Degradation of S-RNase in compatible pollen tubes ofSolanum chacoenseinferred by immunogold labeling

The flowering plant Solanum chacoense uses an S-RNase-based self-incompatibility system in order to reject pollen that shares the same genes at the S-locus (S-haplotype) with the style (an incompatible reaction). Two different models have been advanced to explain how compatible pollen tubes are protected from the cytotoxic effects of the S-RNase, sequestration of the S-RNase in a vacuolar compartment or degradation of the S-RNase in the cytoplasm. Here, we examine the subcellular distribution of an S11-RNase 18 and 24 h post pollination (hpp) in compatible and incompatible crosses by immunogold labeling and transmission electron microscopy. We find that the S-RNase is present in the cytoplasm of both compatible and incompatible crosses by 18 hpp, but that almost all the cytoplasmic S-RNase is degraded by 24 hpp in compatible crosses. These results provide compelling evidence that S-RNases are degraded in compatible but not in incompatible pollen tubes.