displayed increased GUS expression on the lower side in an extended region along the
elongation zone of its stems (Nadella et al. 2006). These results suggest that the gpsmu-
tants fail to properly establish a lateral auxin gradient across their inflorescence stems
after cold gravistimulation, supporting a role for the corresponding genes in early phases
of gravity signal transduction.
Initial results in the molecular genetic analysis of three GPSgenes appear to support
their involvement in gravity signal transduction (Sarah Wyatt, personal communication).
GPS1encodes a cytochrome P450 of unknown function. Although GPS1is not func-
tional in the roots, a root-specific family member has been identified that is up-regulated
in response to gravistimulation. Initial experiments indicate that these P450s may be in-
volved in synthesis of flavonoids and, thus, the regulation of auxin transport through that
pathway (Buer and Muday 2004; Withers and Wyatt, unpublished data). GPS2encodes a
hypothetical synaptobrevin/vesicle-associated membrane protein, v-SNARE (McCallister
and Wyatt, unpublished data). GPS2 protein may be involved in transport of the PIN ef-
flux carriers or other regulatory molecules in the inflorescence stem (see Chapter 5).
Finally, GPS3encodes a transcription factor with a B3 DNA-binding domain similar to
auxin response factors (ARFs). However, GPS3 protein lacks the C-terminal dimerization
domain common among ARF proteins (see Chapter 5). Initial subcellular localizations of
GPS3 using a GFP fusion support nuclear localization for the protein, but its role in grav-
itropic signal transduction is as yet unknown (Nadella and Wyatt, unpublished data).
Hence, these GPSgenes hold great promise to further our understanding of the molecu-
lar mechanisms that govern gravity signal transduction in shoots.
2.2.5 Global ‘-omic’ approaches to the study of root gravitropism
Although the genetic approach has been successful at identifying new gravity signal
transducers, it also has limitations due to functional redundancy associated with frequent
gene duplications in plants and from pleiotropy, both of which mask function in gravi-
tropism. Trying to bypass such difficulties, several groups have recently used techniques
derived from genomics and proteomics to identify genes or proteins whose expression
varies early in response to gravistimulation. Their hope is that some of these candidates
will contribute to gravitropism.
Knowledge of the Arabidopsis thaliana genome sequence (Arabidopsis-Genome-
Initiative 2000) allowed the design of partial- and whole-genome microarray chips that
can be used for global analyses of gene expression in response to environmental stimuli.
A first attempt at identifying genes whose expression varies early in response to gravi-
stimulation was reported in 2002, when scientists at the University of California–Berkeley
used expression profiling with an Arabidopsis thaliana8,300-gene microarray to demon-
strate that 1.7% of the genes analyzed are differentially expressed in entire seedlings
within 30 min of a gravistimulus. Thirty-nine percent of these differentially expressed
genes were also regulated by gentle mechanical perturbation, unveiling the extreme sen-
sitivity of plants to mechano-stimulation (see Chapter 5). Most of the gravity-regulated
genes fell into only a few functional categories, including Oxidative stress/Plant defense
(22.7%); Metabolism (14.9%); Transcription (8.5%); Cell wall/Plasma membrane
(7.1%), and Signal transduction (6.4%). Many of the gravity up-regulated genes con-