References

78 S.G. Thomas et al.
6.2
The Actin Cytoskeleton and Self-Incompatibility
6.2.1
Actin as a Sensor of Environmental Stimuli
A network of polymers, the actin cytoskeleton, provides a dynamic framework
for multiple cell functions. At the protein level, it is composed of actin
filaments (F-actin) polymerized from monomeric, globular actin (G-actin).
The actin cytoskeleton has been shown to play an essential role in numerous
processes, including cell motility, organelle movement, vesicle trafficking
and cytoplasmic streaming (reviewed by McCurdy et al. 2001; Staiger and
Hussey 2004). Actin filaments reorganize in response to various signalling
events and this mediates diverse responses to external environmental cues.
A major focus of research on the cytoskeleton is the dissection of signalling
cascades that regulate actin dynamics (reviewed by Staiger 2000). In incompatible
Papaver pollen, F-actin reorganizes rapidly (Geitmann et al. 2000;
Snowman et al. 2002). Actin rearrangements in response to specific stimuli
have also been described in stomatal guard cells responding to abscisic acid
and light (Eun and Lee 1997), root hairs responding to Nod factors from
Rhizobium bacteria (Cardenas et al. 1998; Miller et al. 1999) and epidermal
cells responding to fungal and oomycete pathogens (Kobayashi et al. 1992,
1993).
The dynamic nature of the actin cytoskeleton depends on the spatial distribution
and the local activity of a complex mixture of ABPs. The number
of ABPs identified in plants is growing rapidly and includes profilin, actindepolymerizing
factor (ADF)/cofilin, villin/gelsolin-like proteins, fimbrin,
the Arp2/3 complex, AIP1, EF1α, and capping protein (reviewed by Staiger
and Hussey 2004). The activity and regulation of these ABPs by second
messengers suggest that they play key roles as transducers of extracellular
stimuli; however, an involvement in specific cellular signalling processes
has yet to be demonstrated.
Pollen tube tip growth is particularly amenable to studying the signals
responsible for mediating changes to the actin cytoskeleton. It is widely
assumed that the actin cytoskeleton helps guide or regulate the delivery
of secretory vesicles to the pollen tube apex, but exactly how this is accomplished
remains to be established. Furthermore, it is thought that the
control of pollen tube growth involves a complex interplay between the cytoskeleton
and signalling cascades, although there is currently little direct
evidence for this. However, early studies showed that increasing [Ca 2+ ]i artificially
can disrupt actin filaments in pollen tubes (Kohno and Shimmen
1987, 1988), suggesting that Ca 2+ signals to alterations in actin organization.
Analysis of a family of Rho-related GTPases in plants (ROPs) (reviewed