Modulation of K+ translocation by AKT1 and AtHAK5 in Arabidopsis plants

Root cells take up K+ from the soil solution, and a fraction of the absorbed K+ is translocated to the shoot after being loaded into xylem vessels. K+ uptake and translocation are spatially separated processes. K+ uptake occurs in the cortex and epidermis whereas K+ translocation starts at the stele. Both uptake and translocation processes are expected to be linked, but the connection between them is not well characterized. Here, we studied K+ uptake and translocation using Rb+ as a tracer in wild‐type Arabidopsis thaliana and in T‐DNA insertion mutants in the K+ uptake or translocation systems. The relative amount of translocated Rb+ to the shoot was positively correlated with net Rb+ uptake rates, and the akt1 athak5 T‐DNA mutant plants were more efficient in their allocation of Rb+ to shoots. Moreover, a mutation of SKOR and a reduced plant transpiration prevented the full upregulation of AtHAK5 gene expression and Rb+ uptake in K+‐starved plants. Lastly, Rb+ was found to be retrieved from root xylem vessels, with AKT1 playing a significant role in K+‐sufficient plants. Overall, our results suggest that K+ uptake and translocation are tightly coordinated via signals that regulate the expression of K+ transport systems. AtHAK5 and AKT1 are expressed in root vascular tissues of K+‐sufficient plants and contribute to Rb+ (K+) translocation in different ways: AtHAK5 promotes Rb+ translocation whereas AKT1 has a negative role on this process by unloading Rb+ from xylem sap.