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. 2002 May 28;99(11):7792-6.
doi: 10.1073/pnas.112040999.

Induced instability of two Arabidopsis constitutive pathogen-response alleles

Trevor L Stokes  1 , Eric J Richards
Affiliations

Induced instability of two Arabidopsis constitutive pathogen-response alleles

Trevor L Stokes et al. Proc Natl Acad Sci U S A. .

Abstract

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.

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Figures

Figure 1
Figure 1
bal and cpr1–1 map to the same genetic position on chromosome 4.
Figure 2
Figure 2
Genetic interaction of bal and cpr1–1 in F1 hybrids. The morphology of a representative Columbia wild-type individual is compared with homozygous bal and cpr1–1 individuals. CPR1/cpr1–1 individuals have a wild-type phenotype, whereas BAL/bal plants exhibit mild dwarfing and leaf twisting. In contrast, cpr1–1 × bal F1 hybrids are dwarfed. The plants shown were grown in an 18 h light/6 h dark cycle under 70% humidity in an environmental growth chamber. All plants are 22 days old.
Figure 3
Figure 3
Northern blot shows that an R-like message is not overexpressed in the cpr1–1 background.
Figure 4
Figure 4
Recovery of phenotypically normal plants in the bal × cpr1–1 F2 generation.
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