gravity or waviness response (Lu and Federoff 2000; Kang et al. 2002). On the other
hand, it has been postulated that ABA maintains a higher growth rate on the side with
lower water potential in hydrotropically responsive roots. This suggestion arose from the
observation that the hydrotropic response of roots in an agar KCl system of two
ArabidopsisABA mutants, aba1-1andabi2-1, was slightly reduced compared with those
of wild type (Takahashi et al. 2002). In contrast, both aba1-1andabi2-1mutant roots re-
sponded positively to hydrotropism in the screening system with a water potential gradi-
ent (Eapen et al. 2003). However, there are some differences between these two hy-
drotropic systems, namely, light conditions, seedling age, and the substrate of the water
potential gradient, which may account for this discrepancy.
Light has been shown to influence the gravity response in roots, shoots, and other or-
gans (Hangarter 1997). So far, there are no studies that have examined an interaction of
ABA with the development of an asymmetric auxin gradient for differential growth,
which may be an important factor during hydrofacilitation. Yet, it was recently reported
that cells in the columella and quiescent center of Arabidopsisshowed low levels of ABA,
which were not increased by water stress, suggesting a non-stress-related role for ABA in
these cell types (Christmann et al. 2005). Further, there are some reports of ABA-
activated gene expression in the root cap (Hong et al. 1988; Nylander et al. 2001), and so
the root cap might be required for proper regulation of the ABA-dependent processes of
cell division (Dewitte and Murray 2003). Therefore, an important role of ABA in devel-
opmental programs such as tropisms is anticipated. A detailed analysis of more ABA mu-
tants and the cloning of the NHR1gene might thus provide evidence as to whether ABA
functions on gravitropic and/or hydrotropic response in roots.
6.2.5 Future experiments
Our understanding of hydrotropism has lagged many years behind that of gravitropism
and phototropism. However, the future characterization of the genes in the hydrotropic
mutants isolated so far might help to unravel the players in this particular growth re-
sponse. Many questions remain open, particularly those related with the sensing system
for moisture gradient and the mechanism that merges and assesses the diverse stimuli im-
pacting on the root in order to generate the proper tropic response.
Thus far, the NHR1gene seems to block root gravitropic growth and allow roots to di-
rect their growth toward water, since in nhr1mutant roots gravity response is enhanced
(Fig. 6.3). Further, it remains to be determined whether signals such as Ca2+, calmodulin,
pH increases, reactive oxygen species, inositol 1,4,5-trisphosphate, auxin, ethylene,
flavonoids, cytokinins, and brassinosteroids participate in hydrotropism as they seem to do
in gravitropism (reviewed in Chapter 2). Thus, there is considerable potential for further
research to uncover the mystery of how roots are able to amplify a signal, such as water.
6.3 Electrotropism
The direction of growth of certain plant cells or organs can be modified by an applied
electric field. This phenomenon, known as electrotropism, has been observed in fungi