Plant cells must pass a K+ threshold to re-enter the cell cycle

Potassium is an inevitable component of plant life, and potassium channels play a pivotal role in plant growth and development. The role of potassium and of K+ channels in plant cell division and cell-cycle progression, however, has not been determined so far. K+ channel blocker studies with synchronized tobacco BY-2 cells revealed that K+ uptake is required for proper cell-cycle progression during the transition from G(1) to S phase. Electrophysiological studies (patch-clamp and voltage-clamp techniques) showed a cell-cycle dependency of K+ channel activities and reduced driving force for K+ uptake in dividing cells. Among the four Shaker-like K+ channel genes expressed in BY-2 cells, NKT1 represents an inwardly rectifying K+ channel that mediates K+ uptake. NKT1 is transcriptionally induced during G(1) phase, while transcripts of the outward-rectifier NTORK1 dominate S phase. Elongating BY-2 cells appeared hyperpolarized (-101 +/- 11 mV), and had elevated osmotic pressure and approximately twice the turgor pressure when compared with depolarized (-64 +/- 8 mV) dividing cells. This indicates that cells have to gain a threshold K+ level to re-enter the cell cycle. Based on these findings, turgor regulation through modulation of K+ channel density in plant cell division and cell-cycle progression is discussed.