Interestingly, and unlike wild-type plants, hydrostimulated nhr1roots maintained their
amyloplasts in the columella (Ponce and Cassab, unpublished observations). This might sug-
gest that the negative hydrotropic response of nhr1is related to the absence of the signaling
cascade that triggers amyloplast degradation during hydrotropic stimulation. In the screen-
ing system for the isolation of super-hydrotropic response mutants, wild-type seedlings
ceased growing after six days in the WSM and lacked amyloplasts in their columella cells,
supporting the observation made by Takahashi et al. (2003) that water-stressed roots also de-
grade amyloplasts. In contrast, suh1roots exhibited amyloplasts in the columella, indicating
that the root cap seems to combine both a hydrotropic and a gravitropic response in order to
reach the medium with higher water potential in the bottom part of the plate.
Root hydrotropic responsiveness has also been studied by using an agar KCl system in
someArabidopsisagravitropic, auxin-insensitive, and ABA-related mutants (Takahashi et al.
2002). In this study, Arabidopsiswild-type roots began hydrotropic curvature against the
gravity vector after 30 minutes of stimulation and reached 80 to 100 degrees within 24 hours.
In contrast, roots of axr1-3andaxr2-1mutants showed a greater hydrotropic response com-
pared with those of wild type. Both mutants are insensitive to auxin and show altered root
gravitropism, with axr2roots being particularly agravitropic (Lincoln et al. 1990; Nagpal et
al. 2000). axr2roots developed a curvature even in the absence of a moisture gradient, which
might suggest that this response is a consequence of their random root growth direction.
Additionally, mutants affected in basipetal polar auxin transport from the root cap to
the root elongation zone, such as wav6andaux1(Blancaflor and Masson 2003; Swarup
et al. 2005), showed hydrotropic curvature. However, it was previously shown that an
auxin transport inhibitor blocked the hydrotropic curvature of ageotropum roots
(Takahashi 1994). Therefore, this analysis suggests that auxin may regulate gravitropism
and hydrotropism differently, although both tropisms might depend on the formation of
an asymmetric auxin gradient for differential growth. Furthermore, the relatively random
root growth direction of these mutants makes the interpretation of their hydrotropic re-
sponse complex, and the results do not provide strong evidence for or against a role of
auxin and auxin transport in the hydrotropic response.
6.2.4 ABA and the hydrotropic response
Abscisic acid (ABA) functions mostly in plant responses to dehydrating stresses
(Finkelstein et al. 2002), and hence a change in ABA homeostasis could take place under
hydrotropic stimulation. Interestingly, mild water stress stimulates primary root elonga-
tion (Sharp 2002). This implies that ABA might enhance root growth in search of water
under these conditions. Root growth of nhr1mutants is insensitive to ABA in NM (Eapen
et al. 2003), but is highly stimulated by this hormone when seedlings are growing in the
screening medium with a water potential gradient (Ponce and Cassab, unpublished obser-
vations). This suggests that orthogravitropic growth of mutant roots is stimulated under
severe water stress conditions in the presence of exogenous ABA. A putative role of ABA
in root gravitropism has been discussed (Feldman et al. 1985), but has not been thor-
oughly analyzed further (LaMotte and Pickard 2004). Previously, we suggested that ABA
might antagonize the early transduction of gravi-induction of hydrotropic-responsive
roots (Eapen et al. 2005). In fact, there are some ABA mutants affected either in their