Arabidopsis ankyrin-repeat kinase APK2 negatively regulates salt tolerance by degrading the sugar transporter STP11

Soil salinity is one of the major abiotic stressors that seriously affects plant growth and yield. Soluble sugars activated by salt stress serve as the osmoprotectants to improve salt tolerance for plants, and meanwhile they could provide energy sources supporting plant growth and development. However, the molecular mechanism that negatively regulates sugar accumulation under salt stress is not fully understood. Here, we provide evidences that the functions of the ankyrin-repeat kinase gene 2 ( APK2) under salt stress in Arabidopsis. The protein APK2 interacts with and phosphorylates the sugar transporter STP11 during salt stress response. The phosphorylated STP11 is degraded and affects its glucose transport ability. As expected, the apk2 mutant increased salt tolerance. Importantly, salt sensitive phenotype of the stp11 mutant was recovered by the mutation of phosphorylation site Thr . These results totally indicate that APK2 regulates salt tolerance by phosphorylating STP11 protein, which then modulates STP11 protein stability and transport activity. Thus, the study reveals a new molecular mechanism underlying salt-induced stress adaptation by balancing the cellular content of soluble sugar via enhanced degradation of sugar transporter.