tion in Arabidopsisinflorescences. They then constructed Arabidopsisplants expressing
a human inositol polyphosphate 5-phosphatase that should attenuate these InsP 3 levels.
These plants showed 90% reduction in basal InsP 3 content and a disruption of gravitropic
response kinetics, further implicating InsP 3 , and so perhaps Ca2+release, in gravitropic
signaling/response.
Despite this strong circumstantial evidence for a role of Ca2+in gravisignaling, direct
measurements for this change remain ambiguous. Although Gehring et al. (1990) re-
ported gravistimulation-induced Ca2+increases in maize coleoptiles, it has been difficult
to unequivocally relate these changes to Ca2+increases associated with the gravity re-
sponse (Firn and Digby 1990). Legue et al. (1997), using plants loaded with the Ca2+-
sensing fluorescent dye Indo-1, and Sedbrook et al. (1996), using plants expressing ae-
quorin, were unable to detect Ca2+increases upon gravistimulation, yet Legue et al.
(1997) saw clear touch-induced changes under identical conditions. In contrast, Plieth
and Trewavas (2002) have reported Ca2+increases induced by gravistimulation. These
measurements were made in Arabidopsisseedlings transformed with the Ca2+sensor ae-
quorin and gravistimulated by rotation through 135 degrees. The kinetics of these
changes were different from plants rapidly rotated through 360 degrees, which was used
to provide a control for the mechanical stimulation inherent in the rotation. However, con-
sidering the exquisite sensitivity of plants to mechanical stimulation, it still remains pos-
sible that the Ca2+increases associated with the gravistimulation were also reflecting the
mechanical stimulation of the rotation to 135 degrees. These kinds of caveats about ex-
perimental results highlight the difficulties of separating touch from gravity signaling
and response. This problem is not only limited to experimental design but also to the bi-
ological responses to these stimuli, a theme we will discuss in more detail in the section
describing transcriptional responses to touch and gravity below.
If the circumstantial evidence so strongly points to a role for Ca2+signaling in the
gravity response, why has it been so hard to clearly demonstrate the change? One possi-
bility is suggested by the experiments of Plieth and Trewavas (2002). In order to detect
Ca2+changes upon reorientation of their aequorin-expressing plants, these researchers
had to resort to making simultaneous measurements on 500 to 1,000 seedlings and
reconstituting the aequorin with the most sensitive version of its cofactor known (Cp-
coelentrazine). The need for such high sensitivity and numbers suggests the Ca2+signal
is either localized to a very few cells and/or localized within those sensory cells. The el-
evated levels of CaM and CaM-like proteins in the root cap gravisensory cells will likely
sensitize them to very small changes in Ca2+that may be at the limits of current Ca2+de-
tection systems.
Similarly, in animal cells it is well characterized that highly localized Ca2+fluxes,
which are extremely difficult to detect, can elicit dramatic effects on cellular response. For
example, in neurons Ca2+flux specifically through L-type Ca2+channels at the plasma
membrane allows activation of MAP kinase and the CREB transcription factor cascade.
Such activation is mediated by CaM tightly bound to the inner face of the pore of the chan-
nel (Dolmetsch et al. 2001). This spatially restricted signaling system means that very
small Ca2+fluxes are funneled to the appropriate signal transduction chain. In this case,
large-scale changes in cytosolic Ca2+cannot even trigger the response. Thus, it may well
be that limitations on the resolution of current Ca2+measurement technology to detect
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