Growth, Differentiation and Sexuality

with cytochalasin D (inhibiting actin polymerization)
causes a 4-h delay of the gravitropic bending,
whereas rhodamin-phalloidin (inhibiting actin
depolymerization) enhances the gravitropic bending
rate, and sometimes also the bending angle
(Edwards et al. 1997). Gadolinium chloride, an inhibitor
of plant gravitropism and stretch-activated
ion channels, delays the onset of gravitropism and
diminishes gravitropic curvature. An asymmetric
application of gadolinium, Ca 2+ -chelators, and
compound 48/80 (inhibiting calmoduline) to
the growing zone of the sporangiophore elicits
curvature toward the side to which the inhibitors
were applied (Stecker et al. 1990; Edwards 1991).
These results suggest that gravitropic curvature
requires the participation of the cytoskeleton, and
entails a redistribution of Ca 2+ and calmodulin.
4. Sine Law and Exponential Law
Gravitropic bending of shoots and roots of plants
obeys the so-called sine rule or sine law (Sachs
1879), which states that the gravitropic stimulus
canbedescribedbytherelation:
S = g × sin γ (13.4)
where S is the gravitropic stimulus, g the earth
gravitational acceleration (9.81 m/s 2 ), and γ the inclination
angle ( ◦ ) of the plant organ.
Sporangiophores of Phycomyces obey this
classical sine law (Galland et al. 2002). When sporangiophores
are irradiated unilaterally, they bend
toward the light source and a photogravitropic
equilibrium is established. The photogravitropic
bending angle is the result of two antagonistic
responses, i.e., positive phototropism and negative
gravitropism. The irradiance of unilateral light
required to compensate the ensuing gravitropic
response is well described by a novel exponential
law (Grolig et al. 2000; Galland et al. 2002):
I = I0 exp −(kλ g sin γ) (13.5)
where I is the irradiance of the unilateral light that
compensates the gravitropic response elicited at an
inclination angle γ, I0 the absolute threshold irradiance
(ca. 10 −9 W/m 2 , 450 nm), kλ a wavelengthdependent
constant, g the earth’s gravitational
acceleration, and γ the inclination angle of the
sporangiophore (deviation from the vertical). The
exponential law states that the light intensity that
compensates a gravitropic stimulus needs to be
raised exponentially when the gravitropic stimulus
Photomorphogenesis and Gravitropism 251
(g sin γ) is raised linearly. Because this novel law
is valid also for coleoptiles of Avena, it appears to
describe a universal relationship in the interaction
of gravi- and phototropic stimuli (Galland 2002).
5. Gravitropism Mutants
Mutants of Phycomyces with defects in the
genes madD,E,F,G,J are gravitropically partially
defective (so-called stiff mutants). They are
highly pleiotropic, because they show reduced
light-growth and phototropic responses, and
a reduced avoidance response (Bergman et al.
1973; Campuzano et al. 1996). Mycelial responses
such as photocarotenogenesis and photoinitiation
of sporangiophores are, however, unaffected (see
above). System analysis employing Wiener white
noise or sum-of-sinusoid stimuli showed that
these mutants have a lower gain than do the corresponding
wild-type strains or mutants that are
affected only in the phototropism genes madA-C
(Lipson 1975; Palit et al. 1989). The threshold for
photogravitropic equilibrium is raised at least
6 orders of magnitude, and the corresponding
action spectra are highly abnormal, a feature
indicating that these mutations directly affect the
photoreceptor system (Campuzano et al. 1996).
The various features indicate that (1) the photoreceptor
system is affected in these gravitropism
mutants, (2) photo- and graviperception interact
at early steps of the transduction chain, and (3)
the madD,E,F,G gene products interact with those
of the madA,B,C gene products.
Another class of mutants with defects in the
gene madH show enhanced gravitropic and phototropic
bending, and also an enhanced avoidance
response (Lipson et al. 1983; López-Díaz and Lipson
1983). Although the locations of the corresponding
mad genes on the genetic map of Phycomyces have
been determined (Alvarez et al. 1992), their molecular
nature remains unknown.
E. Glomeromycota
Hyphae of the endomycorrhizal fungus Gigaspora
margarita display either negative or positive
gravitropism. The germ tubes grow upward
(negative gravitropism) whereas secondary (i.e.,
branching) hyphae, which are the sites for the
formation of enichulate vesicles, grow downward
(positive gravitropism; Watrud et al. 1977; Hong
et al. 2001). Calcium, an important signal element