K Transporter AtCHX17 with Its Hydrophilic C Tail Localizes to Membranes of the Secretory/Endocytic System: Role in Reproduction and Seed Set

Four cation/H exchanger (CHX) proteins localize to prevacuolar compartments and plasma membrane, and sorting of AtCHX17 to its destination is dependent on the hydrophilic C tail. Quadruple mutants show reduced seed set implying a role of K transport and pH homeostasis in endosome trafficking important for reproduction.The importance of sorting proteins and wall materials to their destination is critical for plant growth and development, though the machinery orchestrating membrane trafficking is poorly understood. Transporters that alter the environment across endomembrane compartments are thought to be important players. Using Escherichia coli and yeast, we previously showed that several Arabidopsis Cation/H eXchanger (AtCHX) members were K transporters with a role in pH homeostasis, though their subcellular location and biological roles in plants are unclear. Co-expression of markers with CHX16, CHX17, CHX18, or CHX19 tagged with a fluorescent protein indicated these transporters associated with plasma membrane (PM) and post-Golgi compartments. Under its native promoter, AtCHX17((1820))GFP localized to prevacuolar compartment (PVC) and to PM in roots. Brefeldin A diminished AtCHX17GFP fluorescence at PM, whereas wortmannin caused formation of GFP-labeled ring-like structures, suggesting AtCHX17 trafficked among PVC, vacuole and PM. AtCHX17((1472)) lacking its carboxylic tail did not associate with PVC or PM in plant cells. Single chx17 mutant or higher-order mutants showed normal root growth and vegetative development. However, quadruple (chx16chx17chx18chx19) mutants were reduced in frequency and produced 50%70% fewer seeds, indicating overlapping roles of several AtCHX17-related transporters in reproduction and/or seed development. Together, our results suggest that successful reproduction and seed development depend on the ability to regulate cation and pH homeostasis by AtCHX17-like transporters on membranes that traffic in the endocytic and/or secretory pathways.