Rice OsMYB9 enhances salt stress tolerance by regulating the vacuolar Na⁺/H⁺ antiporter

Salinity is a major hazard to crop plant growth and significantly reduces grain yield. When subjected to high salinity, plants maintain ion homeostasis through Na⁺ compartmentalization and exclusion, enabling them to mitigate salt stress. In this study, we revealed the role of rice (Oryza sativa) MYB9 (OsMYB9) in regulating salt stress tolerance. OsMYB9 was expressed in the vascular bundles of roots and leaves, particularly in the parenchyma cells. The null mutation of OsMYB9 resulted in increased sodium ion accumulation in shoot tissues under salt stress, leading to salt-sensitive phenotypes. Real-Time Quantitative Reverse Transcription PCR analysis indicated the OsMYB9 mutation led to decreased expression of several genes associated with vacuolar Na⁺/H⁺ antiporters (OsNHX1, OsNHX2, OsNHX3, OsNHX4, and OsNHX5), high-K⁺ affinity transporters (OsHKT1;5), and SALT OVERLY SENSITIVE proteins (OsSOS2 and OsSOS3). Among these, OsMYB9 upregulated the expression of OsNHX1 and OsNHX2 by directly binding to their promoter regions. Furthermore, GIGANTEA (OsGI) interacted with OsMYB9, suggesting that OsGI negatively acts upstream of OsMYB9 and regulates the expression levels of OsNHX1. Collectively, OsMYB9 alleviates the excess accumulation of Na⁺ ions in the xylem by retrieving Na⁺ ions from xylem parenchyma cells and compartmentalizing them into vacuoles. These regulatory mechanisms mediated by OsMYB9 are crucial for minimizing Na⁺ toxicity in photosynthetic tissues and enhancing salt stress tolerance in rice.