Abstract:

In many flowering plants self-fertilization is prevented by an intraspecific reproductive barrier known as self-incompatibility (SI). Among the various SI systems, gametophytic SI (GSI) found in the Solanaceae, Plantaginaceae and Rosaceaeis believed to be the most common type, in which the specificity of SI response is controlled by a single multiallelic self-incompatibility (S)-locus, which is organized in a haplotype manner and carries at least two genes determining the self and non-self pollen recognition specificity: S-RNase (S-ribonucleases) expressed in pistil (pistil S) and SLF (S-Locus F-box) /SFB(S-haplotype-specific F-box) genes in pollen (pollen S). S-RNases have been shown to enter the pollen tubes without allelic specificity and appear to function as cytotoxin to specifically inhibit the growth of the self-pollen tube. However, the precise role of the SLF protein is largely unknown. We have identified a pollen-specific novel SKP1-like protein (SSK1, SLF-interacting SKP1-like1) and found that SSK1 serves as a specific adaptor in a SCF^SLF complex. Substantial reduction of SSK1 expression level does not impair the pollen tube growth in SI-defective styles, but results in pollen inhibition in cross-pollination of functional SI-styles, suggesting that it is required for cross pollen compatibility. Our results indicate that the pollen S determinant contributes to inhibiting, rather than maintaining the S-RNase activity, at least in solanaceous plants. In addition, we also show that S-RNases are specifically ubiquitinated during compatible pollination. Based on these findings, a working model is proposed to explain how S-RNase and SLF interact with each other to make “a life or death” decision to control SI signalling response.