Peptides as signals - Intercellular Communication in Plants - page 39

the SI response by facilitating SRK maturation and stability (Dixit et al. , 2000; Dixit
& Nasrallah, 2001).
After the identification of the female determinant, many efforts were directed
toward identifying the male determinant of self-incompatibility. The following cri-
teria are applied to define a male determinant (Takayama & Isogai, 2003): (1) it is
encoded by a gene located at the S locus; (2) it exhibits allelic diversity reflecting
different variants of the S locus (designated S haplotypes); (3) it is expressed in
sporophytic cells because the SI is determined by sporophytic genotypes; (4) it is
localized on the surface of pollen grains; and (5) it physically interacts with SRK.
In a search for the male determinant, a gene located between SLG8 and SRK8
of the Brassica S 8 haplotype was identified as a good candidate (Schopfer et al. ,
1999). This gene exhibits polymorphism associated with different S haplotypes and
is specifically expressed in anthers. It encodes a small cysteine-rich protein with a
molecular weight of approximately 9 kDa. The protein was named S -locus cysteine-
rich protein (SCR). A definitive proof that SCR functions as the male determinant
of SI came from transgenic studies. Pollen of an S 2 haplotype plant is normally
compatible with stigma of an S 6 plant. However, when SCR6 was transformed into
an S 2 haplotype, pollen from the transgenic plants was rejected by S 6 stigma.
The SCR genes are expressed in the microspores and tapetal cells, the diploid
maternal tissues surrounding the developing male gametophytes (pollen) (Takayama
et al. , 2000). The tapetal cells undergo programmed cell death during microgameto-
genesis and provide their constituents to pollen coating. The SCR expression pattern
explains why the Brassica SI system is sporophytically determined. Further studies
revealed that SCR is localized mainly in the pollen coat (Takayama & Isogai, 2003).
SCR has a cleavable leader sequence and is secreted. SCRs encoded by dif-
ferent alleles generally share approximately 40% sequence identity. This indicates
an extensive divergence of this protein, consistent with their role as the SI speci-
ficity determinants. SCRs generally contain eight cysteine residues that form four
disulphide bonds (Takayama et al. , 2001). It is speculated that specificity of SI
might be determined by other residues. Following pollination, SCR is diffused from
pollen coats through the papilla wall and binds to the cognate SRK (Kachroo et al. ,
2001; Takayama et al. , 2001). The ligand binding induces a conformational change
of SRK, which activates the kinase through autophosphorylation. Activated SRK
phosphorylates downstream targets to initiate a signaling cascade and activate spe-
cific biochemical response, leading to the rejection of self-pollen.
The extensive studies in the last decade have generated a fairly clear picture of
the molecular mechanism involved in pollen recognition of the SI system. How-
ever, the way SCR-SRK interaction is translated into self-pollen arrest remains
largely unknown. One possible scenario (Kachroo et al. , 2002) is that the SCR-
SRK signaling pathway results in release of calcium from the papillar cells to the
pollen-papillae interface. Pollen maintains a calcium gradient. Uptake of calcium
by the pollen grain changes the calcium gradient, leading to blockage of pollen ger-
mination. Another possibility is that the SCR-SRK interaction leads to activation
of a physiological response in the papillae cells, which is analogous to the disease