whelming. Both processes working in tandem might be part of a redundant mechanism
to ensure the plant’s best chances for survival in its environment.
In conclusion, although our view about gravity perception in higher plants continues
to revolve around the idea of sedimenting plastids, one of the more pressing issues is how
the gravity-induced mechanical stimulus of falling statoliths or, alternatively, the pressure
exerted by the entire protoplast, is converted to a chemical signal that subsequently reg-
ulates a physiological response. As discussed in Chapter 2, studies involving the interac-
tions between amyloplasts and other cell organelles such as the vacuole, the cytoskeleton,
and the endoplasmic reticulum are fruitful avenues for research that will help address this
issue. In addition, studies of gravitropism in other plant organs might provide useful ad-
ditional information about the gravitropic response. For example, the peanut gynophore
could be developed into a useful model to discern how the gravity signal is translated into
a positive (i.e., downward) or negative (i.e., upward) response. Moreover, transcript and
protein profiling are helping to identify additional molecular players in the gravitropic re-
sponse (Moseyko et al. 2002; Kimbrough et al. 2004). An exciting avenue for future re-
search will be to observe changes in transcript, protein, and metabolite levels specifically
in the cells that are presumed to sense gravity.
1.7 Acknowledgment
Funding from the Samuel Roberts Noble Foundation Inc. and the National Science
Foundation (DBI-0400580) is gratefully acknowledged.
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