ilarity in the growth responses these stimuli elicit and/or the multiple pathways within
which each protein operates. Indeed, many of the transcripts identified as touch- and/or
gravity-responsive are also known to be regulated by other stimuli such as cold, light, and
pathogens. However, a small subset were found to be either touch- or gravity-specific. For
example, these researchers found 65 genes that changed rapidly and selectively in re-
sponse to gravistimulation, with changes in five being evident within 1 to 2 minutes
(Kimbrough et al. 2004). Similarly, 26 genes showed a mechanostimulation-specific pro-
file. These gravity- or touch-specific genes covered a wide range of functional categories,
from transcription factors to transporters and wall-modifying proteins. Correspondingly
similar studies profiling mechanoresponsive genes in seedlings (Moseyko et al. 2002) or
aerial tissues (Lee et al. 2005) led to similar conclusions. Thus, Moseyko et al. (2002)
found that of the 8,300 genes they probed, 183 were significantly altered after 30 minutes
of gentle mechanical stimulation, with a significant overlap to those regulated by gravi-
stimulation. When comparing transcriptional profiles in aerial tissues after 30 minutes of
touch or darkness, Lee et al. (2005) found that although 2.5% of the total genes were
touch-inducible (589 genes had touch-inducible expression; 171 had reduced expression),
53% were also altered upon transfer of seedlings to darkness. Indeed, all but 3 of the 68
genes most strongly up-regulated by darkness were also touch-inducible. Again, the range
of gene functions in these groups was diverse, including putative signaling elements, wall
modification, and defense responses. These widespread changes in transcription suggest
an exquisitely sensitive mechanical response system that feeds into much of the physiol-
ogy and developmental pathways that are shared by many other signal/response systems.
It seems likely that changes in mRNA stability as well as transcriptional regulation are
playing some role in governing message abundance, especially over the very short (1- to
2-min) time frames where Kimbrough et al. (2004) reported alterations in transcript level.
Indeed, Gutierrez et al. (2002) found that although only approximately 1% of Arabidopsis
genes have unstable transcripts, touch-induced genes were among the most highly repre-
sented group in their analysis. Unstable transcripts are thought to be the hallmark of
genes requiring rapid changes in steady-state transcript abundance, consistent with the
rapid and widespread transcriptional changes seen in response to touch.
The alterations in the five most rapidly changing gravity-responsive transcripts were
abolished in plants where InsP 3 signaling is likely curtailed through ectopic expression
of a human inositol 5-phosphatase (Salinas-Mondragon et al. 2005). However, the other
widespread changes in gene expression induced by gravistimulation were unaffected in
these plants. Thus, there may well be a functional link between rapid InsP 3 -dependent
signaling (Perera et al. 1997, 1999, 2001) and very rapid mRNA abundance changes seen
upon gravistimulation. There must also be an alternative pathway acting to modulate the
expression of the large number of other responsive genes. Equivalent analysis with re-
spect to touch-related gene expression has yet to be reported.
These extensive microarray data now also offer the possibility to look for promoter el-
ements related to responsiveness to touch and gravity. By analyzing the five most highly
gravitropically up-regulated transcripts, Kimbrough et al. (2004) identified TCATTAA as
a potential gravity regulation motif in the promoter sequences of these genes. However,
functional testing of this possibility has yet to be reported. Similarly, a 102-base-pair re-
gion of the TCH4promoter confers touch, dark, cold, heat, and brassinosteroid sensitiv-
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