Toll and interleukin-1 receptor (TIR) domains were originally described from comparisons of proteins found in mammals and Drosophila. They are now known to occur in several organisms, with the most TIR proteins being found in Arabidopsis: our analysis of the sequenced Arabidopsis genome has revealed the presence of at least 135 proteins containing TIR domains. Several novel types of TIR-domain-containing proteins are found in Arabidopsis that are not found in other genomes. Here, we discuss the roles of TIR-domain-containing proteins in pathogen resistance and as candidate signaling modules.
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