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author:("Sun, penguin")
1.  Insight into S-RNase-based self-incompatibility in Petunia: recent findings and future directions 
S-RNase-based self-incompatibility in Petunia is a self/non-self recognition system that allows the pistil to reject self-pollen to prevent inbreeding and to accept non-self pollen for outcrossing. Cloning of S-RNase in 1986 marked the beginning of nearly three decades of intensive research into the mechanism of this complex system. S-RNase was shown to be the sole female determinant in 1994, and the first male determinant, S-locus F-box protein1 (SLF1), was identified in 2004. It was discovered in 2010 that additional SLF proteins are involved in pollen specificity, and recently two S-haplotypes of Petunia inflata were found to possess 17 SLF genes based on pollen transcriptome analysis, further increasing the complexity of the system. Here, we first summarize the current understanding of how the interplay between SLF proteins and S-RNase in the pollen tube allows cross-compatible pollination, but results in self-incompatible pollination. We then discuss some of the aspects that are not yet elucidated, including uptake of S-RNase into the pollen tube, nature, and assembly of SLF-containing complexes, the biochemical basis for differential interactions between SLF proteins and S-RNase, and fate of non-self S-RNases in the pollen tube.
PMCID: PMC4318427
Petunia; SCFSLF complex; self-incompatibility; S-locus F-box protein; Solanaceae; S-RNase
2.  S-RNase-based self-incompatibility in Petunia inflata 
Annals of Botany  2010;108(4):637-646.
For the Solanaceae-type self-incompatibility, also possessed by Rosaceae and Plantaginaceae, the specificity of self/non-self interactions between pollen and pistil is controlled by two polymorphic genes at the S-locus: the S-locus F-box gene (SLF or SFB) controls pollen specificity and the S-RNase gene controls pistil specificity.
This review focuses on the work from the authors' laboratory using Petunia inflata (Solanaceae) as a model. Here, recent results on the identification and functional studies of S-RNase and SLF are summarized and a protein-degradation model is proposed to explain the biochemical mechanism for specific rejection of self-pollen tubes by the pistil.
The protein-degradation model invokes specific degradation of non-self S-RNases in the pollen tube mediated by an SLF, and can explain compatible versus incompatible pollination and the phenomenon of competitive interaction, where SI breaks down in pollen carrying two different S-alleles. In Solanaceae, Plantaginaceae and subfamily Maloideae of Rosaceae, there also exist multiple S-locus-linked SLF/SFB-like genes that potentially function as the pollen S-gene. To date, only three such genes, all in P. inflata, have been examined, and they do not function as the pollen S-gene in the S-genotype backgrounds tested. Interestingly, subfamily Prunoideae of Rosaceae appears to possess only a single SLF/SFB gene, and competitive interaction, observed in Solanaceae, Plantaginaceae and subfamily Maloideae, has not been observed. Thus, although the cytotoxic function of S-RNase is an integral part of SI in Solanaceae, Plantaginaceae and Rosaceae, the function of SLF/SFB may have diverged. This highlights the complexity of the S-RNase-based SI mechanism. The review concludes by discussing some key experiments that will further advance our understanding of this self/non-self discrimination mechanism.
PMCID: PMC3170144  PMID: 21193481
Competitive interaction; Petunia inflata; Plantaginaceae; protein degradation; Rosaceae; self-incompatibility; S-locus F-box protein; Solanaceae; S-RNase; ubiquitination
3.  The amino terminal F-box domain of Petunia inflata S-locus F-box protein is involved in the S-RNase-based self-incompatibility mechanism 
AoB Plants  2011;2011:plr016.
We showed that the F-box domain of PiSLF, Petunia inflata S-locus F-box protein, is essential for SI function and that a P. inflata ortholog of Antirrhinum SSK1 does not interact with PiSLF, consistent with our previous finding that PiSLF might be in a novel E3 complex.
Background and aims
Pistils of flowering plants possessing self-incompatibility (SI) can distinguish between self and non-self pollen, and only allow non-self pollen to effect fertilization. For Petunia inflata, the S-RNase gene encodes pistil specificity and multiple S-locus F-box (SLF) genes encode pollen specificity. Each SLF produced in pollen interacts with a subset of non-self S-RNases to mediate their ubiquitination and degradation by the 26S proteasome.
S-locus F-box has been proposed to function as a component of the conventional SCF (SKP1-CULLIN-F-box protein) complex, based on the finding that two SKP1-like proteins, AhSSK1 (Antirrhinum hispanicum SLF-interacting SKP1-like1) and PhSSK1 (Petunia hybrida SSK1), interact with the F-box domain of some allelic variants of SLF. However, we previously showed that PiSLF (P. inflata SLF) did not interact with any SKP1 of P. inflata or Arabidopsis thaliana, but instead interacted with a RING-finger protein, PiSBP1 (P. inflata S-RNase-Binding Protein1), which may also play the role of SKP1. Thus, the biochemical nature of the SLF-containing complex is as yet unclear.
Principal results
To examine whether the F-box domain of PiSLF is required for SI function, we expressed a truncated PiSLF2 (S2 allelic variant) without this domain in S2S3 plants and showed that, unlike the full-length PiSLF2, it did not cause breakdown of SI in S3 pollen. We identified PiSSK1 (P. inflata SSK1) and found that it did not interact with PiSLF1, PiSLF2 or PiSLF3.
The finding that the truncated PiSLF2 did not cause breakdown of SI in S3 transgenic pollen suggests that the F-box domain of PiSLF2 is required for mediating degradation of S3-RNase, a non-self S-RNase, in S3 pollen, and thus is required for SI function. The finding that PiSSK1 did not interact with three allelic variants of PiSLF is consistent with our previous finding that PiSLF might not be in a conventional SCF complex.
PMCID: PMC3158857  PMID: 22476486

Results 1-3 (3)