References

76 S.G. Thomas et al.
vitro (Franklin-Tong et al. 1988) has allowed dissection of the signalling
cascades triggered by SI (Franklin-Tong and Franklin 2003). Here we describe
dramatic alterations to the actin cytoskeleton that occur during SI
and the characterization of several actin-binding proteins (ABPs) that may
play central roles. We also show that SI involves signalling to programmed
celldeath(PCD)cascades.Wehavebeguntoexplorewhetherthesignalling
cascades for actin alterations and PCD are linked and whether the actin
cytoskeleton functions as a sensor of cellular stress and can initiate PCD.
6.1.1
Pollen–Pistil Interactions
Sexual reproduction is an excellent example of a process whereby plant cells
respond to many different stimuli and utilize extensive signalling cascades.
Pollination involves the deposition of pollen grains on a receptive stigma.
This triggers numerous pollen–pistil interactions, including hydration of
the pollen grain, germination and directional growth of the pollen tube
through the pistil, and delivery of the sperm cells to the ovule, where they
effect fertilization resulting in seed production. These events are tightly
controlled at both the genetic and the biochemical level. Two recent reviews
discuss pollination in more detail (Edlund et al. 2004; Sanchez et al. 2004).
The pollen tube elongates via tip growth, which involves the precise
control of exocytosis and delivery of new plasma membrane and cell wall
materials to the tube apex. It responds to chemical and physical signals
during its journey to the ovule (reviewed by Franklin-Tong 1999a). For
example, signals from the style include arabinogalactan proteins (Cheung
et al. 1995; de Graaf et al. 2003) triacylglycerides (Wolters-Arts et al.
1998), chemocyanin (Kim et al. 2003), and γ-amino butyric acid (GABA)
(Palanivelu et al. 2003). These signal molecules are believed to be involved
in the guidance of the pollen tube through the pistil. Within the pollen
tube, cytosolic free calcium ([Ca 2+ ]i) hasbeenshowntobeanimportant
second messenger regulating pollen tube growth (reviewed by Franklin-
Tong 1999b). Growing pollen tubes exhibit a tip-focused [Ca 2+ ]i gradient
(Rathore et al. 1991; Miller et al. 1992) that oscillates and is coordinated with
growth (Holdaway-Clarke et al. 1997; Messerli et al. 1999, 2000). Although
our understanding of this signalling cascade is incomplete, it is clear that
[Ca 2+ ]i oscillations allow spatio-temporal control of key processes involved
in pollen tube growth (reviewed by Holdaway-Clarke and Hepler 2003).