Abstract
Auxin is one of the crucial regulators of plant growth and development. The discovered auxin cytosolic receptor (TIR1) is not involved in the perception of the hormone signal at the plasma membrane. Instead, another receptor, related to the ABP1, auxin binding protein1, is supposed to be responsible for the perception at the plasma membrane. One of the fast and sensitive auxin-induced reactions is an increase of Ca2+ cytosolic concentration, which is suggested to be dependent on the activation of Ca2+ influx through the plasma membrane. This investigation was carried out with a plasmalemma enriched vesicle fraction, obtained from etiolated maize coleoptiles. The magnitude of Ca2+ efflux through the membrane vesicles was estimated according to the shift of potential dependent fluorescent dye diS-C3-(5). The obtained results showed that during coleoptiles ageing (3rd, 4th and 5th days of seedling etiolated growth) the magnitude of Ca2+ efflux from inside-out vesicles was decreased. Addition of ABP1 led to a recovery of Ca2+ efflux to the level of the youngest and most sensitive cells. Moreover, the efflux was more sensitive, responding from 10−8 to 10−6 M 1-NAA, in vesicles containing ABP1, whereas native vesicles showed the highest efflux at 10−6 M 1-NAA. We suggest that auxin increases plasma membrane permeability to Ca2+ and that ABP1 is involved in modulation of this reaction.
Highlights
One of the most frequent questions, which are faced by investigators of auxin effects is: How can this simple molecule trigger such a huge diversity of physiological reactions? During the last decades, an enormous amount of data on auxin signaling has accumulated
The intensity of Ca2+ transport through vesicle membranes, obtained from maize coleoptiles of different ages was estimated as ΔMP, determined by a shift in fluorescence of diS-C3-(5) dye, commonly used to test transmembrane potential in purified vesicles, and at whole cell level, like protoplast or bacterial cell [45,46]
We assume that right-side-out vesicles do not participate in ΔMP generation because transport of Ca2+ out of the cell is carried out by active systems like Ca2+-ATPase and by the Ca2+/proton antiporter systems
Summary
One of the most frequent questions, which are faced by investigators of auxin effects is: How can this simple molecule trigger such a huge diversity of physiological reactions? During the last decades, an enormous amount of data on auxin signaling has accumulated. A fast ABP1-related auxin-induced shift in the membrane potential (MP) was shown in a similar model system, by use of a sensitive fluorescent dye [38]. The advantage of the latter investigation was the ascertainment that the effect was triggered even by the C-terminal peptide of ABP1 and was blocked by antibodies against it. One of the fast and sensitive reactions triggered by auxin is an elevation of Ca2+ concentration in the cytosol This reaction was estimated for different plant cells, including maize coleoptile parenchyma cells [9,41] Most probably it reflects the activation of plasma membrane channels, permeable for. The current investigation focuses on the involvement of a plasma membrane Ca2+-transport system in auxin signal perception under the control of ABP1
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