A novel model system to study the rapid diversification of R genes

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Plants use disease resistance(R) genes, most of which encode nucleotide-binding leucine-rich repeat(NB-LRR) protein, to recognize pathogens. Each R protein recognizes the specific effector protein. To counter the rapid diversification of pathogen effector genes, it thought that R genes also evolve rapidly. This idea is supported that high degree of polymorphism is observed in R-genes. However, little is known about the mechanisms underlying the R-gene diversification. We analyzed Arabidopsis uni-1D mutant, harbors a semi-dominant and gain-of-function allele of UNI gene, an R gene that has a NB-LRR-related structure. The uni-1D has a constitutively active R protein to induce resistance responses without any pathogen infection. Furthermore, uni-1D heterozygous mutant(hereafter uni1D/+) shows rapidly consume stem cells in the shoot apical meristem of the inflorescence stem, resulting in formation of very short stem. Interestingly, under normal growth condition, we infrequently but repeate … More dly observed that uni-1D/+produced chimeric sectors display the morphology of wild-type-like long inflorescence stem. When we checked nucleotide sequences in this chimeric stem, we always found additional mutations, which presumably disrupted the uni-1D protein function. This reversion event occurs less than 0. 5% of individuals among the population. When we tried with EMS, an alkylating agent, we succeeded to increase the reversion frequency about 30%. Furthermore, when we treated uni-1D/+with zeocin, which causes DNA double-strand breaks, or hydroxyurea(HU), which induces defects of DNA repair and replication by depletion by depleting deoxynucleotide triphosphate pools, the reversion frequency significantly increased. These suggest that the uni-1D systems can easily and efficiently detect various types of nucleotide alterations in the uni-1D gene. Currently, we are analyzing molecular mechanisms underlying the rapid diversification of R genes using this system and our preliminary results imply the involvement of DNA repair machinery in this phenomenon. Less