to encode transcription factors necessary for the normal development of the endodermis
in roots and shoots. Both mutants failed to develop an endodermal cell layer and, as a re-
sult, lacked sedimentable amyloplasts in their shoots. However, both mutants had normal
columella cells containing sedimentable amyloplasts, indicating that amyloplast forma-
tion or sedimentation itself was not impaired. Importantly, although shoot gravitropism
was defective in these mutants, root gravitropism was not. Thus, it was concluded from
these studies that the presence of a normal endodermal layer is important in shoot grav-
itropism (Fukaki et al. 1998).
The importance of the endodermis for gravitropism was also recently demonstrated in
other plant species. For example, an agravitropic mutant in the Japanese morning glory
(Pharbitis nil) called weepingwas shown to be defective in the formation of the endoder-
mal layer, similar to the sgr1mutant in Arabidopsis(Hatakeda et al. 2003). Interestingly,
the disrupted gene in weepingwas recently shown to be an ortholog of Arabidopsis scr.
When the wild-type scr from morning glory (PnSCR) was introduced into the
Arabidopsis scrmutants for complementation, the agravitropic phenotype of Arabidopsis
was rescued (Kitazawa et al. 2005).
Results from the above studies are also beginning to shed light on the relationship be-
tween gravitropism and circumnutation (i.e., oscillatory plant movements). It has long
been debated whether gravitropism and circumnutation are causally related to each other
(Kiss 2006). Interestingly, it appears that circumnutation and winding movement defects
inweepingcould be attributed to loss of endodermal function since the circumnutation
phenotype in Arabidopsis scr1could be rescued by scrfrom wild-type morning glory.
Although this provided compelling evidence that other types of plant movements (such
as circumnutation) require gravity-sensing cells, recent studies in rice coleoptiles indicate
that the relationship between gravitropism and circumnutation may be more complex. For
example, Yoshihara and Iino (2005) demonstrated that red light abolished circumnutation
in rice coleoptiles without affecting the gravitropic response. Furthermore, coleoptiles of
the rice mutant lazydo not circumnutate but do contain sedimentable amyloplasts, which
suggests that mechanisms independent of gravitropism might operate in plant circumnu-
tations (Yoshihara and Iino 2006).
In addition to proving directly that the endodermis is a major gravity-sensing cell layer,
the screen for shoot gravitropic mutants has helped uncover additional features of the en-
dodermis that might be important for gravity perception. Although much attention has
been given to sedimenting amyloplasts, other cellular compartments in the gravisensitive
cells are likely important. For example, in the above-mentioned screen performed by the
group of Tasaka, several other shoot gravitropic mutants were isolated. Of these, the sgr2,
sgr3, and sgr4/zigzag mutants are notable because they are implicated in vacuolar
membrane trafficking (Kato et al. 2002; Yano et al. 2003). The SGR2gene encodes a
phospholipase-like protein and the ZIGgene encodes a SNARE protein. Both are thought
to be involved in vacuolar membrane dynamics because abnormal vesicular and vacuo-
lar structures were observed in the endodermis and several other tissues of the mutant
plants. In addition, GFP-SGR2localizes to the vacuole and some small organelles.
Interestingly, in the mutant plants, many amyloplasts do not sediment properly to the bot-
tom of the cell and appear to maintain their localization in the peripheral cytoplasm of
the endodermal cells (Morita et al. 2002).
frankie
(Frankie)
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