mop2 Is Required for Multiple Silencing Processes Figure 1. The Mop2-1 mutation prevents b1 paramutation and relieves B’ silencing. (A) The EMS mutagenesis screen used to isolate Mop2-1. The stocks carry distinct alleles for two genes that are linked and flank b1 on chromosome 2S; glossy2 (gl2) and white tip (wt). The glossy and white tip phenotypes are only visible in young seedlings and thus are not apparent in the mature plants shown. B’* is used to indicate a B-I allele that was paramutated to B’ in wild type plants. B-I* is used to signify a B-I allele exposed to B’ in the presence of Mediator of paramutation2 (Mop2-1), which prevents paramutation resulting in the B-I phenotype. Mop2-1 is shown linked to GL2 based on subsequent analysis (Figure S1). (B) Frequencies of plants with different pigmentation levels [Lt (light), Md (medium), and Dk (dark)] in progeny segregating Mop2-1/Mop2-1 and Mop2-1/+. The photo shows that Mop2-1 B’/+ B’ pigmentation remains light (left plant), indistinguishable from B’ in wild type. In contrast, homozygous Mop2-1 B’ plants have increased pigmentation (right plant). For (C) and (D), to test the heritability of the increased B’ pigmentation that is observed in homozygous Mop2-1 B’ plants (D) and to generate larger numbers of progeny to examine the penetrance of Mop2-1/+ on preventing B-I* paramutation (C) dark Mop2-1 B’ plants were crossed with +/+ tester carrying B-I/B-P, (Figure S2). The resulting Mop2-1 B’/+ B-P (C) and Mop2-1 B’/+ B-I (D) progeny were scored for light (Lt), medium (Md) and dark (Dk) pigment. (E) Testcrosses to assay whether B-I* segregates unchanged from + B-I*/Mop2-1 B’ are diagramed in Figure S2. Plant pigmentation is shown from the +B-I*/+ B-P (parental class) and +B’/+B-P (recombinant class). doi:10.1371/journal.pgen.1000725.g001 PLoS Genetics | www.plosgenetics.org 4 November 2009 | Volume 5 | Issue 11 | e1000725