The molecular bases of self-incompatibility have been intensively studied i
n a restricted number of model species, but for most families the expressio
n and distribution of S-proteins is unknown. In this work, pistil cryosecti
ons from apple were used for in situ detection of S-proteins. Two specific
antibodies, one against the S3-protein and another against all apple S-prot
eins were used. S-proteins were shown to be localised in the intercellular
space of the transmitting tissue, both in the stigma and style, which agree
s with the proposed mechanism of action for S-RNases in gametophytic self-i
ncompatibility. Some intracellular labelling was also observed in all ovary
sections, confined to one layer of the nucellus surrounding the embryo sac
, but this labelling was found to be non-S-allele-specific. Nevertheless, t
he signal in the ovary was tissue-specific, which may indicate that some co
mponent not encoded by the S-locus but similar to S-proteins was detected.
To the best of our knowledge this is the first report on the precise distri
bution of S-RNases in a rosaceous species.