Seed-borne endophytic Bacillus amyloliquefaciens RWL-1 produces gibberellins and regulates endogenous phytohormones of Oryza sativa

Some microorganisms are adapted to an endophytic mode, living symbiotically with plants through vertical transmission in seeds. The role of plant growth-promoting endophytes has been well studied, but those of seed-associated endophytic bacteria are less understood. The current study aimed to isolate and identify bacterial endophytes associated with rice (Oryza sativa L. 'Jin so mi') seeds, their potential to produce gibberellins (GAs), and role in improving host-plant physiology. The isolated bacterial endophyte RWL-1 was identified as Bacillus amyloliquefaciens by using 16S rRNA sequencing and phylogenetic analysis. The pure culture of B. amyloliquefaciens RWL-1, supplied with deuterated internal standards, was subjected to gas chromatography and mass spectrometric selected ion monitoring (GC-MS/SIM) for quantification of GAs. Results showed the presence of GAs in various quantities (ng/mL) viz., GA₂₀ (17.88 ± 4.04), GA₃₆ (5.75 ± 2.36), GA₂₄ (5.64 ± 2.46), GA₄ (1.02 ± 0.16), GA₅₃ (0.772 ± 0.20), GA₉ (0.12 ± 0.09), GA₁₉ (0.093 ± 0.13), GA₅ (0.08 ± 0.04), GA₁₂ (0.014 ± 0.34), and GA₈ (0.013 ± 0.01). Since endogenous seed GAs are essential for prolonged seed growth and subsequent plant development, we used exogenous GA₃ as a positive control and water as a negative control for comparative analysis of the application of B. amyloliquefaciens RWL-1 to rice plants. The growth parameters of rice plants treated with endophytic bacterial cell application was significantly increased compared to the plants treated with exogenous GA₃ and water. This was also revealed by the significant up-regulation of endogenous GA₁ (17.54 ± 2.40 ng), GA₄ (310 ± 5.41 ng), GA₇ (192.60 ± 3.32 ng), and GA₉ (19.04 ± 2.49 ng) as compared to results of the positive and negative control treatments. Rice plants inoculated with B. amyloliquefaciens RWL-1 exhibited significantly higher endogenous salicylic acid (1615.06 ± 10.81 μg), whereas endogenous abscisic acid (23.31 ± 2.76 ng) and jasmonic acid (25.51 ± 4.20 ng) were observed to be significantly lower in these inoculated plants than in those treated with exogenous GA₃ and water. Results of the present study suggest that B. amyloliquefaciens RWL-1 has the ability to produce GAs and that its inoculation in seedlings can be beneficial to rice plants. Broader field trials should be conducted to determine its use as an alternative biofertilizer.