Heterotrophic ammonia-oxidizing bacteria promote regeneration of Italian ryegrass via a dual-action mechanism

Abstract:

[Background] Heterotrophic ammonia-oxidizing bacteria (HAOB) promote the regeneration of Italian ryegrass by increasing the content of leaf cytokinins, although the specific mechanism remains to be elucidated. [Objective] To elucidate how HAOB promote the regeneration of Italian ryegrass by regulating the synthesis of cytokinins via a dual-action mechanism (in the rhizosphere soil and outside the rhizosphere environment, respectively. [Methods] A strain (S2_8_1) of HAOB and potted Italian ryegrass were used in this study. The treatments were designed as follows: Regular watering (TA); Addition of blank medium for enrichment culture (TB); Addition of 3,4-dimethylpyrazole phosphate (DMPP) (TN1); Addition of DMPP and 20 mmol/L NO₃⁻-N (TN2); Addition of DMPP and 30 mmol/L NO₃⁻-N (TN3); Addition of DMPP and 40 mmol/L NO₃⁻-N (TN4); Inoculation with the strain S2_8_1 (TI). [Results] NO₃⁻-N of 30–40 mmol/L was most effective in increasing the content of leaf cytokinins and enhancing the transfer of these hormones from the roots to the leaves, thereby promoting leaf regeneration. When the nitration-induced increase in cytokinins in the rhizosphere soil reached the maximum, inoculation with strain S2_8_1 further boosted the synthesis of these hormones in the roots, leading to an increased concentration of cytokinins in the leaves. The HAOB strain S2_8_1 significantly elevated the level of cytokinins in leaves via a dual-action mechanism, thereby promoting the regeneration of Italian ryegrass. [Conclusion] This study unveils the dual-action mechanism by which HAOB strains in the rhizosphere enhance plant growth, offering a direct application value for improving forage yields and sustainable agriculture development.