Agriculture Reference
In-Depth Information
Fig. 4.5
Molecular model of the self-incompatibility (SI) response in the Brassicaceae. The
S-locus consists of three genes, SRK, SP11, and SLG. The SRK receptor kinase is the female
determinant and spans the plasma membrane of the stigma papilla cell. SP11 is the male deter-
minant and is predominantly expressed in the anther tapetum and accumulates in the pollen coat
during pollen maturation. Upon pollination, SP11 penetrates the papilla cell wall and binds SRK
in an S-haplotype-specific manner. This binding induces the autophosphorylation of SRK, trig-
gering a signaling cascade that results in the rejection of self-pollen. SLG is not essential for
the self-/nonself-recognition but localizes in the papilla cell wall and enhances the SI reaction
in some S-haplotypes. The signaling cascade downstream of SRK has not yet been character-
ized, but the essential positive effectors include MLPK and ARC1. MLPK localizes papilla cell
membrane and may form a signaling complex with SRK. ARC1, an E3 ubiquitin ligase, binds to
the kinase domain of SRK in a phosphorylation-dependent manner and may target unknown sub-
strates for ubiquitination. The proteasomal degradation of these substrates could result in pollen
rejection. (Takayama and Isogai
2005
)
The
CS
, present in triploid genotypes is due to irregularities occurring at mi-
cro- and mega-sporogenesis level leading to the formation of unviable pollen and
embryosac. In fruit industry, triploidy might be a character of interest when associ-
ated to parthenocarpy, since it allows the production of seedless fruits. This type of
sterility is present in some varieties of
Citrus
, apple and pear.
The
SI
is one of the most important systems to prevent inbreeding (Sanchez et al.
2004
; Takayama and Isogai
2005
). In many species, the self-/nonself-recognition
of
SI
is controlled by a single polymorphic
locus
, the
S-locus
. Molecular dissection
of the
S-locus
revealed that
SI
represents not one system, but a collection of diver-
gent mechanisms. In the
Brassicaceae
, where a sporophytic type of
SI
is present,
the determinant genes encode a pollen ligand and its stigmatic receptor kinase,
whose interaction induces incompatible signalling(s) within the stigma papilla
cells (Fig.
4.5
). In the
Solanaceae
, where a gametophytic-type of
SI
is present, the
determinants are a ribonuclease and an F-box protein, suggesting the involvement
