ized as yet. In addition, the mechanisms that the root cap utilizes to intermingle different
stimuli to resolve in which direction root growth should occur are unknown.
Nonetheless, the recent isolation of hydrotropic mutants, in combination with analy-
sis of a number of gravitropic mutants, have provided some hints of the signal transduc-
tion mechanisms of these tropisms. For example, the gravitropic and waving response of
thenhr1mutant is increased, suggesting that the lack of a hydrotropic response results in
an enhancement of these other root growth responses (Eapen et al. 2003). Moreover, roots
from the starchless mutant pgm1-1, which has a reduced gravitropic response (Kiss et al.
1989), showed an enhanced responsiveness to moisture gradients (Takahashi et al. 2003).
Gravistimulated nhr1seedlings also attained a root curvature of 80 degrees 6 hours be-
fore wild-type plants (Eapen et al. 2003). In contrast, pgm1-1roots reached a 20-degree
hydrotropic curvature 1 hour before the wild-type (Takahashi et al. 2003).
These observations suggest that once the sensing system of the root cap is affected for
one stimulus, the integration and assessment mechanism for other signals is enhanced,
and thus these other root growth responses can occur faster. However, the differences in
the rate of gravitropic bending of nhr1versus hydrotropic bending of pgm1-1roots seem
also to reflect variations in the timing of perception between both stimuli. It has been
shown that the perception time for a gravity stimulus can be as short as 1 second
(Hejnowicz et al. 1998). In contrast, the perception time for osmotic stimulation is up to
2 minutes (Stinemetz et al. 1996). The variability observed in the perception time of both
tropisms might be the consequence of their distinctive mechanisms of perception. It is
widely accepted that perception of gravity occurs in specialized cells of the root cap (sta-
tocytes or columella), which contain motile amyloplasts that can sediment in response to
gravity and can therefore elicit gravisensing (Sack 1997; see also Chapter 1).
The capacity of the root cap to perceive and respond to moisture gradients apparently
produces a dominant signal that abates the gravity response. Recently, it has been found
that this signal triggers the degradation of amyloplasts in columella cells of both Arabi-
dopsisand radish, and hence roots exhibit hydrotropism with fewer impediments from
gravitropism (Takahashi et al. 2003). Transient touch stimulation of Arabidopsisroot tips
likewise restrains gravitropic growth but, in this case, by limiting amyloplast sedimenta-
tion in columella cells (Massa and Gilroy 2003; see also Chapter 5). Therefore, columella
cells can integrate the signaling triggered by moisture gradients, touch receptors, and
possibly even other stimuli in order to generate the appropriate tropic response.
Columella cells also produce the initial gravity-induced lateral auxin gradient in the root
cap. These cells contain one putative component of the auxin efflux carrier complex (PIN3)
which shows rapid relocalization upon gravistimulation and is thought to drive asymmetri-
cal auxin transport responsible for tropic bending (Friml et al. 2002; see also Chapters 2
and 3). Conceivably, columella cells might utilize the same signaling components that drive
differential growth (such as auxin efflux) for all sensory systems, and so work like a funnel
taking in many stimuli and transducing these toward a single set of response elements in
order to synchronize the various tropistic responses (Eapen et al. 2005). Clearly, more re-
mains to be learned about the functional interactions between sensing mechanisms that take
place during gravity, moisture, and touch perception. Nonetheless, hydrotropic stimulation
exerts a more dramatic effect than touch upon the primary mechanism for gravireception,
which might be associated with the importance of water in the life of most plants.
frankie
(Frankie)
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