The Role of a Pollen-Expressed Cullin1 Protein in Gametophytic Self-Incompatibility in Solanum

Abstract

We previously isolated a pollen factor, ui6.1, which encodes a Cullin1 protein (CUL1) that functions in unilateral interspecific incompatibility (UI) in Solanum. Here we show that CUL1 is also required for pollen function in self-incompatibility (SI). We used RNA interference (RNAi) to reduce CUL1 expression in pollen of Solanum arcanum, a wild SI tomato relative. Hemizygous T 0 plants showed little or no transmission of the transfer DNA (T-DNA) through pollen when crossed onto nontransgenic SI plants, indicating that CUL1-deficient pollen are selectively eliminated. When crossed onto a related self-compatible (SC) accession lacking active S-RNase, pollen transmission of the T-DNA followed Mendelian ratios. These results provide further evidence for functional overlap between SI and UI on the pollen side and suggest that CUL1 mutations will reinforce SI-to-SC transitions in natural populations only if preceded by loss of pistil S-RNase expression.S ELF-INCOMPATIBILITY (SI) is a widespread genetic mechanism in hermaphroditic plants that allows for the recognition and rejection of closely related pollen to prevent inbreeding. The breakdown of SI to self-compatibility (SC) through mutation occurs frequently (Igic et al. 2008), presumably driven by reproductive assurance under conditions where pollen from compatible mates is limiting. Pollen from SC species or populations is typically rejected on pistils of related SI species or populations, while, in the reciprocal crosses (SC pollinated by SI), no pollen rejection occurs. This pattern of unilateral incompatibility (UI) is known as the "SI 3 SC rule" (Lewis and Crowe 1958). While the mechanisms underlying SI have been the subject of much investigation, pollen rejection by UI is less well understood.The cultivated tomato (Solanum lycopersicum) and related wild Solanum species provide a powerful system with which to study these reproductive barriers. They exhibit a wide range of mating systems, including SI-enforced obligate outcrossing, SC with facultative outcrossing, and SC with high levels of inbreeding (Rick 1988). Self-compatible biotypes or accessions of mostly SI species provide a source of natural variation for studying SI-related factors.Self-incompatibility in Solanum and other Solanaceae is the S-RNase based, gametophytic type, in which S-specificity is determined by S-RNases in the pistil (McClure et al. 1989) and S-locus F-box proteins (SLFs) in pollen (Sijacic et al. 2004). F-box proteins, together with Skp1 and Cullin1 proteins, are components of Skp, Cullin, Fbox type (SCF) ubiquitin E3 ligases that mark proteins for degradation by the 26S proteasome (Zheng et al. 2002;Moon et al. 2004). The ubiquitinproteasome pathway is thought to regulate pollen-side SI responses in the Solanaceae (Zhang et al. 2009). In the "collaborative non-self-recognition" model (Kubo et al. 2010), the S-locus encodes multiple SLF proteins that together recognize different suites of S-RNases. In compatible pollinations, SLF/SRNase interactions lead to protection of p...

Unilateral incompatibility gene ui1.1 encodes an S-locus F-box protein expressed in pollen of Solanum species

“…Second, our genetic data established that both ui1.1 and ui6.1 are required to overcome the pistil UI barrier in SC × SI hybrids, which is consistent with the biochemical model in which each gene encodes components of the SCF complex. Third, we showed that CUL1 is required for pollenside SI function only if pistils express active S-RNase (8). Finally, the pollen determinants of SI specificity in the Solanaceae and related families with the S-RNase-based gametophytic self-incompatibility system are SLF proteins (2,6).…”

“…In a compatible pollination, the SLF/S-RNase interaction leads to protection of pollen tubes against cytotoxic S-RNase, whereas in incompatible pollinations, a failure to recognize self-S-RNase results in pollen tube arrest. In addition, modifier genes, such as those encoding HT-B, NaStEP, and 120-kDa proteins in the pistil, and CUL1 and SSK1 proteins in pollen, are required for SI function but do not confer S-specificity (7)(8)(9)(10)(11).…”

“…This unidirectional pattern of pollen rejection is referred to as the "SI × SC rule" (12). Although the mechanisms of pollen recognition and rejection by UI are complex (13), several SI factors, including S-RNase, CUL1, and HT, also function in UI (8,14,15).…”

“…diploid S. lycopersicum × S. lycopersicoides hybrids (19). The ui6.1 locus encodes a pollen specific Cullin1 (CUL1) protein (21) that functions in pollen recognition by UI and SI (8). Pollen lacking ui6.1 are incompatible on pistils expressing active S-RNases, but compatible on pistils expressing a mutant S-RNase lacking RNase activity (8).…”

“…The ui6.1 locus encodes a pollen specific Cullin1 (CUL1) protein (21) that functions in pollen recognition by UI and SI (8). Pollen lacking ui6.1 are incompatible on pistils expressing active S-RNases, but compatible on pistils expressing a mutant S-RNase lacking RNase activity (8). This observation suggested that ui6.1-and by extension, ui1.1-is required for pollen resistance to S-RNase-based rejection in the pistil.…”