Root Response to K+-Deprivation in Wheat (Triticum aestivum L.): Coordinated Roles of HAK Transporters, AKT2 and SKOR K+-Channels, and Phytohormone Regulation

Potassium cation (K+) is essential for wheat (Triticum aestivum L.) growth, but the regulatory mechanisms of root response to K+ deficiency are not well understood. This study examines how varying durations of K+-deprivation affect root K+ transport and homeostasis in two wheat varieties, XN979 and YM68. Field pot experiments over three growing seasons showed that XN979 has significantly higher K uptake and productive efficiency than YM68 at a K fertilizer application rate of 60 kg hm−2. Hydroponic experiments revealed that XN979 has a lower Km (K+ concentrations at which 1/2 of Vmax) and a higher Vmax (maximum rate of K+ uptake) in K+ uptake kinetics, indicating better adaptation to K+-deficient environments. RNA-seq analysis after different durations of K+ deficiency (0, 6, 12, 24, 48 h) showed that genes encoding the Arabidopsis K+ Transporter 1 (AKT1) K+-channel in both varieties were not significantly upregulated. Instead, K+ transport in root primarily depended on high-affinity K+ (HAK) transporters. Genes encoding the Arabidopsis K+ Transporter 2 (AKT2) K+-channel in phloem cells were significantly upregulated under K+-deprivation. KOR1 and KOR2, encoding the Stelar K+ Outward Rectifier (SKOR) K+-channel in xylem cells, were significantly downregulated after 6 h and 12 h of K+-deprivation, respectively. Significant changes in the expression levels of the Calcineurin B-Like protein–CBL-Interacting Protein Kinase (CBL-CIPK) signaling system and phytohormones synthesis-related genes suggest their involvement in the root response to K+-deprivation. These findings clarify the regulation of wheat root responses to K deficiency.