Self-incompatibility (SI) is the inherited ability of a fertile plant to prevent self-fertilization. SI constitutes a prezygotic barrier present in about half of the Angiosperm families, and is one of the most widespread evolutionary devices used by flowering plants to prevent inbreeding depression by promoting gene exchange within a species. This cell-cell recognition mechanism, involves finely tuned pollen-pistil interactions under the control of elements of the S-locus. As a result, self-incompatible plants reject self- but accept non-self pollen. In Solanum chacoense pollen rejection occurs when pollen and pistil share a same S-haplotype. The pistillar product involved in SI is a polymorphic S-RNase, encoded by the S-locus, expressed in the style, and that acts as a cytotoxin by degrading the RNA of incompatible pollen tubes. Immunocytochemical studies in our laboratory have shown that the entry of the S-RNases into pollen tubes is haplotype-independent. The pollen component of SI is an F-box protein, motif usually associated with protein-tagging by ubiquitin and degradation through the 26S proteasome. According to a current model, degradation of S-RNases should occur during compatible crosses, and indeed we have very recently shown degradation of S-RNases in in vivo experiments. According to an alternative SI model, during compatible crosses pollen tubes avoid rejection by sequestering S-RNases in the vacuole.
In collaboration with Drs David Morse and M. Hijri, we use the powerful tools provided by genetics, biochemistry, cell biology and confocal microscopy to test the degradation and the sequestration models. In particular, we want to follow the destiny of S-RNases inside pollen tubes after compatible pollinations.