References

6TargetsofSI 79
by Yang 2002) has provided significant new insights into the interactions
between Ca 2+ signals and actin dynamics. It has been demonstrated that the
pollen-expressed ROP1, through its interacting partners RIC3 and RIC4,
plays a role in two signalling cascades: one involving Ca 2+ signalling, the
other involving actin dynamics (Gu et al. 2003, 2005). These data begin to
explain how Ca 2+ signals are linked to the spatio-temporal control of actin
dynamics and tip growth in pollen tubes.
It is well established that both actin organization and levels of F-actin
modulate pollen tube tip growth (Gibbon et al. 1999; Geitmann and Emons
2000; Fu et al. 2001; Vidali et al. 2001; Lovy-Wheeler et al. 2005). A variety of
cytological approaches have allowed researchers to build a consensus view
of the distribution of the actin cytoskeleton in growing pollen tubes. There
are three “zones” of F-actin in the pollen tube: long arrays of longitudinal
actin filament bundles in the shank; a dense sub-apical meshwork of Factin
in a “basket-like” or ring configuration; and at the tip, a fine array of
dynamic filaments. It is likely that the axial cables support reverse-fountainpattern
cytoplasmic streaming and that the apical arrays regulate secretory
vesicle trafficking. Figure 6.1a shows this arrangement for Papaver pollen
tubes.
6.2.2
Actin as a Target for Self-Incompatibility Signals
in Incompatible Pollen
Evidence that SI signals to the actin cytoskeleton was first provided by the
observation that dramatic alterations to F-actin are triggered by SI induction
in incompatible pollen tubes (Geitmann et al. 2000; Snowman et al.
2002). Detectable alterations were extremely rapid, occurring within 1 min.
Thedistinctivesub-apicalbasket-likeconfigurationdisappearedandalarge
“blob” of F-actin appeared at the pollen tube apical region (Fig. 6.1b). The
overall intensity of phalloidin staining was reduced substantially, indicating
a loss of F-actin, and by 5−10 min after SI induction, the longitudinal
F-actin bundles had largely disappeared (Fig. 6.1c,d). The remaining Factin
had a fine, speckled appearance (Fig. 6.1d,e), suggesting severing or
depolymerization of F-actin and pronounced cortical F-actin was evident
(Fig. 6.1c–e). Further alterations were evident between 10 and 20 min; Factin
appeared as large aggregates or “punctate foci” (Fig. 6.1e,f) and these
persisted for at least 3 h. These alterations were specific to induction of SI
in incompatible pollen tubes, as control pollen tubes showed no changes
to actin organization. Furthermore, the SI-induced rearrangement of actin
wasdemonstratedtobeindependentofgrowtharrest(Geitmannetal.
2000).