ASSESSMENT OF PLANT GROWTH-PROMOTING RHIZOBACTERIA: ANTAGONISTIC ACTIVITIES AND DETERMINATIVE FACTORS

Abstract

Seventy plant growth-promoting rhizobacteria that were isolated from the soils of rhizosphere and rhizoplane of various crops in Uttarakhand's plains were assessed for their antagonistic potential. Antagonistic activity was assessed against four fungal pathogens, namely Bipolaris sorokiniana, Colletotrichum gloeosporioides f. sp. mangiferae, Fusarium oxysporum f. sp. lycopersici, and Rhizoctonia solani, using the dual culture plate technique. Among the seventy isolates, thirty showed inhibition of all four pathogens' radial growth of mycelium in vitro. Considerable diversity in antagonistic behavior was observed among the isolates. Isolates Pfa-50 and Pfa-22 showed more than 50% inhibition against all test pathogens, except Fusarium oxysporum, where inhibition was limited to 39.1% and 46.9%, respectively. 16S rRNA gene amplification confirmed the identity of majority of Gram-negative isolates to be Pseudomonas spp. whereas all the Gram-positive isolates were Bacillus spp. The underlying mechanisms employed by the thirty selected isolates to control these four pathogens in vitro were investigated by evaluating their ability to synthesize hydrogen cyanide (HCN), chitinase and other antimicrobial secondary metabolites. Out of the thirty antagonistic isolates, twenty-two showed positive responses for both HCN production and chitinase activity. Isolates lacking both chitinase and HCN production were further examined for their ability to synthesize antimicrobial secondary metabolites. Thin-layer chromatography (TLC) analysis of extracted metabolites revealed that all fluorescent Pseudomonas isolates produced pyoluteorin (PLT), 2,4-diacetylphloroglucinol (DAPG), and phenazines. Isolates Pfa-50, Pfa-2, and Pfa-65 produced pyrrolnitrin (PRN), while isolates Pfa-50, Pfa-2, Pfa-11, and Pfa-24 also produced indole-3-acetic acid (IAA). Secondary metabolites produced by Bacillus spp. included macrolactins, amicoumacins, surfactins, phenazines, IAA, salicylic acid (SA), and other bioactive compounds, all of which possess well-established antimicrobial properties. The screened rhizobacterial isolates demonstrate significant potential as biological control agents and warrant further evaluation under field conditions. These isolates may be effectively incorporated into integrated pest management strategies and have promising applications in organic farming systems.