Abstract: Drought and salinity are two major factors limiting soybean productivity worldwide, and both stresses can negatively affect plant growth in similar manners by disrupting cellular redox homeostasis and metabolism. However, the regulatory networks coordinating transcriptional control and metabolic adaptation under both types of stress remain poorly understood. Here, we demonstrate that the R2R3–MYB transcription factors GmMYB60a and GmMYB60b serve as core negative regulators of drought and salt tolerance in soybean. Loss-of-function mutations in GmMYB60a/b clearly increased stress tolerance, which was associated with reduced reactive oxygen species accumulation and increased antioxidant capacity. The GmMYB60 proteins directly bind to and transcriptionally repress GmCYS20, a key gene involved in cysteine biosynthesis, thereby reducing cellular cysteine and glutathione pools. Notably, the GmCYS20 protein physically interacts with GmMYB60 in the nucleus and weakens its ability to bind DNA, resulting in the formation of a transcription–metabolic feedback loop that stabilizes redox homeostasis under stress. Furthermore, the stress-responsive NAC transcription factor GmNAC3 directly suppresses GmMYB60 expression, thereby linking environmental signals to metabolic regulation. Together, these findings reveal a hierarchical GmNAC3–GmMYB60–GmCYS20 regulatory module that coordinates cysteine homeostasis and oxidative stress responses by integrating transcriptional repression with metabolic feedback. This work provides mechanistic insights into soybean stress adaptation and identifies promising genetic targets for improving soybean resistance to drought and salinity.

Key words: cysteine, GmCYS20, GmMYB60, GmNAC3, ROS homeostasis, soybean, tolerance to drought and salt