Unveiling the GA₄-Ferulic Acid Regulatory Axis: Redox-Mediated Suberization Governs Adventitious Rooting Recalcitrance in <i>Pinus massoniana</i>

<i>Pinus massoniana</i>, a critically important afforestation species in subtropical China, shows severe adventitious rooting recalcitrance linked to endogenous gibberellin (GA) dysregulation. Our study reveals a GA₄-mediated regulatory network that coordinates hormonal crosstalk, redox homeostasis, and cell wall remodeling. Treatment with the GA biosynthesis inhibitor paclobutrazol (PBZ, 100 mg·L⁻¹) shortened rooting time by 32.5% and increased rooting success by 79.5%. We found that PBZ redirected GA flux by upregulating GA₃-oxidase (GA₃OX), leading to GA₄ accumulation. However, elevated GA₄ levels impaired root development by triggering suberization through ferulic acid (FA)-mediated redox imbalance. Application of GA₄ (100 mg·L⁻¹) reduced caffeoyl alcohol content by 54.4% but increased FA and caffeic acid levels 2.4–3.9-fold, shifting lignin precursors toward suberin biosynthesis. FA modulated H₂O₂ flux in a dose-dependent manner: 200 mg·L⁻¹ optimized redox homeostasis (93.7% lower H₂O₂ influx), whereas 1000 mg·L⁻¹ suppressed mitosis. The combination of PBZ (100 mg·L⁻¹) and FA (200 mg·L⁻¹) synergistically enhanced rooting success by 34.4% and achieved 95.8% field survival after two years (vs. 68.5% in controls), challenging the traditional view that lignification alone limits rooting in woody plants. This work provides the first evidence that the GA₄-FA axis controls adventitious root formation in conifers via a Reactive oxygen species (ROS)-dependent switch between suberin and lignin metabolism, offering new strategies to overcome rooting barriers. The PBZ + FA protocol enables scalable clonal propagation of recalcitrant conifers, with potential applications in molecular breeding and forest restoration.