baseline levels within 8 to 10 min (Scott and Allen 1999; Fasano et al. 2001; Boonsirichai
et al. 2003; Young et al. 2006). Although Scott and Allen suggested distinct kinetics in
the cytoplasmic alkalinization of layer-2 cells (layers of columella cells are represented
in Figure 2.1A) between upper and lower statocytes of gravistimulated roots, and an acid-
ification of layer-3 cells, Fasano and collaborators described similar alkalinization in all
statocytes within both columella tiers (Fasano et al. 2001). Gravity-induced alkalinization
of the root statocytes was dramatically attenuated in pgm, a mutant that contains starch-
less plastids and displays altered gravisensitivity (Fasano et al. 2001). It was also accom-
panied by an acidification of the apoplast in wild-type roots, suggesting it might result
from the activation of plasma membrane or vacuolar H+transporters (Fasano et al. 2001;
Li et al. 2005).
Interestingly, altering the cytoplasmic pH of root statocytes by releasing preloaded
caged protons resulted in a delay in the gravitropic response (Fasano et al. 2001).
Furthermore, treating root caps with agents that acidify the apoplast and the cytoplasm at
concentrations that do not alter the overall rate of root growth resulted in an enhancement
of the gravitropic response, whereas treatments with alkalinizing agents delayed it (Scott
and Allen 1999). Likewise, treatments with agents that disrupt actin filaments resulted in
both sustained cytoplasmic alkalinization upon short periods of gravistimulation and en-
hanced gravicurvature, as discussed in Chapter 1 (Hou et al. 2004). Even though the ini-
tial studies differed in the details of their observations, current data converge to suggest
an important role for cytosolic pH in gravity signal transduction in both coleoptiles and
roots. It should be cautioned that the data obtained so far do not demonstrate an essential
role for pH in this pathway, as none of the cytosolic pH manipulations performed so far
have led to a complete elimination of gravitropism.
In conclusion, cytoplasmic pH changes may have a universal role in the early signaling
phases of gravitropism. What might they be doing in this process? We currently have no
definite answer to this important question, partly because we have only a rudimentary un-
derstanding of the molecular mechanisms that govern it. We also have a limited knowl-
edge of the locale of these gravity-induced pH changes within individual statocytes. It has
been proposed that gravity-induced pH changes in the statocytes might facilitate auxin
transport (Fasano et al. 2002). Indeed, such pH changes may be related to the asymmetric
pH responses that were observed at the surface of gravistimulated roots by proton-
selective microelectrodes. These asymmetric surface-pH changes originated at the root cap
and progressed along the root tip at a rate comparable with polar auxin transport
(Monshausen and Sievers 2002). It is possible that surface-pH changes and polar auxin
transport are related, and that the gravity-induced pH changes in columella cells regulate
the activity or cellular distribution of auxin transporters in the statocytes (Fasano et al.
2002). In agreement with this model, mutant and transgenic Arabidopsisplants with altered
expression of a H+-pyrophosphatase (AVP1) display altered auxin transport along with al-
tered expression and mislocalization of the PIN1 auxin efflux facilitator (Li et al. 2005).
2.2.4 Proteins implicated in gravity signal transduction
Recent genetic analyses of gravitropism have contributed to the identification of several
gravity signal transducers. Although most gravitropism mutations turned out to affect as-