Exploring the Plant Growth Promoting Potential of Phosphate Solubilizing Aspergillus niger PSMH-2 and Acinetobacter pitti PSMM-8 Isolated from the Green Gram (Vigna radiata L.) Rhizosphere in Three Distinct Agro-Climatic Zones of Karnataka, India

Abstract

Phosphate solubilizing microorganisms (PSMs) play a crucial role in enhancing phosphorus availability to plants, thereby promoting agricultural productivity. This study aimed to isolate, characterize, and evaluate the efficacy of phosphate solubilizing microorganisms from the rhizosphere soil of green gram (Vigna radiata) cultivated in different agro-climatic zones of Karnataka, India. A total of 30 PSM isolates were obtained from soil samples collected from Mandya, Hassan, and Bengaluru districts. These isolates were identified as either bacterial or fungal and were screened for their phosphate solubilization efficiency using Pikovskaya’s agar medium. The solubilization index (SI) of the isolates was calculated. Among fungal isolates, PSMH-2 showed the highest phosphate solubilizing efficiency in agar (152.17%) and broth (146.77 mg L-1). Among bacterial isolates, PSMM-8 showed the highest phosphate solubilizing efficiency in agar (115.34%) and broth (113.42 mg L-1). The biochemical characterization of selected bacterial isolates was conducted using HiMedia biochemical test kits. The isolates displayed varied biochemical profiles, indicating the presence of diverse enzyme systems. Additionally, the selected isolates were further assessed for their plant growth-promoting attributes, including phosphatase activity, organic acid production, siderophore production, and phytohormone (IAA, GA3, ABA, and SA) synthesis. Notably, the fungal isolate PSMH-2 and the bacterial isolate PSMM-8 showed superior performance in phosphate solubilization and plant growth-promoting traits compared to other isolates and the reference strains. These isolates were identified as Aspergillus niger and Acinetobacter pitti respectively, by molecular characterization. The inoculation of these two identified isolates for growth and yield of green gram under glasshouse study resulted in the maximum plant height, root length, number of nodules, number of pods, grain weight and nutrient uptake. The study highlights the potential of these efficient phosphate solubilizing isolates as bio-inoculants for sustainable agriculture, particularly in enhancing phosphorus availability and promoting the growth of green gram.