The amino terminal F-box domain of Petunia inflata S-locus F-box protein is involved in the S-RNase-based self-incompatibility mechanism

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. RATIONALE: 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 PiSLF(2) (S(2) allelic variant) without this domain in S(2)S(3) plants and showed that, unlike the full-length PiSLF(2), it did not cause breakdown of SI in S(3) pollen. We identified PiSSK1 (P. inflata SSK1) and found that it did not interact with PiSLF(1), PiSLF(2) or PiSLF(3). CONCLUSIONS: The finding that the truncated PiSLF(2) did not cause breakdown of SI in S(3) transgenic pollen suggests that the F-box domain of PiSLF(2) is required for mediating degradation of S(3)-RNase, a non-self S-RNase, in S(3) 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.