In addition, CKs were found to exert a negative effect on expression of SULTR1;1 and SULTR1;2, resulting in a reduction of sulfate uptake in roots. AHK3 and AHK4 are also involved in the root iron uptake machinery in Arabidopsis by negatively regulating the expression of genes which are induced by iron deficiency. Taken together, these studies demonstrate that CKs play a role in the response to the limitations of various nutrients in plants. However, the roles of CKs in low K signaling are still unclear at the present time. Here, we show that CK receptor mutants lose their responsiveness to low K signaling through the measurement of ROS accumulation and root growth under low K conditions. Additionally, we found that CKs affected the induction of HAK5 expression and function under low K conditions. Finally, we provide evidence that CKs negatively regulate low K response. Previous studies have shown that multiple phytohormones regulate low-K signaling, that can lead to effects on gene expression, reduced primary root growth, reduced lateral root growth, and increased root hair growth. One typical phenotype of low-K-grown Arabidopsis plants is the reduction of lateral root numbers. Auxin is known to be a positive regulator of lateral root development. Independent from primary and lateral root growth, BAY 43-9006 284461-73-0 ethylene has been reported to act as a positive regulator of low K-dependent ROS accumulation, HAK5 expression and the induction of root hair growth. Low K also leads to a decrease in primary root growth that may be regulated by ABA. Jasmonate was also shown to regulate low K-dependent gene expression. In this study, we found that CKs function differently from other hormones by acting as negative regulators in low K-dependent HAK5 expression, primary root growth and root hair growth. These data suggest that CKs might function in parallel with ethylene but in an antagonistic behavior in low K signaling. CKs are known to exert influence on the acquisition of several macronutrients, such as nitrogen, phosphorus and sulfur. Specifically, the expression of genes encoding multiple macronutrient transporters, including nitrate transporters, sulfate transporters and phosphate transporters, were decreased by CKs. In the case of nitrogen, CKs and nitrogen are reciprocally influenced. CK content was tightly regulated by nitrogen supply. Higher nitrate-grown Arabidopsis had higher CK levels than low nitrategrown Arabidopsis. In addition, CKs act as long distance root-toshoot signals and local signals for nitrate sensing. CKs could negatively regulate nitrogen uptake via the control of nitrate and ammonium transporter gene expression. Other phosphate and sulfate transporters were regulated similar to nitrate transporters. In our study, we also showed that CKs negatively regulate the gene expression of the high-affinity K transporter HAK5. Moreover, the levels of bioactive CKs were reduced in both roots and shoots with the most drastic reduction observed in roots after 3 days of K deprivation. Collectively, our results support that CKs function as negative regulators of HAK5 gene expression; a regulation that is similar to that of other macronutrient transporters. In this study, we have also demonstrated that CKs control the response to low K conditions through CK signaling by functional analyses of the ahk mutants in response to K deficiency. The results of root growth assays indicated that among the three CK receptor kinases.