ing study, which ties auxin-induced gene expression to differential growth during pho-
totropism and gravitropism, utilized Brassica oleracea (Esmon et al. 2006). These
seedlings were of sufficient size to isolate opposite flanks of hypocotyls after exposure to
lateral light stimulation or after being placed horizontally. mRNA was isolated from each
flank and used to hybridize an Arabidopsismicroarray. A number of genes were shown to
be differentially expressed on the two sides of the stimulated hypocotyls. Additionally, the
expression of these same genes was enhanced in the hypocotyls of auxin-treated etiolated
Arabidopsisseedlings in a NPH4/ARF7-dependent fashion (Esmon et al. 2006). Several
of these genes were shown to encode proteins that are directly tied to growth, including
two expansin genes, EXPA1andEXPA8(Esmon et al. 2006).
Finally, the functional significance of differences in auxin-regulated gene expression
across gravity-stimulated tissues needs to be evaluated in the context of differences in
gravity response between roots and shoots. In light-grown shoot tissues, auxin is gener-
ally limiting for growth (Yang et al. 1993; Gray et al. 1998), so the elevated auxin con-
centrations on the lower side of shoots reoriented relative to gravity could operate to in-
duce transcription of genes encoding proteins that enhance growth, consistent with the
report described above (Esmon et al. 2006). In contrast, although roots also redirect auxin
to the lower side, the opposite response is initiated, resulting in growth with the gravity
vector. Although root growth is negatively regulated by auxin under most conditions
(Pickett et al. 1990), it is not completely clear whether this growth response is due to the
elevated auxin on the lower side or the reduced auxin levels on the upper side. Detailed
kinetic analysis of roots after gravitropic reorientation in many species indicates that
most exhibit enhanced growth on the upper side, rather than the predicted growth inhibi-
tion on the lower side (as reviewed in Wolverton et al. 2002). These kinetic studies also
revealed that shortly after gravitropic reorientation, there is enhanced growth on both
sides of the root, which is sustained on the upper side of the root, followed by a reduced
growth rate on the lower side of the root once curvature initiates (Buer and Muday 2004).
The complexity of this response has led to the suggestion that parts of the response
may be auxin-independent (Wolverton et al. 2002), but it is also possible that the signal-
ing mechanisms (including transcription changes described above) could control this
process. Since auxin positively and negatively regulates the expression of responsive
genes, depending on the complexes of ARF and AUX/IAA proteins involved (Hagen and
Guilfoyle 2002), scenarios can be envisioned to support this more complex response of
roots. The lower auxin levels on the upper side of a reoriented root could relieve auxin’s
repression of genes that encode proteins that positively regulate growth, just as the en-
hanced auxin levels on the lower side could repress synthesis of growth-inducing pro-
teins. Additional experiments will be needed to determine the complex interplay of sig-
nals that change in response to gravity stimulation and their mechanisms for controlling
the complex process of root gravitropism.
3.8 Conclusions
The ability of plants to respond to gravity is tied to the redistribution of auxin. In the last
decade, the proteins that are involved in auxin redistribution have been elucidated by a