Bioprospecting of indigenous fermented organic manures for plant beneficial microorganisms

Abstract

The research work on “Bioprospecting of indigenous fermented organic manures for plant beneficial microorganisms” was performed at the Department of Microbiology, College of Agriculture, Vellayani, Thiruvananthapuram during 2022- 2024, with the objective to isolate plant growth promoting microorganisms from indigenous fermented organic manures and to develop a microbial consortium for growth promotion and suppression of leaf blight in red amaranthus (Amaranthus tricolor L.). In the present study, the total culturable microbial population in twelve samples of indigenous fermented organic manures, collected from farmers of Thiruvananthapuram, Pathanamthitta, and Kottayam districts of Kerala, was assessed using the serial dilution and plating technique. The highest bacterial population was recorded in the panchagavya sample (RAPG), while the highest fungal population was observed in fermented green leaf decoction (SAFC), and the highest actinomycete population was found in fermented cow dung slurry (MIFC). Thirty-eight morphologically distinct bacterial isolates obtained from indigenous fermented organic manures and two bacterial isolates obtained from harithakashayam, a fermented organic manure in a previous study conducted at the Department of Microbiology were used for further studies. The Indole acetic acid (IAA) gibberellic acid (GA) and extracellular ammonia (ECA) production of the forty bacterial isolates were assessed. The isolates produced IAA in the range of 6.27 (SAFM 4) to 57.79 (SAJA 2) μg mL-1 of culture filtrate in the absence of tryptophan and 8.85 (SAFM 4) to 53.31μg mL-1 (KAUF 5) in the presence of tryptophan. The isolate SAFM 2 recorded significantly higher gibberellic acid production of 46.52 μg mL-1 and highest extracellular ammonia production of 14.12 μmol mL⁻¹. Twenty-four isolates producing plant growth hormones and/or extracellular ammonia were screened for phosphate solubilization, hydrogen cyanide (HCN) and siderophore production. Of these, 17 solubilized tricalcium phosphate, 16 produced siderophores, and 2 tested positive for HCN production. Among forty bacterial isolates, twenty-two isolates exhibited antagonistic activity against the leaf blight pathogen, Rhizoctonia solani in dual culture plate assay. Maximum inhibition of R. solani was exhibited by KAUF 5 (42.27 per cent) followed by RAPG 3 (36.73 per cent). The twenty-four bacterial isolates which possessed plant growth promotion traits were subjected to a rapid screening for growth promotion of red amaranthus var. Arun. Treatment with SAFM 2 significantly enhanced germination, seedling vigour, plant height, leaf number, and biomass of amaranthus seedlings. RAPG 6 and KAUF 5 also promoted seedling growth at 14 days of sowing. The antagonistic activity of twenty-two isolates against R. solani in detached leaves of red amaranthus inoculated with the pathogen was assessed. On the 3rd day after pathogen inoculation, the smallest lesion area (0.61 cm²) was observed in leaves treated with the isolate RAPG 3 (83.98 % reduction over control), indicating the highest biocontrol activity. Based on the results of the studies on plant growth promotion, the isolates SAFM 2, RAPG 6, and KAUF 5 were selected for preparing the PGP consortium. The isolates RAPG 3, KAUF 5, and FAHK 7 were selected for preparing the antagonistic consortium based on better performance in studies on antagonistic activity against R. solani. All the five bacterial isolates were used for preparing the total consortium. The compatibility of the isolates was confirmed by cross streak assay. Morphological and biochemical characterization of the isolates revealed that all the five isolates were Gram-positive, endospore-forming rods. The 16S rRNA gene sequencing showed maximum sequence similarity of the isolates with Bacillus tequilensis (KAUF 5), Bacillus altitudinis (SAFM 2), Bacillus subtilis (RAPG 6), Calidifontibacillus erzurumensis (RAPG 3), and Bacillus stercoris (FAHK 7). The evaluation of microbial consortia for growth promotion in red amaranthus under pot culture conditions revealed that drenching the soil with PGP consortium significantly enhanced plant height (22.17 cm), no. of leaves (9.87), fresh root weight (8.72 g), dry root weight (1.24g), fresh shoot weight (35.06 g) and dry shoot weight (4.36g). This was statistically on par with the treatment involving drenching and foliar spray with the total consortium. Treatment with the antagonistic consortium recorded the lowest percentage disease index in red amaranthus inoculated with R. solani resulting in a 56.51% disease suppression compared to the pathogen inoculated control. The findings from the research highlight that the consortium of KAUF 5, SAFM 2, RAPG 6, and FAHK 7 significantly enhanced growth parameters and suppressed leaf blight disease in red amaranthus.