Agriculture Reference
In-Depth Information
growth. In contrast, actinomycin D does partially alleviate pollen inhibition by self-
S-protein (Franklin-Tong
et al.
, 1990), suggesting that gene expression is required
for a full SI response. Comparison of the transcripts produced by pollen treated
with self- or non-self-S-protein using an
in vitro
translation system has led to the
identification of several proteins specific to an incompatible reaction (Franklin-Tong
et al.
, 1990). Whether these genes are important mechanistically to the operation
of SI or are expressed as a result of the incompatibility response is not known, but
their induction suggests that signal transduction events in this system extend as far
as the activation of transcription.
10.3.3.1 Ca
2
+
signalling in the SI response
[Ca
2
+
]
i
is well established as a second messenger in many signal transduction pro-
cesses in plants (Hepler & Wayne, 1985; Trewavas & Gilroy, 1991). Signalling in
pollen germination and tube growth is no exception and calcium imaging studies
have amply demonstrated that an oscillating [Ca
2
+
]
i
gradient is not only present at
the tip of growing pollen tubes but is also essential for tip extension and growth
(Obermeyer & Weisensel, 1991; Miller
et al.
, 1992; Franklin-Tong, 1999). Imaging
[Ca
2
+
]
i
within pollen tubes using Ca
2
+
-selective dyes and concomitantly challeng-
ing them with self-S-protein has demonstrated that the inhibition of pollen tube
growth is preceded by a transient increase in [Ca
2
+
]
i
of up to
1.5 mM in the
shank of the pollen tube (Franklin-Tong
et al.
, 1993, 1995, 1997). This increase is
seen within seconds of application of S-protein, peaks after 4-6 min and gradually
declines to basal levels by
∼
10-12 min. Ca
2
+
imaging suggests that the elevation
appears to originate from the nuclear complex and the endoplasmic reticulum asso-
ciated with it, perhaps suggesting that [Ca
2
+
]
i
may be involved in regulating gene
expression (Franklin-Tong
et al.
, 1993). These increases in [Ca
2
+
]
i
are thought to
initiate the SI signalling cascade, which ultimately results in death of the incompat-
ible pollen. Elevation of [Ca
2
+
]
i
in the shank of the tubes is likely in its own right
to be sufficient to lead to the initial cessation of tube growth as it disrupts the apical
[Ca
2
+
]
i
gradient. Low levels of apical [Ca
2
+
]
i
are associated with loss of growth,
and it is likely that vesicle fusion at the tip requires high [Ca
2
+
]
i
and is rapidly
inhibited in the SI response, leading to pollen tube inhibition. However, as [Ca
2
+
]
i
returns to basal levels after 10-12 min, it is clear that additional events must take
place that prevent pollen tubes from recovering and resuming tip growth.
Normal growth of pollen tubes has also been shown to be inhibited by inositol
trisphosphate (Ins[1,4,5]P
3
) mediated Ca
2
+
release (Franklin-Tong
et al.
, 1996)
and a recent study indicates that there are brief but reproducible decreases of
PtdIns(4,5)P
2
in incompatible
P. rhoeas
pollen, which may indicate that InsP
3
is
released (Straatman
et al.
, 2001). However, there is still no conclusive evidence that
an inositide signalling pathway is involved in the SI response and further studies are
required.
As mentioned above, imaging data suggest that the source of the ions that lead to
the increase in [Ca
2
+
]
i
is intracellular; however, evidence is emerging that import
of extracellular calcium may be involved. Using a calcium ion selective vibrating
∼
