Overexpression of the potassium channel TPKb in small vacuoles confers osmotic and drought tolerance to rice

Potassium (K+) is the most important cationic nutrient for all living organisms. Vacuolar two-pore K+ (TPK) channels are important players in the regulation of cellular levels of K+ but have not been characterised in rice. In order to assess the role of OsTPKb, a K+ selective ion channel predominantly expressed in the tonoplast of small vacuoles, we generated overexpressing (OX) lines using a constitutive promoter and compared their phenotypes with control plants. Relative to control plants, OX lines showed better growth when exposed to low-K+ or water stress conditions. K+ uptake was greater in OX lines which may be driven by increased AKT1 and HAK1 activity. The enhanced K+ uptake led to tissue K+ levels that were raised in roots and shoots. Furthermore, energy dispersive X-ray (EDX) analyses showed a higher cytoplasm: vacuole K+ ratio which is likely to contribute to the increased stress tolerance. In all, the data suggest that TPKb can alter the K+ status of small vacuoles, which is important for general cellular K+ homeostasis which, in turn, affects stress tolerance.