The effect of DC electric field on the elongation growth, proton extrusion and membrane potential of Zea mays L. coleoptile cells; a laboratory study

Abstract

Background: In this study, we investigated the effect of an electric field, with an intensity similar to that of the Earth's field, on plant cells growth. The molecular mechanism underlying this effect remains unclear.

Results: It was found that the electric field, depending on the applied voltage, its duration and the polarization of the maize seedlings, stimulated or inhibited the growth of the seedling organs (root, mesocotyl and coleoptile). Moreover, it was also noticed that the gravitropic response of maize seedlings was inhibited at all voltages studied. Simultaneous measurements of growth and external medium pH show that auxin(IAA, indole-3-acetic acid)- and fusicoccin(FC)-induced elongation growth and proton extrusion of maize coleoptile segments were significantly inhibited at higher voltages. The ionic current flowing through the single coleoptile segment during voltage application was 1.7-fold lower in segments treated with cation channel blocker tetraethylammonium chloride (TEA-Cl) and 1.4-fold higher with IAA compared to the control. The electrophysiological experiments show that the electric field caused the depolarization of the membrane potential of parenchymal coleoptile cells, which was not reversible over 120 min.

Conclusion: It is suggested that a DC electric field inhibits the plasma membrane H⁺ pump activity and K⁺ uptake through voltage-dependent, inwardly rectifying ZMK1 channels (Zea mays K⁺ channel 1). The data presented here are discussed, taking into account the "acid growth hypothesis" of the auxin action and the mechanism of gravitropic response induction.

Keywords: Coleoptile segments; Electric field; Elongation growth; External medium pH; Gravitropic response; Membrane potential; Zea mays L..

Fig. 1

Effect of electric field (EF) on the lengths of maize (Zea mays L.) seedling organs (root, mesocotyl and coleoptile), shown as a percent of the control (seedlings arranged in an electric-application setup but not treated with voltage). Twenty three-day-old seedlings were transferred into an electric-application setup (see Methods, Fig. 5) in which an electric field was applied. After treatment with EF, the lengths of the seedling organs were measured (± 1 mm), and seedlings were transferred for 24 h into a hydroponic container containing a solution of the following composition (control medium): 1 mM KCl, 0.1 mM NaCl, 0.1 mM CaCl₂; pH 5.8-6.0. The differences between the lengths of seedling organs 24 hours after treatment with the EF and the lengths of seedling organs measured immediately after treatment with the EF are expressed as a percent of the control (100%, an increase in length within 24 h of seedling organs untreated with EF). Mean length in the control: root, 11.2 mm; mesocotyl, 5.6 mm; coleoptyl, 8.0 mm. The experiments were repeated four times (80 seedlings). Values are means ± SE. Means followed by the same letter are not significantly different from each other (LSD test P < 0.05)

Fig. 2

Bending of coleoptiles in seedlings untreated (A) and treated (B) with EF (+10 V/180 min) in response to 90° gravistimulation. After the displacement of maize seedlings into a horizontal position (90°) the gravitropic bending of coleoptiles was measured in 30 min intervals for 300 min (see Methods)

Fig. 3

Effect of electric field (±15 V/15 min) on IAA-induced elongation growth (µm/segment) of maize coleoptile segments (A) and simultaneously with growth measured medium pH (B). A⁺and A^- mean the positive and negative polarization of the segment’s apical part, respectively. After excision, the coleoptile segments were preincubated (within 1 h) in the control medium, whereupon they were placed in the setup (Scheme 3) used for electric stimulation. After electric treatment, the coleoptile segments were arranged in an apparatus, which allowed simultaneous measurements of the elongation growth and pH of the incubation medium (see Methods). Data (mean ± S. E.) are means of at least eight independent experiments

Fig. 4

Effect of electric field on the IAA- and FC-induced elongation growth (µm/segment) of maize coleoptile segments (A) and simultaneously with growth measured medium pH (expressed as the difference between H⁺concentration per coleoptile segment at 360 and 60 min, ∆[H⁺]/segment) (B), expressed as % of control (100 %, IAA- or FC-induced elongation growth or proton extrusion of maize coleoptile segments untreated with EF: mean values 1682.7 or 1551.2 µm/segment for growth and 1.83 or 47.54 nM/segment for proton extrusion, respectively. Because of that, the differences between control (FC-induced growth and proton extrusion of the segments untreated with EF) and FC-induced growth and proton extrusion of the segments treated with EF at  ±5 V are statistically insignificant and not shown. Data (mean ± S. E.) are means of at least nine independent experiments. Means followed by the same letter are not significantly different from each other (LSD test P < 0.05)

Fig. 5

A schematic drawing of the electric-application setups that were used to determine the effects of a DC  electric field on the elongation growth of maize seedling organs (Scheme 1) and the gravitropic response of seedling coleoptiles (Scheme 2). In the first setup (Scheme 1) twenty 72-hour-old seedlings (S) were arranged on two plexiglass plates (P) covered with agar (3 % agar prepared on the control medium) in which a platinum wire (PW) in the form of a flattened spiral was placed. In the second setup (Scheme 2), one 96-hour-old maize seedling, after treatment with EF, was used to study the effect of an electric field on the gravitropic response of the maize seedling. In the second setup, the seedling (S) was supported by a carcass made of plexiglass (C), which in turn was connected to the chamber containing the seedling root system immersed in the control medium (CM). The current from a DC power supply (Protek, DC power supply, model 3003, Korea) was fed to the coleoptile using a platinum wire (PW), the end of which in the form of 2-3 coils (slightly larger in diameter than the diameter of the coleoptile) which was placed over the tip of the coleoptile. In order to obtain contact between the platinum wire and the coleoptile, the gap between them was filled with agar (A, 3% prepared on the control solution). The voltage and resultant current from the DC power supply were measured by the volte (V) and ampere (A) meter. A similar procedure for studying the effects of an electric field on the growth of maize seedlings (Fig. 5, Scheme 2) has been previously used by Desrosiers and Bandurski [19]. The setup (Scheme 1) is our idea

Fig. 6

A schematic drawing of the electric-application setup that was used to study the effects of a DC electric field on the elongation growth of maize coleoptile segments. In this setup, twenty 10-mm-long coleoptile segments (CS), excised from 96-hour-old etiolated seedlings, were collected in an intensively aerated control medium for 1 hr. After this period, the coleoptile segments were arranged on two plexiglass plates (P) covered with agar (3 % agar prepared on the control medium), in which a platinum wire (PW) in the form of a flattened spiral was placed. The coleoptile segments were treated with an electric field over 15 min. The setup shown in Fig. 6  is our idea