mentation in gravity susception, and uncovered an important role for vacuolar biogene-
sis and function in gravity perception or signal transduction in shoots (Saito et al. 2005).
In addition, other sgrmutants also identified new gravity signal transducers in Arabi-
dopsisshoots.
sgr5andsgr6are among the shoot gravitropism mutants that are most likely to be de-
fective in early phases of gravity signal transduction within the shoot statocytes. Indeed,
amyloplasts in mutant shoot endodermal cells sediment almost like wild type, indicat-
ing that gravity susception is not affected (Morita et al. 2006; Yano et al., unpublished
results). SGR5 is a zinc-finger protein that is localized in the nucleus and is mainly ex-
pressed in the endodermis. In addition, endodermis-specific expression of wild-type
SGR5in the sgr5-1mutant restored shoot gravitropism to wild-type levels. Hence,
SGR5 is probably a transcription factor that contributes to early events of gravity per-
ception and/or signaling in the statocytes (Morita et al. 2006). Further analyses, such as
exploration of downstream target genes of SGR5, should clarify its function in shoot
gravitropism.
A similar analysis of sgr6mutants suggested that the SGR6 protein is also involved in
an early step of gravity signal transduction in stem statocytes, subsequent to amyloplast
sedimentation. Unfortunately, the molecular function of SGR6 remains unknown, consid-
ering that its amino acid sequence is conserved with predicted orthologous proteins of
unknown function in higher eukaryotes (Yano et al., unpublished results).
An alternative screening approach was also developed to isolate additional shoot grav-
itropism mutants with defects in gravity perception or early phases of signal transduction
(Wyatt et al. 2002). Arabidopsisinflorescence stems show no response to gravistimula-
tion at 4°C. However, stems that are gravistimulated by horizontal placement at 4°C can
execute a bending response to the cold gravistimulus if returned to the vertical position
at room temperature within the next hour (Fukaki et al. 1996). It has been demonstrated
that basipetal auxin transport is abolished in the inflorescence stems of wild-type plants
at 4°C (Nadella et al. 2006).
Taking advantage of this unusual behavior, Wyatt and collaborators (2002) isolated
several gravity persistence signal (gps) mutants that exhibit normal shoot gravitropism at
room temperature but display abnormal bending responses to stimuli provided in the
cold.gps1does not bend, gps2bends in the wrong direction, and gps3over-responds
when returned to room temperature after cold gravistimulation (Wyatt et al. 2002).
Amyloplasts sediment in the direction of gravity in all gpsmutants during cold gravis-
timulation, indicating that gravity susception is not affected.
To investigate a possible effect of the gpsmutations on the ability of inflorescence
stems to “remember” a cold stimulus by developing a lateral auxin gradient upon return
to vertical position at room temperature, Wyatt and her collaborators studied expression
of the auxin-dependent pIAA2::GUSgene in cold gravistimulated wild-type and gpsin-
florescence stems. As expected, wild-type stems showed asymmetrical activation of GUS
expression on the lower side of a section of its stem elongation zone. On the other hand,
gpsmutant plants displayed patterns of GUS expression that were consistent with the
characteristics of their respective bending defects: gsp1stems displayed no evidence of
asymmetrical activation of pIAA2::GUSexpression after cold pretreatment, whereas gps2
showed enhanced GUSexpression on the upper side of cold-gravistimulated stems. gps3
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