Regulatory Mechanism of the GmMYB14 Transcription Factor on Auxin-Related Proteins in Soybean

In a previous study, <i>GmMYB14</i> overexpressing (<i>GmMYB14-OX</i>) transgenic soybean plants displayed a semi-dwarfism and compact phenotype, which was regulated by the brassinosteroid (BR) pathway. However, the phenotype of <i>GmMYB14-OX</i> plants could be partly rescued after spraying them with exogenous BR. This indicates that other hormones, in addition to BR, also play a role in regulating the architecture of <i>GmMYB14-OX</i> plants. We observed a significant decrease in the content of endogenous indole-3-acetic acid (IAA) in transgenic soybean lines (OX9 and OX12) compared to wild type (WT) plants. The plant height, leaf area, leaf petiole length, and leaf petiole angle of <i>GmMYB14-OX</i> plants could also be partly rescued after applying exogenous IAA for two weeks. Transcriptome sequencing analysis revealed that the expression of many genes within the Aux/IAA gene family underwent alterations in the <i>GmMYB14-OX</i> transgenic soybean plants. Among them, <i>Glyma.02G000500</i> (<i>GmIAA1</i>) showed the highest expression in <i>GmMYB14-OX</i> plants. Furthermore, the results of electrophoretic mobility shift assay and dual-luciferase reporter indicate that GmMYB14 protein could bind to the promoter of <i>GmIAA1</i>. In summary, a decrease in endogenous IAA content may be one of the factors contributing to the compact and dwarfed architecture of <i>GmMYB14-OX</i> plants. GmMYB14 also acts as a transcriptional activator of <i>GmIAA1</i> to potentially block IAA effects. Our findings provide a theoretical basis for further investigation of the regulatory mechanism of GmMYB14 on soybean plant architecture.