Abstract
The fungal toxin fusicoccin (FC) induces rapid cell elongation, proton extrusion and plasma membrane hyperpolarization in maize coleoptile cells. Here, these three parameters were simultaneously measured using non-abraded and non-peeled segments with the incubation medium having access to their lumen. The dose–response curve for the FC-induced growth was sigmoidal shaped with the maximum at 10−6 M over 10 h. The amplitudes of the rapid growth and proton extrusion were significantly higher for FC than those for indole-3-acetic acid (IAA). The differences between the membrane potential changes that were observed in the presence of FC and IAA relate to the permanent membrane hyperpolarization for FC and transient hyperpolarization for IAA. It was also found that the lag times of the rapid growth, proton extrusion and membrane hyperpolarization were shorter for FC compared to IAA. At 30 °C, the biphasic kinetics of the IAA-induced growth rate could be changed into a monophasic (parabolic) one, which is characteristic for FC-induced rapid growth. It has been suggested that the rates of the initial phase of the FC- and IAA-induced growth involve two common mechanisms that consist of the proton pumps and potassium channels whose contribution to the action of both effectors on the rapid growth is different.
Highlights
Research on the effects of auxin (IAA) on plant growth has been conducted for quite some time, the molecular mechanism of auxin action on ion transport in growing cells has still not been precisely explained
The results presented here show that the rate of the initial phase of the FC- and indole-3-acetic acid (IAA)-induced growth involves two common mechanisms that consist of the proton pumps and potassium channels whose contribution to the action of both effectors on the rapid growth is different
Maize coleoptile segments respond to the optimal concentrations of FC (10−6 M) and IAA (10−4 M) by an increase in the elongation growth to a similar level over 10 h, both effectors differ in the kinetics of initial growth
Summary
Research on the effects of auxin (IAA) on plant growth has been conducted for quite some time, the molecular mechanism of auxin action on ion transport in growing cells has still not been precisely explained. Patch-clamp techniques that were applied to maize coleoptile protoplasts showed that auxin-induced growth in maize coleoptile cells involves K+ uptake via voltage-dependent, inwardly rectifying K+ channels (ZMK1, Zea mays K+ channel 1), the activity of which contributes to water uptake and to cell expansion (reviewed in [5,9]). It has been shown that, apart from the posttranslational, auxin-dependent upregulation of the K+ uptake channels, auxin regulates the expression of the maize K+ uptake channel gene ZMK1 [10]. This leads to an increase in the number of active K+ channels in the plasma membrane [10]. Experiments that were performed using the patch-clamp technique confirmed earlier studies that showed that auxin-induced growth strictly depends on the external K+
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