AKT5 is a bona fide potassium channel and controls petiole growth in Arabidopsis.

Potassium ion (K+) is essential for plant growth and development. Despite decades of study, the Shaker-type K+ channel AKT5 has remained functionally unassigned. Here, we report that AKT5 functions as a bona fide voltage-dependent K+ channel and serves as an essential regulator of petiole elongation. Structural analyses of AKT5 in a closed state and the AKT5-D403 variant in a pre-open state provide direct structural evidence for functional K+ channel activity. AKT5 assembled into a canonical tetramer with two-fold symmetry and, upon phosphorylation, operated as an inward-rectifying K+ channel activated at strongly hyperpolarized membrane potentials. AKT5 was predominantly expressed in young petioles, and loss of AKT5 function largely suppressed petiole elongation throughout the diurnal cycle, resulting in compact rosettes and reduced biomass under crowded growth conditions. These findings show that AKT5 is involved in a regulator of plant competitive growth and high-density performance.