To prevent inbreeding, some plants have evolved self-incompatibility (SI) mechanisms to recognize and reject "self" pollen. In self-incompatible plants, pollen from the same plant or other plant with the same genotype is recognized and rejected by the pistil. In sporophytic SI interaction, the genotype of the plant from which the pollen is derived determines whether the pollen will be rejected or not; in gametophytic SI interaction, the genotype of the male gametophyte (pollen) determines whether the pollen will be rejected or not. Genetic studies indicated that SI is controlled by a highly polymorphic locus called S-locus. A decade long research has identified the S-RNase as the female determinant for gametophytic self-incompatibility (Kao and Tsukamoto, 2004). Whereas the male determinant, the Pollen-S gene, remains to be identified.

   It is believed that S-RNase acts to degrade RNA of self-pollen tubes, resulting inhibition of self-pollen tube growth. To explain the specificity during SI interaction, an inhibitor model and a receptor model have been proposed. The inhibitor model predicts that the Pollen-S gene encodes an S-RNAse inhibitor whose function is to block S-RNase activity in non-self pollen tubes. Whereas the receptor model suggests that Pollen-S acts as a receptor to control the entry of S-RNase of the same haplotype. The presence of S-RNase in both self and non-self pollen tube favors the inhibitor model. What is the inhibitor? In their long search for the Pollen-S gene, Dr. Yongbiao Xue and co-workers at the Institute of Genetics and Developmental Biology, Beijing, China recently identified a potential candidate gene in Antirrhinum, the S-locus F-box gene (AhSLF-S2) (Lai, et al, 2002, Zhou, et al., 2003). Similar genes were later identified in Prunus (Entani, et al., 2003). In recent issue of The Plant Cell (16:582-595, 2004), Qiao et al. from Dr. Xue's laboratory provided strong evidence that AhSLF is the S-RNase inhibitor. By using coimmunoprecipitation and yeast two-hybrid assays, Qiao et al. demonstrated that AhSLF-S2 physically interacts with S-RNases and also ASK1- and CULLIN1-like proteins of the SCF complex. They further showed that proteasomal inhibitors specifically block compatible interaction but not SI interaction, and S-RNases were indeed ubiquitinated when incubated with pollen extracts in compatible interaction but not in SI interaction. All these data strongly suggest that AhSLF-S2 acts to recruit S-RNases for degradation via the ubiquitin/26S proteasome pathway in compatible interaction, but not in SI interaction. This also implicated that the AhSLF-S2 most likely is the Pollen-S gene. Qiao et al. (Plant Cell, 16:2307-2322, 2004) more recently showed that self-incompatible petunia plants became self-compatible when transformed with the AhSLF-S2 gene. This undoubtedly demonstrated that AhSLF-S2 is indeed the pollen determinant of self-incompatibility in Antirrhinum. Similar mechanisms are also found in petunia by Dr. Te-Hui Kao's group at Pennsylvania State University, USA (Sijacic, et al., 2004).

    Dr. Xue's work in Antirrhinum clearly showed that the male Pollen-S determinant specifically interacts with its female partner, the S-RNase in a genotype-dependent manner that leads to the degradation of S-RNase in self-compatible interaction via ubiquitination/26S proteosome pathway. In my view, the findings from Dr. Xue's laboratory represents an exciting achievement in the field of plant sexual plant reproduction.

REFERENCES

    Entani, T., Iwano, M., Shiba. H., Che, F.S., Isogai, A., and Takayama, S. (2003). Comparative analysis of the self-incompatibility (S-) locus region of Prunum mune: Identification of a pollen-expressed F-box gene with allelic diversity. Genes Cells 8:203-213

    Kao, T.-H. and Tsukamoto, T. (2004). The molecular and genetic basis of S-RNAse based self-incompatibility. Plant Cell 16 (Suppl.):S72-S83

    Lai, Z., Ma, W., Han, B., Liang, L., Zhang, Y., Hong, G., and Xue, Y. (2002). An F-box gene linked to the self-compatibility (S) locus of Antirrhinum is expressed specifically in pollen and tapetum. Plant Mol. Biol. 50:29-42

    Qiao, H., Wang, H., Zhao, L., Zhou, J., Huang, J., Zhang, Y., and Xue, Y. (2004). The F-box protein AhSLF-S2 physically interacts with S-RNAses that may be inhibited by the ubiquitin/26S proteasome pathway of protein degradation during compatible pollination in Antirrhinum. Plant Cell 16:582-595

    Qiao, H., Wang, F., Zhao, L., Zhou, J., Lai, Z., Zhang, Y., Robbins, T.P. and Xue, Y. (2004). The F-box protein AhSLF-S2controls the pollen function of S-RNase-based self-incompatibility. Plant Cell 16:2307-2322

    Sijacic, P., Wang, X., Skirpan, A.L., Wang, Y., Dowd, P.E., McCubbin, A.G., Huang, S., Kao, T.H. (2004). Identification of the pollen determinant of S-RNase-mediated self-incompatibility. Nature 429(6989):302-305

    Zhou, J., Wang, F., Ma, W. Zhang. Y., Han, B., and Xue, Y. (2003). Structural and transcriptional analysis of S-locus F-box genes in Antirrhinum. Sex. Plant Reprod. 16:165-177.

    Wei-Cai Yang, Ph.D.

    Center for Developmental Biology

    The Institute of Genetics and Developmental Biology

    Chinese Academy of Sciences