of fluorescent pseudomonads showed in vitro antifungal activity against Rhizoctonia solani. In contrast none of Flavobacterium isolates showed growth inhibition against the tested fungus. Results of this study showed that fluorescent pseudomonads which are native to soils of Iran had the potential to be used for promotion of plant growth and suppression of soil-borne plant pathogens. In contrast, Flavobacterium seem to be an inefficient rhizobacterium against plant pathogens because of incapability to production of some important secondary metabolites in plant growth promotion and antifungal activity. There is a need to study their effects on different agricultural crops.
Highlights
The bacteria that exert some beneficial effects on plant growth are two general types, those that form a symbiotic relationship with the plant and those that are free-living in the soil
Antibiotic synthesis (Kaaijmakers and Weller, 2001), siderophore production (Alexander and Zuberer, 1991), extra cellular enzymes such as, chitinase, β-1,3glucanase production (Nagarajkumar et al, 2004) and Competition for nutrients and space in rhizosphere are some indirect mechanisms used by bacteria (Schippers et al, 1990)
Hydrogen cyanide (HCN) production from glycine was tested by growing the bacteria in 10% tryptic soy agar (TSA) supplemented with glycine (4.4 g.L-1) and cyanogenesis was revealed using picric acid and Na2CO3 (0.5 and 2%, respectively) impregnated filter paper fixed to the underside of the Petri dish lids
Summary
The bacteria that exert some beneficial effects on plant growth are two general types, those that form a symbiotic relationship with the plant and those that are free-living in the soil. Beneficial free-living soil bacteria are usually referred to as plant growth-promoting rhizobacteria or PGPR (Kloepper et al, 1989). These bacteria can stimulate plant growth either directly or indirectly (Glick, 1995). Mechanisms of biological control of plant pathogens by fluorescent pseudomonads generally involve production of bacterial metabolites such as siderophores, hydrogen cyanide, extra cellular lytic enzymes and induced systemic resistance (O,Sullivan and O,Gara, 1992). Antifungal activity of these isolates was investigated against Rhizoctonia solani as a major soil-borne pathogen
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