water availability. Therefore, we assumed it feasible to screen for super-hydrotropic re-
sponse mutants in Arabidopsis. For this approach, we used the model of the screening
system for nhrmutants, but the WSM is placed in the upper part and the NM in the lower
section of the dish. Roots from one mutant line isolated in this system named super hy-
drotropic response(suh1) continuously grow under water deficit for 10 days in order to
reach the moderate water potential conditions present in the lower section of the dish
(Saucedo and Cassab, unpublished results; Fig. 6.2B and Color Section). Thus, both the
screen for impaired and enhanced hydrotropic response appear to be fruitful avenues of
research toward dissecting the complex signaling phenomena behind the hydrotropic
response.
6.2.3 Perception of Moisture Gradients and Gravity Stimuli by the Root Cap and the
Curvature Response
The sensitivity of the root cap and the root curvature response to hydrostimulation im-
plies that roots have moisture gradient receptors which transform this information into a
physiological signal, and so trigger differential growth. Furthermore, the course followed
by roots through the soil is directed by extraordinarily complex and diverse stimuli and,
thus, the root cap needs to integrate multiple signals to ultimately generate the most ad-
vantageous growth direction. However, neither the sensing cells for moisture gradients
nor the molecular mechanism that transduces the stimulus to a signal has been character-
126 PLANT TROPISMS
Figure 6.2 (also see Color Section). Genetic approaches for examining root hydrotropism in Arabidopsis.
The screening systems in (A) and (B) are composed of a vertically oriented Petri dish containing two differ-
ent media. In (A), a NM is poured in the upper part and a WSM [containing 2.5% (v/v) glycerol, 0.5% (w/v)
alginic acid] in the lower part. The water potential (wp) in the upper part of the dish gradually dropped off
due to diffusion of the glycerol over time, becoming more negative in positions closer to the WSM. Roots of
wild-type plants usually develop a curvature in the NM when the water potential declines from –0.4 to –0.5
MPa after 5 to 6 days. The asterisks denote putative no hydrotropic response mutants. In (B), WSM is poured
in the upper part and a NM in the lower part, which also develops a gradient in water potential over time, but
in opposite direction to that in (A). Roots of wild type plants stop growing after 4 days, but putative super
hydrotropic mutants (suh) grow into the NM after 8 to 9 days (asterisks). Roots of suhmutants continue their
downward growth, which is never arrested.