A Shade Intensity Threshold Triggers Gibberellin Metabolic Reprogramming to Drive Shade Avoidance in Soybean

Shade stress severely constrains soybean yield in soybean-maize intercropping systems, yet the intensity threshold triggering shade avoidance responses and the underlying hormonal mechanisms remain elusive. Here, through two-year field experiments, we demonstrate that shading coverage rate exceeding 80% is the critical threshold initiating shade avoidance, increasing plant height by 16–61%. The seventh internode is the initial responsive site where epidermal cells elongate by 69.86% longitudinally, while radial growth is broadly suppressed (cell area reduced by approximately 41%). Furthermore, integrated hormonal profiling and gene expression analyses demonstrate that shade promotes active gibberellin (GA) accumulation via dual metabolic reprogramming: upregulating biosynthetic genes GmGA20ox1 and GmGA3ox1 and downregulating catabolic genes GmGA2ox–7a and GmGA2ox–7b, leading to a 56.87% increase in GA1 content. Notably, exogenous GA fully mimics the shade-induced phenotype, with genotype sensitivity ranking BX10 > BD2 > W82, indicating that GA plays a central role in mediating the shade response. Yield analysis shows that shade inhibits soybean biomass accumulation and grain yield and preferentially suppresses reproductive rather than vegetative growth, reducing effective pod number and seeds per plant by ~50% whereas 100-seed weight only decreases by 6.55%. Together, these findings reveal that shade modulates GA homeostasis via “enhanced biosynthesis and suppressed catabolism” to remodel internodes, providing a theoretical basis for shade-tolerant soybean breeding and optimizing intercropping systems.