occurred only in the lower pulvinus halves and correlated with the bending response.
These changes in InsP 3 levels were functionally relevant because treatments with an in-
hibitor of phospholipase C, an enzyme that contributes to InsP 3 synthesis, blocked the in-
crease in InsP 3 levels and inhibited the bending response in oat (Perera et al. 2001).
Recently, the same investigating team detected similar biphasic changes of InsP 3 lev-
els upon gravistimulation in Arabidopsisinflorescence stems (Perera et al. 2006). In both
cereal pulvini and dicot inflorescence stems, the second increase in InsP 3 levels preceded
visible bending responses, suggesting its involvement in early phases of the pathway.
To further investigate the potential role of InsP 3 in gravity signal transduction, trans-
genic Arabidopsis thaliana plants expressing human type I inositol polyphosphate
5-phosphatase, an enzyme that specifically hydrolyzes InsP 3 , were generated. These
plants grew like wild type despite containing very low levels of InsP 3 (10% of wild-type
levels). However, they exhibited reduction in the kinetics of inflorescence-stem,
hypocotyl, and root gravitropism, enhanced root gravitropic sensitivity to extracellular
Ca2+, decrease in basipetal auxin transport along the root, and delay in the development
of lateral auxin gradients upon gravistimulation, as deduced from expression analyses of
auxin-sensitive reporter constructs (Perera et al. 2006). Because InsP 3 is a soluble second
messenger that propagates localized Ca2+fluxes through the cell and to neighboring cells,
and the levels of both molecules display parallel, biphasic increases upon gravistimula-
tion, it appears likely that both Ca2+and InsP 3 contribute to gravity signal transduction
in most or all organs of the plant, possibly by modulating auxin transport (Plieth and
Trewavas 2002; Perera et al. 2006). Determination of the tissue(s) within responding or-
gans where both InsP 3 and Ca2+changes occur upon gravistimulation should provide im-
portant insights into their mode of action.
2.2.3 Do pH changes contribute to gravity signal transduction?
Although an involvement of cytosolic Ca2+as a second messenger in gravity signal trans-
duction remains speculative, better evidence exists for a contribution of cytoplasmic pH
in this phase of the response.
If cytosolic pH contributes to gravity signal transduction, its level in the statocytes
should change upon gravistimulation, and interference with such changes should affect the
response. Indeed, both assumptions were recently validated. Rapid cytoplasmic pH
changes upon gravistimulation were observed in the statocytes of both maize pulvini and
Arabidopsis roots (Scott and Allen 1999; Fasano et al. 2001; Johannes et al. 2001;
Boonsirichai et al. 2003; Hou et al. 2004; Young et al. 2006). By using longitudinal maize
stem sections loaded with a pH indicator, Johannes and collaborators were able to moni-
tor the cytoplasmic pH of both gravity-sensing bundle-sheath and parenchyma cells upon
gravistimulation. They found that gravistimulation promotes a fast alkalinization of the
bundle-sheath statocytes without altering the pH of parenchyma cells. They also found that
the gravity-induced cytoplasmic alkalinization in the pulvinus statocytes occurs only in a
restricted region of the cytoplasm where the sedimenting amyloplasts accumulated.
Similar experiments in which the cytoplasmic pH of the root statocytes was measured
during gravistimulation of live Arabidopsisseedlings indicated that a transient alkaliniza-
tion occurs early after gravistimulation onset, peaks within 1 to 2 min, and returns to