SOS2 phosphorylates FREE1 to regulate multi-vesicular body trafficking and vacuolar dynamics under salt stress.

Salt stress causes ion toxicity in plant cells and limits plant growth and crop productivity. Sodium ions (Na+) are transported out of the cell and sequestered in the vacuole for detoxification under salt stress. The salt excretion system is controlled by the SALT OVERLY SENSITIVE (SOS) pathway, which consists of the calcium sensors SOS3 and SOS3-LIKE CALCIUM-BINDING PROTEIN 8, the protein kinase SOS2, and the plasma membrane Na+/H+ antiporter SOS1. Although much is known about salt responses in plants at the molecular level, it remains unclear if and how plants respond to salt stress through endomembrane remodelling. In this study, we describe a mechanism of salt tolerance in Arabidopsis (Arabidopsis thaliana) involving the modulation of FREE1 levels, which impacts multivesicular body (MVB) trafficking. Specifically, the ESCRT-I (endosomal sorting complex required for transport-I) component FREE1 (FYVE DOMAIN PROTEIN REQUIRED FOR ENDOSOMAL SORTING 1) regulates vacuole fragmentation to enhance salt tolerance. SOS2 phosphorylates FREE1, leading to its degradation and affecting MVB maturation, thereby reducing MVB-vacuole fusion and regulating endomembrane dynamics in response to salt stress. These findings highlight the adaptive role of the plant endomembrane system in coping with salt stress.