| Our laboratory studies the molecular mechanism of gametophytic self-incompatibility (GSI) in Petunia hybrida, a genetic barrier to inbreeding which is based on communication between the pollen tube and the style. In GSI, the floral pistil recognizes and rejects self-pollen. GSI is controlled in the style by S-locus encoded glycoproteins, which are functional ribonucleases. Understanding the interaction between the pollen and the style is difficult since the pollen determinant of gametophytic self-incompatibility has not yet been identified. Currently, there are two models that describe the nature of the pollen determinant of gametophytic self-incompatibility. According to one model, the stylar S-ribonuclease enters the “self” pollen tube through a yet unknown receptor located on the surface of the pollen tube and degrades its RNA, resulting in the inhibition of protein synthesis and pollen tube growth arrest. An alternative model proposes that pollen tubes express an intracellular inhibitor of non-self S-ribonucleases. In this case, any S-ribonuclease can enter the pollen tube, but if it does not match the haplotype of the style, it is inhibited.; To investigate protein-protein interactions in gametophytic self-incompatibility, we used the yeast two-hybrid system to identify proteins that could interact with the S-ribonuclease protein. These assays identified a pollen-expressed protein, which we have named PhSBP1, that appears to bind with a high degree of specificity to the Petunia hybrida S-ribonuclease. Although PhSBP1 activates reporter gene expression only when expressed in tandem with an S-ribonuclease protein, binding is not allele specific, and PhSBP1 does not cosegregate with the S-locus. Sequence analysis demonstrated that PhSBP1 contained a C-terminal cysteine-rich region that includes a RING-HC domain. Because many such RING-finger domain proteins appear to function as E3 ubiquitin ligases, our results suggest that ubiquitination and protein degradation may play a role in regulating self-incompatibility interactions. Together, these results suggest that PhSBP1 may be a candidate for the recently proposed general inhibitor (RI) of self-incompatibility ribonucleases. |
