Lessons on signalling in plant self-incompatibility systems - Intercellular Communication in Plants - page 259

which is activated downstream of the Ca 2 + signals after pollen tube growth arrest,
suggesting that it probably is not involved in the initial cessation of tip growth. The
precise nature of the signalling components that lead to the proposed activation of
PCD requires further study. To date, the Papaver SI system appears to be the most
complex and tiered yet studied, with multiple phenomena that each might easily lead
to the initial reversible cessation of tube growth and ultimately a separate distinct
mechanism through which pollen tubes are ultimately terminated.

10.4 Sporophytic self-incompatability

In sporophytic self-incompatibility (SSI), the pollen phenotype is determined by
the genotype of the parent plant and hence the pollen whilst being haploid behaves
as diploid. SSI has been most intensively studied in members of the Brassicacae,
though it is not restricted to this family. The stigmas of plants in this group are made
up of papillate cells that are covered by a cuticle and are classified as 'dry'. In a
compatible pollination, there is a rapid series of events, which include adhesion to the
stigma, hydration and germination of the pollen grains, and subsequent penetration
of the stigma by the emerging pollen tube (Elleman et al. , 1992). The phenomena
associated with incompatible pollination vary somewhat according to the strength of
the alleles involved. In the strongest alleles, adhesion and even hydration are poor,
whereas in weaker alleles pollen tubes may emerge and even penetrate the stigmatic
cuticle, but fail to fully penetrate the stigma. This contrasts with the situation in GSI
where incompatible pollen tube growth ceases approximately one third of the way
down the style.
The fact that in sporophytic SI the pollen though haploid behaves genetically
as diploid, and carries the specificity of both maternal S -haplotypes, suggests that
recognition between the pollen and stigma is likely to involve the surface coating
of pollen grains (pollen coat). Pollen coat is derived largely from the cells of the
tapetum, which nourish developing microspores and, importantly, are diploid. Cy-
tological studies using electron microscopy have revealed that on pollination the
pollen coat rapidly flows into microchannels in the stigmatic cuticle and establishes
molecular continuity with the stigma (Dickinson, 1995). Interaction of the SI deter-
minants and inhibition of self-pollen occurs very rapidly after pollination and though
variable between haplotypes, usually occurs within 45 min (Dickinson, 1995).
As with GSI, identification of the female component of SSI initially focused on
the identification of stigmatic proteins that co-segregate with S -haplotypes. These
initial efforts identified SLGs ( S -locus glycoproteins), which are abundant in stigma
extracts and polymorphic in their isoelectric points (demonstrated by isoelectric
focusing) (Nishio & Hinata, 1977). Like S-RNases, they exhibit characteristics
expected of proteins that would determine female S -haplotype specificity. They
localise to the walls of stigmatic papillae, (the cells that receive pollen on pollination)
and their temporal regulation coincides with the acquisition of SI, being barely
detectable in immature self-compatible buds and increasing dramatically just prior