Plant Electrophysiology Theory & Methods (ed. by Volkov) Springer-Verlag Berlin Heidelberg 2006
Earths gravitational field influences plant growth, morphology, and develop-ment. The vector of the gravity force is powerful enough to largely dominatethe other directional tropic stimuli to which plants respond. Both roots andshoots respond to gravistimulation with differential, directional growth,through processes known as positive and negative gravitropism, respectively(Darwin 1896). Some of the cellular events that underlie gravitropicresponses are known (recently reviewed by Morita and Tasaka 2004); thecompletion of a mechanistic model of plant gravity responses remains anelusive objective.
In roots, the gravity-sensing cells are the columella cells located in the rootcap. These cells, known as stacocytes, contain starch-filled amyloplasts thatsediment in the direction of gravity (Masson 1995). In shoots, the gravity-perceptive tissue is the endodermis, which contains sedimentable amylo-plasts (Tasaka et al. 1999). In both roots and shoots, the perceived gravitropicstimulus is transduced to cells that start exhibiting differential growth, result-ing in organ bending and reorientation. Little is known about the signalingpathway linking gravity perception to differential growth responses, either inthe root, or in the shoot.
Plant cells exhibit a spectrum of bioelectric characteristic including electri-cal potentials, conductance, impedance, and permeability. Plant physiologicalfunctions are closely intertwined with cells electric properties, throughprocesses that involve energy maintenance and ion exchange with the envi-ronment. Steady-state electrical potentials can be measured across the plasmamembrane, using microelectrodes and patch-clamping techniques. On theexternal plant surface, electrical potentials and ion fluxes are monitored usingsurface-contact electrodes and vibrating probes.
A variety of abiotic stimuli induce electrical activity in plants. The best-characterized electrical signals in plants are the action potentials and thevariation potentials (slow waves). Voltage-gated, mechanosensitive, andligand-activated ion channels, as well as proton pumps, are involved in
18 Electrophysiology of Plant Gravitropism
Brinks Hofer Gilson & Lione, 455 North Cityfront Plaza Drive, Chicago, IL 60611, USA
generation and maintenance of these bioelectric potentials. Action poten-tials and variation potentials are both local and intercellularly propagatedelectrical signals. Transmitted to distant regions, these signals trigger anarray of systemic molecular and cellular responses (reviewed by Davies andStankovic 2005). The information that the electrical signals carry and theresponses that they evoke depend on either the ions traversing the membrane,the change in membrane potential, or both.
Despite the long-documented existence of gravity-induced electrical activ-ity in plants, this field is marked with a surprising dearth of investigation.Discovered a century ago in the petioles of Tropaeolum as a differentialchange in extracellular electric potential (Bose 1907), the phenomenon ofgravielectricity has been rarely studied by either electrophysiologists or byresearchers studying gravitropism. Therefore, coming forth with a hypothe-sis to describe gravielectrical responses in plants is a speculative endeavor.
This review summarizes the state of knowledge related to the role of extra-and intra-cellular electrical activity in gravistimulated higher plants. The seminal studies concerning the electrophysiology of plant gravitropism arehighlighted. A few ideas on the correlation of electrical activity with responsesto gravity are presented. In conclusion, a prospect for future research on theelectrophysiology of gravitropism is suggested.
18.2 Extracellular gravielectric potentials
The early studies on involvement of electrical potentials in plant gravitro-pism involved measurements of extracellular electric potentials. In the heroicage of discovery of plant electrical activity, measurements were done usingextracellular, surface contact electrodes, using ionic bridges typically consist-ing of diluted potassium chloride. Following the studies of Bose (1907), pio-neering investigations in the field were conducted by Brauner (1927), Clark(1937), and Schrank (1947). These studies provided evidence that reorienta-tion of plants induces transient electrical activity, a phenomenon that wasdubbed geolectric effect. Decades had to pass before that phenomenonreceived further attention from plant biologists.
Plants are electrically active. They generate characteristic steady-state trans-membrane potential differences and extracellular ionic current patterns.Bioelectricity may be involved in the establishment of plant cell and organpolarity (Nechitailo and Gordeev 2001). For example, electrical current flowsalong the surface of upright-growing epicotyls (Toko et al. 1989, 1990). On thephysically lower end of the organ, the plasma membrane is hyperpolarized by
424 Bratislav Stankovic
12 mV. This hyperpolarization is presumably correlated to spatial information(Etherton and Dedolph 1972).
