progresses. Only a few of the peripheral cells of the root cap are in contact with the bar-
rier and these must be transmitting touch information to the systems that control root
growth to suppress gravitropic response (Massa and Gilroy, 2003a; Figure 5.6). Upon
reaching the end of the obstruction and the mechanical stimulation, gravitropism domi-
nates and the root resumes normal downward growth.
The root tip-to-barrier angle is intermediate between vertical and flat (44 degrees in
wild-typeArabidopsis; Massa and Gilroy 2003b), suggesting a compromise between rap-
idly moving around the object by growing across its surface and the gravitropic response
sending the tip of the root vertically downward. A complete down-regulation of the
gravity-sensing system during contact with the barrier would result in growth flat against
its surface. Therefore, it is likely that the intermediate angle forms as an adaptive com-
promise between the graviresponse and the touch stimulation to successfully allow navi-
gation around an obstacle (Fasano et al. 2002). Indeed, a brief touch to the root cap de-
sensitizes roots to gravity, as assayed by inhibition of subsequent gravitropic growth on a
clinostat (Massa and Gilroy 2003a). Likewise, Mullen et al. (2000) showed that mechan-
ical stress during gravistimulation can delay the development of gravitropic curvature.
Further data in support of the idea that gravisignaling is modulated by mechanosignal-
ing come from experiments observing root growth along a barrier where some or all of
the cells in the root cap are killed via laser ablation, thereby removing the gravisensing
cells. Ablating the whole cap or only the graviperceptive columella cells causes a loss of
the DEZ curvature phase upon encountering the barrier, while the initial CEZ curvature
is unaffected (Massa and Gilroy 2003a). Such observations suggest that the differential
growth in the DEZ comes about because of a modified gravitropic response, as this is de-
pendent on the presence of an intact columella. However, an intact root cap is not needed
for the initial CEZ curvature, suggesting that the differential growth in this region occurs
as a result of cells sensing strain caused by the compressive forces that occur after initial
contact (Evans 2003; Massa and Gilroy 2003a). That is, the cells in the CEZ that respond
by producing tropic curvature may also be the cells experiencing the mechanical stimu-
lation, rather than secondarily responding to a signal produced in the cap, where the di-
rect touch stimulation is occurring. Although the precise mechanism for the interaction
between touch and gravity signaling during these responses remains to be determined,
there are clues to potential components responsible for such signal processing/integration
in the physical machinery of the gravity sensing system.
The first step in the graviresponse is the perception of the gravity vector by the plant.
There are two schools of thought about how this initial event is achieved: the statolith the-
ory and the gravitational pressure theory (see Chapters 1 and 2 for a complete descrip-
tion). The statolith theory states that intracellular sedimenting particles are responsible
for sensing gravity. In higher plants, statoliths are dense, starch-filled amyloplasts inside
specialized cells (Sack 1997; Kiss 2000). In contrast, the gravitational pressure theory
states that the entire protoplast acts as the gravity sensor and the tension and compression
by the protoplast against the extracellular matrix initiates the graviresponse (Wayne and
Staves 1996; Staves et al. 1997a, 1997b).
At present the preponderance of experimental data support the statolith theory of
gravisensing (see Chapter 1) and indeed, starch statolith motility likely plays a central
role in the integration of touch and gravity responses. Thus, when the plant is mechani-
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