Agriculture Reference
In-Depth Information
component of the CLV1 complex is a Rho/Rac-GTPase-related protein. Members
of this family such as Ras are important components in relaying signals through a
series of intracellular biochemical reactions including the kinase cascade.
2.2.6 S-locus cysteine-rich proteins determine specificity
of self-incompatibility in the Brassicaceae
During a compatible pollen-pistil interaction, a desiccated pollen grain adheres
tightly to the surface of stigma, where it hydrates and germinates. The pollen tube
then elongates into the female tissues. The pollen tube elongation is guided toward
individual ovules where sperm cells are released to fertilize the egg cell and central
cells, leading to development of embryo and endosperm (Pruitt, 1999).
Self-incompatibility (SI) is a phenomenon in which self-pollen is recognized and
rejected by pistil whereas non-self pollen is accepted (see also Chapter 10). SI is
used by many plant species to prevent inbreeding and maintain genetic variability.
The SI system is genetically controlled by a single locus called the S locus, which
contains multiple genes that could be multi-allelic. Different mechanisms govern
pollen recognition and rejection. The SI system of the Solanaceae species is called
gametophytic self-incompatibility. In this system, if the single S allele carried by
the haploid pollen matches either of the two S alleles present in the diploid tissues
of the pistil, the pollen will be rejected (Matton et al. , 1994). Usually self-pollen
can germinate but pollen tube elongation is inhibited after they enter the style.
In contrast, in the sporophytic self-incompatibility system used by the Bras-
sicaceae species, the genotype of the male parent, not that of the pollen itself,
determines the outcome. If either of the two S alleles present in the male parent
matches either of the two S alleles carried by the pistil, pollen hydration and germi-
nation is arrested. The self-incompatible response in this SI system occurs on the
stigma surface by blocking the very early events of pollen-pistil interactions, sug-
gesting that cell surface molecules are involved in pollen recognition. The molecular
mechanism that determines sporophytic self-incompatibility in the Brassicaceae is
described here.
Genetic and molecular studies carried out a decade ago identified two multi-allelic
genes associated with the S locus in Brassica (Nasrallah et al. , 1987; Stein et al. ,
1991). One of them encodes S -locus glycoprotein (SLG), and the other encodes
S -locus receptor kinase (SRK). SLG is a glycoprotein that is localized to the cell
wall of papillae, the epidermal cells of the pistil where pollen-pistil interaction
occurs (Kandasamy et al. , 1989). SRK is a transmembrane protein which contains
an extracellular domain (S-domain) with high sequence similarity to SLG and an
intracellular serine/threonine kinase domain. SLG and SRK are tightly linked and
highly polymorphic. Like SLG, SLK is predominantly expressed in the papilla
cells. Suppression of LSG or LRK was found to convert an otherwise incompatible
interaction into self-compatible (Takayama & Isogai, 2003), demonstrating that
both proteins function in pollen recognition. It is believed that SRK is the female
determinant of SI specificity in the stigma, and that SLG enhances the strength of
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