<scp>KUP</scp> 9 maintains root meristem activity by regulating K + and auxin homeostasis in response to low K

Potassium (K) is essential for plant growth and development. Here, we show that the KUP/HAK/KT K+ transporter KUP9 controls primary root growth in Arabidopsis thaliana. Under low-K+ conditions, kup9 mutants displayed a short-root phenotype that resulted from reduced numbers of root cells. KUP9 was highly expressed in roots and specifically expressed in quiescent center (QC) cells in root tips. The QC acts to maintain root meristem activity, and low-K+ conditions induced QC cell division in kup9 mutants, resulting in impaired root meristem activity. The short-root phenotype and enhanced QC cell division in kup9 mutants could be rescued by exogenous auxin treatment or by specifically increasing auxin levels in QC cells, suggesting that KUP9 affects auxin homeostasis in QC cells. Further studies showed that KUP9 mainly localized to the endoplasmic reticulum (ER), where it mediated K+ and auxin efflux from the ER lumen to the cytoplasm in QC cells under low-K+ conditions. These results demonstrate that KUP9 maintains Arabidopsis root meristem activity and root growth by regulating K+ and auxin homeostasis in response to low-K+ stress.