II.3. BRASSICA CROPS (BRASSICA SPP.) – 217
There are three highly polymorphic genes involved in the SI response. The
two female determinants consist of the S-locus glycoproteins (SLGs) and the S-locus
receptor kinase (SRK). SRK consists of an SLG-like extracellular domain, a
transmembrane domain and an intercellular serine/threonine domain. SLG and SCR
expression occurs just before the flower opens, primarily in the stigma papilla cells. They
also exhibit allelic sequence diversity (Takayama and Isogai, 2005). The male
determinant genes named SP11 (S-locus protein 11) or SCR (S-locus cysteine rich) code
for the secretion of small, cystine-rich proteins, SP11/SCR, in anther tapetum cells and
gametophytically in the microspores (Takayama et al., 2000). These genes are tightly
linked and behave as a single Mendelian locus, displaying multiple allelic versions
(Takayama and Isogai, 2003). The SI-response occurs when stigma and pollen share at
least one allele. Upon pollination, SP11, carried in the pollen coat, penetrates the papilla
cell wall and binds with SRK. The binding induces autophosphorylation of SRK starting
a signalling cascade that causes the rejection of self-pollen by preventing hydration and
further development of the pollen tube (Takayama and Isogai, 2005). SLGs are not
present or active in all members of the mustard family (Kusaba et al., 2001). If there is a
compatible reaction, the papilla cells provide moisture for pollen germination; however,
with self-pollination, the absorption of water and germination are disrupted (Dickinson,
1995) and a callus deposition may occur at the attachment site (see Hinata, Isogai and
Isuzugawa, 1994 for a review). If the incompatible pollen is able to germinate, the pollen
tube growth is slowed or inhibited due to the inability of the pollen tube to grow through
the papilla cell wall.
For vegetable crops, the National Vegetable Research Station at Wellesbourne,
England, maintains a collection of all known S alleles together with their internationally
accepted nomenclature (Dickson and Wallace, 1986). The genotypes of most
self-incompatible Brassica plants will be heterozygous at the S locus, since
cross-fertilisation is mandated by the self-incompatibility specificities of the S alleles
present. Dominant and recessive interactions occur between S-haplotypes (Thompson and
Taylor, 1966). The interaction is complex with the S-haplotypes classified as class I or
class II, based on the nucleotide sequences of SGL and SRK alleles (Nasrallah, Nishio and
Nasrallah, 1991). The class I S-haplotypes are normally dominant over class II
S-haplotypes in the pollen. The S allele specificities of the pollen and the stigma can be
co-dominant, which occurs more frequently than the dominance/recessive.
Dominance/recessive relationships occur more frequently in the pollen than the stigma
and are not identical for S alleles between the stigma and pollen (Watanabe and Hinata,
1999). Among the SP11/SCR alleles in Class I S-haplotypes, the dominance relationship
is non-linear whereas Class II S-haplotypes exhibit linear dominance (Takayama and
Isogai, 2003; Hatakeyama et al., 1998). The molecular mechanism of the dominance
relationship in the stigma is an active area of investigation and is not fully understood
(Takayama and Isogai, 2003; Fujimoto et al., 2006). Selfed seed of most incompatible
plants can be obtained through bud pollination i.e. applying pollen to the stigma one to
four days before the flower opens since the SGLs and SRK are not expressed until just
prior to the flower opening (Takayama and Isogai, 2005). Various other methods have
been utilised to overcome the SI system including stigma mutilation, stigma treatment
with various organic acids, solvents, oils and ionic solutions, thermally aided pollination
as well as elevated carbon dioxide treatment and momentary high temperature application
(Hinata, Isogai and Isuzugawa,1994).
The SI system of S-haplotypes has been used by vegetable breeders to capture
heterosis by producing top cross, double or three-way F 1 hybrids. However, from the