ity to expression (Iliev et al. 2002), as does a similar sequence for CBF2(a transcriptional
activator with a central role in cold response; Zarka et al. 2003). The microarray data de-
scribed above now present the possibility of assessing how widespread these touch- and
gravity-related transcriptional regulatory motifs might be. Such analysis might provide
one avenue to start to dissect the transcriptional machinery responsible for these genome-
wide changes in expression in response to either gravity or mechanical perturbation.
The general conclusion from all these transcriptional profiling analyses is that both
mechanical- and gravistimulation induce very rapid and extensive changes in mRNA pro-
files. In addition, these stimuli share a huge overlap in the transcriptional changes they
cause, likely reflecting the similarity in developmental responses they elicit. This is in
contrast to recent work on plant hormonal regulation of growth and development where
Nemhauser et al. (2006) concluded that these regulators controlled similar developmen-
tal outcomes through largely non-overlapping transcriptional responses.
The other striking feature is the differences in the precise suites of genes reported as
being under mechanical or gravitational regulation in each study. This may in part reflect
differences in timing and tissues under analysis. However, for the case of mechanical
stimulation it is important to note that touch is a complex stimulus to quantify and apply,
and so the variation is almost certainly also reflecting the different ways mechanical stim-
ulation was conducted in each study. It seems likely that the transcriptional response is
highly tailored to the kind of mechanical stimulation, hinting at a signaling system capa-
ble of encoding information such as magnitude, duration, and location of the touch stim-
ulation coupled to an extremely adaptable response circuit.
An additional theme from such studies is that genes for putative Ca2+-binding proteins
are disproportionately up-regulated upon touch stimulation (Lee et al. 2005), suggesting
an alteration in the Ca2+response system, perhaps as part of an adaptation mechanism to
the initial Ca2+signals associated with touch sensing. In general, as the levels of Ca2+sig-
naling components are elevated, two outcomes are likely: (1) the sensitivity of the system
to future Ca2+increases should be increased, and (2) the emphasis of Ca2+-dependent
cellular responses will be shifted toward those involving the now-elevated Ca2+response
elements. Thus, the touch history of the plant may well shift both its sensitivity and pre-
cise response to future touch stimulation. However, as a note of caution, although CML24
mRNA has been shown to be highly induced by touch (nine-fold increase at 30 minutes
after stimulation) and to be expressed in regions of the plant likely experiencing mechan-
ical strain (such as branch points and organs undergoing rapid elongation), recent analy-
sis indicates no detectable change in protein abundance upon touch stimulation (Delk et
al. 2005).
As an alternative approach to monitor protein changes related to touch and gravistim-
ulation, Young et al. (2006) have used proteomic profiling of the gravistimulated root tip.
This analysis confirms rapid changes in protein levels/modifications. For example,
adenosine kinase changes 1.8-fold during the first 12 minutes of a graviresponse but not
in response to mechanostimulation. Interestingly, one of the early gravitropic response
genes identified by Kimbrough et al. (2004) is S-adenosyl-L-methionine:carboxyl
methyltransferase. Both of these enzymes contribute to the AdoMet cycle, which is in-
volved in the synthesis of a host of cell regulators and components ranging from ethyl-
ene and IAA to lignin (Schoor and Moffatt 2004). Thus, again this analysis suggests that
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