Gene silencing and homology-dependent gene silencing in Arabidopsis: genetic modifiers and DNA methylation.

Transgenes inserted into the plant genome can become inactive (gene silencing) or result in silencing of homologous cellular genes [homology-dependent gene silencing (HDG silencing)]. In an earlier study we reported HDG silencing of chalcone synthase (CHS) in Arabidopsis. This study concerns genetic revertants of one of the CHS HDG-silencing transgenic homozygotes. Two monogenic recessive trans-acting mutations (hog1 and ddm1) that impair gene silencing and HDG silencing were identified. These mutations reduce genomic DNA methylation and affect the quantity and size of CHS mRNA. These results imply that DNA methylation is necessary for both gene silencing and HDG silencing. Two further monogenic, trans-acting, recessive mutations (sil1 and sil2) reduce gene silencing but not HDG silencing. The existence of this mutant class shows that gene silencing involves genes that are not necessary for HDG silencing. A further mutant (Catt) was isolated and has an attenuated HDG-silencing T-DNA.