Lessons on signalling in plant self-incompatibility systems - Intercellular Communication in Plants - page 265

targeting for degradation, including intercellular targeting, recruitment to molecular
complexes (Kachroo et al. , 2002) and transcriptional regulation (Conaway et al. ,
2002).
In addition to ARC1, the calcium-binding protein calmodulin has also been shown
to interact with the SRK kinase domain (Vanoosthuyse et al. , 2003). Calmodulin is a
known component of many signalling pathways in both plants and animals (Sanders
et al. , 1999; Chin & Means, 2000) and calcium fluxes have been reported in both the
cytoplasm and cell walls of stigmatic papillae after both compatible and incompatible
pollination (Dearnaley et al. , 1997; Elleman & Dickinson, 1999). These fluxes occur
directly under the point of contact with the pollen grains and small localised peaks
in [Ca 2 + ] i are correlated with pollen hydration (Dearnaley et al. , 1997; Goring,
2000). As a result, calmodulin has been proposed to mediate cross-talk between
calcium signalling in pollination and SI signal transduction (Vanoosthuyse et al. ,
2003).

10.4.6 Model for the action of SSI in Brassica

As has been discussed above, SSI in Brassica is mediated in the stigma by a receptor
kinase (SRK) and SI in this system is mediated by a recognition event that leads to
a signal transduction cascade (Fig. 10.4). There appear to be multiple mechanisms
to hold SRK in an inactive state, including thioredoxin (THL1) and protein phos-
phatase (KAPP). Upon pollination the pollen S -haplotype determinant, SCR/SP11,
diffuses rapidly from the pollen coat and interacts in an S -haplotype-specific manner
with the extracellular ( S -) domain of SRK. This interaction causes derepression of
the kinase domain of SRK within the cytoplasm of the papillar cell, and phosphory-
lation of specific substrates. Haplotype-specific interaction between SCR/SP11 and
SRK has been demonstrated using two different biochemical approaches (Takayama
et al. , 2001; Kachroo et al. , 2002). This interaction was shown to induce transpho-
sphorylation ('autophosphorylation') of serine and threonine residues in the kinase
domains of, presumably dimerised, SRKs (Schopfer & Nasrallah, 2001; Takayama
et al. , 2001). The exact molecular make-up of the stigmatic receptor is a matter
of some speculation. The majority of (though not all) animal receptor kinases are
active as dimers. It is clear that at least one molecule of SRK must be present in the
receptor complex and also that as the S -haplotypes function independently, dimeri-
sation between SRKs of different haplotypes is unlikely. However, the presence
of the SLGs of each haplotype complicates the situation. Given the high level of
sequence identity between the S-domain of SRK and SLG within an S -haplotype,
it is certainly possible that both SRK homodimers and SRK/SLG heterodimers are
formed and that both could be active receptors for SCR/SP11. This scenario might
explain the enhancement of the SRK-mediated SI response by SLG observed by
Takasaki et al. (2000), as it would lead to an increase in the total number of re-
ceptors present at the stigma surface. One argument countering this is that if SLG
can bind to SP11/SCR, presumably the presence of large amounts of SLG could
titrate the ligand away from the receptor, causing breakdown of SI. As SLG is