References

80 S.G. Thomas et al.
Fig.6.1. Self-incompatibility (SI) induces dramatic rearrangements of actin cytoskeleton
organization in Papaver pollen. a The actin cytoskeleton of a normally growing pollen tube
haslongitudinalarraysofF-actininthepollentubeshank,adensesub-apicalnetworkof
F-actin and a fine array of dynamic filaments at the tip. b Following SI induction rapid
reorganization is observed. Within 1 min, F-actin accumulates in the tip of the pollen tube.
c,d By 20 min after SI induction, the F-actin has disappeared from the cortex and is localized
to the periphery of grains and pollen tubes and “punctate foci” of F-actin begin to appear.
e,f The punctate foci persist for greater than 60 min and increase in size over time. a,b,d–f
are epifluorescence microscope images of rhodamine–phalloidin-labelled pollen tubes. c
is a single optical section from confocal microscope imaging of rhodamine–phalloidinlabelled
pollen grain. Bars represent 10 µm
6.2.3
Self-Incompatibility Stimulates Rapid
and Sustained Depolymerization of F-Actin
Because the actin reorganization observed during the early stages of the SI
response suggested that actin depolymerization had occurred, a quantitativeapproachwasusedtoinvestigatethisfurther.Atechniquebasedon
the number of fluorescent-phalloidin binding sites (Howard and Oresajo
1985; Lillie and Brown 1994) was used to determine the concentration of
actin in filamentous form in pollen (Gibbon et al. 1999; Snowman et al.
2002). We found that the levels of F-actin in pollen throughout hydration,
germination and early growth were constant (Snowman et al. 2002). Upon
SI induction, a rapid and sustained decrease in F-actin levels was induced.
Within 1 min there was a significant reduction in F-actin. These levels continued
to decrease, so that by 5 min after SI induction, F-actin levels were
less than 50% of the controls and by 1 h were reduced by 74% (Snowman et