hydrotropically but their growth was not affected (Jaffe et al. 1985; Takahashi and Scott
1993). Cytoplasmic Ca2+has been postulated to be a transducer for the gravity signal
(Plieth and Trewavas 2002). However, changes in this ion after amyloplast sedimentation
have not been properly documented (Blancaflor and Masson 2003; see also Chapters 2
and 5). Even so, hydrotropic response was completely blocked in ageotropumroots with
a Ca2+chelator and lanthanum, a Ca2+channel blocker (Takano et al. 1997). These ob-
servations suggest that Ca2+may function in the hydrotropic response, independently of
amyloplast sedimentation. It has been proposed that mechanotransductive Ca2+channels
(Chapter 5) might be triggered by temperature, gravity, touch, and water stress (Pickard
and Ding 1993), which might regulate tropic modification of growth (Pickard and Ding
1992).
Pollen tube guidance on the stigma has been also considered the most frequently oc-
curring hydrotropic response in higher plants (Lush et al. 1998). Guidance toward the
stigma by a water gradient may be the first step in a multistage process of guidance to the
ovules.
6.2.2 Genetic analysis of hydrotropism
Up to now, various screening procedures have been implemented to isolate mutants af-
fected in response to gravity, light, and obstacle touching. However, hydrotropism has not
been common in genetic studies because of the complexity of establishing a large-scale
screening system that offers an appropriate stimulus–response interaction (Eapen et al.
2005). For this reason, the design of a screening method for the isolation of Arabidopsis
mutants with abnormal responses to a water potential gradient is noteworthy (Eapen et al.
2003). The screening method consists of a vertically oriented square Petri dish with a nor-
mal nutrient medium (NM) in the upper part, in which Arabidopsisseeds are plated, and
a water stress medium (WSM) in the lower part. A gradient in water potential develops
over time, and wild-type Arabidopsisroots stopped their downward growth and devel-
oped a hydrotropic curvature when the water potential was 0.53 MPa. By developing this
hydrotropic response, Arabidopsisroots avoided the substrate with lower water potential;
that is, they never reached the area containing the WSM and consequently arrested their
gravitropic growth (Fig. 6.2A). Mutants were selected on two conditions: by their contin-
uous root gravitropic response into the medium with lower water potential (lack of hy-
drotropic response), and by their inability to sustain continuous growth into the severe
water-deficit conditions of the WSM (Fig. 6.2A). With this selection, hydrotropic mu-
tants were distinguished from mutants resistant to severe water deficit conditions. The
initial screening resulted in the isolation of two negative hydrotropic mutants, which were
namedno hydrotropic response(nhr). Importantly, in a different system with an air hu-
midity gradient, nhr1roots responded negatively to this stimulus, developing a curvature
in response to gravity instead, confirming that their directional growth toward water is
impaired (Eapen et al. 2003).
Plants in habitats such as dunes or deserts develop extensive root systems that pene-
trate large volumes of soil in search of moisture, either by means of widely spreading
roots or by roots stimulated to grow toward the water table (Larcher 1995). Hence, many
plants likely possess the capacity for such an enhanced responsiveness to gradients in