Isolation and partial structure elucidation of gibberellin produced by plant growth promoting bacteria (PGPB) and its effect on the growth of hybrid rice (Oryza sativa L.)

Forty plant growth promoting bacteria (PGPB) were screened for gibberellin (GA) production through the dwarf rice (cv. Tanginbozu) bioassay. Among the GA-producing isolates, TCeR 48 produced the highest amount of GA(81.70 ug GA3 equiv /mL culture). The peak of GA production by TCeR 48 was at 15 days after inoculation. Extracts from TCeR 48 were purified by high performance liquid chromatography (HPLC) and analyzed through gas chromatography coupled with mass spectrometry (GC-MS). The production of ent-kaur-16-ene, an intermediate in the biosynthesis of gibberellin, is indicated by the peaks at m/z 91 and m/z 105. There is a hint that GAsub20 is present due to peaks at m/z 375 and 418 which are similar to the structure of GAsub20, a 13-hydroxy gibberellin. Further analysis by ultraviolet (UV) spectroscopy revealed the presence of a strong peak at lambda max of 215 nm which indicates the presence of unconjugated -OH bond and a low intensity single bond at lambda max of 260 nm which suggest the presence of isolated C=O and -COOH. Fourier transform infrared (FTIR) spectroscopy confirmed the functional groups present in the unknown GA structure. Peaks at 3377.95/cm and 1411.99/cm suggest the presence of -COOH and -CH3 groups, respectively. All of the observed functional groups in the UV and FTIR spectra are characteristics of the structure of GAsub20. However, a distinct peak at 1116.98/cm of the TCeR 48 spectrum indicates the presence of a double bond in the structure of the unknown GA which is absent in GAsub20. Application of the PGPB-derived GA was explored in hybrid rice seed production. AT five days after sowing (DAS), germination of seeds treated with strains TCeR 48, TCeRe 37 w, BTToRe and farmers' practice (incubation at 60 deg C for 5 days) were significantly higher compared to the control. Seedling height of rice treated with the PGPB isolates was higher than the control and the oven-incubated seeds at 21 DAS. Shoot and root dry weights of the seedlings treated with BTToRe were higher compared to the control. Transplanted seedlings showed significant increase in plant height when treated with GA3, and the PGPB isolates at 3 months after transplanting. Tillering was significantly enhanced by treatment with TCeR 48.