Abstract
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.
Highlights
Plants have developed a wide range of adaptive mechanisms to control growth and development under various abiotic stresses
A Protein BLAST search against the National Center for Biotechnology Information protein database revealed that AtUNC-93 shared high similarity with other UNC-93 orthologs from various species (Supplementary Figure S1)
Phylogenetic analysis of the UNC-93 sequences of 18 species indicated that these UNC-93 orthologs were classified from plants and animals separately, and the plants were clustered into two clades, dicotyledons and monocotyledons (Supplementary Figure S2)
Summary
Plants have developed a wide range of adaptive mechanisms to control growth and development under various abiotic stresses. The maintenance of K+/Na+ homeostasis is one of the important mechanisms which plants use to increase their adaptation to abiotic stresses. K+ is essential for stress responses, and functions as an important role in cell expansion regulated by turgor pressure. The K+ transport systems triple mutants, kup268 and kup gork, exhibited enhanced cell expansion and significantly decreased drought tolerance in Arabidopsis, suggesting that these K+ transporters act as key factors in K+ homeostasis in both drought stress responses and cell growth (Osakabe et al, 2013). The Na+/H+ antiporters NHX5 and NHX6 play important roles in cellular K+/Na+ homeostasis as the nhx double knockout mutant reduced Arabidopsis tolerance to salt tress and salinity stress and exhibited suppressed growth, with smaller and fewer cells (Bassil et al, 2011)
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