in Chapter 2, biochemical and cell biological approaches have identified several signal-
ing molecules that are produced in response to a changing gravity vector and may mod-
ulate auxin transport. It has long been suspected that calcium is a signaling molecule that
changes in concentration in response to changes in the vector of gravity (as reviewed in
Sinclair and Trewavas 1997). Attempts to demonstrate changes in cytoplasmic calcium
concentration using calcium ratio imaging in Arabidopsisroots did not detect calcium
concentration changes in response to gravitropic stimulation (Legue et al. 1997). Evi-
dence in support of calcium as a signal in gravitropic response comes from young seed-
lings of Arabidopsisexpressing aequorin in the cytoplasm (Plieth and Trewavas 2002).
When a population of seedlings is reoriented relative to gravity, there is an enhanced
aequorin signal consistent with elevated cytoplasmic calcium concentration (Plieth and
Trewavas 2002). Several experiments have suggested that gravity stimulation may be am-
plified by cascades involving Ca2+/calmodulin (Sinclair et al. 1996; Lu and Feldman
1997), and a number of older studies suggested a relationship between auxin transport
and calcium concentration (dela Fuente 1984; Allan and Rubery 1991). Therefore, the
possibility that calcium signals are integral to gravitropic response remains intriguing but
requires additional experimental test.
The role of several other signaling molecules has been more clearly demonstrated, as
discussed in Chapter 2. Changes in pH have been observed after gravitropic stimulation
in both Arabidopsisroots (Scott and Allen 1999; Fasano et al. 2001; Boonsirichai et al.
2003) and the maize pulvinus (Johannes et al. 2001). Proton movements have been tied
to auxin transport through examination of mutants and transgenics with altered expres-
sion of the H+-pyrophosphatase, AVP1(Li et al. 2005). Altered AVP1expression changes
vacuolar pH and alters IAA transport and PIN1 localization (Li et al. 2005)
Additionally, inositol lipids have been implicated in the gravity signal transduction
pathway in maize and Arabidopsis. A transient increase in the InsP 3 lipid signal have been
observed in gravity-stimulated maize and oat pulvini (Perera et al. 1999; Perera et al.
2001). Gravity-stimulated pulvini undergo rapid initial changes in InsP 3 levels on both
sides, followed by a greater and more persistent elevation on the lower side (Perera et al.
1999). This later InsP 3 elevation on the lower side is necessary for gravitropic bending of
the pulvinus, as treatment with phospholipase C inhibitors prevent formation of the InsP 3
gradient and reduced gravitropic bending (Perera et al. 1999). In maize, free IAA has
been measured and shown to develop an asymmetry across the gravity-stimulated pulvi-
nus that follows the changes in InsP 3 levels (Long et al. 2002).
More recently, additional support for the role of InsP 3 comes from transgenic studies
inArabidopsiswith plants constitutively expressing the human type I inositol polyphos-
phate 5-phosphatase (InsP 5-ptase), an enzyme that specifically hydrolyzes InsP 3 (Perera
et al. 2006). In the transgenic plants, basal InsP 3 levels are reduced by greater than 90%
compared to wild-type plants. With gravistimulation, InsP 3 levels in inflorescence stems
of transgenic plants show no detectable change, whereas in wild-type plant inflores-
cences, InsP 3 levels increase approximately threefold within the first 5 to 15 min of grav-
istimulation, preceding visible bending (Perera et al. 2006). Gravitropic bending of the
roots, hypocotyls, and inflorescence stems of InsP 5-ptase transgenic plants is reduced
(Perera et al. 2006). These transgenics also exhibit reductions in root basipetal IAA trans-
port and delays in formation of lateral asymmetries in auxin-induced gene expression as
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