Transcriptome and Targeted Hormone Metabolome Reveal the Mechanism of Flower Abscission in Soybeans Under Shade

Shade-induced flower abscission in soybean plants is a significant factor limiting yield improvement. Under shaded conditions, significant differences exist in the flower abscission rates among different soybean varieties, but the regulatory mechanisms remain unclear. This study selected Tiedou 44 (T44) and Liaodou 32 (L32) as experimental materials. Results indicate that under shaded conditions, the flower abscission rate of T44 was significantly higher than that of L32. Physiological analysis revealed that cell wall degradation enzyme activity in T44 pedicels was significantly higher than in L32. Furthermore, compared to L32, T44 flowers under shade conditions exhibited significantly higher levels of IAA, IAA-amino acid conjugates, and ABA. The expression levels of PIN family genes (GMPIN3C, GMPIN3D, PIN3A, GMPIN1A, GMPIN1B, GMPIN1C, GMPIN1D, and GMPIN1E) in T44 were downregulated. These results suggest that the obstruction of auxin polar transport leads to auxin accumulation in flowers. This accumulation, in turn, triggers flower abscission. Additionally, GH3 gene expression was upregulated in T44 compared to L32. GH3 proteins catalyze the conjugation of free auxin (IAA) with amino acids, forming inactive IAA-amino acid complexes. This significantly reduces the concentration of free IAA capable of inhibiting abscission in T44, making flowers more prone to abscission. This study provides crucial insights into the molecular regulatory mechanisms underlying flower abscission in soybean.