The intracellular potassium (K(+) ) homeostasis, which is crucial for plant survival in saline environments, is modulated by K(+) channels and transporters. Some members of the high-affinity K(+) transporter (HAK) family are believed to function in the regulation of plant salt tolerance, but the physiological mechanisms remain unclear. Here, we report a significant inducement of OsHAK21 expression by high-salinity treatment and provide genetic evidence of the involvement of OsHAK21 in rice salt tolerance. Disruption of OsHAK21 rendered plants sensitive to salt stress. Compared with the wild type, oshak21 accumulated less K(+) and considerably more Na(+) in both shoots and roots, and had a significantly lower K(+) net uptake rate but higher Na(+) uptake rate. Our analyses of subcellular localizations and expression patterns showed that OsHAK21 was localized in the plasma membrane and expressed in xylem parenchyma and individual endodermal cells (putative passage cells). Further functional characterizations of OsHAK21 in K(+) uptake-deficient yeast and Arabidopsis revealed that OsHAK21 possesses K(+) transporter activity. These results demonstrate that OsHAK21 may mediate K(+) absorption by the plasma membrane and play crucial roles in the maintenance of the Na(+) /K(+) homeostasis in rice under salt stress.

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Potassium (K+) influx into pollen tubes via K+ transporters is essential for pollen tube growth; however, the mechanism by which K+ transporters are regulated in pollen tubes remains unknown. Here, we report that Arabidopsis thaliana Ca2+-dependent protein kinase11 (CPK11) and CPK24 are involved in Ca2+-dependent regulation of the inward K+ (K+  in) channels in pollen tubes. Using patch-clamp analysis, we demonstrated that K+  in currents of pollen tube protoplasts were inhibited by elevated [Ca2+]cyt. However, disruption of CPK11 or CPK24 completely impaired the Ca2+-dependent inhibition of K+  in currents and enhanced pollen tube growth. Moreover, the cpk11 cpk24 double mutant exhibited similar phenotypes as the corresponding single mutants, suggesting that these two CDPKs function in the same signaling pathway. Bimolecular fluorescence complementation and coimmunoprecipitation experiments showed that CPK11 could interact with CPK24 in vivo. Furthermore, CPK11 phosphorylated the N terminus of CPK24 in vitro, suggesting that these two CDPKs work together as part of a kinase cascade. Electrophysiological assays demonstrated that the Shaker pollen K+  in channel is the main contributor to pollen tube K+  in currents and acts as the downstream target of the CPK11-CPK24 pathway. We conclude that CPK11 and CPK24 together mediate the Ca2+-dependent inhibition of K+  in channels and participate in the regulation of pollen tube growth in Arabidopsis.

Remodeling of auxin distribution during the integration of plant growth responses with the environment requires the precise control of auxin influx and efflux transporters. The plasma membrane-localized PIN-FORMED (PIN) proteins facilitate auxin efflux from cells, and their activity is regulated by reversible phosphorylation. How PIN modulates plant cellular responses to external stresses and whether its activity is coordinated by phospholipids remain unclear. Here, we reveal that, in Arabidopsis (Arabidopsis thaliana), the phosphatidic acid (PA)-regulated PINOID (PID) kinase is a crucial modulator of PIN2 activity and auxin redistribution in response to salt stress. Under salt stress, loss of phospholipase D function impaired auxin redistribution and resulted in markedly reduced primary root growth; these effects were reversed by exogenous PA. The phospholipase D-derived PA interacted with PID and increased PID-dependent phosphorylation of PIN2, which activated auxin efflux and altered auxin accumulation, promoting root growth when exposed to salt stress. Ablation of the PA binding motif not only diminished PID accumulation at the plasma membrane but also abolished PA-promoted PID phosphorylation of PIN2 and its function in coping with salt stress; however, this ablation did not affect inflorescence and cotyledon development or PIN2-dependent gravitropic and halotropic responses. Our data indicate a role for PA in coupling extracellular salt signaling to PID-directed PIN2 phosphorylation and polar auxin transport, highlighting the importance of lipid-protein interactions in the spatiotemporal regulation of auxin signaling.