Horizontal reorientation induces electrical activity in both lower andhigher plants (reviewed by Weisenseel and Meyer 1997). Gravistimulationalters the patterns of extracellular ionic currents, causing current asymmetry.Gravistimulation also changes the patterns of electrical potential along rootsand shoots. For example, within minutes of horizontal reorientation of maizecoleoptiles, the electrical potential became more positive on the lower sidethan on the upper side. The maximal change in potential was 2025 mV, andit was reversible when the coleoptiles were rotated back to vertical (Grahmand Hertz 1962).
In soybean hypocotyls, directional change in the gravity vector inducedfast electrical field changes (Tanada and Vinten-Johansen 1980). Thesechanges were reflected as increase in positive electrical potential in the lowerside of the hypocotyl, occurring rapidly, approximately one minute after hor-izontal placement. The increase in positive electrical potential was maximalin the region undergoing gravity-induced curvature, in a zone 12 cm belowthe hook. The maximal amplitude of the transient change in electrical potentialwas about 17 mV (Tanada and Vinten-Johansen 1980).
Similarly, in bean epicotyls, electrical potential on the lower sideincreased, whereas the potential on the upper side decreased for about onehour after gravistimulation (Imagawa et al. 1991; Fig. 18.1). The electricalactivity was monitored as soon as the plants were rotated. The amplitudes of
Electrophysiology of Plant Gravitropism 425
Upper sideLower side
5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 9020
Fig. 18.1. Changes in extracellular potentials in gravistimulated shoots (approximate data areredrawn from Imagawa et al. 1991). Kinetics of the change in surface electrical potential in theupper and lower sides of gravistimulated adzuki bean (Phaseolus angularis) epicotyl is shown.The measured area was approximately 20 mm from the base of the first leaves. At zero time, theepicotyl was rotated to horizontal position
the electrical potential changes were 1025 mV, and were dependent on theposition of the electrodes attached to the epicotyls (Imagawa et al. 1991).
More recently, Shigematsu et al. (1994) monitored rapid surface potentialchanges in gravistimulated bean epicotyls. In a limited region on the upperside of the epicotyl, surface electrical potential decreased soon after gravis-timulation. The magnitude of the transient potential change was about 10 mV,and it occurred 30120 s following gravistimulation. At the same time, thesurface potentials on the lower side of the epicotyls scarcely changed. Therapid change in potential on the upper side was highly correlated to the earlydownward curvature (transient positive gravitropic response) that wassimultaneously monitored (Shigematsu et al. 1994).
It has been suggested that the electrical asymmetry across the epicotyl isrelated to asymmetric auxin distribution and contributes to induction of H+
secretion, which is thought to initiate differential growth in shoot gravitro-pism (Wright and David 1983). Indeed, because the electrical changes occurprior to the plausible movement of auxin, it was postulated that the signalsmediate the asymmetric auxin distribution or asymmetric Ca2+ distributionduring gravitropism (Imagawa et al. 1991).
The growing root tip also exhibits strong electrical activity. Detailed meas-urements of endogenous currents surrounding roots were reported over 40years ago (Scott and Martin 1962). Since then, several related studies haveindicated that the pattern of endogenous ionic currents around verticallygrowing roots is consistent across species. Ionic currents typically enter themeristem and the younger parts of the elongation zone, and leave theremainder of the elongation zone and the more mature parts of the root(Behrens et al. 1982; Weisenseel et al. 1992). An area of variable current existsaround the root cap; an area of large inward current is present around themeristem and apical part of the elongation zone; and an area of moderateoutward current is present in the remainder of the elongation zone and themature root tissue (Collings et al. 1992).
Horizontal reorientation of plants induces changes in the current patternaround the root. The change in current pattern is rapid, and is initiated at theroot cap; it also appears at the meristem and the elongation zone. Along cressroots, differential current pattern was monitored within just 35 min of reori-entation (Behrens et al. 1982; Iwabuchi et al. 1989). Also in cress roots, theshifts in electrical current pattern were correlated to changes in the growthrate following gravistimulation (Iwabuchi et al. 1989).
Using a vibrating probe, Bjrkman and Leopold (1985, 1987) studiedthe gravistimulus-induced electrophysiological asymmetry in maize roots.They discovered that gravistimulus induced a transient increase in currentflow on the upper side of the root. The increase began 26 min following
426 Bratislav Stankovic
gravistimulation, had a magnitude of approximately 1 A cm2, and lastedapproximately 10 min. A consistent change in current was observed only inthe region adjacent to the statocytes (columella cells). Gravistimulation hadlittle effect on currents in the elongation zone and at the tip of the root cap(Bjrkman and Leopold 1985).
