Close Temporal Relationship between Oscillating Cytosolic K⁺ and Growth in Root Hairs of Arabidopsis

Root hair elongation relies on polarized cell expansion at the growing tip. As a major osmotically active ion, potassium is expected to be continuously assimilated to maintain cell turgor during hair tip growth. However, due to the lack of practicable detection methods, the dynamics and physiological role of K⁺ in hair growth are still unclear. In this report, we apply the small-molecule fluorescent K⁺ sensor NK3 in Arabidopsis root hairs for the first time. By employing NK3, oscillating cytoplasmic K⁺ dynamics can be resolved at the tip of growing root hairs, similar to the growth oscillation pattern. Cross-correlation analysis indicates that K⁺ oscillation leads the growth oscillations by approximately 1.5 s. Artificially increasing cytoplasmic K⁺ level showed no significant influence on hair growth rate, but led to the formation of swelling structures at the tip, an increase of cytosolic Ca²⁺ level and microfilament depolymerization, implying the involvement of antagonistic regulatory factors (e.g., Ca²⁺ signaling) in the causality between cytoplasmic K⁺ and hair growth. These results suggest that, in each round of oscillating root hair elongation, the oscillatory cell expansion accelerates on the heels of cytosolic K⁺ increment, and decelerates with the activation of antagonistic regulators, thus forming a negative feedback loop which ensures the normal growth of root hairs.