Agriculture Reference
In-Depth Information
Figure 10.1
Genetics of pollen rejection in gametophytic and sporophytic SI. In gametophytic SI,
pollen is rejected based on the matching of its haploid
S
-genotype with one of the two
S
-alleles of the
pistil. In sporophytic SI the pollen behaves as diploid, and is rejected if either of the
S
-alleles matches
either of the two alleles in the pistil.
pollen
S
-gene product has recently been identified as an F-box protein (Sijacic
et al.
, 2004). In the Papaveraceae, SI single-locus gametophytic incompatability
involves a complex series of events including changes in calcium ion concentration,
phosphorylation of specific proteins, transcription of pollen genes and DNA frag-
mentation in nuclei (Jordan
et al.
, 2000; Snowman
et al.
, 2000). The pistil
S
-gene
has been cloned but bears no significant homology to any gene of known function
(Foote
et al.
, 1994; Ride
et al.
, 1999). In the single-locus sporophytic SI system
of
Brassica
, both pollen and pistil
S
-genes have been identified, the pollen
S
-gene
product encoding an extracellular ligand that mediates SI through activation of a pro-
tein receptor kinase - the stigmatic
S
-gene product (Schopfer
et al.
, 1999; Takasaki
et al.
, 2000). This chapter focuses on our current knowledge regarding the signalling
events and mechanisms of action of these three different SI systems.
10.2
S-RNase-based single-locus gametophytic SI
S
-allele products were first identified in members of the Solanaceae by elec-
trophoretic analysis of pistil proteins (Bredemeijer & Blass, 1981). A polymorphic
series of basic glycoproteins was identified which vary in molecular weight (
∼
22-
∼
35 kDa) and isoelectric point (
8-10) and segregate with
S
-alleles. The location
and developmental expression of these proteins also correlates with the SI response,
they are largely confined to the extracellular space in the upper third of the stylar
transmitting tract (the site of self-pollen tube inhibition), and are absent 1 day prior
