Agriculture Reference
In-Depth Information
how self-pollen is inhibited and or killed is still unclear. Pollen rejection can occur
at different stages. In the strongest
S
-haplotypes self-pollen fail to even hydrate,
and the decision to deny water must be extremely rapid to account for the speed of
this rejection response (Dickinson, 1995). In weaker
S
-haplotypes rejection takes
place after the initiation of pollen tube growth. It seems likely, therefore, that SI
in
Brassica
can act at several levels as appears to be the case in
Papaver
,butin
Brassica
the primary molecular mechanism leading to rejection may vary between
haplotypes.
Clearly, considerably more work is required to elucidate the precise mechanism
of pollen rejection in this system. A recent development that is likely to speed
this process has been the successful introduction of SSI into
Arabidopsis thaliana
(Nasrallah
et al.
, 2002). Importantly, the availability of SSI
A. thaliana
not only
provides direct access to the genomic resources available for this species, but also
makes it feasible to conduct large-scale mutant screens for loss of SI, and it is likely
these tools will significantly speed the identification of downstream components of
the SRK signalling cascade.
10.5
Summary
Research into SI has a long and distinguished history spanning well over 100 years.
Our understanding of SI systems and how they function has understandably surged
forward with each new wave of technological advances over that period. The molec-
ular data now available implicate a wide variety of signalling processes and mech-
anisms. In terms of signalling complexity the most simple system appears to be
S-RNase-based gametophytic SI, which is also the most phylogenetically widespread
and is most likely the most ancient (Steinbachs & Holsinger, 2002). Both the game-
tophytic system found in
Papaver
and the sporophytic system in
Brassica
appear
to involve extensive signal transduction cascades, which are capable of inhibiting
pollen germination and/or tube growth through multiple mechanisms. While the
precise signalling events appear to be specific to each system, some common mech-
anistic themes are emerging. Calcium signalling has been been strongly implicated
in the SI mechanism of
Papaver
and may also be involved in both
Brassica
and S-
RNase-based systems. Protein phosphorylation is involved in
Papaver
and
Brassica
and possibly the S-RNase-based system. Ubiquitin-mediated protein degradation
has been implicated in
Brassica
and the S-RNase-based systems.
In the systems described, most of the
S
-locus-specific factors have now been
identified. Many of the molecular components downstream of these factors may
well be involved in other signalling processes in plant cells. Of particular note are
the number of parallels that can be drawn between SI recognition systems and host-
pathogen responses from SI systems (Hodgkin
et al.
, 1988). As work progresses
over the next few years, it will be interesting to see how much overlap is found
between the molecules involved in SI responses and signalling cascades involved in
other plant responses.
