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SUMMARY

Within the last 10 years, numerous R genes have been cloned from natural genetic variation in model as well as crop plants, and these have been classified according to their motifs. Some of the downstream signalling components have also been identified by artificial mutagenesis. Recently, cloning of three of these signalling genes (COI1, RAR1 and SGT1b) from Arabidopsis, barley and tobacco have helped uncover the physiological link between defence signalling and ubiquitin-mediated protein degradation. The physical association of COI1 and SGT1b with the components of ubiquitin–ligase complexes has been shown. In addition, post-transcriptional silencing of some of the subunits of the ubiquitin-ligase complex has led to a loss of resistance, indicating that protein degradation may also act as a regulatory mechanism in plant defence. Over the next few years, we should expect to see more examples of the interplay between the defence response and protein degradation in plants.