ing of the trap (Ziegler 1962). However, nothing is known about any of the chemorecep-
tors involved.
Since the early work by Molisch and DuBary in the 1890s (Hart 1990), the directional
growth of the pollen tube down the style and into the ovary has usually been regarded
as the classic example of chemotropic growth in higher plants. Two molecules that have
neurotransmitter properties in animals were recently found to be involved in pollen tube
growth, GABA (-amino butyric acid) and nitric oxide (NO). Through genetic analysis
inArabidopsis, a gradient of GABA was shown to be involved in the final stages of
pollen tube guidance to the ovule (Palanivelu et al. 2003). GABA is a four-carbon
-amino acid, best known for its role in animal neuronal synapses, but was actually first
discovered in plants (Steward et al. 1949). In the pistil, GABA concentrations increase
along the pollen tube path, reaching maximal concentrations in the inner integument
cells directly surrounding the micropyle, the target of the pollen tube for delivery of
sperm cells (Palanivelu et al. 2003). This gradient is interrupted in pollen-pistil interac-
tion 2(pop2) mutants, resulting in aberrant pollen tube growth and guidance. POP2en-
codes a transaminase, which converts GABA to succinate. pop2pistils have more than
100-fold greater GABA levels than do wild-type tissues, thus reducing the magnitude of
the gradient. This elevation apparently results in improper targeting (Palanivelu et al.
2003) (Fig. 6.4A and Color Section). Hence, intracellular degradation of GABA in wild-
type pollen tubes by POP2presumably increases the GABA gradient, allowing tubes to
distinguish the micropyle from the rest of the ovule. The fact that a GABA gradient ex-
ists along the path of pollen tube supports a chemotropic growth of pollen tubes. In an-
imals, GABA binds to G-protein-coupled GABA receptors; homologs of these proteins
have been identified in plants. The heterotrimeric G-protein alpha subunit, GPA1, is ex-
pressed in the pollen tube, although it is not known whether it participates in pollen tube
guidance (Ma 2003).
A role of nitric oxide (NO) was recently demonstrated in the regulation of pollen tube
growth in Lilium longiflorum, especially in the reorientation response (Prado et al. 2004)
(Fig. 6.4B and Color Section). A NO gradient may play a role in finding a suitable path
for the pollen tube, suggesting that this may be another directional response. Nonetheless,
so far nothing is known about the nature of the NO chemoreceptor.
A female gamethophyte protein from maize has also been shown to be required for
pollen tube attraction (Márton et al. 2005), indicating that a wide range of different sub-
stances can elicit a chemoresponse in pollen tubes. Furthermore, directional guidance of
pollen tubes into the style does not appear to be influenced by gravity, indicating that
their chemotropic response might only interact with their hydrotropic, electrotropic, and
oxytropic ones (see following section).
6.5 Thermotropism and oxytropism
Temperature exerts clear effects on all aspects of plant growth and development. Root
thermotropism was first detected by Barthèlèmy (1884), who performed experiments
with hyacinth bulbs floating in water-filled containers (reviewed in Aletsee, 1962).
Wortmann (1885) documented both positive and negative thermotropism in several