Isolation and characterisation of plant growth-promoting bacteria from paddy (Oryza sativa) rhizosphere

Plant growth-promoting bacteria (PGPB) are bacteria that inhabit the plant rhizosphere and can promote plant growth by aiding in the acquisition essential nutrients and plant growth regulators through various mechanisms, including solubilisation of insoluble phosphate (P) in the soil, fixation of atmospheric nitrogen suitable for plant uptake, and production of phytohormone such as indole-3-acetic acid (IAA). This study aims to isolate and identify PGPB isolates from paddy rhizosphere, characterise plant growth- promoting traits such as P solubilising activity, assess IAA phytohormone production and nitrogen-fixing activity, and validate plant growth-promoting activity via in vitro seed vigour index (SVI) test on MR297 paddy seeds. Fifteen PGPB were isolated and identified by 16s rRNA sequence have close relatedness with bacterial species as follows: UMK-PPS2 (Burkholderia vietnamiensis, 99.86%); UMK-PPS3 (Frateuria aurantia, 99.73%); UMK-PPS4, UMK-PPS6, and UMK-PPS8 (Paraburkholderia kururiensis, 100%, 100%, and 98.50%); UMK-PNF2 (Bacillus proteolyticus, 100%); UMK-PNF3 (Pseudarthrobacter niigatensis, 98.97%); UMK-PNF5 (Pseudarthrobacter enclensis, 98.17%); UMK-PNF6 (Bacillus zanthoxyli, 99.26%); UMK-PNF10 (Microbacterium hominis, 98.63%); UMK-PM1 and UMK-PM2 (Methylorubrum thiocyanatum, 96.92% and 98.35%); UMK-PM3 and UMK-PM4 (Methylorubrum salsuginis, 97.73% and 94.05%); and UMK-PM5 (Methylobacterium radiotolerants, 99.29%). It was discovered that all isolates could solubilise P, with Frateuria sp. UMK-PPS3 exhibiting the highest solubilisation index (SI); P solubilisation activity; and phytase activity from SI 1.14 ± 0.01 to SI 8.00 ± 1.00; 0.058 ± 0.013 mg/mL to 0.657 ± 0.079; and 24.180 ± 1.225 U/mL to 204.667 ± 17.546 U/mL. Contrarily, the isolates could produce IAA with the addition of L-tryptophan (Trp) as an IAA precursor varying between 4.378 ± 0.437 μg/mL to 54.704 ± 11.457 μg/mL, with Pseudarthrobacter sp. UMK-PNF5 as the highest IAA producer. Furthermore, all isolates can fix nitrogen due to the good growth on Burk’s media. Other than that, paddy seeds treated with PGPB isolates showed improved seed germination; better shoot production; and root elongation, except for Bacillus sp. UMK- PNF2. The highest SVI was shown by Pseudarthrobacter sp. UMK-PNF3 (686.067 ± 15.079). The results suggest that all PGPB isolates have plant growth-promoting traits and are good potential candidates to be exploited as plant growth promoter biofertiliser replacing chemical fertiliser.