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.

Root K+ acquisition is a key process for plant growth and development, extensively studied in the model plant Arabidopsis thaliana. Because important differences may exist among species, translational research supported by specific studies is needed in crops such as tomato. Here we present a reverse genetics study to demonstrate the role of the SlHAK5 K+ transporter in tomato K+ nutrition, Cs+ accumulation and its fertility. slhak5 KO lines, generated by CRISPR‐Cas edition, were characterized in growth experiments, Rb+ and Cs+ uptake tests and root cells K+‐induced plasma membrane depolarizations. Pollen viability and its K+ accumulation capacity were estimated by using the K+‐sensitive dye Ion Potassium Green 4. SlHAK5 is the major system for high‐affinity root K+ uptake required for plant growth at low K+, even in the presence of salinity. It also constitutes a pathway for Cs+ entry in tomato plants with a strong impact on fruit Cs+ accumulation. SlHAK5 also contributes to pollen K+ uptake and viability and its absence produces almost seedless fruits. Knowledge gained into SlHAK5 can serve as a model for other crops with fleshy fruits and it can help to generate tools to develop low Cs+ or seedless fruits crops.

The high-affinity K+ transporter HAK5 from Arabidopsis is essential for K+ acquisition and plant growth at low micromolar K+ concentrations. Despite its functional relevance in plant nutrition, information about functional domains of HAK5 is scarce. Its activity is enhanced by phosphorylation via the AtCIPK23/AtCBL1-9 complex. Based on the recently published 3D-structure of the bacterial ortholog KimA from Bacillus subtilis, we have modeled AtHAK5 and, by a mutational approach, identified residues G67, Y70, G71, D72, D201, and E312 as essential for transporter function. According to the structural model, residues D72, D201, and E312 may bind K+, whereas residues G67, Y70, and G71 may shape the selective filter for K+, which resembles that of K+shaker-like channels. In addition, we show that phosphorylation of residue S35 by AtCIPK23 is required for reaching maximal transport activity. Serial deletions of the AtHAK5 C-terminus disclosed the presence of an autoinhibitory domain located between residues 571 and 633 together with an AtCIPK23-dependent activation domain downstream of position 633. Presumably, autoinhibition of AtHAK5 is counteracted by phosphorylation of S35 by AtCIPK23. Our results provide a molecular model for K+ transport and describe CIPK-CBL-mediated regulation of plant HAK transporters.

Regulation of root transport systems is essential under fluctuating nutrient supply. In the case of potassium (K + ), HAK/KUP/KT K + transporters and voltage-gated K + channels ensure root K + uptake in a wide range of K + concentrations. In Arabidopsis, the CIPK23/CBL1-9 complex regulates both transporter-and channel-mediated root K + uptake. However, research about K + homeostasis in crops is in demand due to species-specific mechanisms. In the present manuscript, we studied the contribution of the voltage-gated K + channel LKT1 and the protein kinase SlCIPK23 to K + uptake in tomato plants by analysing gene-edited knockout tomato mutant lines, together with two-electrode voltage-clamp experiments in Xenopus oocytes and proteinprotein interaction analyses. It is shown that LKT1 is a crucial player in tomato K + nutrition by contributing approximately 50% to root K + uptake under K + -sufficient conditions. Moreover, SlCIPK23 was responsible for approximately 100% of LKT1 and approximately 40% of the SlHAK5 K + transporter activity in planta. Mg +2 and Na + compensated for K + deficit in tomato roots to a large extent, and the accumulation of Na + was strongly dependent on SlCIPK23 function. The role of CIPK23 in Na + accumulation in tomato roots was not conserved in Arabidopsis, which expands the current set of CIPK23-like protein functions in plants.