Phosphorylation Promotes Liquid-Liquid Phase Separation of GRP8 and Its Assembly into Stress Granules Upon Salinity Stress in Arabidopsis

Drought and salinity are major environmental stresses affecting plant development and growth. SNF1-related protein kinases type 2 (SnRK2s) are key regulators of the plant responses to water deficit and salt stress. Here, we show that Arabidopsis thaliana Glycine-Rich RNA-Binding Protein 8 (GRP8) is a target of abscisic acid (ABA)-non-activated SnRK2s and negatively regulates root growth and seed germination under salt stress. In response to salinity, GRP8 assembles into stress granules (SGs). We show that in addition to the GRP8 C-terminal glycine-rich intrinsically disordered region (IRD), the N-terminal RNA recognition motif (RRM) plays a key role in this process. Phosphorylation of S27 in the RRM by SnRK2s significantly affects the structural dynamics of GRP8, facilitates its dimerization and subsequent liquid-liquid phase separation. Thus, we show that in addition to the known role of IDRs in recruitment into SGs, the RRM plays a decisive role.