Phenotypic instability of Arabidopsis alleles affecting a disease Resistance gene cluster

Background: Three mutations in Arabidopsis thaliana strain Columbia-cpr1, snc1, and bal-map to the RPP5 locus, which contains a cluster of disease Resistance 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 gamma-irradiation, induce high frequency phenotypic instability of the bal allele. In addition, a fraction of the bal and cpr1 alleles segregated from bal x cpr1 F1 hybrids also show signs of phenotypic instability. To gain more insight into the mechanism of phenotypic instability of the bal and cpr1 mutations, we systematically compared the behavior of these unusual alleles with that of the missense gain-of-function snc1 allele in response to DNA damage or passage through F1 hybrids. Results: We found that the cpr1 allele is similar to the bal allele in its unstable behavior after EMS mutagenesis. For both the bal and cpr1 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 cpr1 x bal F1 hybrids, cpr1 x snc1 F1 hybrids and bal x snc1 F1 hybrids exhibited dwarf morphology. While only dwarf F2 plants were produced from bal x snc1 F1 hybrids, about 10% wild-type F2 progeny were produced from cpr1 x snc1 F1 hybrids, as well as from cpr1 x bal hybrids. Segregation analysis suggested that the cpr1 allele in cpr1 x snc1 crosses was destabilized during the late F1 generation to early F2 generation. Conclusion: With exposure to EMS or different F1 hybrid contexts, phenotypic instability is induced for the bal and cpr1 alleles, but not for the snc1 allele. Our results suggest that the RPP5 locus can adopt different metastable genetic or epigenetic states, the stability of which is highly susceptible to mutagenesis and pairing of different alleles.