Auxin-induced growth and its linkage to potassium channels

We investigated the potassium dependence of rapid auxin-induced growth of scrubbed maize (Zea mays L. cv. Garant) coleoptile segments by means of positional angular transducers. Our setup permitted six simultaneous high-resolution measurements. Provided a buffer with a high concentration of calcium was used in conjunction with well-abraded coleoptiles, (i) there was no auxin-induced growth in the absence of K(+)ions, whereas in the presence of 1-10 mM KCl auxin-induced growth could be detected after a lag phase of 20 min; (ii) auxin-pretreated plants, which were not growing in the absence of K(+)ions displayed an instantaneous growth burst if 1-10 mM KCl was added; (iii) this burst of growth did not occur in the absence of auxin, excluding the possibility that it was due to ''stored'' endogenous growth and (iv) it could not be triggered by Cl-, confirming that it was not an anion effect. The effect was specific for monovalent cations and showed an apparent selectivity for K+ and Rb+ but was not observed with Li+ or Na+. The action of 1 mM K+ was reversibly blocked by 10-30 mM tetraethylammonium (TEA) and 5 mM Ba2+ Measurements of auxin-induced proton secretion using a computer-controlled pH-stat revealed a similar dependency on the ionic composition of the bathing medium and a similar inhibition by TEA. We suggest that both auxin-induced growth and proton secretion strictly depend on extracellular K(+)ions and the uptake of K+ mediated by K+ channels at the plasma membrane. These results generally support the acid-growth theory of auxin action, and also concur with evidence that auxin modulation of K+ channels plays a role in the signalling cascade leading to auxin-induced cell elongation.