Ca2+signal transduction pathway as being differentially represented in the root tip of
Arabidopsis thalianaseedlings upon 0.5 and 3 hours of gravistimulation. Three other pro-
teins were found to change transiently their molecular weight, but not their pI, upon
gravistimulation, suggesting post-translational modification (Kamada et al. 2005). The
stimulation times used in this study allowed significant gravitropic curvature, implying
that the identified proteins have the potential of functioning in any phase of gravitropism,
from gravity perception or signal transduction to the curvature response.
In an attempt to focus on early phases of gravity perception and signal transduction,
another study analyzed the protein profiles of 12-min gravistimulated Arabidopsisroot-
tip samples (Murthy, Young, Sabat, and Masson, in preparation). This time point was cho-
sen because it is sufficient to promote productive gravity signal transduction (as deter-
mined by the ability of 12-min gravistimulated root tips to develop tip curvatures after
subsequent clinorotation; see Chapter 1), but insufficient for curvature initiation.
Control and 12-min gravistimulated root tips were dissected, and proteins were ex-
tracted using a three-step fractionation procedure. Protein fractions were subjected to 2D-
GE, followed by silver-staining of the corresponding gels. The protein profiles of control
and gravistimulated samples were compared. Fifty-seven protein spots were uncovered
whose staining intensity was altered after 12 min of gravistimulation relative to unstim-
ulated controls, and the corresponding proteins were identified by mass spectrometry. An
additional control was included in this experiment, in which Arabidopsisseedlings were
gently rotated to the horizontal, then immediately returned to the vertical for an addi-
tional 12 min, as a way to control for the mechano-stimulus that accompanies gravistim-
ulation. Only 4 of the 57 graviresponding proteins also showed differential regulation in
response to the mechano-stimulus control. Hence, a vast majority of these proteins re-
sponded specifically to gravistimulation.
Most of the proteins identified in the latter study fell into the following functional cat-
egories: Unknown function (24%); Metabolism (17%); Stress and detoxification (13%);
Defense (10%); and Energy (10%) (Murthy et al., in preparation). It is striking that only
one of these differentially represented proteins is encoded by a gene also found to be tran-
scriptionally regulated by gravistimulation (Kimbrough et al. 2004). This difference be-
tween root-tip transcriptional- and proteomic-response profiles may again reflect differ-
ences in the experimental procedures. Indeed, transcriptional profiling was carried out on
dark-grown seedlings, whereas analysis by Murthy et al. involved light-grown material
(Kimbrough et al. 2004; Murthy et al. 2007). On the other hand, 6 of the 16 proteins iden-
tified by Sakamoto et al. (2005), or their paralogs, were also identified as differentially
represented by Murthy and collaborators, indicating some consistency between independ-
ent proteomic studies.
Among the 53 gravity-responding root-tip proteins identified by Murthy and collabo-
rators, 3 function in the S-adenosylmethionine (AdoMet) methyl-donor pathway. This
pathway generates precursors for ethylene and polyamine synthesis, and provides methyl
groups for transmethylation reactions that target a number of plant regulatory molecules
such as auxin, cytokinin, jasmonate, salicylate, etc. This result suggested an involvement
of the AdoMet cycle in gravity signal transduction (Young et al. 2006).
A reverse genetic approach was used to investigate this possibility. One of two
Arabidopsisgenes encoding adenosine kinase (ADK1) was shown to contribute to root
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