Recent studies provide potential auxin-related explanations for the surprising character-
istics of the initial phases of root gravicurvature, including early gravity-induced increases
in the levels of flavonoids at the root tip, which inhibit general auxin transport (Buer and
Muday 2004); and an indirect effect of decreased auxin levels on the vesicular trafficking
and degradation of the AGR1/EIR1/PIN2/WAV6 auxin efflux facilitator at the top flank
(Abas et al. 2006). However, independent observations suggest that cytokinin might also
contribute to at least some aspects of root gravicurvature. Using a cytokinin-sensitive
ARR5-GUS reporter construct to indirectly follow changes in cytokinin levels within the
root tip, investigators were able to demonstrate that gravistimulation promotes a fast asym-
metrical increase in reporter expression within the bottom lateral cap flank, potentially re-
flecting differential increases in cytokinin levels at that flank (Aloni et al. 2004). The same
authors also showed that application of exogenous cytokinin to one side of the elongation
zone of vertically oriented roots results in curvature development in the direction of cy-
tokinin application. Hence, they suggested that gravistimulation might promote a lateral
transport of cytokinin across the cap, with accumulation on the lower flank and consequent
differential cellular elongation resulting in initial curvature response (Aloni et al. 2004).
Although this model is attractive at first glance, it should be cautioned that genetic sup-
port for it is currently lacking. The cytokinin-deficient mutants analyzed so far have not
shown dramatic changes in root gravitropism. However, it is true that none of these mu-
tants showed complete obliteration of cytokinin sensitivity (Aloni et al. 2004). Further-
more, expression of one of the genes that contribute mainly to cytokinin biosynthesis in
theArabidopsisroot cap, isopentenyl transferase 5 (IPT5), is auxin-sensitive (Miyawaki et
al. 2004), suggesting the possibility that the asymmetrical activation of ARR5-GUS ex-
pression on the lower flank of gravistimulated root caps might simply be a consequence of
lateral auxin transport, rather than reflecting an effect of gravity signal transduction on the
lateral transport of cytokinins across the cap. The kinetics of differential ARR5-GUS ex-
pression across gravistimulated root tips appear to precede the asymmetrical activation of
the auxin-sensitive DR5-GUS reporter (Aloni et al. 2004). However, these two genes are
indirect reporters of cytokinin and auxin levels, respectively, functioning at the end of their
respective pathways. Hence, data derived from comparative analyses of their expression
kinetics upon gravitimulation should be interpreted with great caution.
Future work will be needed to test asymmetrical activation of ARR5-GUS expression
across the root tip in different mutants, such as arg1-2,arl2-1,adk1-1, or pgm, in order to
test whether this response lies in one of the known gravity signal transduction pathways.
Investigation of this response in a variety of mutants affected in diverse aspects of cy-
tokinin synthesis and response is also crucial. For instance, an analysis of the simple, dou-
ble, and multiple mutants carrying defects in the three cytokinin receptor genes found in
Arabidopsis thaliana(Higuchi et al. 2004) should be carried out to evaluate their relative
contributions to the initial phases of the root gravitropic curvature.
2.3 Gravity signal transduction in organs that do not grow vertically
The organs discussed in this chapter tend to grow vertically, either upward or downward.
However, other plant organs may display distinct growth behavior. For instance, lateral