A Two-Dimensional Vibrating Probe Study of Currents around Lateral Roots of Raphanus sativus Developing in Culture

Electric current precedes emergence of a lateral root in higher plants

Stable electrochemical patterns appear spontaneously around roots of higher plants and are closely related to growth. An electric potential pattern accompanied by lateral root emergence was measured along the surface of the primary root of adzuki bean (Phaseolus angularis) over 21 h using a microelectrode manipulated by a newly developed apparatus. The electric potential became lower at the point where a lateral root emerged. This change preceded the emergence of the lateral root by about 10 h. A theory is presented for calculating two-dimensional patterns of electric potential and electric current density around the primary root (and a lateral root) using only data on the one-dimensional electric potential measured near the surface of the primary root. The development of the lateral root inside the primary root is associated with the influx of electric current of about 0.7 muA.cm(-2) at the surface.

Growth, Gravitropism, and Endogenous Ion Currents of Cress Roots (Lepidium sativum L.) : Measurements Using a Novel Three-Dimensional Recording Probe

A novel, three-dimensional recording, vibrating probe was used for measuring the density and direction of the endogenous ionic current of cress roots (Lepidium sativum L.) bathed in low salt media (artificial pond water, APW). Roots submerged in regular APW and growing vertically show the following current pattern. Current of 0.7 microampere/square centimeter density enters or leaves the root cap; the current changes direction frequently. Current of 1.6 microamperes/square centimeter enters the meristem zone most of the time. Maximum current with a density of 2.2 microamperes/square centimeter enters the apical elongating zone, i.e. between 0.8 and 1.2 millimeters behind the root tip. The current density decreases to 1.4 microamperes/square centimeter at 2 millimeters, i.e. in the central elongating zone, and to 1.0 microampere/square centimeter at 3 millimeters, i.e. in the basal elongating zone. The current direction changes from inward to predominantly outward between 1.2 and 3 millimeters behind the tip. Measurements on opposite flanks of the roots indicate that the current pattern is fairly symmetrical. After placing the roots horizontally, the density of the endogenous current remains stable, but the current direction changes at the root cap and in the meristem zone. The current leaves the root on the upper side and enters on the lower side, causing a highly asymmetrical current pattern at the very tip. The current pattern at the upper and lower side further away from the tip remains the same as in vertical roots. Roots submerged in low Ca(2+) APW show a very different current pattern, no gravitropism, and no change of the current pattern after horizontal orientation. In these roots current enters the root cap and the basal elongating zone and leaves the apical elongating zone. Three conclusions are drawn from these results: First, plant roots elongate by two different modes of growth that are correlated with different current directions. They grow by cytoplasmic enlargement at sites of inward current and by turgor-driven elongation at sites of outward current. Second, a change in the current pattern at the root cap and in the meristem zone is a clear indicator of later gravitropism. Third, Ca(2+) ions are involved in the gravistimulated change in the current pattern, probably affecting the activity of plasmalemma H(+)-ATPases.

The neural tube of the Xenopus embryo maintains a potential difference across itself

In Xenopus embryos, the ectodermal epithelium generates a substantial transepithelial potential (TEP) during certain periods of early development. In this study, we have found that the neural tube (which is derived from the embryonic ectodermal epithelium) of stage 21-25 Xenopus embryos also maintains a potential across itself, with the lumen being, on average, 18 +/- 1 mV negative relative to the interstitial spaces. This transneural tube potential (TNTP) declines gradually from a maximum of -21 +/- 2 mV at stage 23 to a minimum of -14 +/- 2 mV at stage 25. Vibrating probe measurements on transected embryos suggest that the neural tube is capable of driving a current. Large outward currents ranging from 10 to 26 microA/cm2 were detected just dorsal to the center of wounds in transected stage 21-24 embryos, but near the dorsal margin of the wound, in the region corresponding to the cut face of the neural tube, outward current densities were less than half the maximum, ranging from 3 to 9 microA/cm2. The reduced outward current near the dorsal margin suggests a locus of inward current in this region that is subtracted from the much larger outcurrents. Such greatly reduced outward currents were not detected near the ventral margin of the wound.