Unfortunately, the aequorin-derived signal could only be detected if hundreds of
Arabidopsisseedlings were examined in bulk, and the luminescence was too weak for
identification of its source. In the future, it will be necessary to use a similar detection
strategy to analyze gravity-induced Ca2+transients in seedlings that express aequorin
specifically within the root and hypocotyl statocytes to determine whether the signal de-
rives from these cells. It will also be important to determine whether such a signal disap-
pears in mutants that are defective in early phases of gravity signal transduction, such as
the starch-deficient and signal-transduction mutants described below.
In separate experiments using either the system described above or Ca2+-sensitive
fluorophores, other researchers were unable to detect changes in cytosolic Ca2+levels in
the statocytes upon gravistimulation (Sedbrook et al. 1996; Legue et al. 1997; Fasano et
al. 2002; Massa and Gilroy 2003). Although these negative results cast doubt on the pos-
sible involvement of Ca2+in gravity signal transduction within the statocytes, they may
reflect a lack of sensitivity of the Ca2+detection approaches used, or a highly localized,
yet functionally significant, Ca2+pulse undetectable by these sensors. It is interesting to
note that even small and/or highly localized changes in cytosolic Ca2+levels within the
root statocytes might be functionally relevant because these cells express high levels of
calmodulin (Sinclair et al. 1996; see also Chapter 5).
Additional complexity arises in studies that investigate the role of cytosolic Ca2+in
gravity signal transduction within the root statocytes. As emphasized in Chapter 1, touch
stimulation of the root tip promotes a fast increase in cytosolic Ca2+levels within the
stimulated peripheral cap cells, which propagates to surrounding cells to eventually reach
the columella region. There, the corresponding Ca2+wave appears to inhibit gravitropism
by interfering with amyloplast sedimentation (Massa and Gilroy 2003). Hence, if a Ca2+
wave signals an inhibition of gravitropic sensitivity in the statocytes in response to a
touch stimulus at the cap, a role for gravity-induced Ca2+flux in gravity signal transduc-
tion within the statocytes would require a corresponding Ca2+signal that displays a dis-
tinctive signature (Massa and Gilroy 2003).
Models postulating the involvement of mechano-sensitive ion channels as gravity re-
ceptors in plant statocytes currently suffer from a major roadblock: such channels have
not been identified. This may be because most of the ion channels identified as mechano-
sensitive in animals and yeast do not have obvious orthologs in plants (Barritt
and Rychkov 2005; Haswell and Meyerowitz 2006; for a more detailed discussion,
see Chapter 5). The recent discovery of a family of 10 Arabidopsisgenes encoding
membrane-spanning proteins related to the bacterial MscS channels, which serve to pro-
tect the bacteria against cellular lysis during osmotic downshock (Kung and Blount
2004), may constitute an important breakthrough (Haswell and Meyerowitz 2006).
Indeed, initial studies on one of these ArabidopsisMscS-like proteins (MSL3) demon-
strated its ability to rescue the osmotic sensitivity of an E. colimutant lacking mechano-
sensitive ion channel activity. This suggests the channel is mechano-sensitive, at least in
this bacterial system. Interestingly, MSL3 and the highly related MSL2 protein were tar-
geted to the plastid membranes in plants, where they seemed to localize at focal sites as-
sociated with proteins implicated in plastid division. Furthermore, double mutants
showed abnormal plastid morphology, suggestive of defects in plastid division. These ex-
citing results were used to suggest an involvement of these related proteins in regulating
frankie
(Frankie)
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