Gravity-induced current changes along maize roots were also monitored byCollings et al. (1992). Within 1015 min upon reorientation, currents above theroot changed from inward to outward, resulting in asymmetries of up to 1.5 Acm2. That asymmetry is significant, considering that the maximum averagecurrent density in vertical roots was approximately 1.62 A cm2. In similarstudies, the lag of the onset of current change occurred prior to the initiation ofbending. This lag has been correlated to the so-called presentation time forgravity sensing (Behrens et al. 1982; Monshausen and Sievers 2002).
18.3 Intracellular gravielectric potentials
Despite the suggested role for cytosolic ions in responses to gravity, thenature of membrane potential changes in response to gravistimulation is notwell understood. Different types of ion fluxes have been correlated to gravit-ropism and to the concomitant electrical responses. Most frequently, thegravity-induced fluxes of Ca2+ and H+ have been investigated. A few studieshave suggested that calcium may not be involved, or may not be necessary ingravitropism. Examining the gravity-induced bending of maize roots,Collings et al. (1992) discovered that lanthanum had little effect on either thecurrent asymmetry or the growth response. Even a thorough imaging studyfailed to identify a change in cytosolic Ca2+ in gravistimulated roots (Legueet al. 1997). Some researchers have opined that much of the differential elec-tric current upon gravistimulation may be carried by protons instead of Ca2+
(Behrens et al. 1982; Collings et al. 1992).
Adding to the controversy as to the possible role of Ca2+, several pharmaco-logical studies using Ca2+ inhibitors suggest involvement of Ca2+ in gravitro-pism (Lee et al. 1983a; Belyavskaya 1996; Philosoph-Hadas et al. 1996). Forexample, maize roots cultured in EDTA and EGTA lost their ability torespond to gravity; the response was restored by addition of CaCl2 but notMgCl2. Furthermore, asymmetric application of Ca
2+ solution to vertical rootsinduced curvature toward the side of high Ca2+ concentration (Hepler andWayne 1985).
Significant trans-organ fluxes of Ca2+ are triggered by gravitropic stimula-tion. In oat coleoptiles, Ca2+ fluxes were monitored within 10 min following
Electrophysiology of Plant Gravitropism 427
gravistimulation, preceding the initiation of organ bending (Roux et al.1983). Calcium ions were asymmetrically distributed in oat coleoptiles duringand after gravistimulation (Daye et al. 1984). Calcium redistribution was alsomonitored in the graviresponding pulvini of Mimosa (Roblin and Fleurat-Lessard 1987). Studying transgenic Arabidopsis expressing aequorin, a rolefor cytoplasmic Ca2+ was recently affirmed in the gravity transduction mech-anism. In these plants, distinct calcium signaling was observed in response togravistimulation, with kinetics of increases in intracellular Ca2+ being verydifferent from Ca2+ transients evoked by other abiotic stimuli. The cytoplas-mic Ca2+ transients had duration of many minutes, and were correlated to thestrength of the displacement stimulus (Plieth and Trewavas 2002).
Extracellular Ca2+ is also needed for gravitropism (Bjrkman and Cleland1991). Oat coleoptiles incubated in 1 mM EGTA did not exhibit a gravitropicresponse. However, when the EGTA solution was displaced with a solutioncontaining Ca2+, gravitropism was restored (Daye et al. 1984). Similarly, ele-vated concentration of apoplastic Ca2+ was observed in the slower growingparts of gravistimulated organs (Lee et al. 1983b). Thus, increases in apoplasticCa2+ concentration are correlated with the gravitropic response.
In addition to calcium, fluxes of other ions occur upon reorientation.Redistribution of other ions such as potassium and phosphorus wasobserved during gravity-induced curvature in sunflower hypocotyls andmaize coleoptiles (Goswami and Audus 1976). Redistribution of K+ and Cl
was also monitored during the gravitropic response of Mimosa pulvini(Roblin and Fleurat-Lessard 1987). These findings are particularly interest-ing, because the current belief is that both action potentials and variationpotentials in plants involve calcium influx followed by chloride and potas-sium efflux. Transient fluxes of Ca2+, Cl, and K+, and the concomitant elec-trical signals, might play a significant role in transducing the c...