| Summary: | <p>Abstract</p> <p>Background</p> <p>Three mutations in <it>Arabidopsis thaliana </it>strain Columbia – <it>cpr1</it>, <it>snc1</it>, and <it>bal </it>– map to the <it>RPP5 </it>locus, which contains a cluster of disease <it>Resistance </it>genes. The similar phenotypes, gene expression patterns, and genetic interactions observed in these mutants are related to constitutive activation of pathogen defense signaling. However, these mutant alleles respond differently to various conditions. Exposure to mutagens, such as ethyl methanesulfonate (EMS) and γ-irradiation, induce high frequency phenotypic instability of the <it>bal </it>allele. In addition, a fraction of the <it>bal </it>and <it>cpr1 </it>alleles segregated from <it>bal </it>× <it>cpr1 </it>F1 hybrids also show signs of phenotypic instability. To gain more insight into the mechanism of phenotypic instability of the <it>bal </it>and <it>cpr1 </it>mutations, we systematically compared the behavior of these unusual alleles with that of the missense gain-of-function <it>snc1 </it>allele in response to DNA damage or passage through F1 hybrids.</p> <p>Results</p> <p>We found that the <it>cpr1 </it>allele is similar to the <it>bal </it>allele in its unstable behavior after EMS mutagenesis. For both the <it>bal </it>and <it>cpr1 </it>mutants, destabilization of phenotypes was observed in more than 10% of EMS-treated plants in the M1 generation. In addition, exceptions to simple Mendelian inheritance were identified in the M2 generation. Like <it>cpr1 </it>× <it>bal </it>F1 hybrids, <it>cpr1 </it>× <it>snc1 </it>F1 hybrids and <it>bal </it>× <it>snc1 </it>F1 hybrids exhibited dwarf morphology. While only dwarf F2 plants were produced from <it>bal </it>× <it>snc1 </it>F1 hybrids, about 10% wild-type F2 progeny were produced from <it>cpr1 </it>× <it>snc1 </it>F1 hybrids, as well as from <it>cpr1 </it>× <it>bal </it>hybrids. Segregation analysis suggested that the <it>cpr1 </it>allele in <it>cpr1 </it>× <it>snc1 </it>crosses was destabilized during the late F1 generation to early F2 generation.</p> <p>Conclusion</p> <p>With exposure to EMS or different F1 hybrid contexts, phenotypic instability is induced for the <it>bal </it>and <it>cpr1 </it>alleles, but not for the <it>snc1 </it>allele. Our results suggest that the <it>RPP5 </it>locus can adopt different metastable genetic or epigenetic states, the stability of which is highly susceptible to mutagenesis and pairing of different alleles.</p>
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