These results suggest that Cl(-) plays a role in the IAA-induced growth of maize coleoptile segments. A possible mechanism for Cl(-) uptake during IAA-induced growth is proposed in which uptake of K(+) and Cl(-) ions in concert with IAA-induced plasma membrane H(+)-ATPase activity changes the membrane potential to a value needed for turgor adjustment during the growth of maize coleoptile cells.

BackgroundAuxin (IAA) is a central player in plant cell growth. In contrast to the well-established function of the plasma membrane in plant cell expansion, little is known about the role of the vacuolar membrane (tonoplast) in this process.ResultsIt was found that under symmetrical 100 mM K+ and 100 μM cytoplasmic Ca2+ the macroscopic currents showed a typical slow activation and a strong outward rectification of the steady-state currents. The addition of IAA at a final concentration of 1 μM to the bath medium stimulated the SV currents, whereas at 0.1 and 10 μM slight inhibition of SV currents was observed. The time constant, τ, decreased in the presence of this hormone. When single channels were analyzed, an increase in their activity was recorded with IAA compared to the control. The single-channel recordings that were obtained in the presence of IAA showed that auxin increased the amplitude of the single-channel currents. Interestingly, the addition of IAA to the bath medium with the same composition as the one that was used in the patch-clamp experiments showed that auxin decreased the volume of the vacuoles.ConclusionsIt is suggested that the SV channels and the volume of red beet taproot vacuoles are modulated by auxin (IAA).

In contrast to the well-studied effect of auxin on the plasma membrane K+ channel activity, little is known about the role of this hormone in regulating the vacuolar K+ channels. Here, the patch-clamp technique was used to investigate the effect of auxin (IAA) on the fast-activating vacuolar (FV) channels. It was found that the macroscopic currents displayed instantaneous currents, which at the positive potentials were about three-fold greater compared to the one at the negative potentials. When auxin was added to the bath solution at a final concentration of 1 µM, it increased the outward currents by about 60%, but did not change the inward currents. The imposition of a ten-fold vacuole-to-cytosol KCl gradient stimulated the efflux of K+ from the vacuole into the cytosol and reduced the K+ current in the opposite direction. The addition of IAA to the bath solution with the 10/100 KCl gradient decreased the outward current and increased the inward current. Luminal auxin reduced both the outward and inward current by approximately 25% compared to the control. The single channel recordings demonstrated that cytosolic auxin changed the open probability of the FV channels at the positive voltages to a moderate extent, while it significantly increased the amplitudes of the single channel outward currents and the number of open channels. At the positive voltages, auxin did not change the unitary conductance of the single channels. We suggest that auxin regulates the activity of the fast-activating vacuolar (FV) channels, thereby causing changes of the K+ fluxes across the vacuolar membrane. This mechanism might serve to tightly adjust the volume of the vacuole during plant cell expansion.

The role of K+ and Ca2+ in plant growth and development is complex and needs extensive physiological studies. Here, the effects of K+ and Ca2+ ions on growth and membrane potential in the presence of either auxin (IAA) or fusicoccin (FC) were studied in maize coleoptiles. The results presented in this article demonstrate that the inhibitory effect of Ca2+ on growth is significantly restored in the presence of K+. This effect is probably due to competitive inhibition of K+ channels by Ca2+ ions, although other possibilities should be taken into consideration.

No abstract