(McGillavray and Gow 1986) and algae (Brower and Giddings 1980), as well as in pollen
tubes (Marsh and Beams 1945), roots (Fondren and Moore 1987; Schrank 1959), and
shoots (Schrank 1959; Lee et al. 1983) of higher plants. Upon gravistimulation, the cur-
rent flow along the upper flank of the distal elongation zone (DEZ) reversed to efflux
from the root (Iwabuchi et al. 1989; Collings et al. 1992), and changes in intracellular po-
tentials in this zone occurred within a minute (Ishikawa and Evans 1990a). These changes
arose before the development of the gravitropic curvature. Gravistimulation also modi-
fied the pattern of electric current surrounding the root tip (Behrens et al. 1982;
Björkman and Leopold 1987; Iwabuchi et al. 1989), and within the root cap triggered
rapid depolarization of statocytes (Behrens et al. 1985), suggesting that electrical/ionic
signals may be an important component of the gravity sensing/response system.
Electrotropism was enhanced by treatments that interfere with gravitropism, like de-
capping the roots or pretreating them with Ca2+chelator. Likewise, roots of ageotropum
were more responsive to electrotropic stimulation than were roots of normal peas
(Ishikawa and Evans 1990b), suggesting that the early steps of gravitropism and elec-
trotropism occur by independent mechanisms. Nonetheless, the motor mechanisms of the
two responses may have features in common since auxin and auxin transport inhibitors
reduced both gravitropism and electrotropism (Moore et al. 1987).
The kinetics of electrotropic curvature in Vigna mungoL. roots revealed that curvature
130 PLANT TROPISMS
Figure 6.3. Root hydrotropism opposes gravitropism in Arabidopsis. A. Perception of gravity (a weak sig-
nal) appears to occur in columella cells of the root cap, triggered by amyloplasts that can sediment because
of gravitational force. After the sensory system identifies this stimulus, it connects with the hormone system
(perhaps by transient Ca2+fluxes and/or alkalinization of columella cytoplasm) and an asymmetrical signal
is originated within the columella cells, leading to the lateral movement of auxin. B. Perception of moisture
gradients (a strong signal) might take place anywhere in the root cap, but assessment of both gravity and
water signals most probably occurs in columella cells, which in turn elicits amyloplast degradation and ulti-
mately initiates the appropriate bending response. How the sensory system for humidity gradients connects
with the hormone system remains to be uncovered. NHR1might inhibit the root gravitropic response when
both humidity gradients and gravity sensitiveness merge. ABA and ethylene may positively regulate hy-
drotropism and inhibit gravi-facilitation. Shaded bar denotes a moisture gradient; solid bar symbolizes lack
of a moisture gradient; gray arrows indicate the direction of the auxin transport, and their width correlates
with levels of transported auxin; white arrow represents an increase in Ca2+fluxes and pH in a columella cell.