Osmotic adjustment plays a fundamental role in water stress responses and growth in plants; however, the molecular mechanisms governing this process are not fully understood. Here, we demonstrated that the KUP potassium transporter family plays important roles in this process, under the control of abscisic acid (
ABA) and auxin. We generated
Arabidopsis thaliana multiple mutants for K
+ uptake transporter 6 (KUP6),
KUP8,
KUP2/SHORT HYPOCOTYL3, and an
ABA-responsive potassium efflux channel, guard cell outward rectifying K
+ channel (GORK). The triple mutants,
kup268 and
kup68 gork, exhibited enhanced cell expansion, suggesting that these KUPs negatively regulate turgor-dependent growth. Potassium uptake experiments using
86radioactive rubidium ion (
86Rb
+) in the mutants indicated that these KUPs might be involved in potassium efflux in
Arabidopsis roots. The mutants showed increased auxin responses and decreased sensitivity to an auxin inhibitor (1-
N-naphthylphthalamic acid) and
ABA in lateral root growth. During water deficit stress,
kup68 gork impaired
ABA-mediated stomatal closing, and
kup268 and
kup68 gork decreased survival of drought stress. The protein kinase SNF1-related protein kinases 2E (SRK2E), a key component of
ABA signaling, interacted with and phosphorylated KUP6, suggesting that KUP functions are regulated directly via an
ABA signaling complex. We propose that the KUP6 subfamily transporters act as key factors in osmotic adjustment by balancing potassium homeostasis in cell growth and drought stress responses.
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