pects of polar auxin transport or auxin response, a few have been obtained that affect ear-
lier phases of gravity signal transduction that occur in the statocytes. For instance, muta-
tions that affect starch biosynthesis, such as pgm,have been shown to affect gravitropism.
As discussed in Chapter 1, this is not surprising since starch is a dense material that in-
creases the weight of amyloplasts, enabling their sedimentation in the favorable environ-
ment presented by the statocytes’ cytoplasm (Kiss et al. 1989).
Other mutations that affect gravitropism without altering phototropism, starch synthe-
sis, amyloplast sedimentation, or growth responses to auxin, other phytohormones, or
polar auxin transport inhibitors have also been identified. This class of mutants likely af-
fects genes involved specifically in gravity signal transduction. The first such mutants
isolated in Arabidopsis thalianaaffected the ARG1locus. The rhg/arg1mutants dis-
played altered root and hypocotyl gravitropism while maintaining wild-type root-growth
responses to phytohormones and polar auxin transport inhibitors, as well as normal pho-
totropism (Fukaki et al. 1997; Sedbrook et al. 1999). The latter observation was particu-
larly revealing because it indicated that the mutant organs retained their ability to curve
in response to other directional cues. Hence, the defect was likely to lie in the early
phases of gravity signal transduction.
TheARG1gene encodes a J-domain protein that is conserved between plants and the
worm Caenorhabditis elegans, but is absent in yeast or other animals (Sedbrook et al.
1999). This protein was found to contain a J-domain at its N-terminus, a central hy-
drophobic region and a C-terminal domain predicted to form a coiled coil structure (typ-
ically involved in protein–protein interactions). The C-terminal region shares sequence
similarity with proteins that interact with the cytoskeleton, although strong evidence for
cytoskeleton interaction is currently lacking (Sedbrook et al. 1999; Boonsirichai et al.
2003).
In other, better-characterized, J-domain proteins, the highly conserved J-domain di-
rectly interacts with the HSP70 chaperone, modulating its ATPase activity. The residues
needed for this interaction are conserved in ARG1, suggesting that this protein might also
function in association with HSP70 in the folding, trafficking, localization, and/or regu-
lation of gravity signal transducers in the statocyte (Sedbrook et al. 1999).
A combination of biochemical fractionation and functional GFP-fusion localization
studies demonstrated that ARG1 is a peripheral membrane protein that associates with
multiple components of the vesicle trafficking pathway in all plant cells. Targeting its ex-
pression to the root or hypocotyl statocytes of an arg1-2null-mutant rescued the gravi-
tropic phenotype of the corresponding organ (root or hypocotyls, respectively), demon-
strating ARG1’s role in early phases of gravity signal transduction.
A more thorough phenotypic analysis of arg1-2demonstrated an inability for mutant
root statocytes to respond to gravistimulation by cytoplasmic alkalinization and by relo-
calization of the PIN3 auxin efflux facilitator to their lower membrane, confirming a role
for ARG1 in gravity signal transduction (Boonsirichai et al. 2003; Harrison and Masson
2006). Supporting this conclusion, the auxin-responsive DR5-GUS construct demon-
strated an inability for mutant roots to develop a lateral auxin gradient across their cap
upon gravistimulation (Boonsirichai et al. 2003). Taken together, these data suggest that
ARG1 modulates the trafficking and/or activity of auxin efflux facilitators and/or other
membrane-associated proteins needed for lateral auxin transport in the statocytes.
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