Lessons on signalling in plant self-incompatibility systems - Intercellular Communication in Plants - page 254

lack of building materials available for pollen tube growth (Rudd & Franklin-Tong,
2003).
A second phosphoprotein, designated p68 (68 kDa, pI of 6.10-6.45), is also
phosphorylated in response to self-S-protein, but in a Ca 2 + -independent manner.
Phosphorylation of p68 occurs somewhat later than that of p26, being barely de-
tectable at 240 s but much increased at 400 s (Rudd et al. , 1997). This suggests that
p68 is likely to be downstream of p26 in the signal cascade. Inhibition of the pollen
tube growth occurs within 1-2 min of challenge with self-S-protein (coinciding with
the phosphorylation of p26), and is well in advance of p68 phosphorylation. This
might suggest that p68 is not involved in the initial inhibition of tube growth, but
perhaps in later events that cement the fate of incompatible pollen tubes.
A third pollen protein that is involved in phosphorylation events associated with
the SI response is a putative MAPK, p56. This protein was identified using in-gel
kinase assays employing the classic MAPK substrate myelin basic protein (MBP)
(Rudd et al. , 2003). p56 exhibits a basal level of protein kinase activity in growing
pollen tubes, which is significantly enhanced in incompatible pollen tubes. p56
has not been cloned, but several lines of evidence suggest that it is a MAPK having
the ability to phosphorylate MBP in the absence of Ca 2 + ions, evidence that activated
p56 is phosphorylated on a tyrosine residue in vivo , cross-reactivity to a phospho-
MAPK-specific antibody and inhibition by apigenin (Rudd et al. , 2003). p56 is not
envisaged to be involved in the initial inhibition of tip growth as its SI-stimulated
activation peaks several minutes after initial inhibition of pollen tube growth (Rudd
et al. , 2003; Rudd & Franklin-Tong, 2003). Inhibited pollen remains viable for
some 40-60 min however, and p56 may be involved in later, downstream events
involved in making the growth inhibition irreversible (Rudd et al. , 2003). In regard
to putative sites of action for p56, there are two plausible possibilities. The first
is the actin cytoskeleton, which is critical for pollen tube growth (see below). In
animal systems, MAPK activation has been linked to changes in the cytoskeleton
and evidence that this may also occur in plants is emerging (Staiger, 2000; Samaj
et al. , 2002). A second possibility is that p56 feeds into a PCD signalling cascade
as emerging data have demonstrated that MAPK activation plays an important role
in this process in plants (Zhang et al. , 2000; Yang et al. , 2001; Zhang & Liu, 2001;
Ren et al. , 2002).

10.3.4 S-protein-binding proteins in pollen

A pollen protein, which interacts with the pistil S-protein, has been identified by
Western ligand blotting. This protein, termed S-protein binding protein (SBP), binds
to pistil S-proteins of all S -haplotypes examined, suggesting that it is unlikely to be
the pollen S -specificity component (Hearn et al. , 1996). SBP is, however, an integral
plasma membrane protein, as one might expect of a cell surface receptor. SBP is
70-120 kDa in size, this wide variation apparently being due to variable glycosy-
lation. The glycan moieties appear to be required for the interaction between SBP
and S-proteins as the interaction is abolished by periodate treatment of the pollen