Ubiquitin–proteasome‐mediated degradation of S‐RNase in a solanaceous cross‐compatibility reaction

Many plants have a self‐incompatibility (SI) system in which the rejection of self‐pollen is determined by multiple haplotypes at a single locus, termed S. In the Solanaceae, each haplotype encodes a single ribonuclease (S‐RNase) and multiple S‐locus F‐box proteins (SLFs), which function as the pistil and pollen SI determinants, respectively. S‐RNase is cytotoxic to self‐pollen, whereas SLFs are thought to collaboratively recognize non‐self S‐RNases in cross‐pollen and detoxify them via the ubiquitination pathway. However, the actual mechanism of detoxification remains unknown. Here we isolate the components of a SCF^SLF (SCF = SKP1‐CUL1‐F‐box‐RBX1) from Petunia pollen. The SCF^SLF polyubiquitinates a subset of non‐self S‐RNases in vitro. The polyubiquitinated S‐RNases are degraded in the pollen extract, which is attenuated by a proteasome inhibitor. Our findings suggest that multiple SCF^SLF complexes in cross‐pollen polyubiquitinate non‐self S‐RNases, resulting in their degradation by the proteasome.