The Arabidopsis AtUNC-93 Acts as a Positive Regulator of Abiotic Stress Tolerance and Plant Growth via Modulation of ABA Signaling and K+ Homeostasis

Potassium (K+) is one of the essential macronutrients required for plant growth and development, and the maintenance of cellular K+ homeostasis is important for plants to adapt to abiotic stresses and growth. However, the mechanism involved has not been understood clearly. In this study, we demonstrated that AtUNC-93 plays a crucial role in this process under the control of abscisic acid (ABA). AtUNC-93 was localized to the plasma membrane and mainly expressed in the vascular tissues in Arabidopsis thaliana. The atunc-93 mutants showed typical K+-deficient symptoms under low-K+ conditions. The K+ contents of atunc-93 mutants were significantly reduced in shoots but not in roots under either low-K+ or normal conditions compared with wild type plants, whereas the AtUNC-93-overexpressing lines still maintained relatively higher K+ contents in shoots under low-K+ conditions, suggesting that AtUNC-93 positively regulates K+ translocation from roots to shoots. The atunc-93 plants exhibited dwarf phenotypes due to reduced cell expansion, while AtUNC-93-overexpressing plants had larger bodies because of increased cell expansion. After abiotic stress and ABA treatments, the atunc-93 mutants was more sensitive to salt, drought, osmotic, heat stress and ABA than wild type plants, while the AtUNC-93-overexpressing lines showed enhanced tolerance to these stresses and insensitive phenotype to ABA. Furthermore, alterations in the AtUNC-93 expression changed expression of many ABA-responsive and stress-related genes. Our findings reveal that AtUNC-93 functions as a positive regulator of abiotic stress tolerance and plant growth by maintaining K+ homeostasis through ABA signaling pathway in Arabidopsis.