Induced instability of two Arabidopsis constitutive pathogen-response alleles.

Paramutation is an example of a non-Mendelian-directed allelic interaction that results in the epigenetic alteration of one allele. We describe a paramutation-like interaction between two alleles, bal and cpr1-1 (constitutive expressor of PR genes 1), which map to a complex R-like gene cluster on Arabidopsis chromosome 4. Both alleles cause dwarfing and constitutive defense responses, similar to another dwarf variant, ssi1 (suppressor of SA-insensitivity 1). Previous work has demonstrated that the bal and ssi1 phenotypes are caused by overexpression of an R-like gene from the cluster, which activates an salicylic acid-dependent defense pathway. Here, we show that the cpr1-1 variant does not alter gene expression from the R-like gene cluster. The bal and cpr1-1 alleles did not complement each other in F(1) hybrids, but F(2) populations that segregated bal and cpr1-1 alleles contained plants with normal morphology at a frequency of 20%. By using molecularly marked bal and cpr1-1 lines, we found that the majority of the normal phenotypes were correlated with inheritance of an altered cpr1-1 allele. Our observation that cpr1-1 is a metastable allele suggests that cpr1-1 is an epigenetic allele. The cpr1-1 allele is the third candidate epigenetic allele originating from this R-like gene cluster, making the region a possible hotspot of epigenetic variation.