Ionic Current Changes Associated with the Gravity-Induced Bending Response in Roots of Zea mays L.

Abstract

A vibrating probe was used to measure the changes in ionic currents around gravistimulated roots of Zea mays L. in an effort to determine whether these currents are involved in stimulus transduction from the root cap to the elongation zone. We did not observe a migration of the previously reported auxin-insensitive current efflux associated with gravity sensing (T. Bjorkman, A.C.Leopold [1987] Plant Physiol 84: 841-846) back from the root cap.Instead, beginning 10 to 15 min after gravistimulation, an asymmetry in current developed simultaneously along the root around the meristem and apical regions of the elongation zone. This asymmetry comprised a proton efflux from the upper surface, which was superimposed on the symmetrical pattern around the vertical root. The gravity-induced proton efflux was inhibited by the application of the auxin transport inhibitor, 2,3,5-triiodobenzoic acid, whereas the calcium channel blocker, lanthanum, had little effect. Because the onset of the gravity-induced current asymmetry coincided both spatially and temporally with the onset of the differential growth response, we suggest that this current efflux may result from auxin-requiring acid-growth phenomena in the upper root tissue. The implications of this simultaneous onset of both proton efflux and elongation for theories about gravity stimulus transduction are discussed.Studies of root gravitropism have focused on assessing the first stages of gravity perception by the sensing tissue (thought to be the columella cells in the root cap) and determining steps in the response that immediately precede discernible growth asymmetries in the elongation zone (8 and refs. therein). Recently, there has been considerable attention given to changes in root electrophysiology following gravity stimulation (4,5,19). The growing root tip is electrically active, and when it is placed in a conducting solution, ionic currents, measurable with the vibrating probe (14), typically enter the meristem and younger parts of the elongation zone and leave the remainder of the elongation zone and the more mature parts of the root (29).A gravitropic stimulus modifies the symmetrical current pattern found around the vertical root. Behrens et al. (4) measured changes in current parallel to the upper surface of Lepidium sativum roots. These changes were interpreted as

Wound-Induced Changes of Membrane Voltage, Endogenous Currents, and Ion Fluxes in Primary Roots of Maize

“…This current pattern compares well with previous measurements of maize roots and roots of about 20 other species (Hush and Overall, 1989;Miller and Gow, 1989a, 198913;Collings et al, 1992; . Weisenseel et al, 1992).…”

“…Weisenseel et al, 1992). The fact that the current densities measured in our study were generally larger than those measured by Miller and Gow (1989b) and Collings et al (1992) with maize roots can be explained by two factors. First, the use of a three-dimensional recording probe mea-, sures the true current density much better than one-or two-dimensional probes, which record only part of the current-density vectors.…”

Critical consideration on the relationship between auxin transport and calcium transients in gravity perception of Arabidopsis seedlings

“…TIBA and BFA also inhibited the first [Ca 2+ ] c -increase that appears to be independent of changes in the gravity vector, 23 suggesting that these inhibitors affect directly/indirectly the molecules responsible for the [Ca 2+ ] c -increase, such as Ca 2+ channels rather than for the auxin-related gravitropic responses. Indeed, TIBA greatly reduces ionic currents in maize roots, 32 and BFA abolishes a [Ca 2+ ] c -gradient and -oscillation in pollen tubes, indicating that these inhibitors have multiple side effects as previously pointed out. 21 Polar auxin transport was detected in tomato hypocotyls 5 between 5 and 10 minutes after gravistimulation as well as in maize coleoptiles between 30 and 40 minutes.…”

Changes in Root Cap pH Are Required for the Gravity Response of the Arabidopsis Root

“…The DEZ is defined as being centered on the region of the root elongation zone showing 30% maximal growth rate, and the CEZ is defined as being centered on the region of maximal growth rate (Ishikawa and Evans, 1993). The current evidence suggests that the growth of the CEZ, but not the DEZ, is regulated in part by auxin (Ishikawa and Evans, 1993;Evans et al, 1994;Mullen et al, 1998) and likely is mediated, at least in part, via acid growth phenomena (Edwards and Scott, 1974;Evans, 1976;O'Neill and Scott, 1983;Collings et al, 1992; Taylor et al, 1996;Büntemeyer et al, 1998;Felle, 1998;Peters and Felle, 1999). For example, both auxins and auxin antagonists cause root growth and the pH of the medium around the root to change in a correlated manner, implying a role for changes in wall pH in growth control (Evans et al, 1980;Moloney et al, 1981;.…”