CHROMOMETHYLTRANSFERASE3/ KRYPTONITE maintain the sulfurea paramutation in Solanum lycopersicum

Paramutation involves the transfer of a repressive epigenetic mark from a silent allele to an active homologue and, consequently, non-Mendelian inheritance. In tomato the sulfurea ( sulf ) paramutation is associated with a high level of CHG hypermethylation in a region overlapping the transcription start site of the SlTAB2 gene that affects chlorophyll synthesis. The CCG sub-context hypermethylation is under-represented at this region relative to CTG or CAG implicating the CHROMOMETHYLTRANSFERASE3 (CMT3) in paramutation at this locus. Consistent with this interpretation, loss of CMT 3 function leads to loss of the sulf chlorosis, the associated CHG hypermethylation and paramutation. Loss of KRYPTONITE (KYP) histone methyl transferase function has a similar effect linked to reduced H3K9me2 at the promoter region of SlTAB2 and a shift in higher order chromatin structure at this locus. Mutation of the largest subunit of RNA polymerase V ( PolV ) in contrast does not affect sulf paramutation. These findings indicate the involvement of a CMT3/KYP dependent feedback rather than the PolV-dependent pathway leading to RNA directed DNA methylation (RdDM) in the maintenance of paramutation.