Mechanism Analyses Of Potassium Channel AKT1 In Low Potassium Sensing And Root Growth Regulation In Arabidopsis

As one of essential macronutrients, potassium (K) plays crucial roles in plant growth and development.Plant root growth is affected by external K~+ concentrations and also controlled by auxin accumulated in root tips. It is hypothesized that there may exist crosstalk between low-K~+ sensing and auxin signaling in regulation of root growth and morphology. The Shaker K~+ channel AKT1 and the KT/KUP/HAK K~+transporter HAK5 are two major components that mediate K~+ uptake in Arabidopsis root. The previous study has demonstrated that the aktl mutant, but not wild type and the hak5 mutant, can keep primary root growth after transferred from MS medium to low-K~+ (LK) medium, suggesting that AKT1 may somehow involve in root growth regulation. The focus of this dissertation work is to investigate the function and mechanism of AKT1 in low-K~+ sensing and root growth regulation as well as potential crosstalk between low-K~+ signals and auxin signaling.Along with the decrease of external K~+ concentrations, the primary root growth of both wild-type plants and the hak5 mutant showed increased inhibition, while the primary root growth of the aktl mutant was not significantly affected. The split-root experiments and K~+ content results indicated that the aktl mutant maintained primary root growth under low-K~+ conditions was due to the defect in low-K~+ sensing.rather than K~+ accumulation in root. We used CsCl and TEA to block AKT1 function in wild-type plants and the hak5 mutant on LK medium, which can stimulate the primary root growth of wild-type plants and the hak5 mutant to mimic the aktl phenotype. Moreover, there was no difference of primary root growth among materials when under low phosphorus, low nitrogen and salt stress. These data indicate that the low-K~+ perception in Arabidopsis root requires AKT1 function.Addition of auxin analogue NAA in LK medium can inhibit aktl root growth,but promote the primary root growth of wild-type plants and the hak5 mutant. The signal strength of ProDR5:GFP in wild-type and the hak5 mutant, which can indicate auxin concentration, was depended on K~+concentration in medium. We also observed that the GFP fluorescence at the root maturation zone of wild-type and the hak5 mutant was increased under low-K~+ stress, indicating that the root acropetal auxin transport was inhibited, while the auxin level in the aktl root tip was completely unaffected. The auxin efflux carrier PIN1 is considered as the important component that is responsible for root acropetal auxin transport. In the ProPIN1:PIN1-GFP marker lines, PIN1 proteins were rapidly degraded in the root of wild-type plants and the hak5 mutant under low-K~+ condition, but not affected in the aktl mutant root. It is known that PIN 1 proteins continually cycle between the plasma membrane and endosomal compartments. The experiments using FM4-64 and BFA indicate that both PIN1 endocytosis and exocytosis were inhibited by low-K~+ stress in the root of wild-type plants and the hak5 mutant, which promoted PIN1 degradation. However, this phenomenon was not observed in the aktl mutant. These results indicate that PIN1 may act as a key component involving in AKT1-mediated low-K~+ sensing.The results presented in this dissertation demonstrate that AKT1 is required for Arabidopsis response to external K~+ changes, and subsequently regulates the K~+-dependent root growth by modulating PIN1 degradation and auxin redistribution in root. This study provides new insight into the mechanism of plant response to low-K~+ stress and root growth regulation.